Quantitative chemical exchange saturation transfer (CEST) MRI of glioma using Image Downsampling Expedited Adaptive Least-squares (IDEAL) fitting

B1 inhomogeneity corrected CEST MRI based on direct saturation removed omega plot model at 5T

PURPOSE

CEST can image macromolecules/compounds via detecting chemical exchange between labile protons and bulk water. B1 field inhomogeneity impairs CEST quantification. Conventional B1 inhomogeneity correction methods depend on interpolation algorithms, B1 choices, acquisition number or calibration curves, making reliable correction challenging. This study proposed a novel B1 inhomogeneity correction method based on a direct saturation (DS) removed omega plot model.

METHODS

Four healthy volunteers underwent B1 field mapping and CEST imaging under four nominal B1 levels of 0.75, 1.0, 1.5, and 2.0 μT at 5T. DS was resolved using a multi-pool Lorentzian model and removed from respective Z spectrum. Residual spectral signals were used to construct the omega plot as a linear function of 1/B 1 2 $$ {B}_1^2 $$ , from which corrected signals at nominal B1 levels were calculated. Routine asymmetry analysis was conducted to quantify amide proton transfer (APT) effect. Its distribution across white matter was compared before and after B1 inhomogeneity correction and also with the conventional interpolation approach.

RESULTS

B1 inhomogeneity yielded conspicuous artifact on APT images. Such artifact was mitigated by the proposed method. Homogeneous APT maps were shown with SD consistently smaller than that before B1 inhomogeneity correction and the interpolation method. Moreover, B1 inhomogeneity correction from two and four CEST acquisitions yielded similar results, superior over the interpolation method that derived inconsistent APT contrasts among different B1 choices.

CONCLUSION

The proposed method enables reliable B1 inhomogeneity correction from at least two CEST acquisitions, providing an effective way to improve quantitative CEST MRI.

Nuclear Overhauser enhancement imaging at -1.6 ppm in rat brain at 4.7T

PURPOSE

A new nuclear Overhauser enhancement (NOE)-mediated saturation transfer signal at around -1.6 ppm, termed NOE(-1.6), has been reported at high fields of 7T and 9.4T previously. This study aims to validate the presence of this signal at a relatively low field of 4.7T and evaluate its variations in different brain regions and tumors.

METHODS

Rats were injected with monocrystalline iron oxide nanoparticles to reduce the NOE(-1.6) signal. CEST signals were measured using different saturation powers before and after injection to assess the presence of this signal. Multiple-pool Lorentzian fits, with/without inclusion of the NOE(-1.6) pool, were performed on CEST Z-spectra obtained from healthy rat brains and rats with 9L tumors. These fits aimed to further validate the presence of the NOE(-1.6) signal and quantify its amplitude.

RESULTS

The NOE(-1.6) signal exhibited a dramatic change following the injection of monocrystalline iron oxide nanoparticles, confirming its presence at 4.7T. The NOE(-1.6) signal reached its peak at a saturation power of ∼0.75 μT, indicating an optimized power level. The multiple-pool Lorentzian fit without the NOE(-1.6) pool showed higher residuals around -1.6 ppm compared to the fit with this pool, further supporting the presence of this signal. The NOE(-1.6) signal did not exhibit significant variation in the corpus callosum and caudate putamen regions, but it showed a significant decrease in tumors, which aligns with previous findings at 9.4T.

CONCLUSION

This study successfully demonstrated the presence of the NOE(-1.6) signal at 4.7T, which provides valuable insights into its potential applications at lower field strengths.

Fast and robust pulsed chemical exchange saturation transfer (CEST) MRI using a quasi-steady-state (QUASS) algorithm at 3 T

Chemical exchange saturation transfer (CEST) has emerged as a powerful technique to image dilute labile protons. However, its measurement depends on the RF saturation duration (Tsat) and relaxation delay (Trec). Although the recently developed quasi-steady-state (QUASS) solution can reconstruct equilibrium CEST effects under continuous-wave RF saturation, it does not apply to pulsed-CEST MRI on clinical scanners with restricted hardware or specific absorption rate limits. This study proposed a QUASS algorithm for pulsed-CEST MRI and evaluated its performance in muscle CEST measurement. An approximated expression of a steady-state pulsed-CEST signal was incorporated in the off-resonance spin-lock model, from which the QUASS pulsed-CEST effect was derived. Numerical simulation, creatine phantom, and healthy volunteer scans were conducted at 3 T. The CEST effect was quantified with asymmetry analysis in the simulation and phantom experiments. CEST effects of creatine, amide proton transfer, phosphocreatine, and combined magnetization transfer and nuclear Overhauser effects were isolated from a multi-pool Lorentzian model in muscles. Apparent and QUASS CEST measurements were compared under different Tsat/Trec and duty cycles. Paired Student's t-test was employed with P < 0.05 as statistically significant. The simulation, phantom, and human studies showed the strong impact of Tsat/Trec on apparent CEST measurements, which were significantly smaller than the corresponding QUASS CEST measures, especially under short Tsat/Trec times. In comparison, the QUASS algorithm mitigates such impact and enables accurate CEST measurements under short Tsat/Trec times. In conclusion, the QUASS algorithm can accelerate robust pulsed-CEST MRI, promising the efficient detection and evaluation of muscle diseases in clinical settings.

A Pilot Study of Ratiometric Creatine CEST MRI Assessment of Rabbit Skeletal Muscle Energy Metabolism at 3 T

BACKGROUND

pH MRI may provide useful information to evaluate metabolic disruption following ischemia. Radiofrequency amplitude-based creatine chemical exchange saturation transfer (CrCEST) ratiometric MRI is pH-sensitive, which could but has not been explored to examine muscle ischemia.

PURPOSE

To investigate skeletal muscle energy metabolism alterations with CrCEST ratiometric MRI.

STUDY TYPE

Prospective.

ANIMAL MODEL

Seven adult New Zealand rabbits with ipsilateral hindlimb muscle ischemia.

FIELD STRENGTH/SEQUENCE

3 T/two MRI scans, including MRA and CEST imaging, were performed under two B1 amplitudes of 0.5 and 1.25 μT after 2 hours of hindlimb muscle ischemia and 1 hour of reperfusion recovery, respectively.

ASSESSMENT

CEST effects of two energy metabolites of creatine and phosphocreatine (PCrCEST) were resolved with the multipool Lorentzian fitting approach. The pixel-wise CrCEST ratio was quantified by calculating the ratio of the resolved CrCEST peaks under a B1 amplitude of 1.25 μT to those under 0.5 μT in the entire muscle.

STATISTICAL TESTS

One-way ANOVA and Pearson's correlation. P < 0.05 was considered statistically significant.

RESULTS

MRA images confirmed the blood flow loss and restoration in the ischemic hindlimb at the ischemia and recovery phases, respectively. Ischemic muscles exhibited a significant decrease of PCr at the ischemia (under both B1 amplitudes) and recovery phases (under B1 amplitude of 0.5 μT) and significantly increased CrCEST from normal tissues at both phases (under both B1 levels). Specifically, CrCEST decreased, and PCrCEST increased with the CrCEST ratio. Significantly strong correlations were observed among the CrCEST ratio, and CrCEST and PCrCEST under both B1 levels (r > 0.80).

DATA CONCLUSION

The CrCEST ratio altered substantially with muscle pathological states and was closely related to CEST effects of energy metabolites of Cr and PCr, suggesting that the pH-sensitive CrCEST ratiometric MRI is feasible to evaluate muscle injuries at the metabolic level.

EVIDENCE LEVEL

2 TECHNICAL EFFICACY STAGE: 1.

Deep Learning-Based Denoising of CEST MR Data: A Feasibility Study on Applying Synthetic Phantoms in Medical Imaging

Chemical Exchange Saturation Transfer (CEST) magnetic resonance imaging (MRI) provides a novel method for analyzing biomolecule concentrations in tissues without exogenous contrast agents. Despite its potential, achieving a high signal-to-noise ratio (SNR) is imperative for detecting small CEST effects. Traditional metrics such as Magnetization Transfer Ratio Asymmetry (MTRasym) and Lorentzian analyses are vulnerable to image noise, hampering their precision in quantitative concentration estimations. Recent noise-reduction algorithms like principal component analysis (PCA), nonlocal mean filtering (NLM), and block matching combined with 3D filtering (BM3D) have shown promise, as there is a burgeoning interest in the utilization of neural networks (NNs), particularly autoencoders, for imaging denoising. This study uses the Bloch-McConnell equations, which allow for the synthetic generation of CEST images and explores NNs efficacy in denoising these images. Using synthetically generated phantoms, autoencoders were created, and their performance was compared with traditional denoising methods using various datasets. The results underscored the superior performance of NNs, notably the ResUNet architectures, in noise identification and abatement compared to analytical approaches across a wide noise gamut. This superiority was particularly pronounced at elevated noise intensities in the in vitro data. Notably, the neural architectures significantly improved the PSNR values, achieving up to 35.0, while some traditional methods struggled, especially in low-noise reduction scenarios. However, the application to the in vivo data presented challenges due to varying noise profiles. This study accentuates the potential of NNs as robust denoising tools, but their translation to clinical settings warrants further investigation.

Chemical exchange saturation transfer imaging of creatine, phosphocreatine, and protein arginine residue in tissues

Chemical exchange saturation transfer (CEST) MRI has become a promising technique to assay target proteins and metabolites through their exchangeable protons, noninvasively. The ubiquity of creatine (Cr) and phosphocreatine (PCr) due to their pivotal roles in energy homeostasis through the creatine phosphate pathway has made them prime targets for CEST in the diagnosis and monitoring of disease pathologies, particularly in tissues heavily dependent on the maintenance of rich energy reserves. Guanidinium CEST from protein arginine residues (i.e. arginine CEST) can also provide information about the protein profile in tissue. However, numerous obfuscating factors stand as obstacles to the specificity of arginine, Cr, and PCr imaging through CEST, such as semisolid magnetization transfer, fast chemical exchanges such as primary amines, and the effects of nuclear Overhauser enhancement from aromatic and amide protons. In this review, the specific exchange properties of protein arginine residues, Cr, and PCr, along with their validation, are discussed, including the considerations necessary to target and tune their signal effects through CEST imaging. Additionally, strategies that have been employed to enhance the specificity of these exchanges in CEST imaging are described, along with how they have opened up possible applications of protein arginine residues, Cr and PCr CEST imaging in the study and diagnosis of pathology. A clear understanding of the capabilities and caveats of using CEST to image these vital metabolites and mitigation strategies is crucial to expanding the possibilities of this promising technology.

Generalization of quasi-steady-state reconstruction to CEST MRI with two-tiered RF saturation and gradient-echo readout-Synergistic nuclear Overhauser enhancement contribution to brain tumor amide proton transfer-weighted MRI

PURPOSE

While amide proton transfer-weighted (APTw) MRI has been adopted in tumor imaging, there are concurrent APT, magnetization transfer, and nuclear Overhauser enhancement changes. Also, the APTw image is confounded by relaxation changes, particularly when the relaxation delay and saturation time are not sufficiently long. Our study aimed to extend a quasi-steady-state (QUASS) solution to determine the contribution of the multipool CEST signals to the observed tumor APTw contrast.

METHODS

Our study derived the QUASS solution for a multislice CEST-MRI sequence with an interleaved RF saturation and gradient-echo readout between signal averaging. Multiparametric MRI scans were obtained in rat brain tumor models, including T₁ , T₂ , diffusion, and CEST scans. Finally, we performed spinlock model-based multipool fitting to determine multiple concurrent CEST signal changes in the tumor.

RESULTS

The QUASS APTw MRI showed small but significant differences in normal and tumor tissues and their contrast from the acquired asymmetry calculation. The spinlock model-based fitting showed significant differences in semisolid magnetization transfer, amide, and nuclear Overhauser enhancement effects between the apparent and QUASS CEST MRI. In addition, we determined that the tumor APTw contrast is due to synergistic APT increase (+3.5 ppm) and NOE decrease (-3.5 ppm), with their relative contribution being about one third and two thirds under a moderate B₁ of 0.75 μT at 4.7 T.

CONCLUSION

Our study generalized QUASS analysis to gradient-echo image readout and quantified the underlying tumor CEST signal changes, providing an improved elucidation of the commonly used APTw MRI.

Dual-Peak Lorentzian CEST MRI for antiretroviral drug brain distribution

Objectives

Spatial-temporal biodistribution of antiretroviral drugs (ARVs) can now be achieved using MRI by utilizing chemical exchange saturation transfer (CEST) contrasts. However, the presence of biomolecules in tissue limits the specificity of current CEST methods. To overcome this limitation, a Lorentzian line-shape fitting algorithm was developed that simultaneously fits CEST peaks of ARV protons on its Z-spectrum.

Case presentation

This algorithm was tested on the common first line ARV, lamivudine (3TC), that has two peaks resulting from amino (-NH2) and hydroxyl (-OH) protons in 3TC. The developed dual-peak Lorentzian function fitted these two peaks simultaneously, and used the ratio of -NH2 and -OH CEST contrasts as a constraint parameter to measure 3TC presence in brains of drug-treated mice. 3TC biodistribution calculated using the new algorithm was compared against actual drug levels measured using UPLC-MS/MS. In comparison to the method that employs the -NH2 CEST peak only, the dual-peak Lorentzian fitting algorithm showed stronger correlation with brain tissue 3TC levels, signifying estimation of actual drug levels.

Conclusions

We concluded that 3TC levels can be extracted from confounding CEST effects of tissue biomolecules resulting in improved specificity for drug mapping. This algorithm can be expanded to measure a variety of ARVs using CEST MRI.

calf - Software for CEST Analysis with Lorentzian Fitting

Analysis of chemical exchange saturation transfer (CEST) MRI data requires sophisticated methods to obtain reliable results about metabolites in the tissue under study. CEST generates z-spectra with multiple components, each originating from individual molecular groups. The individual lines with Lorentzian line shape are mostly overlapping and disturbed by various effects. We present an elaborate method based on an adaptive nonlinear least squares algorithm that provides robust quantification of z-spectra and incorporates prior knowledge in the fitting process. To disseminate CEST to the research community, we developed software as part of this study that runs on the Microsoft Windows operating system and will be made freely available to the community. Special attention has been paid to establish a low entrance threshold and high usability, so that even less experienced users can successfully analyze CEST data.

Chemical exchange saturation transfer (CEST) magnetic resonance imaging (MRI) quantification of transient ischemia using a combination method of 5-pool Lorentzian fitting and inverse Z-spectrum analysis

Background

Chemical exchange saturation transfer (CEST) is a promising method for the detection of biochemical alterations in cancers and neurological diseases. However, the sensitivity of the currently existing quantitative method for detecting ischemia needs further improvement.

Methods

To further improve the quantification of the CEST signal and enhance the CEST detection for ischemia, we used a quantitative analysis method that combines an inverse Z-spectrum analysis and a 5-pool Lorentzian fitting. Specifically, a 5-pool Lorentzian simulation was conducted with the following brain tissue parameters: water, amide (3.5 ppm), amine (2.2 ppm), magnetization transfer (MT), and nuclear Overhauser enhancement (NOE; -3.5 ppm). The parameters were first calculated offline and stored as the initial value of the Z-spectrum fitting. Then, the measured Z-spectrum with the peak value set to 0 was fitted via the stored initial value, which yielded the reference Z-spectrum. Finally, the difference between the inverse of the Z-spectrum and the inverse of the reference Z-spectrum was used as the CEST definite spectrum.

Results

The simulation results demonstrated that the Z-spectra of the rat brain were well simulated by a 5-pool Lorentzian fitting. Further, the proposed method detected a larger difference than did either the saturation transfer difference or the 5-pool Lorentzian fitting, as demonstrated by simulations. According to the results of the cerebral ischemia rat model, the proposed method provided the highest contrast-to-noise ratio (CNR) between the contralateral and the ipsilateral striatum under various acquisition conditions. The results indicated that the difference of fitted amplitudes generated with a 5-pool Lorentzian fitting in amide at 3.5 ppm (6.04%±0.39%; 6.86%±0.39%) was decreased in a stroke lesion compared to the contralateral normal tissue. Moreover, the difference of the residual of inversed Z-spectra in which 5-pool Lorentzian fitting was used to calculate the reference Z-spectra ( M T R R e x 5 L ) amplitudes in amide at 3.5 ppm (13.83%±2.20%, 15.69%±1.99%) was reduced in a stroke lesion compared to the contralateral normal tissue.

Conclusions

M T R R e x 5 L is predominantly pH-sensitive and is suitable for detecting tissue acidosis following an acute stroke.

Reassembled saturation transfer (REST) MR images at 2 B1 values for in vivo exchange-dependent imaging of amide and nuclear Overhauser enhancement

PURPOSE

Design an efficient CEST scheme for exchange-dependent images with high contrast-to-noise ratio.

THEORY

Reassembled saturation transfer (REST) signals were defined as Δ $$ \Delta $$ r.Z = r.Z_ref - r.Z_CEST and the reassembled exchange-dependen magnetization transfer ratio r.MTR_Rex  = r.1/Z_ref - r.1/Z_CEST , utilizing the averages over loosely sampled reference frequency offsets as Z_ref and over densely sampled target offsets as Z_CEST . Using r.MTR_Rex measured under 2 B_1,sat values, exchange rate could be estimated.

METHODS

The REST approach was optimized and assessed quantitatively by simulations for various exchange rates, pool concentration, and water T₁ . In vivo evaluation was performed on ischemic rat brains at 7 Tesla and human brains at 3 Tesla, in comparison with conventional asymmetrical analysis, Lorentzian difference (LD), an MTR_{Rex_} LD.

RESULTS

For a broad choice of Δ ω ref $$ \Delta {\omega}_{ref} $$ ranges and numbers, Δr.Z and r.MTR_Rex exhibited comparable quantification features with conventional LD and MTR_Rex _LD, respectively, when B_1,sat  ≤ 1 μT. The subtraction of 2 REST values under distinct B_1,sat values showed linear relationships with exchange rate and obtained immunity to field inhomogeneity and variation in MT and water T₁ . For both rat and human studies, REST images exhibited similar contrast distribution to MTR_Rex _LD, with superiority in contrast-to-noise ratio and acquisition efficiency. Compared with MTR_Rex _LD, 2-B_1,sat subtraction REST images displayed better resistance to B₁ inhomogeneity, with more specific enhanced regions. They also showed higher signals for amide than for nuclear Overhauser enhancement effect in human brain, presumably reflecting the higher increment from faster-exchanging species as B_1,sat increased.

CONCLUSION

Featuring high contrast-to-noise ratio efficiency, REST could be a practical exchange-dependent approach readily applicable to either retrospective Z-spectra analysis or perspective 6-offset acquisition.

Guanidinium and amide CEST mapping of human brain by high spectral resolution CEST at 3 T

PURPOSE

To extract guanidinium (Guan) and amide CEST on the human brain at 3 T MRI with the high spectral resolution (HSR) CEST combined with the polynomial Lorentzian line-shape fitting (PLOF).

METHODS

Continuous wave (cw) turbo spin-echo (TSE) CEST was implemented to obtain the optimum saturation parameters. Both Guan and amide CEST peaks were extracted and quantified using the PLOF method. The NMR spectra on the egg white phantoms were acquired to reveal the fitting range and the contributions to the amide and GuanCEST. Two types of CEST approaches, including cw gradient- and spin-echo (cwGRASE) and steady state EPI (ssEPI), were implemented to acquire multi-slice HSR-CEST.

RESULTS

GuanCEST can be extracted with the PLOF method at 3 T, and the optimumB 1 = 0.6 μ T $$ {\mathrm{B}}_1=0.6\kern0.2em \upmu \mathrm{T} $$ was determined for GuanCEST in white matter (WM) and 1.0 μT in gray matter (GM). The optimum B1  = 0.8-1 μT was found for amideCEST. AmideCEST is lower in both WM and GM collected with ssEPI compared to those by cwGRASE (ssEPI = [1.27-1.63]%; cwGRASE = [2.19-2.25]%). The coefficients of variation (COV) of the amide and Guan CEST in both WM and GM for ssEPI (COV: 28.6-33.4%) are significantly higher than those of cwGRASE (COV: 8.6-18.8%). Completely different WM/GM contrasts for Guan and amide CEST were observed between ssEPI and cwGRASE. The amideCEST was found to have originated from the unstructured amide protons as suggested by the NMR spectrum of the unfolded proteins in egg white.

CONCLUSION

Guan and amide CEST mapping can be achieved by the HSR-CEST at 3 T combing with the PLOF method.

In vivo pH mapping with omega plot-based quantitative chemical exchange saturation transfer MRI

PURPOSE

Chemical exchange saturation transfer (CEST) MRI is promising for detecting dilute metabolites and microenvironment properties, which has been increasingly adopted in imaging disorders such as acute stroke and cancer. However, in vivo CEST MRI quantification remains challenging because routine asymmetry analysis (MTRasym ) or Lorentzian decoupling measures a combined effect of the labile proton concentration and its exchange rate. Therefore, our study aimed to quantify amide proton concentration and exchange rate independently in a cardiac arrest-induced global ischemia rat model.

METHODS

The amide proton CEST (APT) effect was decoupled from tissue water, macromolecular magnetization transfer, nuclear Overhauser enhancement, guanidinium, and amine protons using the image downsampling expedited adaptive least-squares (IDEAL) fitting algorithm on Z-spectra obtained under multiple RF saturation power levels, before and after global ischemia. Omega plot analysis was applied to determine amide proton concentration and exchange rate simultaneously.

RESULTS

Global ischemia induces a significant APT signal drop from intact tissue. Using the modified omega plot analysis, we found that the amide proton exchange rate decreased from 29.6 ± 5.6 to 12.1 ± 1.3 s-1 (P < 0.001), whereas the amide proton concentration showed little change (0.241 ± 0.035% vs. 0.202 ± 0.034%, P = 0.074) following global ischemia.

CONCLUSION

Our study determined the labile proton concentration and exchange rate underlying the in vivo APT MRI. The significant change in the exchange rate, but not the concentration of amide proton demonstrated that the pH effect dominates the APT contrast during tissue ischemia.

Average saturation efficiency filter (ASEF) for CEST imaging

PURPOSE

Endogenous CEST signal usually has low specificity due to contamination from the magnetization transfer effect and from fast exchanging labile protons with close Larmor frequencies. We propose to improve CEST signal specificity with an average saturation efficiency filter (ASEF).

METHODS

ASEF measures the difference between CEST signals acquired with similar average irradiation power but largely different duty cycles (DC), for example, a continuous wave or a high DC pulse train versus a low DC one. Simulation and Cr phantom studies were performed to evaluate the characteristics of ASEF for CEST.

RESULTS

Theoretical and simulation studies show that ASEF can suppress fast exchanging processes, with only a small loss of chemical exchange contrast for slow-to-intermediate exchange rates if the difference in DC is large. In the RF offset range of 2 to 5 ppm with an averaged saturation power of 0.8 and 1.6 microteslas, there is a mismatch of ∼0.1% to 2% in the magnetization transfer signal between saturation by continuous wave and a pulse train with DC = 15% and pulse duration of 24 ms, respectively. This mismatch can be minimized by careful selection of saturation power, pulse duration, and DC differences or by applying a small fudge factor between the 2 irradiation powers. Phantom studies of Cr confirmed that ASEF can minimize the magnetization transfer effect and reduce sensitivity to fast exchange processes.

CONCLUSION

ASEF can improve the specificity of slow-to-intermediate exchanging CEST signal with a relatively small loss of sensitivity.

Quantitative chemical exchange saturation transfer imaging of nuclear overhauser effects in acute ischemic stroke

PURPOSE

In chemical exchange saturation transfer imaging, saturation effects between - 2 to - 5 ppm (nuclear Overhauser effects, NOEs) have been shown to exhibit contrast in preclinical stroke models. Our previous work on NOEs in human stroke used an analysis model that combined NOEs and semisolid MT; however their combination might feasibly have reduced sensitivity to changes in NOEs. The aim of this study was to explore the information a 4-pool Bloch-McConnell model provides about the NOE contribution in ischemic stroke, contrasting that with an intentionally approximate 3-pool model.

METHODS

MRI data from 12 patients presenting with ischemic stroke were retrospectively analyzed, as well as from six animals induced with an ischemic lesion. Two Bloch-McConnell models (4 pools, and a 3-pool approximation) were compared for their ability to distinguish pathological tissue in acute stroke. The association of NOEs with pH was also explored, using pH phantoms that mimic the intracellular environment of naïve mouse brain.

RESULTS

The 4-pool measure of NOEs exhibited a different association with tissue outcome compared to 3-pool approximation in the ischemic core and in tissue that underwent delayed infarction. In the ischemic core, the 4-pool measure was elevated in patient white matter ( 1 . 20 ± 0 . 20 ) and in animals ( 1 . 27 ± 0 . 20 ). In the naïve brain pH phantoms, significant positive correlation between the NOE and pH was observed.

CONCLUSION

Associations of NOEs with tissue pathology were found using the 4-pool metric that were not observed using the 3-pool approximation. The 4-pool model more adequately captured in vivo changes in NOEs and revealed trends depending on tissue pathology in stroke.

Quantitative imaging of apoptosis following oncolytic virotherapy by magnetic resonance fingerprinting aided by deep learning

Non-invasive imaging methods for detecting intratumoural viral spread and host responses to oncolytic virotherapy are either slow, lack specificity or require the use of radioactive or metal-based contrast agents. Here we show that in mice with glioblastoma multiforme, the early apoptotic responses to oncolytic virotherapy (characterized by decreased cytosolic pH and reduced protein synthesis) can be rapidly detected via chemical-exchange-saturation-transfer magnetic resonance fingerprinting (CEST-MRF) aided by deep learning. By leveraging a deep neural network trained with simulated magnetic resonance fingerprints, CEST-MRF can generate quantitative maps of intratumoural pH and of protein and lipid concentrations by selectively labelling the exchangeable amide protons of endogenous proteins and the exchangeable macromolecule protons of lipids, without requiring exogenous contrast agents. We also show that in a healthy volunteer, CEST-MRF yielded molecular parameters that are in good agreement with values from the literature. Deep-learning-aided CEST-MRF may also be amenable to the characterization of host responses to other cancer therapies and to the detection of cardiac and neurological pathologies.

Ultra-High-Field MRI in the Diagnosis and Management of Gliomas: A Systematic Review

Importance:

Gliomas, tumors of the central nervous system, are classically diagnosed through invasive surgical biopsy and subsequent histopathological study. Innovations in ultra-high field (UHF) imaging, namely 7-Tesla magnetic resonance imaging (7T MRI) are advancing preoperative tumor grading, visualization of intratumoral structures, and appreciation of small brain structures and lesions.

Objective

Summarize current innovative uses of UHF imaging techniques in glioma diagnostics and treatment.

Methods

A systematic review in accordance with PRISMA guidelines was performed utilizing PubMed. Case reports and series, observational clinical trials, and randomized clinical trials written in English were included. After removing unrelated studies and those with non-human subjects, only those related to 7T MRI were independently reviewed and summarized for data extraction. Some preclinical animal models are briefly described to demonstrate future usages of ultra-high-field imaging.

Results

We reviewed 46 studies (43 human and 3 animal models) which reported clinical usages of UHF MRI in the diagnosis and management of gliomas. Current literature generally supports greater resolution imaging from 7T compared to 1.5T or 3T MRI, improving visualization of cerebral microbleeds and white and gray matter, and providing more precise localization for radiotherapy targeting. Additionally, studies found that diffusion or susceptibility-weighted imaging techniques applied to 7T MRI, may be used to predict tumor grade, reveal intratumoral structures such as neovasculature and microstructures like axons, and indicate isocitrate dehydrogenase 1 mutation status in preoperative imaging. Similarly, newer imaging techniques such as magnetic resonance spectroscopy and chemical exchange saturation transfer imaging can be performed on 7T MRI to predict tumor grading and treatment efficacy. Geometrical distortion, a known challenge of 7T MRI, was at a tolerable level in all included studies.

Conclusion

UHF imaging has the potential to preoperatively and non-invasively grade gliomas, provide precise therapy target areas, and visualize lesions not seen on conventional MRI.

Demonstration of fast and equilibrium human muscle creatine CEST imaging at 3 T

PURPOSE

Creatine chemical exchange saturation transfer (CrCEST) MRI is used increasingly in muscle imaging. However, the CrCEST measurement depends on the RF saturation duration (Ts) and relaxation delay (Td), and it is challenging to compare the results of different scan parameters. Therefore, this study aims to evaluate the quasi-steady-state (QUASS) CrCEST MRI on clinical 3T scanners.

METHODS

T₁ and CEST MRI scans of Ts/Td of 1 s/1 s and 2 s/2 s were obtained from a multi-compartment creatine phantom and 5 healthy volunteers. The CrCEST effect was quantified with asymmetry analysis in the phantom, whereas 5-pool Lorentzian fitting was applied to isolate creatine from phosphocreatine, amide proton transfer, combined magnetization transfer and nuclear Overhauser enhancement effects, and direct water saturation in four major muscle groups of the lower leg. The routine and QUASS CrCEST measurements were compared under two different imaging conditions. Paired Student's t-test was performed with p-values less than 0.05 considered statistically significant.

RESULTS

The phantom study showed a substantial influence of Ts/Td on the routine CrCEST quantification (p = 0.02), and such impact was mitigated with the QUASS algorithm (p = 0.20). The volunteer experiment showed that the routine CrCEST, amide proton transfer, and combined magnetization transfer and nuclear Overhauser enhancement effects increased significantly with Ts and Td (p < 0.05) and were significantly smaller than the corresponding QUASS indices (p < 0.01). In comparison, the QUASS CrCEST MRI showed little dependence on Ts and Td, indicating its robustness and accuracy.

CONCLUSION

The QUASS CrCEST MRI is feasible to provide fast and accurate muscle creatine imaging.

Molecular Imaging of Brain Tumors and Drug Delivery Using CEST MRI: Promises and Challenges

Chemical exchange saturation transfer (CEST) magnetic resonance imaging (MRI) detects molecules in their natural forms in a sensitive and non-invasive manner. This makes it a robust approach to assess brain tumors and related molecular alterations using endogenous molecules, such as proteins/peptides, and drugs approved for clinical use. In this review, we will discuss the promises of CEST MRI in the identification of tumors, tumor grading, detecting molecular alterations related to isocitrate dehydrogenase (IDH) and O-6-methylguanine-DNA methyltransferase (MGMT), assessment of treatment effects, and using multiple contrasts of CEST to develop theranostic approaches for cancer treatments. Promising applications include (i) using the CEST contrast of amide protons of proteins/peptides to detect brain tumors, such as glioblastoma multiforme (GBM) and low-grade gliomas; (ii) using multiple CEST contrasts for tumor stratification, and (iii) evaluation of the efficacy of drug delivery without the need of metallic or radioactive labels. These promising applications have raised enthusiasm, however, the use of CEST MRI is not trivial. CEST contrast depends on the pulse sequences, saturation parameters, methods used to analyze the CEST spectrum (i.e., Z-spectrum), and, importantly, how to interpret changes in CEST contrast and related molecular alterations in the brain. Emerging pulse sequence designs and data analysis approaches, including those assisted with deep learning, have enhanced the capability of CEST MRI in detecting molecules in brain tumors. CEST has become a specific marker for tumor grading and has the potential for prognosis and theranostics in brain tumors. With increasing understanding of the technical aspects and associated molecular alterations detected by CEST MRI, this young field is expected to have wide clinical applications in the near future.

Deep learning-based classification of preclinical breast cancer tumor models using chemical exchange saturation transfer magnetic resonance imaging

Chemical exchange saturation transfer (CEST) magnetic resonance imaging has shown promise for classifying tumors based on their aggressiveness, but CEST contrast is complicated by multiple signal sources and thus prolonged acquisition times are often required to extract the signal of interest. We investigated whether deep learning could help identify pertinent Z-spectral features for distinguishing tumor aggressiveness as well as the possibility of acquiring only the pertinent spectral regions for more efficient CEST acquisition. Human breast cancer cells, MDA-MB-231 and MCF-7, were used to establish bi-lateral tumor xenografts in mice to represent higher and lower aggressive tumors, respectively. A convolutional neural network (CNN)-based classification model, trained on simulated data, utilized Z-spectral features as input to predict labels of different tissue types, including MDA-MB-231, MCF-7, and muscle tissue. Saliency maps reported the influence of Z-spectral regions on classifying tissue types. The model was robust to noise with an accuracy of more than 91.5% for low and moderate noise levels in simulated testing data (SD of noise less than 2.0%). For in vivo CEST data acquired with a saturation pulse amplitude of 2.0 μT, the model had a superior ability to delineate tissue types compared with Lorentzian difference (LD) and magnetization transfer ratio asymmetry (MTR_asym ) analysis, classifying tissues to the correct types with a mean accuracy of 85.7%, sensitivity of 81.1%, and specificity of 94.0%. The model's performance did not improve substantially when using data acquired at multiple saturation pulse amplitudes or when adding LD or MTR_asym spectral features, and did not change when using saliency map-based partial or downsampled Z-spectra. This study demonstrates the potential of CNN-based classification to distinguish between different tumor types and muscle tissue, and speed up CEST acquisition protocols.

Contribution of blood to nuclear Overhauser effect at -1.6 ppm

PURPOSE

A relayed nuclear Overhauser enhancement (rNOE) saturation transfer effect at around -1.6 ppm from water, termed NOE(-1.6), was previously reported in rat and human brain, and some publications suggest that it may be related to blood. Here, we studied whether the NOE(-1.6) arises from blood through in vivo and ex vivo experiments.

METHODS

To evaluate the contribution from in vivo blood to NOE(-1.6), intravascular signals in rat brain were suppressed by two approaches: (1) signal acquisition with a diffusion-weighting of b = 400 s/mm2 ; (2) intravascular injection of 5 mg/kg monocrystalline iron oxide nanoparticle (MION). Ex vivo blood sample was also prepared. The signals were acquired using a chemical exchange saturation transfer (CEST) pulse sequence. Multiple-pool Lorentzian fitting of CEST Z-spectra was performed to quantify the NOE(-1.6) signal.

RESULTS

There are no significant variations in the fitted in vivo NOE(-1.6) signals when measured with or without diffusion-weighting, but significant signal decease does occur after injection of MION. The NOE(-1.6) signal from ex vivo blood is weaker than that from in vivo tissues.

CONCLUSION

Considering the relatively small volume of blood in brain, the in vivo experiments with diffusion weighting and the ex vivo experiments both suggest that the NOE(-1.6) is not mainly from blood. The mechanism for the in vivo experiments with MION are less clear. MION not only suppresses MR signals from intravascular space, but changes the susceptibility in the perivascular space. This result suggests that although the NOE(-1.6) is not mainly from blood, it may be vasculature dependent.

Whole-brain amide CEST imaging at 3T with a steady-state radial MRI acquisition

PURPOSE

To develop a steady-state saturation with radial readout chemical exchange saturation transfer (starCEST) for acquiring CEST images at 3 Tesla (T). The polynomial Lorentzian line-shape fitting approach was further developed for extracting amideCEST intensities at this field.

METHOD

StarCEST MRI using periodically rotated overlapping parallel lines with enhanced reconstruction-based spatial sampling was implemented to acquire Z-spectra that are robust to brain motion. Multi-linear singular value decomposition postprocessing was applied to enhance the CEST SNR. The egg white phantom studies were performed at 3T to reveal the contributions to the 3.5 ppm CEST signal. Based on the phantom validation, the amideCEST peak was quantified using the polynomial Lorentzian line-shape fitting, which exploits the inverse relationship between Z-spectral intensity and the longitudinal relaxation rate in the rotating frame. The 3D turbo spin echo CEST was also performed to compare with the starCEST method.

RESULTS

The amideCEST peak showed a negligible peak B₁ dependence between 1.2 µT and 2.4 µT. The amideCEST images acquired with starCEST showed much improved image quality, SNR, and motion robustness compared to the conventional 3D turbo spin echo CEST method with the same scan time. The amideCEST contrast extracted by the polynomial Lorentzian line-shape fitting method trended toward a stronger gray matter signal (1.32% ± 0.30%) than white matter (0.92% ± 0.08%; P = .02, n = 5). When calculating the magnetization transfer contrast and T₁ -corrected rotating frame relaxation rate maps, amideCEST again was not significantly different for white matter and gray matter.

CONCLUSION

Rapid multi-slice amideCEST mapping can be achieved by the starCEST method (< 5 min) at 3T by combing with the polynomial Lorentzian line-shape fitting method.

Analysis Protocol for the Quantification of Renal pH Using Chemical Exchange Saturation Transfer (CEST) MRI

The kidney plays a major role in maintaining body pH homeostasis. Renal pH, in particular, changes immediately following injuries such as intoxication and ischemia, making pH an early biomarker for kidney injury before the symptom onset and complementary to well-established laboratory tests. Because of this, it is imperative to develop minimally invasive renal pH imaging exams and test pH as a new diagnostic biomarker in animal models of kidney injury before clinical translation. Briefly, iodinated contrast agents approved by the US Food and Drug Administration (FDA) for computed tomography (CT) have demonstrated promise as novel chemical exchange saturation transfer (CEST) MRI agents for pH-sensitive imaging. The generalized ratiometric iopamidol CEST MRI analysis enables concentration-independent pH measurement, which simplifies in vivo renal pH mapping. This chapter describes quantitative CEST MRI analysis for preclinical renal pH mapping, and their application in rodents, including normal conditions and acute kidney injury.This publication is based upon work from the COST Action PARENCHIMA, a community-driven network funded by the European Cooperation in Science and Technology (COST) program of the European Union, which aims to improve the reproducibility and standardization of renal MRI biomarkers. This analysis protocol chapter is complemented by two separate chapters describing the basic concepts and experimental procedure.

Preliminary demonstration of in vivo quasi-steady-state CEST postprocessing-Correction of saturation time and relaxation delay for robust quantification of tumor MT and APT effects

PURPOSE

Chemical exchange saturation transfer (CEST) MRI is versatile for measuring the dilute labile protons and microenvironment properties. However, the use of insufficiently long RF saturation duration (Ts) and relaxation delay (Td) may underestimate the CEST measurement. This study proposed a quasi-steady-state (QUASS) CEST analysis for robust CEST quantification.

METHODS

The CEST signal evolution was modeled as a function of the longitudinal relaxation rate during Td and spin-lock relaxation rate during Ts, from which the QUASS-CEST effect is derived. Numerical simulation and in vivo rat glioma MRI experiments were conducted at 11.7 T to compare the apparent and QUASS-CEST results obtained under different Ts/Td of 2 seconds/2 seconds and 4 seconds/4 seconds. Magnetization transfer and amide proton transfer effects were resolved using a multipool Lorentzian fitting and evaluated in contralateral normal tissue and tumor regions.

RESULTS

The simulation showed the dependence of the apparent CEST effect on Ts and Td, and such reliance was mitigated with the QUASS algorithm. Animal experiment results showed that the apparent magnetization transfer and amide proton transfer effects and their contrast between contralateral normal tissue and tumor regions increased substantially with Ts and Td. In comparison, the QUASS magnetization transfer and amide proton transfer effects and their difference between contralateral normal tissue and tumor exhibited little dependence on Ts and Td. In addition, the apparent magnetization transfer and amide proton transfer were significantly smaller than the corresponding QUASS indices (P < .05).

CONCLUSION

The QUASS-CEST algorithm enables robust CEST quantification and offers a straightforward approach to standardize CEST experiments.

Quasi-steady-state CEST (QUASS CEST) solution improves the accuracy of CEST quantification: QUASS CEST MRI-based omega plot analysis

PURPOSE

CEST MRI omega plot quantifies the labile proton fraction ratio (f_r ) and exchange rate (k_sw ), yet it assumes long RF saturation time (Ts) and relaxation delay (Td). Our study aimed to test if a quasi-steady-state (QUASS) CEST analysis that accounts for the effect of finite Ts and Td could improve the accuracy of CEST MRI quantification.

METHODS

We modeled the MRI signal evolution using a typical CEST EPI sequence. The signal relaxes toward its thermal equilibrium following the bulk water relaxation rate during Td, and then toward its CEST steady state following the spin-lock relaxation rate during Ts from which the QUASS CEST effect is derived. Both f_r and k_sw were solved from simulated conventional apparent CEST and QUASS CEST MRI. We also performed MRI experiments from a Cr-gel phantom under serially varied Ts and Td times from 1.5 to 7.5 s.

RESULTS

Simulation showed that, although k_sw could be slightly overestimated (3%-15%) for the range of Ts and Td, f_r could be substantially underestimated by as much as 67%. In contrast, the QUASS solution provided accurate k_sw and f_r determination within 2%. The CEST MRI experiments confirmed that the QUASS solution enabled robust quantification of k_sw and f_r , superior over the omega plot analysis based on the conventional apparent CEST MRI measurements.

CONCLUSIONS

The QUASS CEST MRI algorithm corrects the effect of finite Ts and Td times on CEST measurements, thereby allowing robust and accurate CEST quantification.

Evaluation of a similarity anisotropic diffusion denoising approach for improving in vivo CEST-MRI tumor pH imaging

PURPOSE

Chemical exchange saturation transfer MRI provides new approaches for investigating tumor microenvironment, including tumor acidosis that plays a key role in tumor progression and resistance to therapy. Following iopamidol injection, the detection of the contrast agent inside the tumor tissue allows measurements of tumor extracellular pH. However, accurate tumor pH quantifications are hampered by the low contrast efficiency of the CEST technique and by the low SNR of the acquired CEST images, hence in a reduced detectability of the injected agent. This work aims to investigate a novel denoising method for improving both tumor pH quantification and accuracy of CEST-MRI pH imaging.

METHODS

An hybrid denoising approach was investigated for CEST-MRI pH imaging based on the combination of the nonlocal mean filter and the anisotropic diffusion tensor method. The denoising approach was tested in simulated and in vitro data and compared with previously reported methods for CEST imaging and with established denoising approaches. Finally, it was validated with in vivo data to improve the accuracy of tumor pH maps.

RESULTS

The proposed method outperforms current denoising methods in CEST contrast quantification and detection of the administered contrast agent at several increasing noise levels with simulated data. In addition, it achieved a better pH quantification in in vitro data and demonstrated a marked improvement in contrast detection and a substantial improvement in tumor pH accuracy in in vivo data.

CONCLUSION

The proposed approach effectively reduces the noise in CEST images and increases the sensitivity detection in CEST-MRI pH imaging.

Redesigned reporter gene for improved proton exchange-based molecular MRI contrast

Reporter gene imaging allows for non-invasive monitoring of molecular processes in living cells, providing insights on the mechanisms underlying pathology and therapy. A lysine-rich protein (LRP) chemical exchange saturation transfer (CEST) MRI reporter gene has previously been developed and used to image tumor cells, cardiac viral gene transfer, and oncolytic virotherapy. However, the highly repetitive nature of the LRP reporter gene sequence leads to DNA recombination events and the expression of a range of truncated LRP protein fragments, thereby greatly limiting the CEST sensitivity. Here we report the use of a redesigned LRP reporter (rdLRP), aimed to provide excellent stability and CEST sensitivity. The rdLRP contains no DNA repeats or GC rich regions and 30% less positively charged amino-acids. RT-PCR of cell lysates transfected with rdLRP demonstrated a stable reporter gene with a single distinct band corresponding to full-length DNA. A distinct increase in CEST-MRI contrast was obtained in cell lysates of rdLRP transfected cells and in in vivo LRP expressing mouse brain tumors ([Formula: see text], n = 10).

Studies of Dynamic Binding of Amino Acids to TiO2 Nanoparticle Surfaces by Solution NMR and Molecular Dynamics Simulations

Adsorption of biomolecules onto material surfaces involves a potentially complex mechanism where molecular species interact to varying degrees with a heterogeneous material surface. Surface adsorption studies by atomic force microscopy, sum frequency generation spectroscopy, and solid-state NMR detect the structures and interactions of biomolecular species that are bound to material surfaces, which, in the absence of a solid-liquid interface, do not exchange rapidly between surface-bound forms and free molecular species in bulk solution. Solution NMR has the potential to complement these techniques by detecting and studying transiently bound biomolecules at the liquid-solid interface. Herein, we show that dark-state exchange saturation transfer (DEST) NMR experiments on gel-stabilized TiO₂ nanoparticle (NP) samples detect several forms of biomolecular adsorption onto titanium(IV) oxide surfaces. Specifically, we use the DEST approach to study the interaction of amino acids arginine (Arg), lysine (Lys), leucine (Leu), alanine (Ala), and aspartic acid (Asp) with TiO₂ rutile NP surfaces. Whereas Leu, Ala, and Asp display only a single weakly interacting form in the presence of TiO₂ NPs, Arg and Lys displayed at least two distinct bound forms: a species that is surface bound and retains a degree of reorientational motion and a second more tightly bound form characterized by broadened DEST profiles upon the addition of TiO₂ NPs. Molecular dynamics simulations indicate different surface bound states for both Lys and Arg depending on the degree of TiO₂ surface hydroxylation but only a single bound state for Asp regardless of the degree of surface hydroxylation, in agreement with results obtained from the analysis of DEST profiles.

Diffusion Tensor Imaging and Chemical Exchange Saturation Transfer MRI Evaluation on the Long-Term Effects of Pulsed Focused Ultrasound and Microbubbles Blood Brain Barrier Opening in the Rat

Blood-brain barrier opening (BBBO) with pulsed Focused Ultrasound (pFUS) and microbubbles (MB) has received increasing interest as a method for neurotherapeutics of the central nervous system. In general, conventional MRI [i.e., T2w, T2^∗w, gadolinium (Gd) enhanced T1w] is used to monitor the effects of pFUS+MB on BBBO and/or assess whether sonication results in parenchymal damage. This study employed multimodal MRI techniques and ¹⁸F-Fludeoxyglucose (FDG) PET to evaluate the effects of single and multiple weekly pFUS+MB sessions on morphology and glucose utilization levels in the rat cortex and hippocampus. pFUS was performed with 0.548 MHz transducer with a slow infusion over 1 min of Optison^TM (5–8 × 10⁷ MB) in nine focal points in cortex and four in hippocampus. During pFUS+MB treatment, Gd-T1w was performed at 3 T to confirm BBBO, along with subsequent T2w, T2^∗w, DTI and glucose CEST (glucoCEST)-weighted imaging by high field 9.4 T and compared with FDG-PET and immunohistochemistry. Animals receiving a single pFUS+MB exhibited minimal hypointense voxels on T2^∗w. Brains receiving multiple pFUS+MB treatments demonstrated persistent T2w and T2^∗ abnormalities associated with changes in DTI and glucoCEST when compared to contralateral parenchyma. Decreased glucoCEST contrast was substantiated by FDG-PET in cortex following multiple sonications. Immunohistochemistry showed significantly dilated vessels and decreased neuronal glucose transporter (GLUT3) expression in sonicated cortex and hippocampus without changes in neuronal counts. These results suggest the importance to standardize MRI protocols in concert with advanced imaging techniques when evaluating long term effects of pFUS+MB BBBO in clinical trials for neurological diseases.

Relayed nuclear Overhauser enhancement imaging with magnetization transfer contrast suppression at 3 T

PURPOSE

To develop a pulsed CEST magnetization-transfer method for rapidly acquiring relayed nuclear Overhauser enhancement (rNOE)-weighted images with magnetic transfer contrast (MTC) suppression at clinical field strength (3 T).

METHODS

Using a pulsed CEST magnetization-transfer method with low saturation powers (B₁ ) and long mixing time (t_mix ) to suppress contributions due to strong MTC from solid-like macromolecules, a low B₁ also minimized direct water saturation. These MTC contributions were further reduced by subtracting the Z-spectral signals at two or three offsets by assuming that the residual MTC is a linear function between -3.5 ppm and -12.5 ppm.

RESULTS

Phantom studies of a lactic acid (Lac) solution mixed with cross-linked bovine serum albumin show that strong MTC interference has a significant impact on the optimum B₁ for detecting rNOEs, due to lactate binding. The MTC could be effectively suppressed using a pulse train with a B₁ of 0.8 μT, a pulse duration (t_p ) of 40 ms, a t_mix of 60 ms, and a pulse number (N) of 30, while rNOE signal was well maintained. As a proof of concept, we applied the method in mouse brain with injected hydrogel and a cell-hydrogel phantom. Results showed that rNOE-weighted images could provide good contrast between brain/cell and hydrogel.

CONCLUSION

The developed pulsed CEST magnetization-transfer method can achieve MTC suppression while preserving most of the rNOE signal at 3 T, which indicates the potential for translation of this technique to clinical applications related to mobile proteins/lipids change.

Analysis of model-based and model-free CEST effect quantification methods for different medical applications

Chemical exchange saturation transfer (CEST) magnetic resonance imaging (MRI) holds great potential to provide new metabolic information for clinical applications such as tumor, stroke and Parkinson's Disease diagnosis. Many active research and developments have been conducted to translate this emerging MRI technique for routine clinical applications. In general, there are two CEST quantification techniques: (i) model-free and (ii) model-based techniques. The reliability of these quantification techniques depends heavily on the experimental conditions and quality of the collected data. Errors such as noise may lead to misleading quantification results and thus inaccurate diagnosis when CEST imaging becomes a standard or routine imaging scan in the future. This paper investigates the accuracy and robustness of these quantification techniques under different signal-to-noise (SNR) levels and magnetic field strengths. The quantified CEST effect before and after adding random Gaussian White Noise using model-free and model-based quantification techniques were compared. It was found that the model-free technique consistently yielded larger average percentage error across all tested parameters compared to its model-based counterpart, and that the model-based technique could withstand SNR of about 3 times lower than the model-free technique. When applied on noisy brain tumor, ischemic stroke, and Parkinson's Disease clinical data, the model-free technique failed to produce significant differences between normal and abnormal tissue whereas the model-based technique consistently generated significant differences. Although the model-free technique was less accurate and robust, its simplicity and thus speed would still make it a good approximate when the SNR was high (>50) or when the CEST effect was large and well-defined. For more accurate CEST quantification, model-based techniques should be considered. When SNR was low (<50) and the CEST effect was small such as those acquired from clinical field strength scanners, which are generally 3T and below, model-based techniques should be considered over model-free counterpart to maintain an average percentage error of less than 44% even under very noisy condition as tested in this work.

Evaluating the feasibility of creatine-weighted CEST MRI in human brain at 7 T using a Z-spectral fitting approach

The current study aims to evaluate the feasibility of creatine (Cr) chemical exchange saturation transfer (CEST)-weighted MRI at 7 T in the human brain by optimizing the saturation pulse parameters and computing contrast using a Z-spectral fitting approach. The Cr-weighted (Cr-w) CEST contrast was computed from phantoms data. Simulations were carried out to obtain the optimum saturation parameters for Cr-w CEST with lower contribution from other brain metabolites. CEST-w images were acquired from the brains of four human subjects at different saturation parameters. The Cr-w CEST contrast was computed using both asymmetry analysis and Z-spectra fitting approaches (models 1 and 2, respectively) based on Lorentzian functions. For broad magnetization transfer (MT) effect, Gaussian and Super-Lorentzian line shapes were also evaluated. In the phantom study, the Cr-w CEST contrast showed a linear dependence on concentration in physiological range and a nonlinear dependence on saturation parameters. The in vivo Cr-w CEST map generated using asymmetry analysis from the brain represents mixed contrast with contribution from other metabolites as well and relayed nuclear Overhauser effect (rNOE). Simulations provided an estimate for the optimum range of saturation parameters to be used for acquiring brain CEST data. The optimum saturation parameters for Cr-w CEST to be used for brain data were around B_1rms  = 1.45 μT and duration = 2 seconds. The Z-spectral fitting approach enabled computation of individual components. This also resulted in mitigating the contribution from MT and rNOE to Cr-w CEST contrast, which is a major source of underestimation in asymmetry analysis. The proposed modified z-spectra fitting approach (model 2) is more stable to noise compared with model 1. Cr-w CEST contrast obtained using fitting was 6.98 ± 0.31% in gray matter and 5.45 ± 0.16% in white matter. Optimal saturation parameters reduced the contribution from other CEST effects to Cr-w CEST contrast, and the proposed Z-spectral fitting approach enabled computation of individual components in Z-spectra of the brain. Therefore, it is feasible to compute Cr-w CEST contrast with a lower contribution from other CEST and rNOE.

Voxel-wise Optimization of Pseudo Voigt Profile (VOPVP) for Z-spectra fitting in chemical exchange saturation transfer (CEST) MRI

Background

Chemical exchange saturation transfer (CEST) MRI is a promising approach for detecting biochemical alterations in cancers and neurological diseases, but the quantification can be challenging. Among numerous quantification methods, Lorentzian difference (LD) is relatively simple and widely used, which employs Lorentzian line-shape as a reference to describe the direct saturation (DS) of water and takes account of difference against experimental CEST spectra data. However, LD often overestimates CEST and nuclear overhauser enhancement (NOE) effects. Specifically, for fast-exchanging CEST species require higher saturation power (B_{1_sat}) or in the presence of strong magnetization transfer (MT) contrast, Z-spectrum appears more like a Gaussian line-shape rather than a Lorentzian line-shape.

Methods

To improve the conventional LD analysis, the present study developed and validated a novel fitting algorithm through a linear combination of Gaussian and Lorentzian function as the reference spectra, namely, Voxel-wise Optimization of Pseudo Voigt Profile (VOPVP). The experimental Z-spectra were pre-fitted with Gaussian and Lorentzian method independently, in order to determine Lorentzian proportionality coefficient (a). To further compensate for the line-shape changes under different B_{1_sat}'s, a B₁-dependent adjustment was applied to the experimental Z-spectra (Z_{_exp}) according to the prior knowledge learned from 5-pool Bloch equation-based simulations at a range of B_{1_sat}'s. Then, the obtained Z-spectra (Z_{_B1adj}) was fitted by the previously defined VOPVP function. Considering the asymmetric component of MT, the positive- and negative-side of Z-spectra were fitted separately, while the middle part (-0.6 to 0.6 ppm, consisted primarily of DS) was fitted using Lorentzian function. Finally, the difference between Z_{_VOPVP} and Z_{_exp} was defined as the CEST and NOE contrast. To validate our VOPVP method, an extensive simulation of CEST Z-spectra was performed using 5-pool model and 6-pool model with greater MT component.

Results

In comparison with LD approach, VOPVP exhibited lower sum of squares due to error (SSE) and higher goodness of fit (R-square) for the experimental Z-spectra at all B_{1_sat}. Moreover, the results indicated that VOPVP fitting improved the overestimated contributions from amide proton transfer (APT) and NOE through LD at all B_{1_sat}. Despite that the relationship for B₁-dependent adjustment was pre-determined using a single 5-pool model, the VOPVP fittings obtained accurate quantification for multiple 6-pool models with a range of T₁w's and T₂w's. The robustness of VOPVP fitting was also proved by simulations using 3T parameters. Furthermore, we assessed VOPVP in vivo in a glioblastoma-bearing mouse. Compared to LD maps, VOPVP quantification maps displayed higher contrast-to-noise ratio between tumor and normal contralateral tissue for APT, glutamate and nuclear overhauser effect (NOE), when B_{1_sat} >1 µT.

Conclusions

As an improvement of LD method, VOPVP fitting can serve as a simple, robust and more accurate approach for quantifying CEST and NOE contrast.

High-resolution creatine mapping of mouse brain at 11.7 T using non-steady-state chemical exchange saturation transfer

The current study aims to optimize the acquisition scheme for the creatine chemical exchange saturation transfer weighted (CrCESTw) signal on mouse brain at 11.7 T, in which a strong magnetization transfer contrast (MTC) is present, and to further develop the polynomial and Lorentzian line-shape fitting (PLOF) method for quantifying CrCESTw signal with a non-steady-state (NSS) acquisition scheme. Studies on a Cr phantom with cross-linked bovine serum albumin (BSA) as well as on mouse brain demonstrated that the maximum CrCESTw signal was reached with a short saturation time determined by the rotating frame relaxation time of the MTC pool instead of the steady-state saturation. The saturation power for the maximal signal was around 1-1.5 μT for Cr with 20% cross-linked BSA and in vivo applications, but 2 μT was found to be most practical for signal stability. For the CrCEST acquisition with strong MTC interference, the optimal saturation power and length are completely different from those on Cr solution alone. This observation could be explained well using R_1ρ theory by incorporating the strong MTC pool. Finally, a high-resolution Cr map was obtained on mouse brain using the PLOF method with the NSS CEST acquisition and a cryogenic coil. The Cr map obtained by CEST showed homogenous intensity across the mouse brain except for regions with cerebrospinal fluid.

Adaptive denoising for chemical exchange saturation transfer MR imaging

High image signal-to-noise ratio (SNR) is required to reliably detect the inherently small chemical exchange saturation transfer (CEST) effects in vivo. In this study, it was demonstrated that identifying spectral redundancies of CEST data by principal component analysis (PCA) in combination with an appropriate data-driven extraction of relevant information can be used for an effective and robust denoising of CEST spectra. The relationship between the number of relevant principal components and SNR was studied on fitted in vivo Z-spectra with artificially introduced noise. Three different data-driven criteria to automatically determine the optimal number of necessary components were investigated. In addition, these criteria facilitate straightforward assessment of data quality that could provide guidance for CEST MR protocols in terms of SNR. Insights were applied to achieve a robust denoising of highly sampled low power Z-spectra of the human brain at 3 and 7 T. The median criterion provided the best estimation for the optimal number of components consistently for all three investigated artificial noise levels. Application of the denoising technique to in vivo data revealed a considerable increase in image quality for the amide and rNOE contrast with a considerable SNR gain. At 7 T the denoising capability was quantified to be comparable or even superior to an averaging of six measurements. The proposed denoising algorithm enables an efficient and robust denoising of CEST data by combining PCA with appropriate data-driven truncation criteria. With this generally applicable technique at hand, small CEST effects can be reliably detected without the need for repeated measurements.

Fast correction of B0 field inhomogeneity for pH-specific magnetization transfer and relaxation normalized amide proton transfer imaging of acute ischemic stroke without Z-spectrum

PURPOSE

The magnetization transfer and relaxation normalized amide proton transfer (MRAPT) analysis is promising to provide a highly pH-specific mapping of tissue acidosis, complementing commonly used CEST asymmetry analysis. We aimed to develop a fast B₀ inhomogeneity correction algorithm for acute stroke magnetization transfer and relaxation normalized amide proton transfer imaging without Z-spectral interpolation.

METHODS

The proposed fast field inhomogeneity correction describes B₀ artifacts with linear regression. We compared the new algorithm with the routine interpolation correction approach in CEST imaging of a dual-pH phantom. The fast B₀ correction was further evaluated in amide proton transfer imaging of normal and acute stroke rats.

RESULTS

Our phantom data showed that the proposed fast B₀ inhomogeneity correction significantly improved pH MRI contrast, recovering over 80% of the pH MRI contrast-to-noise-ratio difference between the raw magnetization transfer ratio asymmetry and that using the routine interpolation-based B₀ correction approach. In normal rat brains, the proposed fast B₀ correction improved pH-specific MRI uniformity across the intact tissue, with the ratio of magnetization transfer and relaxation normalized amide proton transfer ratio being 10% of that without B₀ inhomogeneity correction. In acute stroke rats, fast B₀ inhomogeneity-corrected pH MRI reveals substantially improved pH lesion conspicuity, particularly in regions with nonnegligible B₀ inhomogeneity. The pH MRI contrast-to-noise ratio between the ipsilateral diffusion lesion and contralateral normal tissue improved significantly with fast B₀ correction (from 1.88 ± 0.48 to 2.20 ± 0.44, P < .01).

CONCLUSIONS

Our study established an expedient B₀ inhomogeneity correction algorithm for fast pH imaging of acute ischemia.

Optimization and repeatability of multipool chemical exchange saturation transfer MRI of the prostate at 3.0 T

BACKGROUND

Chemical exchange saturation transfer (CEST) can potentially support cancer imaging with metabolically derived information. Multiparametric prostate MRI has improved diagnosis but may benefit from additional information to reduce the need for biopsies.

PURPOSE

To optimize an acquisition and postprocessing protocol for 3.0 T multipool CEST analysis of prostate data and evaluate the repeatability of the technique.

STUDY TYPE

Prospective.

SUBJECTS

Five healthy volunteers (age range: 24-47 years; median age: 28 years) underwent two sessions (interval range: 7-27 days; median interval: 20 days) and two biopsy-proven prostate cancer patients were evaluated once. Patient 1 (71 years) had a Gleason 3 + 4 transition zone (TZ) tumor and patient 2 (55 years) had a Gleason 4 + 3 peripheral zone (PZ) tumor.

FIELD STRENGTH

3.0 T. Sequences run: T2 -weighted turbo-spin-echo (TSE); diffusion-weighted imaging; CEST; WASABI (for B0 determination).

ASSESSMENT

Saturation, readout, and fit-model parameters were optimized to maximize in vivo amide and nuclear Overhauser effect (NOE) signals. Repeatability (intrasession and intersession) was evaluated in healthy volunteers. Subsequently, preliminary evaluation of signal differences was made in patients. Regions of interest were drawn by two post-FRCR board-certified readers, both with over 5 years of experience in multiparametric prostate MRI.

STATISTICAL TESTS

Repeatability was assessed using Bland-Altman analysis, coefficient of variation (CV), and 95% limits of agreement (LOA). Statistical significance of CEST contrast was calculated using a nonparametric Mann-Whitney U-test.

RESULTS

The optimized saturation scheme was found to be 60 sinc-Gaussian pulses with 40 msec pulse duration, at 50% duty-cycle with continuous-wave pulse equivalent B1 power (B1CWPE ) of 0.92 μT. The magnetization transfer (MT) contribution to the fit-model was centered at -1.27 ppm. Intersession coefficients of variation (CVs) of the amide, NOE, and magnetization transfer (MT) and asymmetric magnetization transfer ratio (MTRasym ) signals of 25%, 23%, 18%, and 200%, respectively, were observed. Fit-metric and MTRasym CVs agreed between readers to within 4 and 10 percentage points, respectively.

DATA CONCLUSION

Signal differences of 0.03-0.10 (17-43%) detectable depending upon pool, with MT the most repeatable (signal difference of 17-22% detectable).

LEVEL OF EVIDENCE

2 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2019;50:1238-1250.

Early identification of neonatal mild hypoxic-ischemic encephalopathy by amide proton transfer magnetic resonance imaging: A pilot study

PURPOSE

This study aimed to evaluate the amide proton transfer (APT) values in neonates with mild hypoxic-ischemic encephalopathy (HIE) using APT imaging.

METHOD

A total of 30 full-term neonates with mild HIE (16 males and 14 females; mean postnatal age 4.2 days, age range 2-7 days) and 12 normal neonates (six males and six females; mean postnatal age 3.3 days, age range 2-5 days) underwent conventional magnetic resonance imaging and APT imaging. APT measurements were performed in multiple regions of interest (ROIs) in the brain. APT values were statistically analyzed to assess for significant differences between the mild HIE and normal neonates in different regions of the brain, and correlation with neonatal gestational age.

RESULTS

In 30 neonates with mild HIE, 10% (3/30) of the HIE patients had normal conventional MRI. There were significant differences in APT values of the HIE group in bilateral caudate, bilateral thalamus, bilateral centrum semiovale and left globus pallidus/putamen (p < 0.05), and no statistical difference was observed in right globus pallidus/putamen (p = 0.051) and brainstem (p = 0.073) between the two groups. Furthermore, APT values in bilateral caudate, bilateral globus pallidus/putamen, bilateral thalamus, and brainstem regions (p < 0.05) exhibited positive linear correlations with gestational age in the control group, except for bilateral centrum semiovale (right: Pearson's r = 0.554, p = 0.062; left: Pearson's r = 0.561, p = 0.058). In the mild HIE groups, no significant correlation with gestational age was found in all regions.

CONCLUSIONS

APT imaging is a feasible and useful technique with diagnostic capability for neonatal HIE.

CEST MR-Fingerprinting: Practical considerations and insights for acquisition schedule design and improved reconstruction

PURPOSE

To understand the influence of various acquisition parameters on the ability of CEST MR-Fingerprinting (MRF) to discriminate different chemical exchange parameters and to provide tools for optimal acquisition schedule design and parameter map reconstruction.

METHODS

Numerical simulations were conducted using a parallel computing implementation of the Bloch-McConnell equations, examining the effect of TR, TE, flip-angle, water T 1 and T 2 , saturation-pulse duration, power, and frequency on the discrimination ability of CEST-MRF. A modified Euclidean distance matching metric was evaluated and compared to traditional dot product matching. L-Arginine phantoms of various concentrations and pH were scanned at 4.7T and the results compared to numerical findings.

RESULTS

Simulations for dot product matching demonstrated that the optimal flip-angle and saturation times are 30 ∘ and 1100 ms, respectively. The optimal maximal saturation power was 3.4 μT for concentrated solutes with a slow exchange rate, and 5.2 μT for dilute solutes with medium-to-fast exchange rates. Using the Euclidean distance matching metric, much lower maximum saturation powers were required (1.6 and 2.4 μT, respectively), with a slightly longer saturation time (1500 ms) and 90 ∘ flip-angle. For both matching metrics, the discrimination ability increased with the repetition time. The experimental results were in agreement with simulations, demonstrating that more than a 50% reduction in scan-time can be achieved by Euclidean distance-based matching.

CONCLUSIONS

Optimization of the CEST-MRF acquisition schedule is critical for obtaining the best exchange parameter accuracy. The use of Euclidean distance-based matching of signal trajectories simultaneously improved the discrimination ability and reduced the scan time and maximal saturation power required.

Determination of multipool contributions to endogenous amide proton transfer effects in global ischemia with high spectral resolution in vivo chemical exchange saturation transfer MRI

PURPOSE

Chemical exchange saturation transfer (CEST) MRI has been used for quantitative assessment of dilute metabolites and/or pH in disorders such as acute stroke and tumor. However, routine asymmetry analysis (MTRasym ) may be confounded by concomitant effects such as semisolid macromolecular magnetization transfer (MT) and nuclear Overhauser enhancement. Resolving multiple contributions is essential for elucidating the origins of in vivo CEST contrast.

METHODS

Here we used a newly proposed image downsampling expedited adaptive least-squares fitting on densely sampled Z-spectrum to quantify multipool contribution from water, nuclear Overhauser enhancement, MT, guanidinium, amine, and amide protons in adult male Wistar rats before and after global ischemia.

RESULTS

Our results revealed the major contributors to in vivo T1 -normalized MTRasym (3.5 ppm) contrast between white and gray matter (WM/GM) in normal brain (-1.96%/second) are pH-insensitive macromolecular MT (-0.89%/second) and nuclear Overhauser enhancement (-1.04%/second). Additionally, global ischemia resulted in significant changes of MTRasym , being -2.05%/second and -1.56%/second in WM and GM, which are dominated by changes in amide (-1.05%/second, -1.14%/second) and MT (-0.88%/second, -0.62%/second). Notably, the pH-sensitive amine and amide effects account for nearly 60% and 80% of the MTRasym changes seen in WM and GM, respectively, after global ischemia, indicating that MTRasym is predominantly pH-sensitive.

CONCLUSION

Combined amide and amine effects dominated the MTRasym changes after global ischemia, indicating that MTRasym is predominantly pH-sensitive and suitable for detecting tissue acidosis following acute stroke.

Toward more reliable measurements of NOE effects in CEST spectra at around -1.6 ppm (NOE (-1.6)) in rat brain

PURPOSE

Recently, a new relayed nuclear Overhauser enhancement (NOE) saturation transfer effect at around -1.6 parts per million, termed NOE(-1.6), and its potential applications in tumor and stroke were reported by several institutes. However, there is a concern of the reproducibility of NOE(-1.6) measurements because it is not reported by many other publications. This paper aims to study the influence of typically overlooked experimental settings on the NOE(-1.6) signal and to build a framework for more reliable measurements of NOE(-1.6) at 9.4T.

METHODS

Z-spectra were obtained in rat brains. A fitting approach was performed to quantify all known saturation transfer effects except NOE(-1.6). Residual signals were obtained by removing these confounding effects from Z-spectra and were then used to quantify NOE(-1.6). Multislice imaging was performed to study the NOE(-1.6) dependence on brain regions. The influences of euthanasia, anesthesia, breathing gases, and RF irradiation power were also evaluated.

RESULTS

Results demonstrate that the NOE(-1.6) signal contributions are often not clearly observable in raw Z-spectra at relatively high irradiation powers due to, for example, the direct water saturation effect, but they can be visualized after removing other nonspecific effects. In addition, the NOE(-1.6) effect depends on brain region, decreases postmortem, shifts after long-duration anesthesia, and may be enhanced by increasing O2 and N2 O breathing air concentrations.

CONCLUSION

Because the NOE(-1.6) effect is more susceptible to the direct water saturation effect and more sensitive to physiological conditions than are other CEST effects, incorporating known sensitivities into the experimental design and data analysis is necessary to ensure more reliable NOE(-1.6) results.

Ratiometric NOE(-1.6) contrast in brain tumors

Recently, a new nuclear Overhauser enhancement (NOE)-mediated saturation transfer effect at around -1.6 ppm from water, termed NOE(-1.6), was reported to show hypointense signals in brain tumors. Similar to chemical exchange saturation transfer or magnetization transfer (MT) effects, which depend on the solute pool concentration, the exchange/coupling rate, the solute transverse relaxation rate, etc., the NOE(-1.6) effect should also depend on these factors. Since the exchange rate is relevant to tissue pH, and the coupling rate and the solute transverse relaxation rate are relevant to the motional property of the coupled molecules, further studies to quantify the contribution from only the exchange/coupling rate and the solute transverse relaxation rate are always interesting. The purpose of this paper is to apply a ratiometric approach to the NOE(-1.6) effect to obtain a metric that is more specific to the NOE coupling rate and the solute transverse relaxation rate than the NOE(-1.6) signal amplitude. Simulations indicate that the ratiometric approach allows us to rule out nearly all of the non-specific factors including the solute pool concentration, solute and water longitudinal relaxation rates, direct water saturation, and semi-solid MT effects, and provides a more specific NOE coupling rate- and solute transverse relaxation rate-weighted signal. Animal studies show that the ratiometric NOE(-1.6) decreases dramatically in brain tumors, which suggests that the change in the NOE(-1.6) coupling rate and/or the solute transverse relaxation rate are major contributors to the previously observed hypointense NOE(-1.6) signal in tumors.

Chemical exchange saturation transfer fingerprinting for exchange rate quantification

PURPOSE

There is an increased interest to determine the exchange rate using CEST to provide pH information. However, current CEST quantification methods require lengthy scan times and do not address magnetization transfer effects. The purpose of this work was to apply the magnetic resonance fingerprinting (MRF) concept to CEST to achieve more efficient and accurate exchange rate quantification.

METHODS

The proposed CEST fingerprinting method used varying saturation powers and saturation times to create unique signal evolutions for different exchange rates. The acquired signal was matched to a predefined dictionary to determine the exchange rate. The magnetization transfer effects were also addressed in the framework of CEST fingerprinting: The simulated dictionary could predict the signal curves without magnetization transfer effects, and comparing the dictionary to the acquired signals allowed the correction of the magnetization transfer effects. The CEST fingerprinting method was compared with the conventional pulsed quantitative CEST method using omega plots in the creatine phantom study.

RESULTS

The CEST fingerprinting method has a significantly reduced scan time (10 minutes versus 50 minutes) while providing more accurate exchange rate quantification using literature values as the reference.

CONCLUSION

In this study, we demonstrate that CEST fingerprinting is more efficient (5 times faster) compared with pulsed quantitative CEST. It is also shown that the results of the proposed CEST fingerprinting technique are much closer to the literature values than pulsed quantitative CEST at 3 T.

Improving B0 Correction for pH-Weighted Amine Proton Chemical Exchange Saturation Transfer (CEST) Imaging by Use of k-Means Clustering and Lorentzian Estimation

Amine chemical exchange saturation transfer (CEST) echoplanar imaging (EPI) provides unique pH and amino acid MRI contrast, enabling sensitive detection of altered microenvironment properties in various diseases. However, CEST contrast is sensitive to static magnetic field (B₀) inhomogeneities. Here we propose 2 new B₀ correction algorithms for use in correcting pH-weighted amine CEST EPI based on k-means clustering and Lorentzian fitting of CEST data: the iterative downsampling estimation using Lorentzian fitting and the 2-stage Lorentzian estimation with 4D polynomial fitting. Higher quality images of asymmetric magnetization transfer ratio (MTR_asym) at 3.0 ppm could be obtained with the proposed algorithms than with the existing B₀ correction methods. In particular, the proposed methods are shown to improve the intertissue consistency, interpatient consistency, and tumor region signal-to-noise ratio of MTR_asym at 3.0 ppm images, with nonexcessive computation time.

Influences of experimental parameters on chemical exchange saturation transfer (CEST) metrics of brain tumors using animal models at 4.7T

PURPOSE

To investigate the dependence of magnetization transfer ratio asymmetry at 3.5 ppm (MTR_asym (3.5 ppm)), quantitative amide proton transfer (APT^# ), and nuclear Overhauser enhancement (NOE^# ) signals or contrasts on experimental imaging parameters.

METHODS

Modified Bloch equation-based simulations using 2-pool and 5-pool exchange models and in vivo rat brain tumor experiments at 4.7T were performed with varied RF saturation power levels, saturation lengths, and relaxation delays. The MTR_asym (3.5 ppm), APT^# , and NOE^# contrasts between tumor and normal tissues were compared among different experimental parameters.

RESULTS

The MTR_asym (3.5 ppm) image contrasts between tumor and normal tissues initially increased with the RF saturation length, and the maxima occurred at 1.6-2 s under relatively high RF saturation powers (>2.1 μT) and at a longer saturation length under relatively low RF saturation powers (<1.3 μT). The APT^# contrasts also increased with the RF saturation length but peaked at longer RF saturation lengths relative to MTR_asym (3.5 ppm). The NOE^# contrasts were either positive or negative, depending on the experimental parameters applied.

CONCLUSION

Tumor MTR_asym (3.5 ppm), APT^# , and NOE^# contrasts can be maximized at different saturation parameters. The maximum MTR_asym (3.5 ppm) contrast can be obtained with a relatively longer RF saturation length (several seconds) at a relatively lower RF saturation power.

Chemical exchange saturation transfer MRI shows low cerebral 2-deoxy-D-glucose uptake in a model of Alzheimer's Disease

Glucose is the central nervous system's only energy source. Imaging techniques capable to detect pathological alterations of the brain metabolism are useful in different diagnostic processes. Such techniques are also beneficial for assessing the evaluation efficacy of therapies in pre-clinical and clinical stages of diseases. Chemical exchange saturation transfer (CEST) magnetic resonance imaging (MRI) is a possible alternative to positron emission tomography (PET) imaging that has been widely explored in cancer research in humans and animal models. We propose that pathological alterations in brain 2-deoxy-D-glucose (2DG) uptake, typical of neurodegenerative diseases, can be detected with CEST MRI. Transgenic mice overexpressing a mutated form of amyloid precusrsor protein (APP23), a model of Alzheimer's disease, analyzed with CEST MRI showed a clear reduction of 2DG uptake in different brain regions. This was reminiscent of the cerebral condition observed in Alzheimer's patients. The results indicate the feasibility of CEST for analyzing the brain metabolic state, with better image resolution than PET in experimental models.

Dual-frequency irradiation CEST-MRI of endogenous bulk mobile proteins

A novel MRI contrast is proposed which enables the selective detection of endogenous bulk mobile proteins in vivo. Such a non-invasive imaging technique may be of particular interest for many diseases associated with pathological alterations of protein expression, such as cancer and neurodegenerative disorders. Specificity to mobile proteins was achieved by the selective measurement of intramolecular spin diffusion and the removal of semi-solid macromolecular signal components by a correction procedure. For this purpose, the approach of chemical exchange saturation transfer (CEST) was extended to a radiofrequency (RF) irradiation scheme at two different frequency offsets (dualCEST). Using protein model solutions, it was demonstrated that the dualCEST technique allows the calculation of an image contrast which is exclusively sensitive to changes in concentration, molecular size and the folding state of mobile proteins. With respect to application in humans, dualCEST overcomes the selectivity limitations at relatively low magnetic field strengths, and thus enables examinations on clinical MR scanners. The feasibility of dualCEST examinations in humans was verified by a proof-of-principle examination of a brain tumor patient at 3 T. With its specificity for the mobile fraction of the proteome, its comparable sensitivity to conventional water proton MRI and its applicability to clinical MR scanners, this technique represents a further step towards the non-invasive imaging of proteomic changes in humans.

PRO-QUEST: a rapid assessment method based on progressive saturation for quantifying exchange rates using saturation times in CEST

PURPOSE

To develop a new MRI technique to rapidly measure exchange rates in CEST MRI.

METHODS

A novel pulse sequence for measuring chemical exchange rates through a progressive saturation recovery process, called PRO-QUEST (progressive saturation for quantifying exchange rates using saturation times), has been developed. Using this method, the water magnetization is sampled under non-steady-state conditions, and off-resonance saturation is interleaved with the acquisition of images obtained through a Look-Locker type of acquisition. A complete theoretical framework has been set up, and simple equations to obtain the exchange rates have been derived.

RESULTS

A reduction of scan time from 58 to 16 minutes has been obtained using PRO-QUEST versus the standard QUEST. Maps of both T₁ of water and B₁ can simply be obtained by repetition of the sequence without off-resonance saturation pulses. Simulations and calculated exchange rates from experimental data using amino acids such as glutamate, glutamine, taurine, and alanine were compared and found to be in good agreement. The PRO-QUEST sequence was also applied on healthy and infarcted rats after 24 hours, and revealed that imaging specificity to ischemic acidification during stroke was substantially increased relative to standard amide proton transfer-weighted imaging.

CONCLUSION

Because of the reduced scan time and insensitivity to nonchemical exchange factors such as direct water saturation, PRO-QUEST can serve as an excellent alternative for researchers and clinicians interested to map pH changes in vivo.