Glutamate imaging (GluCEST) lateralizes epileptic foci in nonlesional temporal lobe epilepsy

Fast 3D chemical exchange saturation transfer imaging with variably-accelerated sensitivity encoding (vSENSE)

PURPOSE

To extend the variably-accelerated sensitivity encoding (vSENSE) method from 2D to 3D for fast chemical exchange saturation transfer (CEST) imaging, and prospectively implement it for clinical MRI.

METHODS

The CEST scans were acquired from 7 normal volunteers and 15 brain tumor patients using a 3T clinical scanner. The 2D and 3D "artifact suppression" (AS) vSENSE algorithms were applied to generate sensitivity maps from a first scan acquired with conventional SENSE-accelerated 2D and 3D CEST data. The AS sensitivity maps were then applied to reconstruct the other CEST frames at higher acceleration factors. Both retrospective and prospective acceleration in phase-encoding and slice-encoding dimensions were implemented.

RESULTS

Applying the 2D AS vSENSE algorithm to a 2-fold undersampled 3.5-ppm CEST frame halved the scan time of conventional SENSE, while generating essentially identical reconstruction errors (p ≈ 1.0). The 3D AS vSENSE algorithm permitted prospective acceleration by up to 8-fold, in total, from phase-encoding and slice-encoding directions for individual source CEST images, and an overall speed-up in scan time of 5-fold. The resulting vSENSE-accelerated amide proton transfer-weighted images agreed with conventional 2-fold-accelerated SENSE CEST results in brain tumor patients and healthy volunteers. Importantly, the vSENSE method eliminated unfolding artifacts in the slice-encoding direction that compromised conventional SENSE CEST scans.

CONCLUSION

The vSENSE method can be extended to 3D CEST imaging to provide higher acceleration factors than conventional SENSE without compromising accuracy.

Microstructural and Neurochemical Changes in the Rat Brain After Diffuse Axonal Injury

BACKGROUND

Diffuse axonal injury (DAI) is one of the devastating types of traumatic brain injury, but is difficult to detect on conventional imaging in its early stages.

PURPOSE

To test the technical feasibility and diagnostic value of diffusion kurtosis imaging (DKI) and glutamate chemical exchange saturation transfer (GluCEST) imaging in the brain after DAI.

STUDY TYPE

Prospective.

ANIMAL MODEL

Sixty Sprague-Dawley rats. The DAI model was induced by using the impact acceleration model of Marmarou et al with modified settings.

FIELD STRENGTH/SEQUENCE

A 7.0T animal MR scanner with a fast spin-echo sequence (T₂ -weighted imaging), fast spin-echo multislice sequence (DKI), echo planar imaging in the signal of the chemical exchange saturation transfer sequence (CEST), and point-resolved spectroscopy sequence (hydrogenproton magnetic resonance spectroscopy, ¹ H-MRS).

ASSESSMENT

Brain MRI scanned before and 2 hours after injury. DKI images were processed with MatLab and MRIcro software, GluCEST images were processed using software routines written in MatLab, and spectroscopic data were postprocessed with LCModel.

STATISTICAL TESTS

The parameters of these techniques were assessed using the independent sample t-test and Pearson correlation.

RESULTS

Mean kurtosis and mean diffusivity values were significantly higher than controls in the parietal lobe, hippocampus, and thalamus (P <  0.01). However, fractional anisotropy was lower only in the parietal lobe, with no detectable changes in the hippocampus and thalamus. GluCEST values of the parietal lobe, hippocampus, and thalamus were significantly higher than controls in DAI rats (P <  0.01). This change was further validated through ¹ H-MRS. A positive correlation was observed between glutamate (Glu) and glutamate compound (Glx) concentrations and GluCEST values (Glu: R²  = 0.589, Glx: R²  = 0.878).

DATA CONCLUSION

DKI and GluCEST might be acceptably sensitive for tracking microstructural and neurochemical changes in the brain following DAI.

LEVEL OF EVIDENCE

1 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2019;49:1069-1077.

Chemical exchange saturation transfer (CEST) as a new method of signal obtainment in magnetic resonance molecular imaging in clinical and research practice

The work describes the physical basis of the chemical exchange saturation transfer (CEST) technique; it presents the beginnings of the implementation of the method and its possible applications. The principles of correct data acquisition and possible solutions used during the design of the CEST sequence are shown. The main problems related to data analysis are indicated, and an example Z-spectrum from in vivo study of the rat brain is introduced. Furthermore, the parameters related to spectrum analyses such as magnetisation transfer asymmetry (MTRasym) and amide proton transfer asymmetry (APTasym) are presented. In the following part, different types of the CEST method often mentioned in the literature are discussed. Subsequently, the possible applications of the CEST method in both clinical and experimental practice are described.

Emerging Technologies to Image Tissue Metabolism

Due to the implication of altered metabolism in a large spectrum of tissue function and disease, assessment of metabolic processes becomes essential in managing health. In this regard, imaging can play a critical role in allowing observation of biochemical and physiological processes. Nuclear imaging methods, in particular positron emission tomography, have been widely employed for imaging metabolism but are mainly limited by the use of ionizing radiation and the sensing of only one parameter at each scanning session. Observations in healthy individuals or longitudinal studies of disease could markedly benefit from non-ionizing, multi-parameter imaging methods. We therefore focus this review on progress with the non-ionizing radiation methods of MRI, hyperpolarized magnetic resonance and magnetic resonance spectroscopy, chemical exchange saturation transfer, and emerging optoacoustic (photoacoustic) imaging. We also briefly discuss the role of nuclear and optical imaging methods for research and clinical protocols.

Glutamate weighted imaging contrast in gliomas with 7 Tesla magnetic resonance imaging

Introduction

Diffuse gliomas are incurable malignancies, which undergo inevitable progression and are associated with seizure in 50–90% of cases. Glutamate has the potential to be an important glioma biomarker of survival and local epileptogenicity if it can be accurately quantified noninvasively.

Methods

We applied the glutamate-weighted imaging method GluCEST (glutamate chemical exchange saturation transfer) and single voxel MRS (magnetic resonance spectroscopy) at 7 Telsa (7 T) to patients with gliomas. GluCEST contrast and MRS metabolite concentrations were quantified within the tumour region and peritumoural rim. Clinical variables of tumour aggressiveness (prior adjuvant therapy and previous radiological progression) and epilepsy (any prior seizures, seizure in last month and drug refractory epilepsy) were correlated with respective glutamate concentrations. Images were separated into post-hoc determined patterns and clinical variables were compared across patterns.

Results

Ten adult patients with a histo-molecular (n = 9) or radiological (n = 1) diagnosis of grade II-III diffuse glioma were recruited, 40.3 +/− 12.3 years. Increased tumour GluCEST contrast was associated with prior adjuvant therapy (p = .001), and increased peritumoural GluCEST contrast was associated with both recent seizures (p = .038) and drug refractory epilepsy (p = .029). We distinguished two unique GluCEST contrast patterns with distinct clinical and radiological features. MRS glutamate correlated with GluCEST contrast within the peritumoural voxel (R = 0.89, p = .003) and a positive trend existed in the tumour voxel (R = 0.65, p = .113).

Conclusion

This study supports the role of glutamate in diffuse glioma biology. It further implicates elevated peritumoural glutamate in epileptogenesis and altered tumour glutamate homeostasis in glioma aggressiveness. Given the ability to non-invasively visualise and quantify glutamate, our findings raise the prospect of 7 T GluCEST selecting patients for individualised therapies directed at the glutamate pathway. Larger studies with prospective follow-up are required.

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7 T GluCEST glioma imaging is feasible, producing high quality quantifiable images.

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Increased peritumoural GluCEST contrast correlates with drug resistant epilepsy.

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Increased tumour GluCEST contrast is associated with prior adjuvant therapy.

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Two GluCEST patterns were identified with distinct clinico-radiological features.

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GluCEST contrast correlates with MRS glutamate in peritumoural regions.

Structural and functional asymmetry of medial temporal subregions in unilateral temporal lobe epilepsy: A 7T MRI study

Mesial temporal lobe epilepsy (TLE) is a common neurological disorder affecting the hippocampus and surrounding medial temporal lobe (MTL). Although prior studies have analyzed whole-brain network distortions in TLE patients, the functional network architecture of the MTL at the subregion level has not been examined. In this study, we utilized high-resolution 7T T2-weighted magnetic resonance imaging (MRI) and resting-state BOLD-fMRI to characterize volumetric asymmetry and functional network asymmetry of MTL subregions in unilateral medically refractory TLE patients and healthy controls. We subdivided the TLE group into mesial temporal sclerosis patients (TLE-MTS) and MRI-negative nonlesional patients (TLE-NL). Using an automated multi-atlas segmentation pipeline, we delineated 10 MTL subregions per hemisphere for each subject. We found significantly different patterns of volumetric asymmetry between the two groups, with TLE-MTS exhibiting volumetric asymmetry corresponding to decreased volumes ipsilaterally in all hippocampal subfields, and TLE-NL exhibiting no significant volumetric asymmetries other than a mild decrease in whole-hippocampal volume ipsilaterally. We also found significantly different patterns of functional network asymmetry in the CA1 subfield and whole hippocampus, with TLE-NL patients exhibiting asymmetry corresponding to increased connectivity ipsilaterally and TLE-MTS patients exhibiting asymmetry corresponding to decreased connectivity ipsilaterally. Our findings provide initial evidence that functional neuroimaging-based network properties within the MTL can distinguish between TLE subtypes. High-resolution MRI has potential to improve localization of underlying brain network disruptions in TLE patients who are candidates for surgical resection.

In vivo metabotropic glutamate receptor type 5 abnormalities localize the epileptogenic zone in mesial temporal lobe epilepsy

OBJECTIVE

Surgical specimens from patients with mesial temporal lobe epilepsy (MTLE) show abnormalities in tissue concentrations of metabotropic glutamate receptor type 5 (mGluR5). To clarify whether these abnormalities are specific to the epileptogenic zone (EZ), we characterized in vivo whole-brain mGluR5 availability in MTLE patients using positron emission tomography (PET) and [¹¹ C]ABP688, a radioligand that binds specifically to the mGluR5 allosteric site.

METHODS

Thirty-one unilateral MTLE patients and 30 healthy controls underwent [¹¹ C]ABP688 PET. We compared partial volume corrected [¹¹ C]ABP688 nondisplaceable binding potentials (BP_ND ) between groups using region-of-interest and whole-brain voxelwise analyses. [¹⁸ F]Fluorodeoxyglucose (FDG) PET was acquired in 15 patients, for whom we calculated asymmetry indices of [¹¹ C]ABP688 BP_ND and [¹⁸ F]FDG uptake to compare lateralization and localization differences.

RESULTS

[¹¹ C]ABP688 BP_ND was focally reduced in the epileptogenic hippocampal head and amygdala (p < 0.001). Patients with hippocampal atrophy showed more extensive abnormalities, including the ipsilateral temporal neocortex (p = 0.006). [¹¹ C]ABP688 BP_ND showed interhemispheric differences of higher magnitude and discriminated the epileptogenic structures more accurately when compared to [¹⁸ F]FDG uptake, which showed more widespread hypometabolism. Among 23 of 25 operated patients with >1 year of follow-up, 13 were seizure-free (Engel Ia) and showed significantly lower [¹¹ C]ABP688 BP_ND in the ipsilateral entorhinal cortex.

INTERPRETATION

[¹¹ C]ABP688 PET provides a focal biomarker for the EZ in MTLE with higher spatial accuracy compared to [¹⁸ F]FDG PET. Focally reduced mGluR5 availability in the EZ might reflect receptor internalization or conformational changes in response to excessive extracellular glutamate, supporting a potential role for mGluR5 as therapeutic target in human MTLE. Ann Neurol 2019; 1-11 ANN NEUROL 2019;85:218-228.

Mapping the Changes of Glutamate Using Glutamate Chemical Exchange Saturation Transfer (GluCEST) Technique in a Traumatic Brain Injury Model: A Longitudinal Pilot Study

Glutamate excitoxicity plays a crucial role in the pathophysiology of traumatic brain injury (TBI) through the initiation of secondary injuries. Glutamate chemical exchange saturation transfer (GluCEST) MRI is a newly developed technique to noninvasively image glutamate in vivo with high sensitivity and spatial resolution. The aim of the present study was to use a rat model of TBI to map changes in brain glutamate distribution and explore the capability of GluCEST imaging for detecting secondary injuries. Sequential GluCEST imaging scans were performed in adult male Sprague-Dawley rats before TBI and at 1, 3, 7, and 14 days after TBI. GluCEST% increased and peaked on day 1 after TBI in the core lesion of injured cortex and peaked on day 3 in the ipsilateral hippocampus, as compared to baseline and controls. GluCEST% gradually declined to baseline by day 14 after TBI. A negative correlation between the GluCEST% of the ipsilateral hippocampus on day 3 and the time in the correct quadrant was observed in injured rats. Immunolabeling for glial fibrillary acidic protein showed significant astrocyte activation in the ipsilateral hippocampus of TBI rats. IL-6 and TNF-α in the core lesion peaked on day 1 postinjury, while those in the ipsilateral hippocampus peaked on day 3. These subsequently gradually declined to sham levels by day 14. It was concluded that GluCEST imaging has potential to be a novel neuroimaging approach for predicting cognitive outcome and to better understand neuroinflammation following TBI.

Creatine and phosphocreatine mapping of mouse skeletal muscle by a polynomial and Lorentzian line-shape fitting CEST method

PURPOSE

To obtain high-resolution Cr and PCr maps of mouse skeletal muscle using a polynomial and Lorentzian line-shape fitting (PLOF) CEST method.

METHODS

Wild-type mice and guanidinoacetate N-methyltransferase-deficient (GAMT-/-) mice that have low Cr and PCr concentrations in muscle were used to assign the Cr and PCr peaks in the Z-spectrum at 11.7 T. A PLOF method was proposed to simultaneously extract and quantify the Cr and PCr by assuming a polynomial function for the background and 2 Lorentzian functions for the CEST peaks at 1.95 ppm and 2.5 ppm.

RESULTS

The Z-spectra of phantoms revealed that PCr has 2 CEST peaks (2 ppm and 2.5 ppm), whereas Cr only showed 1 peak at 2 ppm. Comparison of the Z-spectra of wild-type and GAMT-/- mice indicated that, contrary to brain, there was no visible protein guanidinium peak in the skeletal-muscle Z-spectrum, which allowed us to extract clean PCr and Cr CEST signals. High-resolution PCr and Cr concentration maps of mouse skeletal muscle were obtained by the PLOF CEST method after calibration with in vivo MRS.

CONCLUSIONS

The PLOF method provides an efficient way to map Cr and PCr concentrations simultaneously in the skeletal muscle at high MRI field.

Reproducibility of 2D GluCEST in healthy human volunteers at 7 T

PURPOSE

To investigate the reproducibility of gray and white matter glutamate contrast of a brain slice among a small group of healthy volunteers by using the 2D single-slice glutamate CEST (GluCEST) imaging technique.

METHODS

Six healthy volunteers were scanned multiple times for within-day and between-day reproducibility. One more volunteer was scanned for within-day reproducibility at 7T MRI. Glutamate CEST contrast measurements were calculated for within subjects and among the subjects and the coefficient of variations are reported.

RESULTS

The GluCEST measurements were highly reproducible in the gray and white matter area of the brain slice, whether it was within-day or between-day with a coefficient of variation of less than 5%.

CONCLUSION

This preliminary study in a small group of healthy volunteers shows a high degree of reproducibility of GluCEST MRI in brain and holds promise for implementation in studying age-dependent changes in the brain.

Imaging biomarkers of epileptogenecity after traumatic brain injury - Preclinical frontiers

Posttraumatic epilepsy (PTE) is a major neurodegenerative disease accounting for 20% of symptomatic epilepsy cases. A long latent phase offers a potential window for prophylactic treatment strategies to prevent epilepsy onset, provided that the patients at risk can be identified. Some promising imaging biomarker candidates for posttraumatic epileptogenesis have been identified, but more are required to provide the specificity and sensitivity for accurate prediction. Experimental models and preclinical longitudinal, multimodal imaging studies allow follow-up of complex cascade of events initiated by traumatic brain injury, as well as monitoring of treatment effects. Preclinical imaging data from the posttraumatic brain are rich in information, yet examination of their specific relevance to epilepsy is lacking. Accumulating evidence from ongoing preclinical studies in TBI support insight into processes involved in epileptogenesis, e.g. inflammation and changes in functional and structural brain-wide connectivity. These efforts are likely to produce both new biomarkers and treatment targets for PTE.

Glutamate-sensitive imaging and evaluation of cognitive impairment in multiple sclerosis

BACKGROUND

Cognitive impairment (CI) profoundly impacts quality of life for patients with multiple sclerosis (MS). Dysfunctional regulation of glutamate in gray matter (GM) has been implicated in the pathogenesis of MS by post-mortem pathological studies and in CI by in vivo magnetic resonance spectroscopy, yet GM pathology is subtle and difficult to detect using conventional T₁- and T₂-weighted magnetic resonance imaging (MRI). There is a need for high-resolution, clinically accessible imaging techniques that probe molecular changes in GM.

OBJECTIVE

To study cortical GM pathology related to CI in MS using glutamate-sensitive chemical exchange saturation transfer (GluCEST) MRI at 7.0 Tesla (7T).

METHODS

A total of 20 patients with relapsing-remitting MS and 20 healthy controls underwent cognitive testing, anatomical imaging, and GluCEST imaging. Glutamate-sensitive image contrast was quantified for cortical GM, compared between cohorts, and correlated with clinical measures of CI.

RESULTS AND CONCLUSION

Glutamate-sensitive contrast was significantly increased in the prefrontal cortex of MS patients with accumulated disability (p < 0.05). In addition, glutamate-sensitive contrast in the prefrontal cortex was significantly correlated with symbol digit modality test (r_S = -0.814) and choice reaction time (r_S = 0.772) scores in patients (p < 0.05), suggesting that GluCEST MRI may have utility as a marker for GM pathology and CI.

Imaging Cancer Metabolism: Underlying Biology and Emerging Strategies

Dysregulated cellular metabolism is a characteristic feature of malignancy that has been exploited for both imaging and targeted therapy. With regard to imaging, deranged glucose metabolism has been leveraged using 18F-FDG PET. Metabolic imaging with 18F-FDG, however, probes only the early steps of glycolysis; the complexities of metabolism beyond these early steps in this single pathway are not directly captured. New imaging technologies-both PET with novel radiotracers and MR-based methods-provide unique opportunities to investigate other aspects of cellular metabolism and expand the metabolic imaging armamentarium. This review will discuss the underlying biology of metabolic dysregulation in cancer, focusing on glucose, glutamine, and acetate metabolism. Novel imaging strategies will be discussed within this biologic framework, highlighting particular strengths and limitations of each technique. Emphasis is placed on the role that combining modalities will play in enabling multiparametric imaging to fully characterize tumor biology to better inform treatment.

Glutamate-Weighted Chemical Exchange Saturation Transfer Magnetic Resonance Imaging Detects Glutaminase Inhibition in a Mouse Model of Triple-Negative Breast Cancer

Glutamate is an important metabolite of glutaminolysis, a metabolic pathway used by many aggressive cancers, including triple-negative breast cancer (TNBC). With the exception of the brain, in vivo detection of glutamate in tissues using ¹H magnetic resonance spectroscopy (MRS) is challenging. Compared with MRS, glutamate-weighted chemical exchange saturation transfer MR imaging (GluCEST MRI) offers a more sensitive detection mechanism that is free of glutamine interference. Here, we developed a robust, highly repeatable GluCEST MRI protocol in mice bearing human TNBC xenografts and treated with a potent glutaminase inhibitor, CB-839. In paired studies, treatment with CB-839 for 2 days reduced the GluCEST asymmetry value compared with baseline (P < 0.05, n = 10). The absolute change of the GluCEST asymmetry value was -2.5 percent points after CB-839 treatment versus +0.3 after vehicle (P < 0.01). Correspondingly, treatment with CB-839 reduced tumor glutamate concentrations by 1.5 mmol/L, consistent with prior calibration between changes of the GluCEST value versus tissue glutamate concentration; CB-839, however, did not change tumor intracellular pH. These results demonstrate in a mouse model of breast cancer the utility of GluCEST MRI to detect the early response to glutaminase inhibition.Significance: A sensitive method enables noninvasive detection of tumor response to inhibitors of glutamine metabolism. Cancer Res; 78(19); 5521-6. ©2018 AACR.

GlucoCEST imaging with on-resonance variable delay multiple pulse (onVDMP) MRI

PURPOSE

To examine the detection sensitivity for the rapidly exchanging hydroxyl protons of D-glucose using the recently developed on-resonance variable delay multi-pulse (onVDMP) chemical exchange saturation transfer (CEST) technique.

METHODS

The onVDMP method was applied for the detection of water signal changes upon venous D-glucose infusion in mice with 9L glioma xenografts. The effect size of onVDMP MRI during infusion was compared with that of conventional continuous wave (CW) CEST MRI.

RESULTS

Both methods highlighted the tumor and the blood vessels on D-glucose infusion. In interleaved studies, the mean signal changes detected by onVDMP were found to be 1.8 times higher than those by CW-CEST, attributed to its high labeling efficiency for fast exchanging protons and the labeling of the OH protons over a larger frequency range.

CONCLUSIONS

The onVDMP method is a more sensitive technique for the detection of exogenous CEST agents with fast-exchanging protons compared to CW-CEST MRI.

Nanosensors for the Chemical Imaging of Acetylcholine Using Magnetic Resonance Imaging

A suite of imaging tools for detecting specific chemicals in the central nervous system could accelerate the understanding of neural signaling events critical to brain function and disease. Here, we introduce a class of nanoparticle sensors for the highly specific detection of acetylcholine in the living brain using magnetic resonance imaging. The nanosensor is composed of acetylcholine-catalyzing enzymes and pH-sensitive gadolinium contrast agents co-localized onto the surface of polymer nanoparticles, which leads to changes in T₁ relaxation rate (1/ T₁). The mechanism of the sensor involves the enzymatic hydrolysis of acetylcholine leading to a localized decrease in pH which is detected by the pH-sensitive gadolinium chelate. The concomitant change in 1/ T₁ in vitro measured a 20% increase from 0 to 10 μM acetylcholine concentration. The applicability of the nanosensors in vivo was demonstrated in the rat medial prefrontal cortex showing distinct changes in 1/ T₁ induced by pharmacological stimuli. The highly specific acetylcholine nanosensor we present here offers a promising strategy for detection of cholinergic neurotransmission and will facilitate our understanding of brain function through chemical imaging.

In Vivo Multimodal Magnetic Resonance Imaging Changes After N-Methyl-d-Aspartate-Triggered Spasms in Infant Rats

Objective

Despite the serious neurodevelopmental sequelae of epileptic encephalopathy during infancy, the pathomechanisms involved remain unclear. To find potential biomarkers that can reflect the pathogenesis of epileptic encephalopathy, we explored the neurometabolic and microstructural sequelae after infantile spasms using a rat model of infantile spasms and in vivo magnetic resonance imaging techniques.

Methods

Rats prenatally exposed to betamethasone were subjected to three rounds of intraperitoneal N-methyl-d-aspartate (NMDA) triggering of spasms or received saline injections (controls) on postnatal days (P) 12, 13, and 15. Chemical exchange saturation transfer imaging of glutamate (GluCEST) were performed at P15 and 22 and diffusion tensor imaging and additional spectroscopy (1H-MRI/MRS) of the cingulate cortex were serially done at P16, 23, and 30 and analyzed. Pathological analysis and western blotting were performed with rats sacrificed on P35.

Results

Within 24 h of the three rounds of NMDA-induced spasms, there was an acute increase in the GluCEST (%) in the cortex, hippocampus, and striatum. When focused on the cingulate cortex, mean diffusivity (MD) was significantly decreased during the acute period after multiple spasms with an increase in γ-aminobutyric acid (GABA), glutamate, and glutamine N-acetylaspartate-plus-N-acetylaspartylglutamate (tNAA), total choline, and total creatine. The juvenile rats also showed decreased MD on diffusion tensor imaging and significant decreases in taurine, tNAA, and macromolecules-plus-lipids in the cingulate cortex. Pathologically, there was a significant reduction in glial fibrillary acidic protein, myelin basic protein, and neuronal nuclei expression in the cingulate cortex of rats with NMDA-induced spasms.

Significance

These neurometabolic and microstructural alterations after NMDA-triggered spasms might be potential imaging biomarkers of epileptic encephalopathy.

In vivo GluCEST MRI: Reproducibility, background contribution and source of glutamate changes in the MPTP model of Parkinson's disease

Glutamate Chemical Exchange Saturation Transfer (GluCEST) MRI is a recently developed technique to image glutamate. In the present study, we evaluated the reproducibility and background contamination to the GluCEST and source of the GluCEST changes in a mouse model of Parkinson's disease. Repeated measurements in five mice demonstrated an intra-animal coefficient of variation (CV) of GluCEST signal to be 2.3 ± 1.3% and inter-animal CV of GluCEST to be 3.3 ± 0.3%. Mice were treated with MPTP to create a localized striatal elevation of glutamate. We found an elevation in the GluCEST contrast of the striatum following MPTP treatment (Control: 23.3 ± 0.8%, n = 16; MPTP: 26.2 ± 0.8%, n = 19; p ≤ 0.001). Additionally, the positive association between glutamate concentration measured via 1H MRS and GluCEST signal was used to estimate background contribution to the measured GluCEST. The contribution of signal from non-glutamate sources was found to be ~28% of the total GluCEST. Immunohistochemical analysis of the brain showed co-localization of glutamate with GFAP in the striatum. This suggests that the elevated glutamate present in the striatum in this mouse model reflects astroglial proliferation or reactivity due to the action of MPTP. The potential of GluCEST as a biomarker for imaging inflammation mediated gliosis is discussed.

Separating fast and slow exchange transfer and magnetization transfer using off-resonance variable-delay multiple-pulse (VDMP) MRI

PURPOSE

To develop a method that can separate and quantify the fast (>1 kHz) and slow exchange transfer and magnetization transfer components in Z-spectra.

METHODS

Z-spectra were recorded as a function of mixing time using a train of selective pulses providing variable-delay multipulse build-up curves. Fast and slow transfer components in the Z-spectra were separated and quantified on a voxel-by-voxel basis by fitting the mixing time-dependent CEST signal using a 3-pool model.

RESULTS

Phantom studies of glutamate solution, bovine serum albumin solution, and hair conditioner showed the capability of the proposed method to separate fast and slow transfer components. In vivo mouse brain studies showed a strong contrast between white matter and gray matter in the slow-transferring map, corresponding to an asymmetric component of the conventional semisolid magnetization transfer contrast. In addition, a fast-transferring proton map was found that was homogeneous across the brain and attributed to the total contributions of the fast-exchanging protons from proteins, metabolites, and a symmetric magnetization transfer contrast component.

CONCLUSIONS

This new method provides a simple way to extract fast and slow transfer components from the Z-spectrum, leading to novel MRI contrasts, and providing insight into the different magnetization transfer contrast contributions.

Increased CEST specificity for amide and fast-exchanging amine protons using exchange-dependent relaxation rate

Chemical exchange saturation transfer (CEST) imaging of amides at 3.5 ppm and fast-exchanging amines at 3 ppm provides a unique means to enhance the sensitivity of detection of, for example, proteins/peptides and neurotransmitters, respectively, and hence can provide important information on molecular composition. However, despite the high sensitivity relative to conventional magnetic resonance spectroscopy (MRS), in practice, CEST often has relatively poor specificity. For example, CEST signals are typically influenced by several confounding effects, including direct water saturation (DS), semi-solid non-specific magnetization transfer (MT), the influence of water relaxation times (T_1w ) and nearby overlapping CEST signals. Although several editing techniques have been developed to increase the specificity by removing DS, semi-solid MT and T_1w influences, it is still challenging to remove overlapping CEST signals from different exchanging sites. For instance, the amide proton transfer (APT) signal could be contaminated by CEST effects from fast-exchanging amines at 3 ppm and intermediate-exchanging amines at 2 ppm. The current work applies an exchange-dependent relaxation rate (R_ex ) to address this problem. Simulations demonstrate that: (1) slowly exchanging amides and fast-exchanging amines have distinct dependences on irradiation powers; and (2) R_ex serves as a resonance frequency high-pass filter to selectively reduce CEST signals with resonance frequencies closer to water. These characteristics of R_ex provide a means to isolate the APT signal from amines. In addition, previous studies have shown that CEST signals from fast-exchanging amines have no distinct features around their resonance frequencies. However, R_ex gives Lorentzian lineshapes centered at their resonance frequencies for fast-exchanging amines and thus can significantly increase the specificity of CEST imaging for amides and fast-exchanging amines.

The oxymoron of image-guided resection in 3 T MRI-negative extratemporal epilepsy: Technique and postoperative results

OBJECTIVE

We share our experience with extratemporal MRI-negative epilepsies that received "image-guided" resection with the use of neuronavigation after invasive presurgical video-EEG monitoring. We describe and discuss our technique of image generation, navigation system registration, and surgical resection. In addition, we evaluate seizure outcome with respect to the preoperatively planned versus achieved resection.

PATIENTS AND METHODS

Seven patients with 3 T MRI-negative extratemporal epilepsy received navigation-guided resective surgery. The resection plan was based on electrophysiological data from intracranial EEG recordings. For each case a resection segment was created in the neuronavigation device in a systematic manner. We compared the preoperatively planned segment to the achieved resection and looked for correlation with postoperative seizure outcome according to Engel classification, at last follow-up (mean 2.4 years, range 1-4 years).

RESULTS

Mean volume of planned resections was 23.8 ± 15.3 cm³ and of achieved resections 17 ± 10.4 cm³. There was complete overlap with planned resection in 4 patients and partial overlap in 3. Postoperative seizure outcome was class I in 4 patients (57%), IIIA in 1 patient and IVB in 2 patients. Three patients reached seizure-freedom (Engel IA). Volume of planned resection, volume difference of planned versus achieved resection and level of overlap (complete versus partial) did not significantly correlate to seizure outcome.

CONCLUSION

The use of neuronavigation for planning and executing a tailored resection in MRI-negative extratemporal epilepsy is elaborate but can be an effective procedure.

In Vivo Multidimensional Brain Imaging in Huntington's Disease Animal Models

Huntington's disease (HD) is a genetic neurodegenerative disorder caused by an abnormal expansion of a CAG repeat located in the gene encoding for huntingtin protein. This mutation induces the expression of a polyglutamine stretch in the mutated protein resulting in the modification of various biological properties of the wild-type protein and the progressive appearance of motor, cognitive, and psychiatric disorders that are typically associated to this condition. Although the exact neuropathological mechanisms of degeneration are still not fully understood, HD pathology is characterized by severe neuronal losses in various brain regions including the basal ganglia and many cortical areas. Early signs of astrogliosis may precede actual neuronal degeneration. Early metabolic impairment at least in part associated with mitochondrial complex II deficiency may play a key role in huntingtin-induced mechanisms of neurodegeneration. Clinical trials are actively prepared including various gene-silencing approaches aiming at decreasing mutated huntingtin production. However, with the lack of a specific imaging biomarker capable of visualizing mutated huntingtin or huntingtin aggregates, there is a need for surrogate markers of huntingtin neurodegeneration. MRI and caudate nucleus atrophy is one of the most sensitive imaging biomarkers of HD. As such it can be used as a means to study disease progression and potential halting of the neurodegenerative process by therapeutic intervention, but this marker relies on actual neuronal loss which is a somewhat a late event in the pathology. As a means to develop, characterize and evaluate new, potentially earlier biomarkers of HD pathology we have recently embarked on a series of NMR developments looking for brain imaging techniques that allow for noninvasive longitudinal evaluation/characterization of functional alterations in animal models of HD. This chapter describes an assemblage of innovative NMR methods that have proved useful in detecting pathological cell dysfunctions in various preclinical models of HD.

Direct saturation-corrected chemical exchange saturation transfer MRI of glioma: Simplified decoupling of amide proton transfer and nuclear overhauser effect contrasts

PURPOSE

Chemical exchange saturation transfer (CEST) MRI has shown promise in tissue characterization in diseases like stroke and tumor. However, in vivo CEST imaging such as amide proton transfer (APT) MRI is challenging because of concomitant factors such as direct water saturation, macromolecular magnetization transfer, and nuclear overhauser effect (NOE), which lead to a complex contrast in the commonly used asymmetry analysis (MTRasym). Here, we propose a direct saturation-corrected CEST (DISC-CEST) analysis for simplified decoupling and quantification of in vivo CEST effects.

METHODS

CEST MRI and relaxation measurements were carried out on a classical 2-pool creatine-gel CEST phantom and normal rat brains (N = 6) and a rat model of glioma (N = 8) at 4.7T. The proposed DISC-CEST quantification was carried out and compared with conventional MTRasym and the original three-offset method.

RESULTS

We demonstrated that the DISC-CEST contrast in the phantom had much stronger correlation with MTRasym than the three-offset method, which showed substantial underestimation. In normal rat brains, the DISC-CEST approach revealed significantly stronger APT effect in gray matter and higher NOE effect in white matter. Furthermore, the APT and NOE maps derived from DISC-CEST showed significantly higher APT effect in the tumors than contralateral normal tissue but no apparent difference in NOE.

CONCLUSION

The proposed DISC-CEST method, by correction of nonlinear direct water saturation effect, serves as a promising alternative to both the commonly used MTRasym and the simplistic three-offset analyses. It provides simple yet reliable in vivo CEST quantification such as APT and NOE mapping in brain tumor, which is promising for clinical translation. Magn Reson Med 78:2307-2314, 2017. © 2017 International Society for Magnetic Resonance in Medicine.

Investigation of the contribution of total creatine to the CEST Z-spectrum of brain using a knockout mouse model

The current study aims to assign and estimate the total creatine (tCr) signal contribution to the Z-spectrum in mouse brain at 11.7 T. Creatine (Cr), phosphocreatine (PCr) and protein phantoms were used to confirm the presence of a guanidinium resonance at this field strength. Wild-type (WT) and knockout mice with guanidinoacetate N-methyltransferase deficiency (GAMT-/-), which have low Cr and PCr concentrations in the brain, were used to assign the tCr contribution to the Z-spectrum. To estimate the total guanidinium concentrations, two pools for the Z-spectrum around 2 ppm were assumed: (i) a Lorentzian function representing the guanidinium chemical exchange saturation transfer (CEST) at 1.95 ppm in the 11.7-T Z-spectrum; and (ii) a background signal that can be fitted by a polynomial function. Comparison between the WT and GAMT-/- mice provided strong evidence for three types of contribution to the peak in the Z-spectrum at 1.95 ppm, namely proteins, Cr and PCr, the latter fitted as tCr. A ratio of 20 ± 7% (protein) and 80 ± 7% tCr was found in brain at 2 μT and 2 s saturation. Based on phantom experiments, the tCr peak was estimated to consist of about 83 ± 5% Cr and 17 ± 5% PCr. Maps for tCr of mouse brain were generated based on the peak at 1.95 ppm after concentration calibration with in vivo magnetic resonance spectroscopy.

Hippocampal MRS and subfield volumetry at 7T detects dysfunction not specific to seizure focus

Ultra high-field 7T MRI offers sensitivity to localize hippocampal pathology in temporal lobe epilepsy (TLE), but has rarely been evaluated in patients with normal-appearing clinical MRI. We applied multimodal 7T MRI to assess if focal subfield atrophy and deviations in brain metabolites characterize epileptic hippocampi. Twelve pre-surgical TLE patients (7 MRI-negative) and age-matched healthy volunteers were scanned at 7T. Hippocampal subfields were manually segmented from 600μm isotropic resolution susceptibility-weighted images. Hippocampal metabolite spectra were acquired to determine absolute concentrations of glutamate, glutamine, myo-inositol, NAA, creatine and choline. We performed case-controls analyses, using permutation testing, to identify abnormalities in hippocampal imaging measures in individual patients, for evaluation against clinical evidence of seizure lateralisation and neuropsychological memory test scores. Volume analyses identified hippocampal subfield atrophy in 9/12 patients (75%), commonly affecting CA3. 7/8 patients had altered metabolite concentrations, most showing reduced glutamine levels (62.5%). However, neither volume nor metabolite deviations consistently lateralized the epileptogenic hippocampus. Rather, lower subiculum volumes and glutamine concentrations correlated with impaired verbal memory performance. Hippocampal subfield and metabolic abnormalities detected at 7T appear to reflect pathophysiological processes beyond epileptogenesis. Despite limited diagnostic contributions, these markers show promise to help elucidate mnemonic processing in TLE.

Glutamate imaging (GluCEST) reveals lower brain GluCEST contrast in patients on the psychosis spectrum

Psychosis commonly develops in adolescence or early adulthood. Youths at clinical high risk (CHR) for psychosis exhibit similar, subtle symptoms to those with schizophrenia (SZ). Malfunctioning neurotransmitter systems, such as glutamate, are implicated in the disease progression of psychosis. Yet, in vivo imaging techniques for measuring glutamate across the cortex are limited. Here, we use a novel 7 Tesla MRI glutamate imaging technique (GluCEST) to estimate changes in glutamate levels across cortical and subcortical regions in young healthy individuals and ones on the psychosis spectrum. Individuals on the psychosis spectrum (PS; n=19) and healthy young individuals (HC; n=17) underwent MRI imaging at 3 and 7 T. At 7 T, a single slice GluCEST technique was used to estimate in vivo glutamate. GluCEST contrast was compared within and across the subcortex, frontal, parietal and occipital lobes. Subcortical (χ2 (1)=4.65, P=0.031) and lobular (χ2 (1)=5.17, P=0.023) GluCEST contrast levels were lower in PS compared with HC. Abnormal GluCEST contrast levels were evident in both CHR (n=14) and SZ (n=5) subjects, and correlated differentially, across regions, with clinical symptoms. Our findings describe a pattern of abnormal brain neurochemistry early in the course of psychosis. Specifically, CHR and young SZ exhibit diffuse abnormalities in GluCEST contrast attributable to a major contribution from glutamate. We suggest that neurochemical profiles of GluCEST contrast across cortex and subcortex may be considered markers of early psychosis. GluCEST methodology thus shows promise to further elucidate the progression of the psychosis disease state.

Chemical Exchange Saturation Transfer MRI Signal Loss of the Substantia Nigra as an Imaging Biomarker to Evaluate the Diagnosis and Severity of Parkinson's Disease

The early diagnosis of Parkinson's disease (PD) and the accurate evaluation of disease severity are crucial for intervention and treatment in PD patients. In this study, we applied chemical exchange saturation transfer (CEST) imaging to patients at different stages of PD and explored the clinical value of the CEST signal loss of the substantia nigra as an imaging biomarker of PD. The measured CEST signal intensities (including amide proton transfer-weighted or APTw, and total CEST or CEST_total) of the substantia nigra in PD patients showed a significantly decreased tendency with PD progression. Compared to normal controls, the APTw and CEST_total intensities of PD patients significantly decreased at both the early and advanced or late stages. These APTw and CEST_total values of the substantia nigra were also significantly lower in advanced or late stage PD patients than in early stage PD patients. For PD patients with unilateral symptoms, the APTw and CEST_total values in the substantia nigra on the affected side were significantly lower than those in normal controls. Both the APTw and CEST_total values of PD were significantly correlated with the severity of disease and disease duration. Our findings suggest that the CEST MRI signal of the substantia nigra is a potential imaging biomarker for the diagnosis and monitoring of the severity of PD.

Clinical applications of chemical exchange saturation transfer (CEST) MRI

Chemical exchange saturation transfer (CEST) magnetic resonance imaging (MRI) has been developed and employed in multiple clinical imaging research centers worldwide. Selective radiofrequency (RF) saturation pulses with standard 2D and 3D MRI acquisition schemes are now routinely performed, and CEST MRI can produce semiquantitative results using magnetization transfer ratio asymmetry (MTR_asym ) analysis while accounting for B₀ inhomogeneity. Faster clinical CEST MRI acquisition methods and more quantitative acquisition and analysis routines are under development. Endogenous biomolecules with amide, amine, and hydroxyl groups have been detected during clinical CEST MRI studies, and exogenous CEST agents have also been administered to patients. These CEST MRI tools show promise for contributing to assessments of cerebral ischemia, neurological disorders, lymphedema, osteoarthritis, muscle physiology, and solid tumors. This review summarizes the salient features of clinical CEST MRI protocols and critically evaluates the utility of CEST MRI for these clinical imaging applications.

LEVEL OF EVIDENCE

5 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2018;47:11-27.

CEST imaging of fast exchanging amine pools with corrections for competing effects at 9.4 T

Chemical exchange saturation transfer (CEST) imaging of fast exchanging amine protons at 3 ppm offset from the water resonant frequency is of practical interest, but quantification of fast exchanging pools by CEST is challenging. To effectively saturate fast exchanging protons, high irradiation powers need to be applied, but these may cause significant direct water saturation as well as non-specific semi-solid magnetization transfer (MT) effects, and thus decrease the specificity of the measured signal. In addition, the CEST signal may depend on the water longitudinal relaxation time (T_1w ), which likely varies between tissues and with pathology, further reducing specificity. Previously, an analysis of the asymmetry of saturation effects (MTR_asym ) has been commonly used to quantify fast exchanging amine CEST signals. However, our results show that MTR_asym is greatly affected by the above factors, as well as asymmetric MT and nuclear Overhauser enhancement (NOE) effects. Here, we instead applied a relatively more specific inverse analysis method, named AREX (apparent exchange-dependent relaxation), that has previously been applied only to slow and intermediate exchanging solutes. Numerical simulations and controlled phantom experiments show that, although MTR_asym depends on T_1w and semi-solid content, AREX acquired in steady state does not, which suggests that AREX is more specific than MTR_asym . By combining with a fitting approach instead of using the asymmetric analysis to obtain reference signals, AREX can also avoid contaminations from asymmetric MT and NOE effects. Animal experiments show that these two quantification methods produce differing contrasts between tumors and contralateral normal tissues in rat brain tumor models, suggesting that conventional MTR_asym applied in vivo may be influenced by variations in T_1w , semi-solid content, or NOE effect. Thus, the use of MTR_asym may lead to misinterpretation, while AREX with corrections for competing effects likely enhances the specificity and accuracy of quantification to fast exchanging pools.

Imaging and spectroscopic approaches to probe brain energy metabolism dysregulation in neurodegenerative diseases

Changes in energy metabolism are generally considered to play an important role in neurodegenerative diseases such as Alzheimer's, Parkinson's, and Huntington's diseases. Whether these changes are causal or simply a part of self-defense mechanisms is a matter of debate. Furthermore, energy defects have often been discussed solely in the context of their probable neuronal origin without considering the cellular heterogeneity of the brain. Recent data point towards the existence of a tri-cellular compartmentation of brain energy metabolism between neurons, astrocytes, and oligodendrocytes, each cell type having a distinctive metabolic profile. Still, the number of methods to follow energy metabolism in patients is extremely limited and existing clinical techniques are blind to most cellular processes. There is a need to better understand how brain energy metabolism is regulated in health and disease through experiments conducted at different scales in animal models to implement new methods in the clinical setting. The purpose of this review is to offer a brief overview of the broad spectrum of methodological approaches that have emerged in recent years to probe energy metabolism in more detail. We conclude that multi-modal neuroimaging is needed to follow non-cell autonomous energy metabolism dysregulation in neurodegenerative diseases.

Magnetization Transfer Contrast and Chemical Exchange Saturation Transfer MRI. Features and analysis of the field-dependent saturation spectrum

Magnetization Transfer Contrast (MTC) and Chemical Exchange Saturation Transfer (CEST) experiments measure the transfer of magnetization from molecular protons to the solvent water protons, an effect that becomes apparent as an MRI signal loss ("saturation"). This allows molecular information to be accessed with the enhanced sensitivity of MRI. In analogy to Magnetic Resonance Spectroscopy (MRS), these saturation data are presented as a function of the chemical shift of participating proton groups, e.g. OH, NH, NH₂, which is called a Z-spectrum. In tissue, these Z-spectra contain the convolution of multiple saturation transfer effects, including nuclear Overhauser enhancements (NOEs) and chemical exchange contributions from protons in semi-solid and mobile macromolecules or tissue metabolites. As a consequence, their appearance depends on the magnetic field strength (B₀) and pulse sequence parameters such as B₁ strength, pulse shape and length, and interpulse delay, which presents a major problem for quantification and reproducibility of MTC and CEST effects. The use of higher B₀ can bring several advantages. In addition to higher detection sensitivity (signal-to-noise ratio, SNR), both MTC and CEST studies benefit from longer water T₁ allowing the saturation transferred to water to be retained longer. While MTC studies are non-specific at any field strength, CEST specificity is expected to increase at higher field because of a larger chemical shift dispersion of the resonances of interest (similar to MRS). In addition, shifting to a slower exchange regime at higher B₀ facilitates improved detection of the guanidinium protons of creatine and the inherently broad resonances of the amine protons in glutamate and the hydroxyl protons in myoinositol, glycogen, and glucosaminoglycans. Finally, due to the higher mobility of the contributing protons in CEST versus MTC, many new pulse sequences can be designed to more specifically edit for CEST signals and to remove MTC contributions.

Mapping the alterations in glutamate with GluCEST MRI in a mouse model of dopamine deficiency

Glutamate chemical exchange saturation transfer (GluCEST) MRI was used to measure metabolic changes in mice treated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) by mapping regional cerebral glutamate. The GluCEST contrast following MPTP treatment was correlated with ¹ H-MR spectroscopy, motor function, and immunohistochemical measures. The GluCEST contrast was found to be significantly higher in the striatum and motor cortex of mice treated with MPTP than in controls (p < 0.001), which was confirmed by localized ¹ H-MR spectroscopy. Elevated striatal GluCEST was positively associated with local astrogliosis measured by immunohistochemistry for glial fibrillary acidic protein. Additionally, a negative correlation was found between motor function, measured by the four-limb grip strength test, and GluCEST of the striatum (R = -0.705, p < 0.001) and the motor cortex (R = -0.617, p < 0.01), suggesting a role of elevated glutamate in the abnormal cerebral motor function regulation. The GluCEST contrast and glial fibrillary acidic protein immunostaining were unaltered in the thalamus indicating glutamate elevation was localized to the striatum and the motor cortex. These findings suggest that in addition to measuring spatial changes in glutamate, GluCEST may serve as an in vivo biomarker of metabolic and functional changes that may be applied to the assessment of a broad range of neuropathologies. Read the Editorial Highlight for this article on page 346.

Molecular fMRI

Comprehensive analysis of brain function depends on understanding the dynamics of diverse neural signaling processes over large tissue volumes in intact animals and humans. Most existing approaches to measuring brain signaling suffer from limited tissue penetration, poor resolution, or lack of specificity for well-defined neural events. Here we discuss a new brain activity mapping method that overcomes some of these problems by combining MRI with contrast agents sensitive to neural signaling. The goal of this "molecular fMRI" approach is to permit noninvasive whole-brain neuroimaging with specificity and resolution approaching current optical neuroimaging methods. In this article, we describe the context and need for molecular fMRI as well as the state of the technology today. We explain how major types of MRI probes work and how they can be sensitized to neurobiological processes, such as neurotransmitter release, calcium signaling, and gene expression changes. We comment both on past work in the field and on challenges and promising avenues for future development.

SIGNIFICANCE STATEMENT

Brain researchers currently have a choice between measuring neural activity using cellular-level recording techniques, such as electrophysiology and optical imaging, or whole-brain imaging methods, such as fMRI. Cellular level methods are precise but only address a small portion of mammalian brains; on the other hand, whole-brain neuroimaging techniques provide very little specificity for neural pathways or signaling components of interest. The molecular fMRI techniques we discuss have particular potential to combine the specificity of cellular-level measurements with the noninvasive whole-brain coverage of fMRI. On the other hand, molecular fMRI is only just getting off the ground. This article aims to offer a snapshot of the status and future prospects for development of molecular fMRI techniques.

In vivo imaging of brain glutamate defects in a knock-in mouse model of Huntington's disease

Huntington's disease (HD) is an inherited neurodegenerative disease characterized by motor, cognitive and psychiatric symptoms. Atrophy of the striatum has been proposed for several years as a biomarker to assess disease progression in HD gene carriers. However, it does not provide any information about the biological mechanisms linked to HD pathogenesis. Changes in brain metabolites have been also consistently seen in HD patients and animal models using Magnetic Resonance Spectroscopy (MRS), but metabolite measurements are generally limited to a single voxel. In this study, we used Chemical Exchange Saturation Transfer imaging of glutamate (gluCEST) in order to map glutamate distribution in the brain of a knock-in mouse model (Ki140CAG) with a precise anatomical resolution. We demonstrated that both heterozygous and homozygous mice with pathological CAG repeat expansion in gene encoding huntingtin exhibited an atrophy of the striatum and a significant alteration of their metabolic profile in the striatum as compared to wild type littermate controls. The striatal decrease was then confirmed by gluCEST imaging. Surprisingly, CEST imaging also revealed that the corpus callosum was the most affected structure in both genotype groups, suggesting that this structure could be highly vulnerable in HD. We evaluated for the first time gluCEST imaging as a potential biomarker of HD and demonstrated its potential for characterizing metabolic defects in neurodegenerative diseases in specific regions.

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.

Imaging increased glutamate in children with Sturge-Weber syndrome: Association with epilepsy severity

BACKGROUND

Sturge-Weber syndrome (SWS) is strongly associated with epilepsy. Brain tissue studies have suggested that epileptic activity in SWS is driven by glutamatergic synaptic activity. Here, we used proton magnetic resonance spectroscopic imaging (MRSI) to test if glutamate (GLU) concentrations are increased in the affected hemisphere and if such increases are associated with severity of epilepsy in children with SWS. We also studied the metabolic correlates of MRSI abnormalities, using glucose positron emission tomography (PET) imaging.

METHODS

3T MRI and glucose PET were performed in 10 children (age: 7-78 months) with unilateral SWS and a history of epilepsy. MRSI data were acquired from the affected (ipsilateral) and non-affected (contralateral) hemispheres. GLU, N-acetyl-aspartate (NAA) and creatine (Cr) were quantified in multiple voxels; GLU/Cr and NAA/Cr ratios were calculated and compared to seizure frequency as well as glucose PET findings.

RESULTS

The highest GLU/Cr ratios were found in the affected hemisphere in all children except one with severe atrophy. The maximum ipsilateral/contralateral GLU/Cr ratios ranged between 1.0 and 2.5 (mean: 1.6). Mean ipsilateral/contralateral GLU/Cr ratios were highest in the youngest children and showed a strong positive correlation with clinical seizure frequency scores assessed at the time of the scan (r=0.88, p=0.001) and also at follow-up (up to 1 year, r=0.80, p=0.009). GLU increases in the affected hemisphere coincided with areas showing current or previous increases of glucose metabolism on PET in 5 children. NAA/Cr ratios showed no association with clinical seizure frequency.

CONCLUSIONS

Increased glutamate concentrations in the affected hemisphere, measured by MRSI, are common in young children with unilateral SWS and are associated with frequent seizures. The findings lend support to the role of excess glutamate in SWS-associated epilepsy.

Computational analysis in epilepsy neuroimaging: A survey of features and methods

Epilepsy affects 65 million people worldwide, a third of whom have seizures that are resistant to anti-epileptic medications. Some of these patients may be amenable to surgical therapy or treatment with implantable devices, but this usually requires delineation of discrete structural or functional lesion(s), which is challenging in a large percentage of these patients.

Advances in neuroimaging and machine learning allow semi-automated detection of malformations of cortical development (MCDs), a common cause of drug resistant epilepsy. A frequently asked question in the field is what techniques currently exist to assist radiologists in identifying these lesions, especially subtle forms of MCDs such as focal cortical dysplasia (FCD) Type I and low grade glial tumors. Below we introduce some of the common lesions encountered in patients with epilepsy and the common imaging findings that radiologists look for in these patients. We then review and discuss the computational techniques introduced over the past 10 years for quantifying and automatically detecting these imaging findings. Due to large variations in the accuracy and implementation of these studies, specific techniques are traditionally used at individual centers, often guided by local expertise, as well as selection bias introduced by the varying prevalence of specific patient populations in different epilepsy centers. We discuss the need for a multi-institutional study that combines features from different imaging modalities as well as computational techniques to definitively assess the utility of specific automated approaches to epilepsy imaging. We conclude that sharing and comparing these different computational techniques through a common data platform provides an opportunity to rigorously test and compare the accuracy of these tools across different patient populations and geographical locations. We propose that these kinds of tools, quantitative imaging analysis methods and open data platforms for aggregating and sharing data and algorithms, can play a vital role in reducing the cost of care, the risks of invasive treatments, and improve overall outcomes for patients with epilepsy.

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We introduce common epileptogenic lesions encountered in patients with drug resistant epilepsy.

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We discuss state of the art computational techniques used to detect lesions.

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There is a need for multi-institutional studies that combine these techniques.

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Clinically validated pipelines alongside the advances in imaging and electrophysiology will improve outcomes.