Assessment of frequency drift on CEST MRI and dynamic correction: application to gagCEST at 7 T

Gaining Insight into Updated MR Imaging for Quantitative Assessment of Cartilage Injury in Knee Osteoarthritis

PURPOSE OF THE REVIEW

Knee Osteoarthritis (KOA) entails progressive cartilage degradation, reviewed via MRI for morphology, biochemical composition, and microtissue alterations, discussing clinical advantages, limitations, and research applicability.

RECENT FINDINGS

Compositional MRI, like T2/T2* mapping, T1rho mapping, gagCEST, dGEMRIC, sodium imaging, diffusion-weighted imaging, and diffusion-tensor imaging, provide insights into cartilage injury in KOA. These methods quantitatively measure collagen, glycosaminoglycans, and water content, revealing important information about biochemical compositional and microstructural alterations. Innovative techniques like hybrid multi-dimensional MRI and diffusion-relaxation correlation spectrum imaging show potential in depicting initial cartilage changes at a sub-voxel level. Integration of automated image analysis tools addressed limitations in manual cartilage segmentation, ensuring robust and reproducible assessments of KOA cartilage. Compositional MRI techniques reveal microstructural changes in cartilage. Multi-dimensional MR imaging assesses biochemical alterations in KOA-afflicted cartilage, aiding early degeneration identification. Integrating artificial intelligence enhances cartilage analysis, optimal diagnostic accuracy for early KOA detection and monitoring.

Multi-echo-based fat artifact correction for CEST MRI at 7 T

PURPOSE

CEST MRI is influenced by fat signal, which can reduce the apparent CEST contrast or lead to pseudo-CEST effects. Our goal was to develop a fat artifact correction based on multi-echo fat-water separation that functions stably for 7 T knee MRI data.

METHODS

Our proposed algorithm utilizes the full complex data and a phase demodulation with an off-resonance map estimation based on the Z-spectra prior to fat-water separation to achieve stable fat artifact correction. Our method was validated and compared to multi-echo-based methods originally proposed for 3 T by Bloch-McConnell simulations and phantom measurements. Moreover, the method was applied to in vivo 7 T knee MRI examinations and compared to Gaussian fat saturation and a published single-echo Z-spectrum-based fat artifact correction method.

RESULTS

Phase demodulation prior to fat-water separation reduced the occurrence of fat-water swaps. Utilizing the complex signal data led to more stable correction results than working with magnitude data, as was proposed for 3 T. Our approach reduced pseudo-nuclear Overhauser effects compared to the other correction methods. Thus, the mean asymmetry contrast at 3.5 ppm in cartilage over five volunteers increased from -9.2% (uncorrected) and -10.6% (Z-spectrum-based) to -1.5%. Results showed higher spatial stability than with the fat saturation pulse.

CONCLUSION

Our work demonstrates the feasibility of multi-echo-based fat-water separation with an adaptive fat model for fat artifact correction for CEST MRI at 7 T. Our approach provided better fat artifact correction throughout the entire spectrum and image than the fat saturation pulse or Z-spectrum-based correction method for both phantom and knee imaging results.

Improved postprocessing of dynamic glucose-enhanced CEST MRI for imaging brain metastases at 3 T

BACKGROUND

Dynamic glucose-enhanced (DGE) chemical exchange saturation transfer (CEST) has the potential to characterize glucose metabolism in brain metastases. Since the effect size of DGE CEST is small at 3 T (< 1%), measurements of signal-to-noise ratios are challenging. To improve DGE detection, we developed an acquisition pipeline and extended image analysis for DGE CEST on a hybrid 3-T positron emission tomography/magnetic resonance imaging system.

METHODS

This cross-sectional study was conducted after local ethical approval. Static Z-spectra (from -100 to 100 ppm) were acquired to compare the use of 1.2 versus 2 ppm to calculate static glucose-enhanced (glucoCEST) maps in 10 healthy volunteers before and after glucose infusion. Dynamic CEST images were acquired during glucose infusion. Image analysis was optimized using motion correction, dynamic B0 correction, and principal component analysis (PCA) to improve the detection of DGE CEST in the sagittal sinus, cerebrospinal fluid, and grey and white matter. The developed DGE CEST pipeline was applied to four patients diagnosed with brain metastases.

RESULTS

GlucoCEST was strongest in healthy tissues at 2 ppm. Correcting for motion, B0, and use of PCA locally improved DGE maps. A larger contrast between healthy tissues and enhancing regions in brain metastases was found when dynamic B0 correction and PCA denoising were applied.

CONCLUSION

We demonstrated the feasibility of DGE CEST with our developed acquisition and analysis pipeline at 3 T in patients with brain metastases. This work enables a direct comparison of DGE CEST to 18F-fluoro-deoxy-D-glucose positron emission tomography of glucose metabolism in patients with brain metastases.

RELEVANCE STATEMENT

Contrast between brain metastasis and healthy brain tissue in DGE CEST MR images is improved by including principle component analysis and dynamic magnetic field correction during postprocessing. This approach enables the detection of increased DGE CEST signal in brain metastasis, if present.

KEY POINTS

• Despite the low signal-to-noise ratio, dynamic glucose-enhanced CEST MRI is feasible at 3 T. • Principal component analyses and dynamic magnetic field correction improve DGE CEST MRI. • DGE CEST MRI does not consequently show changes in brain metastases compared to healthy brain tissue. • Increased DGE CEST MRI in brain metastases, if present, shows overlap with contrast enhancement on T1-weighted images.

7 tricks for 7 T CEST: Improving the reproducibility of multipool evaluation provides insights into the effects of age and the early stages of Parkinson's disease

The objective of the current study was to optimize the postprocessing pipeline of 7 T chemical exchange saturation transfer (CEST) imaging for reproducibility and to prove this optimization for the detection of age differences and differences between patients with Parkinson's disease versus normal subjects. The following 7 T CEST MRI experiments were analyzed: repeated measurements of a healthy subject, subjects of two age cohorts (14 older, seven younger subjects), and measurements of 12 patients with Parkinson's disease. A slab-selective, B 1 + -homogeneous parallel transmit protocol was used. The postprocessing, consisting of motion correction, smoothing, B 0 -correction, normalization, denoising, B 1 + -correction and Lorentzian fitting, was optimized regarding the intrasubject and intersubject coefficient of variation (CoV) of the amplitudes of the amide pool and the aliphatic relayed nuclear Overhauser effect (rNOE) pool within the brain. Seven "tricks" for postprocessing accomplished an improvement of the mean voxel CoV of the amide pool and the aliphatic rNOE pool amplitudes of less than 5% and 3%, respectively. These postprocessing steps are: motion correction with interpolation of the motion of low-signal offsets (1) using the amide pool frequency offset image as reference (2), normalization of the Z-spectrum using the outermost saturated measurements (3), B 0 correction of the Z-spectrum with moderate spline smoothing (4), denoising using principal component analysis preserving the 11 highest intensity components (5), B 1 + correction using a linear fit (6) and Lorentzian fitting using the five-pool fit model (7). With the optimized postprocessing pipeline, a significant age effect in the amide pool can be detected. Additionally, for the first time, an aliphatic rNOE contrast between subjects with Parkinson's disease and age-matched healthy controls in the substantia nigra is detected. We propose an optimized postprocessing pipeline for CEST multipool evaluation. It is shown that by the use of these seven "tricks", the reproducibility and, thus, the statistical power of a CEST measurement, can be greatly improved and subtle changes can be detected.

B0 Correction for 3T Amide Proton Transfer (APT) MRI Using a Simplified Two-Pool Lorentzian Model of Symmetric Water and Asymmetric Solutes

Amide proton transfer (APT)-weighted MRI is a promising molecular imaging technique that has been employed in clinic for detection and grading of brain tumors. MTRasym, the quantification method of APT, is easily influenced by B0 inhomogeneity and causes artifacts. Current model-free interpolation methods have enabled moderate B0 correction for middle offsets, but have performed poorly at limbic offsets. To address this shortcoming, we proposed a practical B0 correction approach that is suitable under time-limited sparse acquisition scenarios and for B1 ≥ 1 μT under 3T. In this study, this approach employed a simplified Lorentzian model containing only two pools of symmetric water and asymmetric solutes, to describe the Z-spectral shape with wide and ‘invisible’ CEST peaks. The B0 correction was then performed on the basis of the fitted two-pool Lorentzian lines, instead of using conventional model-free interpolation. The approach was firstly evaluated on densely sampled Z-spectra data by using the spline interpolation of all acquired 16 offsets as the gold standard. When only six offsets were available for B0 correction, our method outperformed conventional methods. In particular, the errors at limbic offsets were significantly reduced (n = 8, p < 0.01). Secondly, our method was assessed on the six-offset APT data of nine brain tumor patients. Our MTRasym (3.5 ppm), using the two-pool model, displayed a similar contrast to the vendor-provided B0-orrected MTRasym (3.5 ppm). While the vendor failed in correcting B0 at 4.3 and 2.7 ppm for a large portion of voxels, our method enabled well differentiation of B0 artifacts from tumors. In conclusion, the proposed approach could alleviate analysis errors caused by B0 inhomogeneity, which is useful for facilitating the comprehensive metabolic analysis of brain tumors.

Dynamic alteration in myocardium creatine during acute infarction using MR CEST imaging

Creatine (Cr) is an essential metabolite in the creatine kinase reaction, which plays a critical role in maintaining normal cardiac function. Chemical exchange saturation transfer (CEST) MRI offers a novel way to map myocardium Cr. This study aims to investigate the dynamic alteration in myocardium Cr during acute infarction using CEST MRI, which may facilitate understanding of the heart remodeling mechanism at the molecular level. Seven adult Bama pigs underwent cardiac cine, Cr CEST, and late gadolinium-enhanced (LGE) T₁ -weighted (T₁ w) imaging three and 14 days after myocardial infarction induction on a 3 T scanner. Cardiac structural and functional indices, including myocardium mass (MM), end-diastolic volume (EDV), end-systolic volume (ESV), stroke volume (SV), and ejection fraction (EF), were measured from cines. Infarct angle was determined from LGE T₁ w images, based on which myocardium was classified into infarct, adjacent, and remote regions. Cr-weighted CEST signal was quantified from a three-pool Lorentzian fitting model and measured within each region and the entire myocardium. Student's t-test was conducted to evaluate any significant differences in measurements between the two time points. Correlation was assessed with Pearson correlation. P values less than 0.05 were considered statistically significant. Over the studied period, MM, EDV, and ESV did not alter significantly (P > 0.05), whereas significant increases of SV and EF and decrease of infarct angle were observed (P < 0.05). Meanwhile, the Cr-weighted CEST signal elevated significantly on Day 14 compared with Day 3 in the infarct (10.00 ± 1.28% versus 6.91 ± 1.54%, P < 0.01), adjacent (11.17 ± 2.00% versus 8.01 ± 1.58%, P = 0.01), and entire myocardium (11.03 ± 1.36% versus 8.19 ± 1.28%, P < 0.01). Moderate negative correlations were shown between the infarct angle and Cr-weighted CEST signals in the infarct (r = -0.80, P < 0.001), adjacent (r = -0.58, P = 0.03), and entire myocardium (r = -0.76, P < 0.01). In conclusion, the dynamic increase of myocardium Cr during acute infarction may interact with cardiac structural and functional recovery. The study provides supplementary insights into the heart remodeling process from the metabolic viewpoint.

Review and consensus recommendations on clinical APT-weighted imaging approaches at 3T: Application to brain tumors

Amide proton transfer-weighted (APTw) MR imaging shows promise as a biomarker of brain tumor status. Currently used APTw MRI pulse sequences and protocols vary substantially among different institutes, and there are no agreed-on standards in the imaging community. Therefore, the results acquired from different research centers are difficult to compare, which hampers uniform clinical application and interpretation. This paper reviews current clinical APTw imaging approaches and provides a rationale for optimized APTw brain tumor imaging at 3 T, including specific recommendations for pulse sequences, acquisition protocols, and data processing methods. We expect that these consensus recommendations will become the first broadly accepted guidelines for APTw imaging of brain tumors on 3 T MRI systems from different vendors. This will allow more medical centers to use the same or comparable APTw MRI techniques for the detection, characterization, and monitoring of brain tumors, enabling multi-center trials in larger patient cohorts and, ultimately, routine clinical use.

GLINT: GlucoCEST in neoplastic tumors at 3 T-clinical results of GlucoCEST in gliomas

Objective

Clinical relevance of dynamic glucose enhanced (DGE) chemical exchange saturation transfer (CEST) imaging has mostly been demonstrated at ultra-high field (UHF) due to low effect size. Results of a cohort study at clinical field strength are shown herein.

Materials and methods

Motion and field inhomogeneity corrected T1ρ‐based DGE (DGE⍴) images were acquired before, during and after a d-glucose injection with 6.3 s temporal resolution to detect accumulation in the brain. Six glioma patients with clear blood–brain barrier (BBB) leakage, two glioma patients with suspected BBB leakage, and three glioma patients without BBB leakage were scanned at 3 T.

Results

In high-grade gliomas with BBB leakage, d-glucose uptake could be detected in the gadolinium (Gd) enhancing region as well as in the tumor necrosis with a maximum increase of ∆DGE⍴ around 0.25%, whereas unaffected white matter did not show any significant DGE⍴ increase. Glioma patients without Gd enhancement showed no detectable DGE⍴ effect within the tumor.

Conclusion

First application of DGE⍴ in a patient cohort shows an association between BBB leakage and DGE signal irrespective of the tumor grade. This indicates that glucoCEST corresponds more to the disruptions of BBB with Gd uptake than to the molecular tumor profile or tumor grading.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10334-021-00982-5.

What do we know about dynamic glucose-enhanced (DGE) MRI and how close is it to the clinics? Horizon 2020 GLINT consortium report

Cancer is one of the most devastating diseases that the world is currently facing, accounting for 10 million deaths in 2020 (WHO). In the last two decades, advanced medical imaging has played an ever more important role in the early detection of the disease, as it increases the chances of survival and the potential for full recovery. To date, dynamic glucose-enhanced (DGE) MRI using glucose-based chemical exchange saturation transfer (glucoCEST) has demonstrated the sensitivity to detect both d-glucose and glucose analogs, such as 3-oxy-methyl-d-glucose (3OMG) uptake in tumors. As one of the recent international efforts aiming at pushing the boundaries of translation of the DGE MRI technique into clinical practice, a multidisciplinary team of eight partners came together to form the “glucoCEST Imaging of Neoplastic Tumors (GLINT)” consortium, funded by the Horizon 2020 European Commission. This paper summarizes the progress made to date both by these groups and others in increasing our knowledge of the underlying mechanisms related to this technique as well as translating it into clinical practice.

Towards robust glucose chemical exchange saturation transfer imaging in humans at 3 T: Arterial input function measurements and the effects of infusion time

Dynamic glucose-enhanced (DGE) magnetic resonance imaging (MRI) has shown potential for tumor imaging using D-glucose as a biodegradable contrast agent. The DGE signal change is small at 3 T (around 1%) and accurate detection is hampered by motion. The intravenous D-glucose injection is associated with transient side effects that can indirectly generate subject movements. In this study, the aim was to study DGE arterial input functions (AIFs) in healthy volunteers at 3 T for different scanning protocols, as a step towards making the glucose chemical exchange saturation transfer (glucoCEST) protocol more robust. Two different infusion durations (1.5 and 4.0 min) and saturation frequency offsets (1.2 and 2.0 ppm) were used. The effect of subject motion on the DGE signal was studied by using motion estimates retrieved from standard retrospective motion correction to create pseudo-DGE maps, where the apparent DGE signal changes were entirely caused by motion. Furthermore, the DGE AIFs were compared with venous blood glucose levels. A significant difference (p = 0.03) between arterial baseline and postinfusion DGE signal was found after D-glucose infusion. The results indicate that the measured DGE AIF signal change depends on both motion and blood glucose concentration change, emphasizing the need for sufficient motion correction in glucoCEST imaging. Finally, we conclude that a longer infusion duration (e.g. 3-4 min) should preferably be used in glucoCEST experiments, because it can minimize the glucose infusion side effects without negatively affecting the DGE signal change.

Clinical translation of amide proton transfer (APT) MRI for ischemic stroke: a systematic review (2003-2020)

Amide proton transfer (APT) magnetic resonance imaging (MRI) is a pH-sensitive imaging technique that can potentially complement existing clinical imaging protocol for the assessment of ischemic stroke. This review aims to summarize the developments in the clinical research of APT imaging of ischemic stroke after 17 years of progress since its first preclinical study in 2003. Three electronic databases: PubMed, Scopus, and Cochrane Library were systematically searched for articles reporting clinical studies on APT imaging of ischemic stroke. Only articles in English published between 2003 to 2020 that involved patients presenting ischemic stroke-like symptoms that underwent APT MRI were included. Of 1,093 articles screened, 14 articles met the inclusion criteria with a total of 282 patients that had been scanned using APT imaging. Generally, the clinical studies agreed APT effect to be hypointense in ischemic tissue compared to healthy tissue, allowing for the detection of ischemic stroke. Other uses of APT imaging have also been investigated in the studies, including penumbra identification, predicting long term clinical outcome, and serving as a biomarker for supportive treatment monitoring. The published results demonstrated the potential of APT imaging in these applications, but further investigations and larger trials are needed for conclusive evidence. Future studies are recommended to report the result of asymmetry analysis at 3.5 ppm along with the findings of the study to reduce this contribution to the heterogeneity of experimental methods observed and to facilitate effective comparison of results between studies and centers. In addition, it is important to focus on the development of fast 3D imaging for full volumetric ischemic tissue assessment for clinical translation.

Glutamate Chemical Exchange Saturation Transfer Imaging and Functional Alterations of Hippocampus in Rat Depression Model: A Pilot Study

BACKGROUND

Adjusting abnormal glutamate neurotransmission is a crucial mechanism in the treatment of depression. However, few non-invasive techniques could effectively detect changes in glutamate neurotransmitters, and no consensus exists on whether glutamate could affect resting-state function changes in depression.

PURPOSE

To study the changes in glutamate chemical exchange saturation transfer (GluCEST) value in the hippocampus of rat model exposed to chronic unpredictable mild stress (CUMS), and to explore the effect of this change on the activity of hippocampal glutamatergic neurons.

STUDY TYPE

Prospective animal study.

ANIMAL MODEL

Twenty male Sprague-Dawley rats (200-300 g).

FIELD STRENGTH/SEQUENCE

7.0 T scanner. Fat rapid acquisition relaxation enhancement sequence for GluCEST, and echo planner imaging sequence for resting-state functional magnetic resonance imaging (rs_fMRI).

ASSESSMENT

Rats were divided into two groups: CUMS group (N = 10) and control group (CTRL, N = 10). The magnetization transfer ratio asymmetry analysis was used to quantify the GluCEST data, and evaluate the rs_fMRI data through the amplitude of low-frequency fluctuation (ALFF) and regional homogeneity (ReHo) analysis.

STATISTICAL TESTS

A t-test was used to compare the difference in GluCEST or rs_fMRI between CUMS and CTRL groups. Spearman's correlation was applied to explore the correlation between GluCEST values and abnormal fMRI values in hippocampus. Statistical significance was set at P < 0.05.

RESULTS

The GluCEST value in the left hippocampus has changed significantly (3.3 ± 0.3 [CUMS] vs. 3.9 ± 0.4 [CTRL], P < 0.05). In addition, the GluCEST value was significantly positively correlated with the ALFF values (r = 0.5, P < 0. 05, df = 7) and negatively correlated with the ReHo values (r = -0.6, P < 0.05, df = 7).

DATA CONCLUSION

GluCEST technique has the feasibility of mapping glutamate changes in rat depression. Glutamate neurotransmitters are important factors affecting the abnormal function of neural activity.

LEVEL OF EVIDENCE

2 TECHNICAL EFFICACY: Stage 1.

A fast and novel method for amide proton transfer-chemical exchange saturation transfer multislice imaging

Amide proton transfer-chemical exchange saturation transfer (APT-CEST) imaging provides important information for the diagnosis and monitoring of tumors. For such analysis, complete coverage of the brain is advantageous, especially when registration is performed with other magnetic resonance (MR) modalities, such as MR spectroscopy (MRS). However, the acquisition of Z-spectra across several slices via multislice imaging may be time-consuming. Therefore, in this paper, we present a new approach for fast multislice imaging, allowing us to acquire 16 slices per frequency offset within 8 s. The proposed fast CEST-EPI sequence employs a presaturation module, which drives the magnetization into the steady-state equilibrium for the first frequency offset. A second module, consisting of a single CEST pulse (for maintaining the steady-state) followed by an EPI acquisition, passes through a loop to acquire multiple slices and adjacent frequency offsets. Thus, the whole Z-spectrum can be recorded much faster than the conventional saturation scheme, which employs a presaturation for each single frequency offset. The validation of the CEST sequence parameters was performed by using the conventional saturation scheme. Subsequently, the proposed and a modified version of the conventional CEST sequence were compared in vitro on a phantom with different T1 times and in vivo on a brain tumor patient. No significant differences between both sequences could be found in vitro. The in vivo data yielded almost identical MTR_asym contrasts for the white and gray matter as well as for tumor tissue. Our results show that the proposed fast CEST-EPI sequence allows for rapid data acquisition and provides similar CEST contrasts as the modified conventional scheme while reducing the scanning time by approximately 50%.

Real-time motion and retrospective coil sensitivity correction for CEST using volumetric navigators (vNavs) at 7T

PURPOSE

To explore the impact of temporal motion-induced coil sensitivity changes on CEST-MRI at 7T and its correction using interleaved volumetric EPI navigators, which are applied for real-time motion correction.

METHODS

Five healthy volunteers were scanned via CEST. A 4-fold correction pipeline allowed the mitigation of (1) motion, (2) motion-induced coil sensitivity variations, ΔB1- , (3) motion-induced static magnetic field inhomogeneities, ΔB₀ , and (4) spatially varying transmit RF field fluctuations, ΔB1+ . Four CEST measurements were performed per session. For the first 2, motion correction was turned OFF and then ON in absence of voluntary motion, whereas in the other 2 controlled head rotations were performed. During post-processing ΔB1- was removed additionally for the motion-corrected cases, resulting in a total of 6 scenarios to be compared. In all cases, retrospective ∆B₀ and - ΔB1+ corrections were performed to compute artifact-free magnetization transfer ratio maps with asymmetric analysis (MTR_asym ).

RESULTS

Dynamic ΔB1- correction successfully mitigated signal deviations caused by head motion. In 2 frontal lobe regions of volunteer 4, induced relative signal errors of 10.9% and 3.9% were reduced to 1.1% and 1.0% after correction. In the right frontal lobe, the motion-corrected MTR_asym contrast deviated 0.92%, 1.21%, and 2.97% relative to the static case for Δω = 1, 2, 3 ± 0.25 ppm. The additional application of ΔB1- correction reduced these deviations to 0.10%, 0.14%, and 0.42%. The fully corrected MTR_asym values were highly consistent between measurements with and without intended head rotations.

CONCLUSION

Temporal ΔB1- cause significant CEST quantification bias. The presented correction pipeline including the proposed retrospective ΔB1- correction significantly reduced motion-related artifacts on CEST-MRI.

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.

Current status of functional MRI of osteoarthritis for diagnosis and prognosis

PURPOSE OF REVIEW

Osteoarthritis is a major source of disability, pain and socioeconomic cost worldwide. The epidemiology of the disorder is multifactorial including genetic, biological and biomechanical components, some of them detectable by MRI. This review provides the most recent update on MRI biomarkers which can provide functional information of the joint structures for diagnosis, prognosis and treatment response monitoring in osteoarthritis trials.

RECENT FINDINGS

Compositional or functional MRI can provide clinicians with valuable information on glycosaminoglycan content (chemical exchange saturation transfer, sodium MRI, T1ρ) and collagen organization (T2, T2, apparent diffusion coefficient, magnetization transfer) in joint structures. Other parameters may also provide useful information, such as volumetric measurements of joint structures or advanced image data postprocessing and analysis. Automated tools seem to have a great potential to be included in these efforts providing standardization and acceleration of the image data analysis process.

SUMMARY

Functional or compositional MRI has great potential to provide noninvasive imaging biomarkers for osteoarthritis. Osteoarthritis as a whole joint condition needs to be diagnosed in early stages to facilitate selection of patients into clinical trials and/or to measure treatment effectiveness. Advanced evaluation including machine learning, neural networks and multidimensional data analysis allow for wall-to-wall understanding of parameter interactions and their role in clinical evaluation of osteoarthritis.

On the interference from agar in chemical exchange saturation transfer MRI parameter optimization in model solutions

Chemical exchange saturation transfer (CEST) MRI is currently set to become part of clinical routine as it enables indirect detection of low concentrated molecules and proteins. Recently, intermediate to fast exchanging functional groups of glucose and its derivatives, glutamate and dextran, have gained attention as promising CEST contrast agents. To increase the specificity of CEST MRI for certain functional groups, the presaturation module is commonly optimized. At an early stage, this is performed in well-defined model solutions, in which, for instance, the relaxation times are adjusted to mimic in vivo conditions. This often involves agar, assuming the substance would not yield significant CEST effects by itself, which the current study proves to be an invalid assumption. Model solutions at different pH values and concentrations of agar were investigated at different temperatures at a 9.4 T human whole body MR scanner. High power presaturation of around 4 μT, optimal for investigating intermediate to fast exchanging groups, was applied. Postprocessing included spatiotemporal corrections for B₀ and spatial corrections for B₁⁺ . CEST effects of up to 3 % of the bulk water signal were observed. From pH, concentration and temperature dependency, it was concluded that the observed behavior reflects a CEST effect of agar. It was also shown how to remove this undesirable contribution from CEST MRI data. It was concluded that if agar is involved in the CEST MRI parameter optimization process, its contribution to the observed effects has to be taken into account. CEST agent concentration must be sufficiently high to be able to neglect the contribution of agar, or a control sample at matched pH is necessary for correction. Experiments on pure agarose showed reduced CEST effects compared with agar but did not provide a neutral baseline either.

High-sensitivity CEST mapping using a spatiotemporal correlation-enhanced method

PURPOSE

To obtain high-sensitivity CEST maps by exploiting the spatiotemporal correlation between CEST images.

METHODS

A postprocessing method accomplished by multilinear singular value decomposition (MLSVD) was used to enhance the CEST SNR by exploiting the correlation between the Z-spectrum for each voxel and the low-rank property of the overall CEST data. The performance of this method was evaluated using CrCEST in ischemic mouse brain at 11.7 tesla. Then, MLSVD CEST was applied to obtain Cr, amide, and amine CEST maps of the ischemic mouse brain to demonstrate its general applications.

RESULTS

Complex-valued Gaussian noise was added to CEST k-space data to mimic a low SNR situation. MLSVD CEST analysis was able to suppress the noise, recover the degraded CEST peak, and provide better CrCEST quality compared to the smoothing and singular value decomposition (SVD)-based denoising methods. High-resolution Cr, amide, and amine CEST maps of an ischemic stroke using MLSVD CEST suggest that CrCEST is also a sensitive pH mapping method, and a wide range of pH changes can be detected by combing CrCEST with amine CEST at high magnetic fields.

CONCLUSION

MLSVD CEST provides a simple and efficient way to improve the SNR of CEST images.

Delayed urea differential enhancement CEST (dudeCEST)-MRI with T1 correction for monitoring renal urea handling

PURPOSE

We demonstrate a method of delayed urea differential enhancement CEST for probing urea recycling action of the kidney using expanded multi-pool Lorentzian fitting and apparent exchange-dependent relaxation compensation.

METHODS

T₁ correction of urea CEST contrast by apparent exchange-dependent relaxation was tested in phantoms. Nine mice were scanned at 7 Tesla following intraperitoneal injection of 2M 150 μL urea, and later saline. T₁ maps and Z-spectra were acquired before and 20 and 40 min postinjection. Z-spectra were fit to a 7-pool Lorentzian model for CEST quantification and compared to urea assay of kidney homogenate. Renal injury was induced by aristolochic acid in 7 mice, and the same scan protocol was performed.

RESULTS

Apparent exchange-dependent relaxation corrected for variable T₁ times in phantoms. Urea CEST contrast at +1 ppm increased significantly at both time points following urea injection in the inner medulla and papilla. When normalizing the postinjection urea CEST contrast to the corresponding baseline value, both urea and saline injection resulted in identical fold changes in urea CEST contrast. Urea assay of kidney homogenate showed a significant correlation to both apparent exchange-dependent relaxation (R² = 0.4687, P = .0017) and non-T₁ -corrected Lorentzian amplitudes (R² = 0.4964, P = .0011). Renal injury resulted in increased T₁ time in the cortex and reduced CEST contrast change upon urea and saline infusion.

CONCLUSION

Delayed urea enhancement following infusion can provide insight into renal urea handling. In addition, changes in CEST contrast at 1.0 ppm following saline infusion may provide insight into renal function.

Characterization of B0-field fluctuations in prostate MRI

Multi-parametric MRI is increasingly used for prostate cancer detection. Improving information from current sequences, such as T2-weighted and diffusion-weighted (DW) imaging, and additional sequences, such as magnetic resonance spectroscopy (MRS) and chemical exchange saturation transfer (CEST), may enhance the performance of multi-parametric MRI. The majority of these techniques are sensitive to B0-field variations and may result in image distortions including signal pile-up and stretching (echo planar imaging (EPI) based DW-MRI) or unwanted shifts in the frequency spectrum (CEST and MRS). Our aim is to temporally and spatially characterize B0-field changes in the prostate. Ten male patients are imaged using dual-echo gradient echo sequences with varying repetitions on a 3 T scanner to evaluate the temporal B0-field changes within the prostate. A phantom is also imaged to consider no physiological motion. The spatial B0-field variations in the prostate are reported as B0-field values (Hz), their spatial gradients (Hz/mm) and the resultant distortions in EPI based DW-MRI images (b-value = 0 s/mm2 and two oppositely phase encoded directions). Over a period of minutes, temporal changes in B0-field values were ≤19 Hz for minimal bowel motion and ≥30 Hz for large motion. Spatially across the prostate, the B0-field values had an interquartile range of ≤18 Hz (minimal motion) and ≤44 Hz (large motion). The B0-field gradients were between -2 and 5 Hz/mm (minimal motion) and 2 and 12 Hz/mm (large motion). Overall, B0-field variations can affect DW, MRS and CEST imaging of the prostate.

Frequency-stabilized chemical exchange saturation transfer imaging with real-time free-induction-decay readout

PURPOSE

To correct the temporal B₀ drift in chemical exchange saturation transfer (CEST) imaging in real-time with extra free-induction-decay (FID) readout.

THEORY AND METHODS

The frequency stabilization module of the recently proposed frequency-stabilized CEST (FS-CEST) sequence was further simplified by replacing the original three k-space lines of gradient-echo (GRE) readout with a single k-space line of FID readout. The B₀ drift was quantified using the phase difference between the odd and even parts of the FID signal in the frequency stabilization module and then used to update the B₀ frequency in the succeeding modules. The proposed FS-CEST sequence with FID readout (FID FS-CEST) was validated in phantoms and 16 human subjects on cross-vendor scanners.

RESULTS

In the Siemens experiments, the FID FS-CEST sequence successfully corrected the user-induced B₀ drift, generating consistent amide proton transfer-weighted (APTw) images and magnetization transfer ratio asymmetry (MTR_asym ) spectra with those from the non-frequency-stabilized CEST (NFS-CEST) sequence without B₀ drift. In the Philips experiments, the FID FS-CEST sequence produced more stable APTw images and MTR_asym spectra than the NFS-CEST sequence in the presence of practical B₀ drift. Quantitatively, the SD of the APTw signal values in the deep gray matter from 15 subjects was 0.26% for the FID FS-CEST sequence compared to 1.03% for the NFS-CEST sequences, with the fluctuations reduced by nearly three-quarters.

CONCLUSIONS

The proposed FS-CEST sequence with FID readout can effectively correct the temporal B₀ drift on cross-vendor scanners.

The role of cartilage glycosaminoglycan structure in gagCEST

Glycosaminoglycan (GAG) chemical exchange saturation transfer (gagCEST) is a potential method for cartilage quality assessment. The aim of this study was to investigate how the gagCEST effect depends on the types and molecular organization of GAG typically found in articular cartilage. gagCEST was performed on different concentrations of GAG in various forms: free chains of chondroitin sulfate (CS) of different types (-A and -C) and GAG bound to protein in aggregated and nonaggregated aggrecan extracted from calf articular cartilage. The measured magnetization transfer ratio asymmetry (MTR_asym ) was compared with known GAG concentrations or GAG concentrations determined through biochemical analysis. The gagCEST effect was assessed through the linear regression coefficient with 95% confidence interval of MTR_asym per GAG concentration. We observed a lower gagCEST effect in phantoms containing a mixture of CS-A and CS-C compared with phantoms containing mainly CS-A. The difference in response corresponds well to the difference in CS-A concentration. GAG bound in aggrecan from calf articular cartilage, where CS-A is assumed to be the major type of GAG, produed a similar gagCEST effect as that observed for free CS-A. The effect was also similar for aggregated (ie, bound to hyaluronic acid) and nonaggregated aggrecan. In conclusion, our results indicate that the aggrecan structure in itself does not impact the gagCEST effect, but that the effect is strongly dependent on GAG type. In phantoms, the current implementation of gagCEST is sensitive to CS-A while for CS-C, the main GAG component in mature human articular cartilage, the sensitivity is limited. This difference in gagCEST sensitivity between GAG types detected in phantoms is a strong motivation to also explore the possibility of a similar effect in vivo.

Magnetic Resonance Imaging of the Musculoskeletal System at 7T: Morphological Imaging and Beyond

In 2017, a whole-body 7T magnetic resonance imaging (MRI) device was given regulatory approval for clinical use in both the EU and United States for neuro and musculoskeletal applications. As 7 Tesla allows for higher signal-to-noise , which results in higher resolution images than those obtained on lower-field-strength scanners, it has attracted considerable attention from the musculoskeletal field, as evidenced by the increasing number of publications in the last decade. Besides morphological imaging, the quantitative MR methods, such as T₂, T₂∗, T_1ρ mapping, sodium imaging, chemical-exchange saturation transfer, and spectroscopy, substantially benefit from ultrahigh field scanning. In this review, we provide technical considerations for the individual techniques and an overview of (mostly) clinical applications for the assessment of cartilage, tendon, meniscus, and muscle. The first part of the review is dedicated to morphological applications at 7T, and the second part describes the most recent developments in quantitative MRI at 7T.

Noninvasive imaging of renal urea handling by CEST-MRI

PURPOSE

Renal function is characterized by concentration of urea for removal in urine. We tested urea as a CEST-MRI contrast agent for measurement of the concentrating capacity of distinct renal anatomical regions.

METHODS

The CEST contrast of urea was examined using phantoms with different concentrations and pH levels. Ten C57BL/6J mice were scanned twice at 7 T, once following intraperitoneal injection of 2M 150 µL urea and separately following an identical volume of saline. Kidneys were segmented into regions encompassing the cortex, outer medulla, and inner medulla and papilla to monitor spatially varying urea concentration. Z-spectra were acquired before and 20 minutes after injection, with dynamic scanning of urea handling performed in between via serial acquisition of CEST images acquired following saturation at +1 ppm.

RESULTS

Phantom experiments revealed concentration and pH-dependent CEST contrast of urea that was both acid- and base-catalyzed. Z-spectra acquired before injection showed significantly higher CEST contrast in the inner medulla and papilla (2.3% ± 1.9%) compared with the cortex (0.15% ± 0.75%, P = .011) and outer medulla (0.12% ± 0.58%, P = .008). Urea infusion increased CEST contrast in the inner medulla and papilla by 2.1% ± 1.9% (absolute), whereas saline infusion decreased CEST contrast by -0.5% ± 2.0% (absolute, P = .028 versus urea). Dynamic scanning revealed that thermal drift and diuretic status are confounding factors.

CONCLUSION

Urea CEST has a potential of monitoring renal function by capturing the spatially varying urea concentrating ability of the kidneys.

A comparison of static and dynamic ∆B0 mapping methods for correction of CEST MRI in the presence of temporal B0 field variations

PURPOSE

To assess the performance, in the presence of scanner instabilities, of three dynamic correction methods which integrate ∆B₀ mapping into the chemical exchange saturation transfer (CEST) measurement and three established static ∆B₀ -correction approaches.

METHODS

A homogeneous phantom and five healthy volunteers were scanned with a CEST sequence at 7 T. The in vivo measurements were performed twice: first with unaltered system frequency and again applying frequency shifts during the CEST acquisition. In all cases, retrospective voxel-wise ∆B₀ -correction was performed using one intrinsic and two extrinsic [prescans with dual-echo gradient-echo and water saturation shift referencing (WASSR)] static approaches. These were compared with two intrinsic [using phase data directly generated by single-echo or double-echo GRE (gradient-echo) CEST readout (CEST-GRE-2TE)] and one extrinsic [phase from interleaved dual-echo EPI (echo planar imaging) navigator (NAV-EPI-2TE)] dynamic ∆B₀ -correction approaches [allowing correction of each Z-spectral point before magnetization transfer ratio asymmetry (MTR_asym) analysis].

RESULTS

All three dynamic methods successfully mapped the induced drift. The intrinsic approaches were affected by the CEST labeling near water (∆ω < |0.3| ppm). The MTR_asym contrast was distorted by the frequency drift in the brain by up to 0.21%/Hz when static ∆B₀ -corrections were applied, whereas the dynamic ∆B₀ corrections reduced this to <0.01%/Hz without the need of external scans. The CEST-GRE-2TE and NAV-EPI-2TE resulted in highly consistent MTR_asym values with/without drift for all subjects.

CONCLUSION

Reliable correction of scanner instabilities is essential to establish clinical CEST MRI. The three dynamic approaches presented improved the ∆B₀ -correction performance significantly in the presence of frequency drift compared to established static methods. Among them, the self-corrected CEST-GRE-2TE was the most accurate and straightforward to implement.

[Biochemical cartilage imaging-update 2019]

BACKGROUND

Cartilage imaging using magnetic resonance imaging (MRI) is increasingly used for early detection of cartilage damage. Biochemical MR methods to assess cartilage damage are essential for optimal treatment planning.

PURPOSE

The aim of this review is to provide an update on advanced cartilage imaging based on biochemical MR techniques. The clinical applications and additional benefits compared to conventional MRI are presented.

MATERIALS AND METHODS

A literature search of PubMed regarding the clinical applications of various biochemical MR methods and morphological MR imaging was performed.

RESULTS

While T2 mapping can be easily implemented on clinical routine MR scanners, the T1rho method is technically more demanding and is not available on all MR scanners. dGEMRIC, which can be performed with all field strengths, is now severely restricted due to the recent decision of the European Medical Agency (EMA) to withdraw linear gadolinium contrast agents from the market because of proven gadolinium deposition in the brain. Sodium imaging is the most sensitive MRI method for glycosaminoglycan (GAG), but is limited to 7 T. In addition to early diagnosis of cartilage degeneration before morphological changes are visible, biochemical MRI offers predictive markers, e.g., effect of lifestyle changes or assessing results of cartilage repair surgery.

CONCLUSION

Cartilage imaging based on biochemical MRI allows a shift from qualitative to quantitative MRI. Biochemical MRI plays an increasingly important role in the early diagnosis of cartilage degeneration for monitoring of disease-modifying drugs and as predictive imaging biomarker in clinical diagnostics. In cartilage repair, monitoring of the efficacy of different cartilage repair surgery techniques to develop hyaline-like cartilage can be performed with biochemical MRI.

Improved chemical exchange saturation transfer imaging with real-time frequency drift correction

PURPOSE

To investigate the effects of frequency drift on chemical exchange saturation transfer (CEST) imaging at 3T, and to propose a new sequence for correcting artifacts attributed to B₀ drift in real time.

THEORY AND METHODS

A frequency-stabilized CEST (FS-CEST) imaging sequence was proposed by adding a frequency stabilization module to the conventional non-frequency-stabilized CEST (NFS-CEST) sequence, which consisted of a small tip angle radiofrequency excitation pulse and readout of three non-phase-encoded k-space lines. Experiments were performed on an egg white phantom and 26 human subjects on a heavy-duty clinical scanner, in order to compare the difference of FS-CEST and NFS-CEST sequences for generating the z-spectrum, magnetization transfer ratio asymmetry (MTR_asym ) spectrum, and amide proton transfer weighted (APTw) image.

RESULTS

The B₀ drift in CEST imaging, if not corrected, would cause APTw images and MTR_asym spectra from both the phantom and volunteers to be either significantly higher or lower than the true values, depending on the status of the scanner. The FS-CEST sequence generated substantially more stable MTR_asym spectra and APTw images than the conventional NFS-CEST sequence. Quantitatively, the compartmental-average APTw signals (mean ± standard deviation) from frontal white matter regions of all 26 human subjects were -0.32% ± 2.32% for the NFS-CEST sequence and -0.14% ± 0.37% for the FS-CEST sequence.

CONCLUSIONS

The proposed FS-CEST sequence provides an effective approach for B₀ drift correction without additional scan time and should be adopted on heavy-duty MRI scanners.

Real-time simultaneous shim and motion measurement and correction in glycoCEST MRI using double volumetric navigators (DvNavs)

PURPOSE

CEST MRI allows for indirect detection of molecules with exchangeable protons, measured as a reduction in water signal because of continuous transfer of saturated protons. CEST requires saturation pulses on the order of a second, as well as repeated acquisitions at different offset frequencies. The resulting extended scan time makes CEST susceptible to subject motion, which introduces field inhomogeneity, shifting offset frequencies and causing distortions in CEST spectra that resemble true CEST effects. This is a particular problem for molecules that resonate close to water, such as hydroxyl group in glycogen. To address this, a technique for real-time measurement and correction of motion and field inhomogeneity is proposed.

METHODS

A CEST sequence was modified to include double volumetric navigators (DvNavs) for real-time simultaneous motion and shim correction. Phantom tests were conducted to investigate the effects of motion and shim changes on CEST quantification and to validate the accuracy of DvNav motion and shim estimates. To evaluate DvNav shim and motion correction in vivo, acquisitions including 5 experimental conditions were performed in the calf muscle of 2 volunteers.

RESULTS

Phantom data show that DvNav-CEST accurately estimates frequency and linear gradient changes because of motion and corrects resulting image distortions. In addition, DvNav-CEST improves CEST quantification in vivo in the presence of motion.

CONCLUSION

The proposed technique allows for real-time simultaneous motion and shim correction with no additional scanning time, enabling accurate CEST quantification even in the presence of motion and field inhomogeneity.