Practical data acquisition method for human brain tumor amide proton transfer (APT) imaging

Chemical exchange saturation transfer for predicting response to stereotactic radiosurgery in human brain metastasis

PURPOSE

The purpose of this work was to determine the predictive value of chemical exchange saturation transfer (CEST) metrics in brain metastases treated with stereotactic radiosurgery (SRS).

METHODS

CEST spectra at a radiofrequency power of 0.52 µT were collected on a 3 Tesla (T) magnetic resonance imaging from 25 patients at three time points: pretreatment, 1 week, and 1 month post-treatment. Amide proton transfer-weighted images and maps of the amplitude and width of Lorentzian-shaped CEST peaks and the relaxation-compensated AREX metric were constructed at the offset frequencies of amide, amine, and relayed nuclear Overhauser effect (NOE) from aliphatic groups as well as the broad magnetization transfer effect. Pretreatment CEST metrics, as well as CEST metric changes at 1 week post-treatment, were compared to changes in tumor volume at 1 month.

RESULTS

Significant (P < 0.05) 1-week predictive metrics included NOE peak amplitude (R = 0.69) in normal-appearing white matter (NAWM) and width (R = -0.55) in tumor. Baseline NOE in contralateral NAWM was negatively correlated (R = -0.69) with volume changes at 1 month. Metrics-defined outside tumor margins had higher correlation with volume changes than tumor regions of interest.

CONCLUSION

CEST metrics, in particular, the NOE peak amplitude, can predict volume changes 1 month post-SRS. Magn Reson Med 78:1110-1120, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Investigation of the performance of variable-density Z-spectrum acquisition scheme in MR chemical exchange saturation transfer effect quantification

Chemical exchange saturation transfer (CEST) MRI emerges as a powerful imaging method to probe chemical exchange between bulk water and labile protons. Conventional CEST imaging needs detailed Z-spectrum acquisition, even at frequencies with limited information for CEST parameter quantification, which harms imaging efficiency and is not favorable in practical applications. In this study, a variable-density Z-spectrum acquisition scheme was proposed. Data with saturation frequencies close to water, labile protons and their negative frequency offset was densely sampled, while the remaining part was sparsely acquired. The accuracy of the proposed method in CEST ratio measurement was investigated with numerical simulation. Compared to the conventionally evenly distributed Z-spectrum acquisition method, the proposed scheme showed significant improvement of CEST ratio characterization with existence of noise and static magnetic field inhomogeneity. With the additional advantage of easy implementation, the developed Z-spectrum acquisition scheme provides a useful framework for accurate CEST parameter quantification without compromising the imaging time.

MR Imaging Biomarkers in Oncology Clinical Trials

The authors discuss eight areas of quantitative MR imaging that are currently used (RECIST, DCE-MR imaging, DSC-MR imaging, diffusion MR imaging) in clinical trials or emerging (CEST, elastography, hyperpolarized MR imaging, multiparameter MR imaging) as promising techniques in diagnosing cancer and assessing or predicting response of cancer to therapy. Illustrative applications of the techniques in the clinical setting are summarized before describing the current limitations of the methods.

Investigating hydroxyl chemical exchange using a variable saturation power chemical exchange saturation transfer (vCEST) method at 3 T

PURPOSE

To develop a chemical exchange saturation transfer (CEST) scheme sensitive to hydroxyl protons at 3 T. Clinical imaging of hydroxyl moieties can have an impact on osteoarthritis, neuropsychiatric disorders, and cancer.

THEORY

By varying saturation amplitude linearly with frequency offset, the direct water saturation component of the Z-spectrum is flattened and can be subtracted to produce a magnetization transfer ratio difference spectrum (MTRdiff ) that isolates solute resonances. Variable saturation power allows for near optimization of hydroxyl and amine/amide moieties in one Z-spectrum.

METHODS

Phantom studies were used to test vCEST performance in two environments: (1) aqueous single-solute (glycogen, glucose); (2) aqueous multiple solute (glycogen with bovine serum albumin). In vivo vCEST imaging of glycosaminoglycan content in patellar-femoral cartilage was performed in a subject with history of cartilage transplant.

RESULTS

In solutions with overlapping resonances, vCEST resolves separate hydroxyl and amine/amide peaks. CEST hydroxyl signal in cartilage is negligible, but with vCEST, hydroxyl signal ranged from 2 to 5% ppm and showed distinct contrast between lesions and normal appearing cartilage.

CONCLUSION

Introduced a variable saturation amplitude CEST (vCEST) scheme to improve sensitivity to exchangeable hydroxyl moieties at 3 T resulting in detection of hydroxyl in the presence of multiple solutes with overlapping resonances. Magn Reson Med 76:826-837, 2016. © 2015 Wiley Periodicals, Inc.

(15)N Heteronuclear Chemical Exchange Saturation Transfer MRI

A two-step heteronuclear enhancement approach was combined with chemical exchange saturation transfer (CEST) to magnify (15)N MRI signal of molecules through indirect detection via water protons. Previous CEST studies have been limited to radiofrequency (rf) saturation transfer or excitation transfer employing protons. Here, the signal of (15)N is detected indirectly through the water signal by first inverting selectively protons that are scalar-coupled to (15)N in the urea molecule, followed by chemical exchange of the amide proton to bulk water. In addition to providing a small sensitivity enhancement, this approach can be used to monitor the exchange rates and thus the pH sensitivity of the participating (15)N-bound protons.

Magnetic Resonance Imaging of Glucose Uptake and Metabolism in Patients with Head and Neck Cancer

Imaging metabolic dysfunction, a hallmark of solid tumors, usually requires radioactive tracers. Chemical exchange saturation transfer (CEST) imaging can potentially detect and visualize glucose uptake and metabolism, without the need for radioisotopes. Here, we tested the feasibility of using glucose CEST (glucoCEST) to image unlabeled glucose uptake in head and neck cancer by using a clinical 3T magnetic resonance imaging (MRI) scanner. The average CEST contrast between tumors and normal tissue in 17 patients was 7.58% (P = 0.006) in the 3–4 ppm offset frequency range and 5.06% (P = 0.02) in 1–5 ppm range. In a subgroup of eight patients, glucoCEST signal enhancement was higher in tumors than in normal muscle (4.98% vs. 1.28%, P < 0.021). We conclude that glucoCEST images of head and neck cancer can be obtained with a clinical 3T MRI scanner.

Comparison of pulsed three-dimensional CEST acquisition schemes at 7 tesla: steady state versus pseudosteady state

PURPOSE

To compare two pulsed, volumetric chemical exchange saturation transfer (CEST) acquisition schemes: steady state (SS) and pseudosteady state (PS) for the same brain coverage, spatial/spectral resolution and scan time.

METHODS

Both schemes were optimized for maximum sensitivity to amide proton transfer (APT) and nuclear Overhauser enhancement (NOE) effects through Bloch-McConnell simulations, and compared in terms of sensitivity to APT and NOE effects, and to transmit field inhomogeneity. Five consented healthy volunteers were scanned on a 7 Tesla Philips MR-system using the optimized protocols at three nominal B₁ amplitudes: 1 μT, 2 μT, and 3 μT.

RESULTS

Region of interest based analysis revealed that PS is more sensitive (P < 0.05) to APT and NOE effects compared with SS at low B₁ amplitudes (0.7-1.0 μT). Also, both sequences have similar dependence on the transmit field inhomogeneity. For the optimum CEST presaturation parameters (1 μT and 2 μT for APT and NOE, respectively), NOE is less sensitive to the inhomogeneity effects (15% signal to noise ratio [SNR] change for a B₁ dropout of 40%) compared with APT (35% SNR change for a B₁ dropout of 40%).

CONCLUSION

For the same brain coverage, spatial/spectral resolution and scan time, at low power levels PS is more sensitive to the slow chemical exchange-mediated processes compared with SS. Magn Reson Med 77:2280-2287, 2017. © 2016 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.

pH-sensitive MRI demarcates graded tissue acidification during acute stroke - pH specificity enhancement with magnetization transfer and relaxation-normalized amide proton transfer (APT) MRI

pH-sensitive amide proton transfer (APT) MRI provides a surrogate metabolic biomarker that complements the widely-used perfusion and diffusion imaging. However, the endogenous APT MRI is often calculated using the asymmetry analysis (MTRasym), which is susceptible to an inhomogeneous shift due to concomitant semisolid magnetization transfer (MT) and nuclear overhauser (NOE) effects. Although the intact brain tissue has little pH variation, white and gray matter appears distinct in the MTRasym image. Herein we showed that the heterogeneous MTRasym shift not related to pH highly correlates with MT ratio (MTR) and longitudinal relaxation rate (R1w), which can be reasonably corrected using the multiple regression analysis. Because there are relatively small MT and R1w changes during acute stroke, we postulate that magnetization transfer and relaxation-normalized APT (MRAPT) analysis increases MRI specificity to acidosis over the routine MTRasym image, hence facilitates ischemic lesion segmentation. We found significant differences in perfusion, pH and diffusion lesion volumes (P<0.001, ANOVA). Furthermore, MRAPT MRI depicted graded ischemic acidosis, with the most severe acidosis in the diffusion lesion (-1.05±0.29%/s), moderate acidification within the pH/diffusion mismatch (i.e., metabolic penumbra, -0.67±0.27%/s) and little pH change in the perfusion/pH mismatch (i.e., benign oligemia, -0.04±0.14%/s), providing refined stratification of ischemic tissue injury.

Amide proton transfer imaging of brain tumors using a self-corrected 3D fast spin-echo dixon method: Comparison With separate B0 correction

PURPOSE

To assess the quantitative performance of three-dimensional (3D) fast spin-echo (FSE) Dixon amide proton transfer (APT) imaging of brain tumors compared with B₀ correction with separate mapping methods.

METHODS

Twenty-two patients with brain tumors (54.2 ± 18.7 years old, 12 males and 10 females) were scanned at 3 Tesla (T). Z-spectra were obtained at seven different frequency offsets at ±3.1 ppm, ± 3.5 ppm, ± 3.9 ppm, and -1560 ppm. The scan was repeated three times at +3.5 ppm with echo shifts for Dixon B₀ mapping. The APT image corrected by a three-point Dixon-type B₀ map from the same scan (3D-Dixon) or a separate B₀ map (2D-separate and 3D-separate), and an uncorrected APT image (3D-uncorrected) were generated. We compared the APT-weighted signals within a tumor obtained with each 3D method with those obtained with 2D-separate as a reference standard.

RESULTS

Excellent agreements and correlations with the 2D-separate were obtained by the 3D-Dixon method for both mean (ICC = 0.964, r = 0.93, P < 0.0001) and 90th-percentile (ICC = 0.972, r = 0.95, P < 0.0001) APT-weighted signals. These agreements and correlations for 3D-Dixon were better than those obtained by the 3D-uncorrected and 3D-separate methods.

CONCLUSION

The 3D FSE Dixon APT method with intrinsic B₀ correction offers a quantitative performance that is similar to that of established two-dimensional (2D) methods. Magn Reson Med 77:2272-2279, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Chemical exchange saturation transfer (CEST) imaging with fast variably-accelerated sensitivity encoding (vSENSE)

PURPOSE

The widespread clinical use of chemical exchange saturation transfer (CEST) imaging is hampered by relatively long scan times due to its requirement that multiple saturation-offset image frames be acquired. Here, a novel variably-accelerated sensitivity encoding (vSENSE) method is proposed that provides faster CEST acquisition than conventional SENSE.

THEORY AND METHODS

The vSENSE method fully samples one CEST saturation frame, then undersamples the other frames variably. The fully-sampled frame, in conjunction with newly proposed incoherence absorption and artifact suppression strategies, improves the accuracy of sensitivity maps and permits higher acceleration factors for the other undersampled frames than regular SENSE. vSENSE is validated in a phantom, a normal volunteer and eight brain tumor patients at 3 Tesla.

RESULTS

vSENSE with an acceleration factor of four generated a 3-6 times smaller error on average than conventional SENSE (P ≤ 0.02), with acceleration factors of 2-4, as compared with full k-space reconstruction. vSENSE permitted four-fold acceleration for amide proton transfer-weighted images, while regular SENSE could only provide a factor of two. When conventional SENSE is used with vSENSE's variable undersampling pattern, erroneous (∼9%) z-spectra result.

CONCLUSION

The vSENSE method enabled twice the acceleration and generated more accurate images than conventional SENSE. Magn Reson Med 77:2225-2238, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Label-free CEST MRI Detection of Citicoline-Liposome Drug Delivery in Ischemic Stroke

Citicoline (CDPC) is a natural supplement with well-documented neuroprotective effects in the treatment of neurodegenerative diseases. In the present study, we sought to exploit citicoline as a theranostic agent with its inherent chemical exchange saturation transfer (CEST) MRI signal, which can be directly used as an MRI guidance in the citicoline drug delivery. Our in vitro CEST MRI results showed citicoline has two inherent CEST signals at +1 and +2 ppm, attributed to exchangeable hydroxyl and amine protons, respectively. To facilitate the targeted drug delivery of citicoline to ischemic regions, we prepared liposomes encapsulating citicoline (CDPC-lipo) and characterized the particle properties and CEST MRI properties. The in vivo CEST MRI detection of liposomal citicoline was then examined in a rat brain model of unilateral transient ischemia induced by a two-hour middle cerebral artery occlusion. The results showed that the delivery of CPDC-lipo to the brain ischemic areas could be monitored and quantified by CEST MRI. When administered intra-arterially, CDPC-lipo clearly demonstrated a detectable CEST MRI contrast at 2 ppm. CEST MRI revealed that liposomes preferentially accumulated in the areas of ischemia with a disrupted blood-brain-barrier. We furthermore used CEST MRI to detect the improvement in drug delivery using CDPC-lipo targeted against vascular cell adhesion molecule (VCAM)-1 in the same animal model. The MRI findings were validated using fluorescence microscopy. Hence, liposomal citicoline represents a prototype theranostic system, where the therapeutic agent can be detected directly by CEST MRI in a label-free fashion.

A theoretical analysis of chemical exchange saturation transfer echo planar imaging (CEST-EPI) steady state solution and the CEST sensitivity efficiency-based optimization approach

Chemical exchange saturation transfer (CEST) MRI is sensitive to dilute labile protons and microenvironmental properties, augmenting routine relaxation-based MRI. Recent developments of quantitative CEST (qCEST) analysis such as omega plots and RF-power based ratiometric calculation have extended our ability to elucidate the underlying CEST system beyond the simplistic apparent CEST measurement. CEST MRI strongly varies with experimental factors, including the RF irradiation level and duration as well as repetition time and flip angle. In addition, the CEST MRI effect is typically small, and experimental optimization strategies have to be carefully evaluated in order to enhance the CEST imaging sensitivity. Although routine CEST MRI has been optimized largely based on maximizing the magnitude of the CEST effect, the CEST signal-to-noise (SNR) efficiency provides a more suitable optimization index, particularly when the scan time is constrained. Herein, we derive an analytical solution of the CEST effect that takes into account key experimental parameters including repetition time, imaging flip angle and RF irradiation level, and solve its SNR efficiency. The solution expedites CEST imaging sensitivity calculation, substantially faster than the Bloch-McConnell equation-based numerical simulation approach. In addition, the analytical solution-based SNR formula enables the exhaustive optimization of CEST MRI, which simultaneously predicts multiple optimal parameters such as repetition time, flip angle and RF saturation level based on the chemical shift and exchange rate. The sensitivity efficiency-based optimization approach could simplify and guide imaging of CEST agents, including glycogen, glucose, creatine, gamma-aminobutyric acid and glutamate. Copyright © 2016 John Wiley & Sons, Ltd.

Insight into the quantitative metrics of chemical exchange saturation transfer (CEST) imaging

PURPOSE

To evaluate the reliability of four CEST imaging metrics for brain tumors, at varied saturation power levels and magnetic field strengths (3-9.4 Tesla (T)).

METHODS

A five-pool proton exchange model (free water, semisolid, amide, amine, and NOE-related protons) was used for the simulations. For the in vivo study, eight glioma-bearing rats were scanned at 4.7 T. The CEST ratio (CESTR), CESTR normalized with the reference value (CESTR^nr ), inverse Z-spectrum-based (MTR_Rex ), and apparent exchange-related relaxation (AREX) were compared.

RESULTS

The simulated CEST signal intensities using MTR_Rex and AREX were substantially increased at relatively high radiofrequency (RF) saturation powers at 3 T and 4.7 T, whereas CESTR and CESTR^nr metrics remained relatively stable. There were tremendously high MTR_Rex and AREX signals around the water frequency at all field strengths because of the small denominators. In the rat tumor study at 4.7 T, both CESTR and CESTR^nr showed clear contrasts in the tumor with respect to the normal tissue across all saturation power levels (0.5-3 μT), whereas the AREX showed negligible to negative insignificant contrasts.

CONCLUSIONS

CEST metrics must be carefully selected based on the different experimental settings. CESTR and CESTR^nr are more reliable at 3 T (a clinical field strength) and 4.7 T. Magn Reson Med 77:1853-1865, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

UCEPR: Ultrafast localized CEST-spectroscopy with PRESS in phantoms and in vivo

PURPOSE

Chemical exchange saturation transfer (CEST) is a contrast mechanism enhancing low-concentration molecules through saturation transfer from their exchangeable protons to bulk water. Often many scans are acquired to form a Z-spectrum, making the CEST method time-consuming. Here, an ultrafast localized CEST-spectroscopy with PRESS (UCEPR) is proposed to obtain the entire Z-spectrum of a voxel using only two scans, significantly accelerating CEST.

THEORY AND METHODS

The approach combines ultrafast nonlocalized CEST spectroscopy with localization using PRESS. A field gradient is applied concurrently with the saturation pulse producing simultaneous saturation of all Z-spectrum frequencies that are also spatially encoded. A readout gradient during data acquisition resolves the spatial dependence of the CEST responses into frequency. UCEPR was tested on a 3T scanner both in phantoms and in vivo.

RESULTS

In phantoms, a fast Z-spectroscopy acquisition of multiple pH-variant iopamidol samples was achieved with four- to seven-fold acceleration as compared to the conventional CEST methods. In vivo, amide proton transfer (APT) in white matter of healthy human brain was measured rapidly in 48 s and with high frequency resolution (≤ 0.2 ppm).

CONCLUSION

Compared with conventional CEST methods, UCEPR has the advantage of rapidly acquiring high-resolution Z-spectra. Potential in vivo applications include ultrafast localized Z-spectroscopy, quantitative, or dynamic CEST studies.

Assessment of Glioma Response to Radiotherapy Using Multiple MRI Biomarkers with Manual and Semiautomated Segmentation Algorithms

BACKGROUND AND PURPOSE

Multimodality magnetic resonance imaging (MRI) can provide complementary information in the assessment of brain tumors. We aimed to segment tumor in amide proton transfer-weighted (APTw) images and to investigate multiparametric MRI biomarkers for the assessment of glioma response to radiotherapy. For tumor extraction, we evaluated a semiautomated segmentation method based on region of interest (ROI) results by comparing it with the manual segmentation method.

METHODS

Thirteen nude rats injected with U87 tumor cells were irradiated by an 8-Gy radiation dose. All MRI scans were performed on a 4.7-T animal scanner preradiation, and at day 1, day 4, and day 8 postradiation. Two experts performed manual and semiautomated methods to extract tumor ROIs on APTw images. Multimodality MRI signals of the tumors, including structural (T₂ and T₁ ), functional (apparent diffusion coefficient and blood flow), and molecular (APTw and magnetization transfer ratio or MTR), were calculated and compared quantitatively.

RESULTS

The semiautomated method provided more reliable tumor extraction results on APTw images than the manual segmentation, in less time. A considerable increase in the ADC intensities of the tumor was observed during the postradiation. A steady decrease in the blood flow values and in the APTw signal intensities were found after radiotherapy.

CONCLUSIONS

The semiautomated method of tumor extraction showed greater efficiency and stability than the manual method. Apparent diffusion coefficient, blood flow, and APTw are all useful biomarkers in assessing glioma response to radiotherapy.

Assessing Amide Proton Transfer (APT) MRI Contrast Origins in 9 L Gliosarcoma in the Rat Brain Using Proteomic Analysis

PURPOSE

To investigate the biochemical origin of the amide photon transfer (APT)-weighted hyperintensity in brain tumors.

PROCEDURES

Seven 9 L gliosarcoma-bearing rats were imaged at 4.7 T. Tumor and normal brain tissue samples of equal volumes were prepared with a coronal rat brain matrix and a tissue biopsy punch. The total tissue protein and the cytosolic subproteome were extracted from both samples. Protein samples were analyzed using two-dimensional gel electrophoresis, and the proteins with significant abundance changes were identified by mass spectrometry.

RESULTS

There was a significant increase in the cytosolic protein concentration in the tumor, compared to normal brain regions, but the total protein concentrations were comparable. The protein profiles of the tumor and normal brain tissue differed significantly. Six cytosolic proteins, four endoplasmic reticulum proteins, and five secreted proteins were considerably upregulated in the tumor.

CONCLUSIONS

Our experiments confirmed an increase in the cytosolic protein concentration in tumors and identified several key proteins that may cause APT-weighted hyperintensity.

Evaluations of Tumor Acidosis Within In Vivo Tumor Models Using Parametric Maps Generated with Acido CEST MRI

PURPOSE

We aimed to develop pixelwise maps of tumor acidosis to aid in evaluating extracellular tumor pH (pHe) in cancer biology.

PROCEDURES

MCF-7 and MDA-MB-231 mouse models were imaged during a longitudinal study. AcidoCEST MRI and a series of image processing methods were used to produce parametric maps of tumor pHe, and tumor pHe was also measured with a pH microsensor.

RESULTS

Sufficient contrast-to-noise for producing pHe maps was achieved by using standard image processing methods. A comparison of pHe values measured with acidoCEST MRI and a pH microsensor showed that acidoCEST MRI measured tumor pHe with an accuracy of 0.034 pH units. The MCF-7 tumor model was found to be more acidic compared to the MDA-MB-231 tumor model. The pHe was not related to tumor size during the longitudinal study.

CONCLUSIONS

These results show that acidoCEST MRI can create pixelwise tumor pHe maps of mouse models of cancer.

Grading diffuse gliomas without intense contrast enhancement by amide proton transfer MR imaging: comparisons with diffusion- and perfusion-weighted imaging

OBJECTIVES

To investigate whether amide proton transfer (APT) MR imaging can differentiate high-grade gliomas (HGGs) from low-grade gliomas (LGGs) among gliomas without intense contrast enhancement (CE).

METHODS

This retrospective study evaluated 34 patients (22 males, 12 females; age 36.0 ± 11.3 years) including 20 with LGGs and 14 with HGGs, all scanned on a 3T MR scanner. Only tumours without intense CE were included. Two neuroradiologists independently performed histogram analyses to measure the 90th-percentile (APT₉₀) and mean (APT_mean) of the tumours' APT signals. The apparent diffusion coefficient (ADC) and relative cerebral blood volume (rCBV) were also measured. The parameters were compared between the groups with Student's t-test. Diagnostic performance was evaluated with receiver operating characteristic (ROC) analysis.

RESULTS

The APT₉₀ (2.80 ± 0.59 % in LGGs, 3.72 ± 0.89 in HGGs, P = 0.001) and APT_mean (1.87 ± 0.49 % in LGGs, 2.70 ± 0.58 in HGGs, P = 0.0001) were significantly larger in the HGGs compared to the LGGs. The ADC and rCBV values were not significantly different between the groups. Both the APT₉₀ and APT_mean showed medium diagnostic performance in this discrimination.

CONCLUSIONS

APT imaging is useful in discriminating HGGs from LGGs among diffuse gliomas without intense CE.

KEY POINTS

• Amide proton transfer (APT) imaging helps in grading non-enhancing gliomas • High-grade gliomas showed higher APT signal than low-grade gliomas • APT imaging showed better diagnostic performance than diffusion- and perfusion-weighted imaging.

Improvement of water saturation shift referencing by sequence and analysis optimization to enhance chemical exchange saturation transfer imaging

PURPOSE

To optimize B0-field inhomogeneity correction for chemical exchange saturation transfer (CEST) imaging by investigating different water saturation shift referencing (WASSR) Z-spectrum shapes and different frequency correction techniques.

METHODS

WASSR Z-spectra were simulated for different B1-fields and pulse durations (PD). Two parameter settings were used for further simulations and experiments (WASSR1: B1=0.1 μT, PD=50ms; WASSR2: B1=0.3 μT, PD=40ms). Four frequency correction techniques were investigated: 1) MinW: Minimum of the spline-interpolated WASSR-spectrum; 2) MSCF: maximum symmetry center frequency algorithm; 3) PMSCF: further development of MSCF algorithm; 4) BFit: fit with Bloch equations. Performance of frequency correction was assessed with Monte-Carlo simulations and in-vivo MR examinations in the brain and intervertebral disks.

RESULTS

Different shapes of WASSR-Z-spectra were obtained by changing B1 and PD including spectra with one (1-Peak) or two (2-Peak) minima. WASSR1 resulted in 1-Peak WASSR-spectrum, whereas WASSR2 resulted in 2-Peak WASSR-spectrum. Both Monte-Carlo simulations and in-vivo MR examinations revealed highest accuracy of field-inhomogeneity correction with WASSR1 combined with PMSCF or BFit.

CONCLUSION

Using a WASSR sequence, which results in a Z-spectrum with a single absorption peak, in combination with advanced postprocessing algorithms enables improved B0-field inhomogeneity correction for CEST imaging.

Amide proton transfer (APT) magnetic resonance imaging of prostate cancer: comparison with Gleason scores

OBJECTIVE

To evaluate the utility of amide proton transfer (APT) imaging in estimating the Gleason score (GS) of prostate cancer (Pca).

MATERIALS AND METHODS

Sixty-six biopsy-proven cancers were categorized into four groups according to the GS: GS-6 (3 + 3); GS-7 (3 + 4/4 + 3); GS-8 (4 + 4) and GS-9 (4 + 5/5 + 4). APT signal intensities (APT SIs) and apparent diffusion coefficient (ADC) values of each GS group were compared by one-way analysis of variance with Tukey's HSD post hoc test.

RESULTS

The mean and standard deviation of the APT SIs (%) and ADC values (×10(-3) mm(2)/s) were as follows: GS-6, 2.48 ± 0.59 and 1.16 ± 0.26; GS-7, 5.17 ± 0.66 and 0.92 ± 0.18; GS-8, 2.56 ± 0.85 and 0.86 ± 0.17; GS-9, 1.96 ± 0.75 and 0.85 ± 0.18, respectively. The APT SI of the GS-7 group was highest, and there were significant differences between the GS-6 and GS-7 groups and the GS-7 and GS-9 groups (p < 0.05). The ADC value of the GS-6 group was significantly higher than each value of the GS-7, GS-8, and GS-9 groups (p < 0.05), but no significant differences were obtained among the GS-7, GS-8, and GS-9 groups.

CONCLUSION

The mean APT SI in Pca with a GS of 7 was higher than that for the other GS groups.

Imaging the accumulation and suppression of tau pathology using multiparametric MRI

Mouse models of Alzheimer's disease have served as valuable tools for investigating pathogenic mechanisms relating to neurodegeneration, including tau-mediated and neurofibrillary tangle pathology-a major hallmark of the disease. In this work, we have used multiparametric magnetic resonance imaging (MRI) in a longitudinal study of neurodegeneration in the rTg4510 mouse model of tauopathy, a subset of which were treated with doxycycline at different time points to suppress the tau transgene. Using this paradigm, we investigated the sensitivity of multiparametric MRI to both the accumulation and suppression of pathologic tau. Tau-related atrophy was discernible from 5.5 months within the cortex and hippocampus. We observed markedly less atrophy in the treated rTg4510 mice, which was enhanced after doxycycline intervention from 3.5 months. We also observed differences in amide proton transfer, cerebral blood flow, and diffusion tensor imaging parameters in the rTg4510 mice, which were significantly less altered after doxycycline treatment. We propose that these non-invasive MRI techniques offer insight into pathologic mechanisms underpinning Alzheimer's disease that may be important when evaluating emerging therapeutics targeting one of more of these processes.

Accelerating chemical exchange saturation transfer (CEST) MRI by combining compressed sensing and sensitivity encoding techniques

PURPOSE

To evaluate the feasibility of accelerated chemical-exchange-saturation-transfer (CEST) imaging using a combination of compressed sensing (CS) and sensitivity encoding (SENSE) at 3 Tesla.

THEORY AND METHODS

Two healthy volunteers and six high-grade glioma patients were recruited. Raw CEST image k-space data were acquired (with varied radiofrequency saturation power levels for the healthy volunteer study), and a sequential CS and SENSE reconstruction (CS-SENSE) was assessed. The MTR_asym (3.5 ppm) signals were compared with varied CS-SENSE acceleration factors.

RESULTS

In the healthy volunteer study, a CS-SENSE acceleration factor of R = 2 × 2 (CS × SENSE) was achieved without compromising the reconstructed MTR_asym (3.5 ppm) image quality. The MTR_asym (3.5 ppm) signals obtained from the CS-SENSE reconstruction with R = 2 × 2 were well preserved compared with the reference image (R = 2 for only SENSE). In the glioma patient study, the MTR_asym (3.5 ppm) signals were significantly higher in the tumor region (Gd-enhancing tumor core) than in the normal-appearing white matter (P < 0.001). There was no significant MTR_asym (3.5 ppm) difference between the reference image and CS-SENSE-reconstructed image in the acceleration factor of R = 2 × 2.

CONCLUSION

Combining the SENSE technique with CS (R = 2 × 2) enables considerable acceleration of CEST image acquisition and potentially has a wide range of clinical applications. Magn Reson Med 77:779-786, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Added value of amide proton transfer imaging to conventional and perfusion MR imaging for evaluating the treatment response of newly diagnosed glioblastoma

OBJECTIVES

To determine the added value of amide proton transfer (APT) imaging to conventional and perfusion MRI for differentiating tumour progression (TP) from the treatment-related effect (TE) in patients with post-treatment glioblastomas.

METHODS

Sixty-five consecutive patients with enlarging contrast-enhancing lesions following concurrent chemoradiotherapy were assessed using contrast-enhanced T1-weighted MRI (CE-T1WI), 90th percentile histogram parameters of normalized cerebral blood volume (nCBV90) and APT asymmetry value (APT90). Diagnostic performance was determined using the area under the receiver operating characteristic curve (AUC) and cross validations.

RESULTS

There were statistically significant differences in the mean APT90 between the TP and the TE groups (3.87-4.01 % vs. 1.38-1.41 %; P < .001). Compared with CE-T1WI alone, the addition of APT90 to CE-T1WI significantly improved cross-validated AUC from 0.58-0.74 to 0.89-0.91 for differentiating TP from TE. The combination of CE-T1WI, nCBV90 and APT90 resulted in greater diagnostic accuracy for differentiating TP from TE than the combination of CE-T1WI and nCBV90 (cross-validated AUC, 0.95-0.97 vs. 0.84-0.91). The inter-reader agreement between the expert and trainee was excellent for the measurements of APT90 (intraclass correlation coefficient, 0.94).

CONCLUSION

Adding APT imaging to conventional and perfusion MRI improves the diagnostic performance for differentiating TP from TE.

KEY POINTS

• APT imaging could provide a reliable distinction between TP and TE • Adding APT imaging to CE-T1WI improved the diagnostic accuracy versus CE-T1WI alone • Multimodal imaging using CE-T1WI, perfusion and APT imaging led to accurate diagnosis • The inter-reader agreement of APT histogram parameters was excellent.

Quantitative assessment of the effects of water proton concentration and water T1 changes on amide proton transfer (APT) and nuclear overhauser enhancement (NOE) MRI: The origin of the APT imaging signal in brain tumor

PURPOSE

To quantify pure chemical exchange-dependent saturation transfer (CEST) related amide proton transfer (APT) and nuclear Overhauser enhancement (NOE) signals in a rat glioma model and to investigate the mixed effects of water content and water T₁ on APT and NOE imaging signals.

METHODS

Eleven U87 tumor-bearing rats were scanned at 4.7 T. A relatively accurate mathematical approach, based on extrapolated semisolid magnetization-transfer reference signals, was used to remove the concurrent effects of direct water saturation and semisolid magnetization-transfer. Pure APT and NOE signals, in addition to the commonly used magnetization-transfer-ratio asymmetry at 3.5 ppm, MTR_asym (3.5ppm), were assessed.

RESULTS

The measured APT signal intensity of the tumor (11.06%, much larger than the value reported in the literature) was the major contributor (approximately 80.6%) to the MTR_asym (3.5ppm) contrast between the tumor and the contralateral brain region. Both the water content ([water proton]) and water T₁ (T_1w ) were increased in the tumor, but there were no significant correlations among APT, NOE, or MTR_asym (3.5ppm) signals and T_1w /[water proton].

CONCLUSION

The effect of increasing T_1w on the CEST signal in the tumor was mostly eliminated by the effect of increasing water content, and the observed APT-weighted hyperintensity in the tumor should be dominated by the increased amide proton concentration. Magn Reson Med 77:855-863, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Applying amide proton transfer-weighted MRI to distinguish pseudoprogression from true progression in malignant gliomas

PURPOSE

To assess amide proton transfer-weighted (APTW) imaging features in patients with malignant gliomas after chemoradiation and the diagnostic performance of APT imaging for distinguishing true progression from pseudoprogression.

MATERIALS AND METHODS

After approval by the Institutional Review Board, 32 patients with clinically suspected tumor progression in the first 3 months after chemoradiation were enrolled and scanned at 3T. Longitudinal routine magnetic resonance imaging (MRI) changes and medical records were assessed to confirm true progression versus pseudoprogression. True progression was defined as lesions progressing on serial imaging over 6 months, and pseudoprogression was defined as lesions stabilizing or regressing without intervention. The APTWmean and APTWmax signals were obtained from three to five regions of interests for each patient and compared between the true progression and pseudoprogression groups. The diagnostic performance was assessed with receiver operating characteristic curve analysis.

RESULTS

The true progression was associated with APTW hyperintensity (APTWmean  = 2.75% ± 0.42%), while pseudoprogression was associated with APTW isointensity to mild hyperintensity (APTWmean  = 1.56% ± 0.42%). The APTW signal intensities were significantly higher in the true progression group (n = 20) than in the pseudoprogression group (P < 0.001; n = 12). The cutoff APTWmean and APTWmax intensity values to distinguish between true progression and pseudoprogression were 2.42% (with a sensitivity of 85.0% and a specificity of 100%) and 2.54% (with a sensitivity of 95.0% and a specificity of 91.7%), respectively.

CONCLUSION

The APTW-MRI signal is a valuable imaging biomarker for distinguishing pseudoprogression from true progression in glioma patients. J. Magn. Reson. Imaging 2016;44:456-462.

Mapping murine diabetic kidney disease using chemical exchange saturation transfer MRI

PURPOSE

Diabetic nephropathy (DN) is the leading cause of renal failure; however, current clinical tests are insufficient for assessing this disease. DN is associated with changes in renal metabolites, so we evaluated the utility of chemical exchange saturation transfer (CEST) imaging to detect changes characteristic of this disease.

METHODS

Sensitivity of CEST imaging at 7 Tesla to DN was evaluated by imaging diabetic mice [db/db, db/db endothelial nitric oxide synthase (eNOS)-/-] that show different levels of nephropathy as well as by longitudinal imaging (8 to 24 weeks). Nondiabetic (db/m) mice were used as controls.

RESULTS

Compared with nondiabetic mice, the CEST contrasts of hydroxyl metabolites that correspond to glucose and glycogen were significantly increased in papilla (P), inner medulla (IM), and outer medulla (OM) in db/db and db/db eNOS-/- kidneys at 16 weeks. The db/db eNOS-/- mice that showed advanced nephropathy exhibited greater CEST effects in OM and significant CEST contrasts were also observed in cortex. Longitudinally, db/db mice exhibited progressive increases in hydroxyl signals in IM+P and OM from 12 to 24 weeks and an increase was also observed in cortex at 24 weeks.

CONCLUSION

CEST MRI can be used to measure changes of hydroxyl metabolites in kidney during progression of DN. Magn Reson Med 76:1531-1541, 2016. © 2015 International Society for Magnetic Resonance in Medicine.

Chemical Exchange Saturation Transfer MR Imaging is Superior to Diffusion-Tensor Imaging in the Diagnosis and Severity Evaluation of Parkinson's Disease: A Study on Substantia Nigra and Striatum

Parkinson's disease (PD) is a neurodegenerative disorder characterized by nigrostriatal cell loss. To date, the diagnosis of PD is still based primarily on the clinical manifestations, which may be typical and obvious only in advanced-stage PD. Thus, it is crucial to find a reliable marker for the diagnosis of PD. We conducted this study to assess the diagnostic efficiency of chemical exchange saturation transfer (CEST) imaging and diffusion-tensor imaging (DTI) in PD at 3 T by evaluating changes on substantia nigra and striatum. Twenty-three PD patients and twenty-three age-matched normal controls were recruited. All patients and controls were imaged on a 3-T MR system, using an eight-channel head coil. CEST imaging was acquired in two transverse slices of the head, including substantia nigra and striatum. The magnetization transfer ratio asymmetry at 3.5 ppm, MTRasym(3.5 ppm), and the total CEST signal intensity between 0 and 4 ppm were calculated. Multi-slice DTI was acquired for all the patients and normal controls. Quantitative analysis was performed on the substantia nigra, globus pallidus, putamen, and caudate. The MTRasym(3.5 ppm) value, the total CEST signal intensity, and fractional anisotropy value of the substantia nigra were all significantly lower in PD patients than in normal controls (P = 0.003, P = 0.004, and P < 0.001, respectively). The MTRasym(3.5 ppm) values of the putamen and the caudate were significantly higher in PD patients than in normal controls (P = 0.010 and P = 0.009, respectively). There were no significant differences for the mean diffusivity in these four regions between PD patients and normal controls. In conclusion, CEST MR imaging provided multiple CEST image contrasts in the substantia nigra and the striatum in PD and may be superior to DTI in the diagnosis of PD.

Highly accelerated chemical exchange saturation transfer (CEST) measurements with linear algebraic modeling

PURPOSE

In clinical studies, compartmental average chemical exchange saturation transfer (CEST) measurements rather than voxel-by-voxel CEST images may suffice for evaluating its diagnostic value. A recently developed method-spectroscopy with linear algebraic modeling, or SLAM-could directly provide compartmental measures with dramatically reduced scan time and optimal signal-to-noise ratios. Here, we test whether SLAM can be adapted to significantly accelerate CEST acquisitions.

THEORY AND METHODS

Conventional anatomical images and raw CEST image k-space data were acquired from seven brain tumor patients. SLAM was applied to the CEST data using acceleration factors of R = 1-45, after segmenting compartments from co-registered images. SLAM-CEST measures were compared with average values from the identical compartments obtained by conventional Fourier transform (FT) CEST.

RESULTS

SLAM generated compartmental average CEST z-spectra that were indistinguishable from conventional FT-CEST for R ≤ 45. SLAM-CEST z-spectra at ±3.5 ppm were highly correlated with FT-CEST measures (r(2)  ≥ 0.98 for R ≤ 9; r ≥ 0.995 for R ≤ 45). The average error of SLAM-CEST versus FT-CEST measures was ≤10% for R ≤ 45, in acquisitions requiring as few as a single k-space phase-encoding step.

CONCLUSION

Applied to patients with brain tumors, SLAM-CEST can yield results that are quantitatively equivalent to conventional CEST up to 45 times faster, which could prove enabling in clinical settings where scan time is limiting. Magn Reson Med 76:136-144, 2016. © 2015 Wiley Periodicals, Inc.

Accuracy and uncertainty of asymmetric magnetization transfer ratio quantification for amide proton transfer (APT) imaging at 3T: a Monte Carlo study

Amide proton transfer (APT) imaging offers a novel and powerful MRI contrast mechanism for quantitative molecular imaging based on the principle of chemical exchange saturation transfer (CEST). Asymmetric magnetization transfer ratio (MTR(asym)) quantification is crucial for Z-spectrum analysis of APT imaging, but is still challenging, particularly at clinical field strength. This paper studies the accuracy and uncertainty in the quantification of MTR(asym) for APT imaging at 3T, by using high-order polynomial fitting of Z-spectrum through Monte Carlo simulation. Results show that polynomial fitting is a biased estimator that consistently underestimates MTR(asym). For a fixed polynomial order, the accuracy of MTR(asym) is almost constant with regard to signal-to-noise ratio (SNR) while the uncertainty decreases exponentially with SNR. The higher order polynomial fitting increases both the accuracy and the uncertainty of MTR(asym). For different APT signal intensity levels, the relative accuracy and the absolute uncertainty keep constant for a fixed polynomial order. These results indicate the limitations and pitfalls of polynomial fitting for MTR(asym) quantification so better quantification technique for MTR(asym) estimation is warranted.

Effect of the saturation pulse duration on chemical exchange saturation transfer in amide proton transfer MR imaging: a phantom study

Amide proton transfer (APT) contrast imaging is based on the chemical exchange saturation transfer (CEST) of protons between the amide groups and bulk water. Here, we demonstrate the effect of the saturation pulse duration on CEST in APT imaging with use of a clinical MR scanner. Four samples were prepared from chicken egg white diluted with H2O. Experiments were performed on a 3T clinical MR scanner with use of a body coil for two-channel parallel radiofrequency transmission. APT images were acquired at six frequency offsets (± 3.0, ± 3.5, ± 4.0 ppm) with respect to the water resonance as well as one far off-resonant frequency (-160 ppm) for signal normalization. The CEST effect was defined as asymmetry of the magnetization transfer ratio at 3.5 ppm. We measured the CEST effects in the egg white samples with different concentrations at seven saturation pulse durations. The influence of the extension of repetition time (TR) on the CEST effect was also evaluated. The CEST effect was not influenced by the change in TR. The CEST effect was increased significantly with the concentration when the duration was ≥1.0 s (P < 0.01). The CEST effect was highly correlated with the concentration at all saturation pulse durations, and its increase ratio was higher at longer saturation pulse durations. In conclusion, a long saturation pulse duration is useful for the sensitive detection of mobile proteins and peptides in APT imaging.

Novel Magnetic Resonance Imaging Techniques in Brain Tumors

Magnetic resonance imaging is a powerful, noninvasive imaging technique with exquisite sensitivity to soft tissue composition. Magnetic resonance imaging is primary tool for brain tumor diagnosis, evaluation of drug response assessment, and clinical monitoring of the patient during the course of their disease. The flexibility of magnetic resonance imaging pulse sequence design allows for a variety of image contrasts to be acquired, including information about magnetic resonance-specific tissue characteristics, molecular dynamics, microstructural organization, vascular composition, and biochemical status. The current review highlights recent advancements and novel approaches in MR characterization of brain tumors.

Quantitative assessment of amide proton transfer (APT) and nuclear overhauser enhancement (NOE) imaging with extrapolated semisolid magnetization transfer reference (EMR) signals: II. Comparison of three EMR models and application to human brain glioma at 3 Tesla

PURPOSE

To evaluate the use of three extrapolated semisolid magnetization transfer reference (EMR) methods to quantify amide proton transfer (APT) and nuclear Overhauser enhancement (NOE) signals in human glioma.

METHODS

Eleven patients with high-grade glioma were scanned at 3 Tesla. aEMR(2) (asymmetric magnetization-transfer or MT model to fit two-sided, wide-offset data), sEMR(2) (symmetric MT model to fit two-sided, wide-offset data), and sEMR(1) (symmetric MT model to fit one-sided, wide-offset data) were assessed. ZEMR and experimental data at 3.5 ppm and -3.5 ppm were subtracted to calculate the APT and NOE signals (APT(#) and NOE(#)), respectively.

RESULTS

The aEMR(2) and sEMR(1) models provided quite similar APT(#) signals, while the sEMR(2) provided somewhat lower APT(#) signals. The aEMR(2) had an erroneous NOE(#) quantification. Calculated APT(#) signal intensities of glioma (∼4%), much larger than the values reported previously, were significantly higher than those of edema and normal tissue. Compared with normal tissue, gadolinium-enhancing tumor cores were consistently hyperintense on the APT(#) maps and slightly hypointense on the NOE(#) maps.

CONCLUSION

The sEMR(1) model is the best choice for accurately quantifying APT and NOE signals. The APT-weighted hyperintensity in the tumor was dominated by the APT effect, and the MT asymmetry at 3.5 ppm is a reliable and valid metric for APT imaging of gliomas at 3T.

Imaging chemical exchange saturation transfer (CEST) effects following tumor-selective acidification using lonidamine

Increased lactate production through glycolysis in aerobic conditions is a hallmark of cancer. Some anticancer drugs have been designed to exploit elevated glycolysis in cancer cells. For example, lonidamine (LND) inhibits lactate transport, leading to intracellular acidification in cancer cells. Chemical exchange saturation transfer (CEST) is a novel MRI contrast mechanism that is dependent on intracellular pH. Amine and amide concentration-independent detection (AACID) and apparent amide proton transfer (APT*) represent two recently developed CEST contrast parameters that are sensitive to pH. The goal of this study was to compare the sensitivity of AACID and APT* for the detection of tumor-selective acidification after LND injection. Using a 9.4-T MRI scanner, CEST data were acquired in mice approximately 14 days after the implantation of 10(5) U87 human glioblastoma multiforme (GBM) cells in the brain, before and after the administration of LND (dose, 50 or 100 mg/kg). Significant dose-dependent LND-induced changes in the measured CEST parameters were detected in brain regions spatially correlated with implanted tumors. Importantly, no changes were observed in T1- and T2-weighted images acquired before and after LND treatment. The AACID and APT* contrast measured before and after LND injection exhibited similar pH sensitivity. Interestingly, LND-induced contrast maps showed increased heterogeneity compared with pre-injection CEST maps. These results demonstrate that CEST contrast changes after the administration of LND could help to localize brain cancer and monitor tumor response to chemotherapy within 1 h of treatment. The LND CEST experiment uses an anticancer drug to induce a metabolic change detectable by endogenous MRI contrast, and therefore represents a unique cancer detection paradigm which differs from other current molecular imaging techniques that require the injection of an imaging contrast agent or tracer.

Molecular MRI differentiation between primary central nervous system lymphomas and high-grade gliomas using endogenous protein-based amide proton transfer MR imaging at 3 Tesla

OBJECTIVES

To show the ability of using the amide proton transfer-weighted (APTW) MRI signals as imaging biomarkers to differentiate primary central nervous system lymphomas (PCNSLs) from high-grade gliomas (HGGs).

METHODS

Eleven patients with lymphomas and 21 patients with HGGs were examined. Magnetization-transfer (MT) spectra over an offset range of ± 6 ppm and the conventional MT ratio (MTR) at 15.6 ppm were acquired. The APTW signals, total chemical-exchange-saturation-transfer signal (integral between 0 and 5 ppm, CEST total), and MTR signal were obtained and compared between PCNSLs and HGGs. The diagnostic performance was assessed with the receiver operating characteristic (ROC) curve analysis.

RESULTS

The PCNSLs usually showed more homogeneous APTW hyperintensity (spatially compared to normal brain tissue) than the HGGs. The APTW max, APTW max-min and CEST total signal intensities were significantly lower (P < 0.05, 0.001 and 0.05, respectively), while the APTW min and MTR were significantly higher (both P < 0.01) in PCNSL lesions than in HGG lesions. The APTW values in peritumoral oedema were significantly lower for PCNSLs than for HGGs (P < 0.01). APTW max-min had the highest area under the ROC curve (0.963) and accuracy (94.1 %) in differentiating PCNSLs from HGGs.

CONCLUSIONS

The protein-based APTW signal would be a valuable MRI biomarker by which to identify PCNSLs and HGGs presurgically.

KEY POINTS

PCNSLs overall showed more homogeneous APTW hyperintensity than HGGs. Maximum APTW signals were lower in PCNSL lesions than in HGG lesions. MTR signals were higher in PCNSLs than in HGGs. APTW heterogeneity had the highest accuracy in differentiating PCNSLs from HGGs.

Histogram Analysis of Amide Proton Transfer Imaging to Identify Contrast-enhancing Low-Grade Brain Tumor That Mimics High-Grade Tumor: Increased Accuracy of MR Perfusion

PURPOSE

To determine whether histogram analysis of amide proton transfer (APT) imaging provides increased accuracy of magnetic resonance (MR) perfusion imaging for the identification of contrast material-enhancing low-grade tumor (World Health Organization grades 1 and 2) that mimics high-grade tumor (World Health Organization grades 3 and 4).

MATERIALS AND METHODS

This retrospective study was approved by the institutional review board. Forty-five patients with pathologically proven, solitary, contrast-enhancing tumors were enrolled in this study. APT-derived signal intensity from the calculated APT asymmetry at the offset frequency of 3.5 ppm and normalized cerebral blood volume (nCBV) were measured on solid portions of the tumor by using a 90% histogram cutoff (denoted as APT90 and nCBV90, respectively). The diagnostic performance of the imaging parameters was determined with leave-one-out cross validation. Interobserver agreement was assessed by using the intraclass correlation coefficient.

RESULTS

APT90 demonstrated a significant difference between contrast-enhancing low-grade and high-grade tumors for both readers (P < .001 for both readers). Compared with nCBV90, adding APT90 significantly improved the area under the receiver operating characteristic curve (AUC) for the identification of contrast-enhancing low-grade tumor from 0.80 to 0.97 for reader 1 (P = .023) and from 0.82 to 0.97 for reader 2 (P = .035), respectively. By using leave-one-out cross-validation, the cross-validated AUC of the combination of nCBV90 and APT90 was 0.95 for reader 1 and 0.96 for reader 2. The intraclass correlation coefficient for the APT90 calculations was 0.89.

CONCLUSION

Histogram analysis of APT imaging provided increased accuracy of MR perfusion imaging for the identification of contrast-enhancing low-grade tumor that mimics high-grade tumor.

Simultaneous detection and separation of hyperacute intracerebral hemorrhage and cerebral ischemia using amide proton transfer MRI

PURPOSE

To explore the capability of amide proton transfer (APT) imaging in the detection of hemorrhagic and ischemic strokes using preclinical rat models.

METHODS

The rat intracerebral hemorrhage (ICH) model (n = 10) was induced by injecting bacterial collagenase VII-S into the caudate nucleus, and the permanent ischemic stroke model (n = 10) was induced by using a 4-0 nylon suture to occlude the origin of the middle cerebral artery. APT-weighted (APTw) MRI was acquired on a 4.7T animal imager and quantified using the magnetization transfer-ratio asymmetry at 3.5 ppm from water.

RESULTS

There was a consistently high APTw MRI signal in hyperacute ICH during the initial 12 h after injection of collagenase compared with the contralateral brain tissue. When hemorrhagic and ischemic stroke were compared, hyperacute ICH and cerebral ischemia demonstrated opposite APTw MRI contrasts-namely, hyperintense versus hypointense compared with contralateral brain tissue, respectively. There was a stark contrast in APTw signal intensity between these two lesions.

CONCLUSION

APT-MRI could accurately detect hyperacute ICH and distinctly differentiate hyperacute ICH from cerebral ischemia, thus opening up the possibility of introducing to the clinic a single MRI scan for the simultaneous visualization and separation of hemorrhagic and ischemic strokes at the hyperacute stage. Magn Reson Med 74:42-50, 2015. © 2014 Wiley Periodicals, Inc.

Scan-rescan reproducibility of parallel transmission based amide proton transfer imaging of brain tumors

PURPOSE

To evaluate the reproducibility of amide proton transfer (APT) imaging of brain tumors using a parallel transmission-based technique.

MATERIALS AND METHODS

Thirteen patients with brain tumors (four low-grade gliomas, three glioblastoma multiforme, five meningiomas, and one malignant lymphoma) were included in the study. APT imaging was conducted at 3T using a 2-channel parallel transmission scheme with a saturation time of 2 seconds and B1 amplitude of 2 μT. A 2D fast spin-echo sequence with driven-equilibrium refocusing was used for imaging. Z-spectra were obtained at 25 frequency offsets from -6 to +6 ppm (step 0.5 ppm). A point-by-point B0 correction was performed with a B0 map. A scan-rescan reproducibility test was performed in two sessions on separate days for each patient. The interval between the two sessions was 4.8 ± 3.5 days. Regions-of-interest (ROIs) were placed to include the whole tumor for each case. A mean and 90-percentile value of APT signal for the whole tumor histogram was calculated for each session. The between-session and within-session reproducibility was evaluated using linear regression analysis, intraclass correlation coefficient (ICC), and a Bland-Altman plot.

RESULTS

The mean and 90-percentile values of the APT signal for whole tumor ROI showed excellent agreements between the two sessions, with R(2) of 0.91 and 0.96 in the linear regression analysis and ICC of 0.95 and 0.97, respectively.

CONCLUSION

Parallel transmission-based APT imaging of brain tumors showed good reproducibility.

Nuclear Overhauser Enhancement imaging of glioblastoma at 7 Tesla: region specific correlation with apparent diffusion coefficient and histology

OBJECTIVE

To explore the correlation between Nuclear Overhauser Enhancement (NOE)-mediated signals and tumor cellularity in glioblastoma utilizing the apparent diffusion coefficient (ADC) and cell density from histologic specimens. NOE is one type of chemical exchange saturation transfer (CEST) that originates from mobile macromolecules such as proteins and might be associated with tumor cellularity via altered protein synthesis in proliferating cells.

PATIENTS AND METHODS

For 15 patients with newly diagnosed glioblastoma, NOE-mediated CEST-contrast was acquired at 7 Tesla (asymmetric magnetization transfer ratio (MTRasym) at 3.3ppm, B1 = 0.7 μT). Contrast enhanced T1 (CE-T1), T2 and diffusion-weighted MRI (DWI) were acquired at 3 Tesla and coregistered. The T2 edema and the CE-T1 tumor were segmented. ADC and MTRasym values within both regions of interest were correlated voxelwise yielding the correlation coefficient rSpearman (rSp). In three patients who underwent stereotactic biopsy, cell density of 12 specimens per patient was correlated with corresponding MTRasym and ADC values of the biopsy site.

RESULTS

Eight of 15 patients showed a weak or moderate positive correlation of MTRasym and ADC within the T2 edema (0.16≤rSp≤0.53, p<0.05). Seven correlations were statistically insignificant (p>0.05, n = 4) or yielded rSp≈0 (p<0.05, n = 3). No trend towards a correlation between MTRasym and ADC was found in CE-T1 tumor (-0.31<rSp<0.28, p<0.05, n = 9; p>0.05, n = 6). The biopsy-analysis within CE-T1 tumor revealed a strong positive correlation between tumor cellularity and MTRasym values in two of the three patients (rSppatient3 = 0.69 and rSppatient15 = 0.87, p<0.05), while the correlation of ADC and cellularity was heterogeneous (rSppatient3 = 0.545 (p = 0.067), rSppatient4 = -0.021 (p = 0.948), rSppatient15 = -0.755 (p = 0.005)).

DISCUSSION

NOE-imaging is a new contrast promising insight into pathophysiologic processes in glioblastoma regarding cell density and protein content, setting itself apart from DWI. Future studies might be based on the assumption that NOE-mediated CEST visualizes cellularity more accurately than ADC, especially in the CE-T1 tumor region.

Quantitative assessment of amide proton transfer (APT) and nuclear overhauser enhancement (NOE) imaging with extrapolated semi-solid magnetization transfer reference (EMR) signals: Application to a rat glioma model at 4.7 Tesla

PURPOSE

To quantify amide proton transfer (APT) and nuclear Overhauser enhancement (NOE) contributions to in vivo chemical exchange saturation transfer MRI signals in tumors.

THEORY AND METHODS

Two-pool (free water and semi-solid protons) and four-pool (free water, semi-solid, amide, and upfield NOE-related protons) tissue models combined with the super-Lorentzian lineshape for semi-solid protons were used to fit wide and narrow frequency-offset magnetization-transfer (MT) data, respectively. Extrapolated semi-solid MT signals at 3.5 and -3.5 ppm from water were used as reference signals to quantify APT and NOE, respectively. Six glioma-bearing rats were scanned at 4.7 Tesla. Quantitative APT and NOE signals were compared at three saturation power levels.

RESULTS

The observed APT signals were significantly higher in the tumor (center and rim) than in the contralateral normal brain tissue at all saturation powers, and were the major contributor to the APT-weighted image contrast (based on MT asymmetry analysis) between the tumor and the normal brain tissue. The NOE (a positive confounding factor) enhanced this APT-weighted image contrast. The fitted amide pool sizes were significantly larger, while the NOE-related pool sizes were significantly smaller in the tumor than in the normal brain tissue.

CONCLUSION

The extrapolated semi-solid magnetization transfer reference provides a relatively accurate approach for quantitatively measuring pure APT and NOE signals.

Relaxation-compensated CEST-MRI of the human brain at 7T: Unbiased insight into NOE and amide signal changes in human glioblastoma

Endogenous chemical exchange saturation transfer (CEST) effects of protons resonating near to water protons are always diluted by competing effects such as direct water saturation and semi-solid magnetization transfer (MT). This leads to unwanted T2 and MT signal contributions that contaminate the observed CEST signal. Furthermore, all CEST effects appear to be scaled by the T1 relaxation time of the mediating water pool. As MT, T1 and T2 are also altered in tumor regions, a recently published correction algorithm yielding the apparent exchange-dependent relaxation AREX, is used to evaluate in vivo CEST effects. This study focuses on CEST effects of amides (3.5ppm) and Nuclear-Overhauser-mediated saturation transfer (NOE, -3.5ppm) that can be properly isolated at 7T. These were obtained in 10 glioblastoma patients, and this is the first comprehensive study where AREX is applied in human brain as well as in human glioblastoma. The correction of CEST effects alters the contrast significantly: after correction, the CEST effect of amides does not show significant contrast between contrast enhancing tumor regions and normal tissue, whereas NOE drops significantly in the tumor area. In addition, new features in the AREX contrasts are visible. This suggests that previous CEST approaches might not have shown pure CEST effects, but rather water relaxation shine-through effects. Our insights help to improve understanding of the CEST effect changes in tumors and correlations on a cellular and molecular level.

A review of optimization and quantification techniques for chemical exchange saturation transfer MRI toward sensitive in vivo imaging

Chemical exchange saturation transfer (CEST) MRI is a versatile imaging method that probes the chemical exchange between bulk water and exchangeable protons. CEST imaging indirectly detects dilute labile protons via bulk water signal changes following selective saturation of exchangeable protons, which offers substantial sensitivity enhancement and has sparked numerous biomedical applications. Over the past decade, CEST imaging techniques have rapidly evolved owing to contributions from multiple domains, including the development of CEST mathematical models, innovative contrast agent designs, sensitive data acquisition schemes, efficient field inhomogeneity correction algorithms, and quantitative CEST (qCEST) analysis. The CEST system that underlies the apparent CEST-weighted effect, however, is complex. The experimentally measurable CEST effect depends not only on parameters such as CEST agent concentration, pH and temperature, but also on relaxation rate, magnetic field strength and more importantly, experimental parameters including repetition time, RF irradiation amplitude and scheme, and image readout. Thorough understanding of the underlying CEST system using qCEST analysis may augment the diagnostic capability of conventional imaging. In this review, we provide a concise explanation of CEST acquisition methods and processing algorithms, including their advantages and limitations, for optimization and quantification of CEST MRI experiments.

Grading glial tumors with amide proton transfer MR imaging: different analytical approaches

Amide proton transfer (APT) magnetic resonance imaging is gaining attention for its capability for grading glial tumors. Usually, a representative slice is analyzed. Different definitions of tumor areas have been employed in previous studies. We hypothesized that the accuracy of APT imaging for brain tumor grading may depend upon the analytical methodology used, such as selection of regions of interest (ROIs), single or multiple tumor slices, and whether or not there is normalization to the contralateral white matter. This study was approved by the institutional review board, and written informed consent was waived. Twenty-six patients with histologically proven glial tumors underwent preoperative APT imaging with a three-dimensional gradient-echo sequence. Two neuroradiologists independently analyzed APT asymmetry (APTasym) images by placing ROIs on both a single representative slice (RS) and all slices including tumor (i.e. whole tumor: WT). ROIs indicating tumor extent were separately defined on both FLAIR and, if applicable, contrast-enhanced T1-weighted images (CE-T1WI), yielding four mean APTasym values (RS-FLAIR, WT-FLAIR, RS-CE-T1WI, and WT-CE-T1WI). The maximum values were also measured using small ROIs, and their differences among grades were evaluated. Receiver operating characteristic (ROC) curve analysis was also conducted on mean and maximum values. Intra-class correlation coefficients for inter-observer agreement were excellent. Significant differences were observed between high- and low-grade gliomas for all five methods (P < 0.01). ROC curve analysis found no statistically significant difference among them. This study clarifies that single-slice APT analysis is robust despite tumor heterogeneity, and can grade glial tumors with or without the use of contrast material.

Magnetization transfer prepared gradient echo MRI for CEST imaging

Chemical exchange saturation transfer (CEST) is an emerging MRI contrast mechanism that is capable of noninvasively imaging dilute CEST agents and local properties such as pH and temperature, augmenting the routine MRI methods. However, the routine CEST MRI includes a long RF saturation pulse followed by fast image readout, which is associated with high specific absorption rate and limited spatial resolution. In addition, echo planar imaging (EPI)-based fast image readout is prone to image distortion, particularly severe at high field. To address these limitations, we evaluated magnetization transfer (MT) prepared gradient echo (GRE) MRI for CEST imaging. We proved the feasibility using numerical simulations and experiments in vitro and in vivo. Then we optimized the sequence by serially evaluating the effects of the number of saturation steps, MT saturation power (B1), GRE readout flip angle (FA), and repetition time (TR) upon the CEST MRI, and further demonstrated the endogenous amide proton CEST imaging in rats brains (n = 5) that underwent permanent middle cerebral artery occlusion. The CEST images can identify ischemic lesions in the first 3 hours after occlusion. In summary, our study demonstrated that the readily available MT-prepared GRE MRI, if optimized, is CEST-sensitive and remains promising for translational CEST imaging.

Evaluations of extracellular pH within in vivo tumors using acidoCEST MRI

PURPOSE

A practical, noninvasive method is needed to measure the extracellular pH (pHe) within in vivo tumors to longitudinally monitor tumor acidosis. We have optimized a biomedical imaging method, termed acidoCEST MRI, to provide noninvasive assessments of tumor pHe in preclinical models of mammary carcinoma.

METHODS

A CEST-FISP MRI method was optimized to detect the chemical exchange saturation transfer (CEST) of two amide protons of a clinically approved CT contrast agent, iopromide. The ratio of the two CEST effects was used to measure pH. Routes of administration of iopromide were evaluated to ensure sufficient delivery of the agent to the tumor. The optimized acidoCEST MRI method was then used to evaluate the change in tumor pHe following alkalinizing bicarbonate treatment.

RESULTS

The acidoCEST MRI protocol measured pH between 6.2 and 7.2 pH units. Greater delivery of iopromide was shown to improve the precision of the measurement of tumor pHe, but the agent did not influence the tumor pHe. AcidoCEST MRI was used to longitudinally monitor the effect of bicarbonate treatment on the pHe of tumors and bladders.

CONCLUSION

This study demonstrates that an optimized acidoCEST MRI method is a practical, noninvasive method for assessing changes in tumor acidosis.

Advantages of chemical exchange-sensitive spin-lock (CESL) over chemical exchange saturation transfer (CEST) for hydroxyl- and amine-water proton exchange studies

The chemical exchange (CE) rate of endogenous hydroxyl and amine protons with water is often comparable to the difference in their chemical shifts. These intermediate exchange processes have been imaged by the CE saturation transfer (CEST) approach with low-power and long-duration irradiation. However, the sensitivity is not optimal and, more importantly, the signal is contaminated by slow magnetization transfer processes. Here, the properties of CEST signals are compared with those of a CE-sensitive spin-lock (CESL) technique irradiating at the labile proton frequency. First, using a higher power and shorter irradiation in CE-MRI, we obtain: (i) an increased selectivity to faster CE rates via a higher sensitivity to faster CEs and a lower sensitivity to slower CEs and magnetization transfer processes; and (ii) a decreased in vivo asymmetric magnetization transfer contrast measured at ±15 ppm. The sensitivity gain of CESL over CEST is higher for a higher power and shorter irradiation. Unlike CESL, CEST signals oscillate at a very high power and short irradiation. Second, time-dependent CEST and CESL signals are well modeled by analytical solutions of CE-MRI with an asymmetric population approximation, which can be used for quantitative CE-MRI and validated by simulations of Bloch-McConnell equations and phantom experiments. Finally, the in vivo amine-water proton exchange contrast measured at 2.5 ppm with ω1 = 500 Hz is 18% higher in sensitivity for CESL than CEST at 9.4 T. Overall, CESL provides better exchange rate selectivity and sensitivity than CEST; therefore, CESL is more suitable for CE-MRI of intermediate exchange protons.

Optimization of 7-T chemical exchange saturation transfer parameters for validation of glycosaminoglycan and amide proton transfer of fibroglandular breast tissue

PURPOSE

To (a) implement simulation-optimized chemical exchange saturation transfer (CEST) measurements sensitive to amide proton transfer (APT) and glycosaminoglycan (GAG) hydroxyl proton transfer effects in the human breast at 7 T and (b) determine the reliability of these techniques for evaluation of fibroglandular tissue in the healthy breast as a benchmark for future studies of pathologic findings.

MATERIALS AND METHODS

All human studies were institutional review board approved, were HIPAA compliant, and included informed consent. The CEST parameters of saturation duration (25 msec) and amplitude (1 μT) were chosen on the basis of simulation-driven optimization for APT contrast enhancement with the CEST effect quantified by using residuals of a Lorentzian fit. Optimized parameters were implemented at 7 T in 10 healthy women in two separate examinations to evaluate the reliability of CEST magnetic resonance (MR) imaging measurements in the breast. CEST z-spectra were acquired over saturation offset frequencies ranging between ±40 ppm by using a quadrature unilateral breast coil. The imaging-repeat imaging reliability was assessed in terms of the intraclass correlation coefficient, which indicates the ratio of between-subject variation to total variation.

RESULTS

Simulations were performed of the Bloch equations with chemical exchange-guided selection of optimal values for pulse duration and amplitude, 25 msec and 1 μT, respectively. Reliability was evaluated by using intraclass correlation coefficients (95% confidence intervals), with acceptable results: 0.963 (95% confidence interval: 0.852, 0.991) and 0.903 (95% confidence interval: 0.609, 0.976) for APT and GAG, respectively.

CONCLUSION

Simulations were used to derive optimal CEST preparation parameters to elicit maximal CEST contrast enhancement in healthy fibroglandular breast tissue due to APT at 7 T. By using these parameters, reproducible values were obtained for both the amide and hydroxyl protons from CEST MR imaging at 7 T and are feasible in the human breast.