Multiple sclerosis is an inflammatory demyelinating condition of the central nervous system affecting approximately 1 million people in the USA. Although standard structural MRI techniques are now the main imaging modality for multiple sclerosis diagnosis and management, they are yet to provide information regarding the metabolic profile of the disease. Ultra-high field 7T MRI systems have provided gains in signal-to-noise ratio (SNR) and spatial resolution for structural MRI as well as larger chemical shifts leading to improvements in specialized imaging sequences, such as nuclear Overhauser effect (NOE) imaging, that can evaluate macromolecular metabolite composition. In this work, NOE images were acquired on a cohort of multiple sclerosis and healthy control subjects to spatially map differences in their lipid metabolites as a result of NOE effects. NOE image data were acquired on a total of 25 subjects {15 multiple sclerosis subjects [10 females, 5 males (21-70 years)] and 10 healthy controls [5 females, 5 males (23-71 years)]} on a 7T MRI system with a frequency offset range of -5 to 5 ppm. A five-pool Lorentzian line fitting model was utilized to fit and quantitatively compare direct saturation (DS), magnetization transfer (MT), amide proton transfer (APT), amine, and relayed NOE (rNOE) and used as a comparison to conventional T1 maps. Grey and white matter tissues were segmented using the T1 maps, while the lesion tissue was segmented manually. Correlations between disease duration and lesion load were performed to investigate any existing relationship to image contrast. The primary findings of this work include statistically significant decreases in the rNOE pool for the normal-appearing white matter (NAWM) (11.4% decrease) and normal-appearing grey matter (NAGM) (10.6% decrease) in multiple sclerosis subjects compared to healthy controls. Additionally, a significant decrease in the amine pool was also observed for NAWM (15.3% decrease) in multiple sclerosis subjects compared to healthy controls. Changes in multiple sclerosis lesion contrast were also observed for several pools (DS, amine, and rNOE). Decreases in both the rNOE and amine pools suggest that in multiple sclerosis, there are diffuse decreases in mobile lipids, such as those found in neuronal cell bodies, as well as a decrease in proteins with amine groups. Furthermore, these measurable contrast changes were not detected in the corresponding T1 maps. NOE imaging can provide complementary metabolic information to conventional MRI methods. Future studies will focus on utilizing this technique for longitudinal tracking of disease progression and investigating similar demyelinating diseases. Metabolic MRI was performed at an ultra-high field to evaluate nuclear Overhauser effect lipid metabolite content in a cohort of multiple sclerosis subjects. Jacobs et al. report metabolite decreases in pathological white and grey matter. These diffuse decreases in mobile lipids and proteins suggest a potential metric for global tissue injury.

PURPOSE:The purpose of this study was to determine the effect of acute nicotinamide riboside (NR) supplementation on cerebral nicotinamide adenine dinucleotide (NAD+) levels in the human brain in vivo by means of downfield proton MRS (DF 1H MRS). METHODS:DF 1H MRS was performed on 10 healthy volunteers in a 7.0 T MRI scanner with spectrally selective excitation and spatially selective localization to determine cerebral NAD+ levels on two back-to-back days: once after an overnight fast (baseline) and once 4 h after oral ingestion of nicotinamide riboside (900 mg). Additionally, two more baseline scans were performed following the same paradigm to assess test-retest reliability of the NAD+ levels in the absence of NR. RESULTS:NR supplementation increased mean NAD+ concentration compared to the baseline (0.458 ± 0.053 vs. 0.392 ± 0.058 mM; p < 0.001). The additional two baseline scans demonstrated no differences in mean NAD+ concentrations (0.425 ± 0.118 vs. 0.405 ± 0.082 mM; p = 0.45), and no difference from the first baseline scan (F(2, 16) = 0.907; p = 0.424). CONCLUSION:These preliminary results confirm that acute NR supplementation increases cerebral NAD+ levels in healthy human volunteers and shows the promise of DF 1H MRS utility for robust detection of NAD+ in humans in vivo.

PurposeIn vivo quantification of lactate has numerous applications in studying the pathology of both cerebral and musculoskeletal systems. Due to its low concentration (~0.5–1 mM), and overlap with lipid signals, traditional 1H MR spectra acquired in vivo using a small voxel and short echo time often result in an inadequate signal to detect and resolve the lactate peak, especially in healthy human volunteers.MethodsIn this study, using a semi‐LASER acquisition with long echo time (TE = 288 ms) and large voxel size (80 × 70 × 20 mm3), we clearly visualize the combined signal of lactate and threonine. Therefore, we call the signal at 1.33 ppm Lac+ and quantify Lac+ concentration from water suppressed spectra in healthy human brains in vivo. Four participants (22–37 years old; mean age = 28 ± 5.4; three male, one female) were scanned on four separate days, and on each day four measurements were taken. Intra‐day values are calculated for each participant by comparing the four measurements on a single day. Inter‐day values were calculated using the mean intra‐day measurements.ResultsThe mean intra‐participant Lac+ concentration, standard deviation (SD), and coefficient of variation (CV) ranged from 0.49 to 0.61 mM, 0.02 to 0.07 mM, and 4% to 13%, respectively, across four volunteers. The inter‐participant Lac+ concentration, SD, and CV was 0.53 mM, ±0.06 mM, and 11%.ConclusionRepeatability is shown in Lac+ measurement in healthy human brain using a long echo time semi‐LASER sequence with a large voxel in about 3.5 min at 3 T.

PURPOSE:Ultrahigh field (≥7 T) MRI is at the cutting edge of medical imaging, enabling enhanced spatial and spectral resolution as well as enhanced susceptibility contrast. However, transmit ( B 1 + $$ {\mathrm{B}}_1^{+} $$ ) field inhomogeneity due to standing wave effects caused by the shortened RF wavelengths at 7 T is still a challenge to overcome. Novel hardware methods such as dielectric pads have been shown to improve the B 1 + $$ {\mathrm{B}}_1^{+} $$ field inhomogeneity but are currently limited in their corrective effect by the range of high-permittivity materials available and have a fixed shelf life. In this work, an optimized metasurface design is presented that demonstrates in vivo enhancement of the B 1 + $$ {\mathrm{B}}_1^{+} $$ field. METHODS:A prototype metasurface was optimized by an empirical capacitor sweep and by varying the period size. Phantom temperature experiments were performed to evaluate potential metasurface heating effects during scanning. Lastly, in vivo gradient echo images and B 1 + $$ {\mathrm{B}}_1^{+} $$ maps were acquired on five healthy subjects on a 7 T system. Dielectric pads were also used as a comparison throughout the work as a standard comparison. RESULTS:The metasurfaces presented here enhanced the average relative SNR of the gradient echo images by a factor of 2.26 compared to the dielectric pads factor of 1.61. Average B 1 + $$ {\mathrm{B}}_1^{+} $$ values reflected a similar enhancement of 27.6% with the metasurfaces present versus 8.9% with the dielectric pads. CONCLUSION:The results demonstrate that metasurfaces provide superior performance to dielectric padding as shown by B 1 + $$ {\mathrm{B}}_1^{+} $$ maps reflecting their direct effects and resulting enhancements in image SNR at 7 T.

IntroductionWith nicotine dependence being a significant healthcare issue worldwide there is a growing interest in developing novel therapies and diagnostic aids to assist in treating nicotine addiction. Glutamate (Glu) plays an important role in cognitive function regulation in a wide range of conditions including traumatic brain injury, aging, and addiction. Chemical exchange saturation transfer (CEST) imaging via ultra-high field MRI can image the exchange of certain saturated labile protons with the surrounding bulk water pool, making the technique a novel tool to investigate glutamate in the context of addiction. The aim of this work was to apply glutamate weighted CEST (GluCEST) imaging to study the dorsal anterior cingulate cortex (dACC) in a small population of smokers and non-smokers to determine its effectiveness as a biomarker of nicotine use.Methods2D GluCEST images were acquired on 20 healthy participants: 10 smokers (ages 29-50) and 10 non-smokers (ages 25-69), using a 7T MRI system. T1-weighted images were used to segment the GluCEST images into white and gray matter tissue and further into seven gray matter regions. Wilcoxon rank-sum tests were performed, comparing mean GluCEST contrast between smokers and non-smokers across brain regions.ResultsGluCEST levels were similar between smokers and non-smokers; however, there was a moderate negative age dependence (R2 = 0.531) in smokers within the cingulate gyrus.ConclusionFeasibility of GluCEST imaging was demonstrated for in vivo investigation of smokers and non-smokers to assess glutamate contrast differences as a potential biomarker with a moderate negative age correlation in the cingulate gyrus suggesting reward network involvement.

We present a review outlining the basic mechanism, background, recent technical developments, and clinical applications of aqueous dielectric padding in the field of MRI. Originally meant to be a temporary solution, it has gained traction as an effective method for correcting B1 + inhomogeneities due to the unique properties of the calcium titanate and barium titanate perovskites used. Aqueous dielectric pads have used a variety of high-permittivity materials over the years to improve the quality of MRI acquisitions at 1.5 and 3 T and more recently for 7 T neuroimaging applications. The technical development and assessment of these pads have been advanced by an increased use of mathematical modeling and electromagnetic simulations. These tools have allowed for a more complete understanding of the physical interactions between dielectric pads and the RF coil, making testing and safety assessments more accurate. The ease of use and effectiveness that dielectric pads offer have allowed them to become more commonplace in tackling imaging challenges in more clinically focused environments. More recently, they have seen usage not only in anatomical imaging methods but also in specialized metabolic imaging sequences such as GluCEST and NOEMTR . New colossally high-permittivity materials have been proposed; however, practical utilization has been a continued challenge due to unfavorable frequency dependences as well as safety limitations. A new class of metasurfaces has been under development to address the shortcomings of conventional dielectric padding while also providing increased performance in enhancing MRI images.

Creatine chemical exchange saturation transfer (CrCEST) MRI is an emerging high resolution and noninvasive method for measuring muscle specific oxidative phosphorylation (OXPHOS). However, CrCEST measurements are sensitive to changes in muscle pH, which might confound the measurement and interpretation of creatine recovery time (τCr). Even with the same prescribed exercise stimulus, the extent of acidification and hence its impact on τCr is expected to vary between individuals. To address this issue, a method to measure pH pre- and post-exercise and its impact on CrCEST MRI with high temporal resolution is needed. In this work, we integrate carnosine 1H- magnetic resonance spectroscopy (MRS) and 3D CrCEST to establish "mild" and "moderate/intense" exercise stimuli. We then test the dependence of CrCEST recovery time on pH using different exercise stimuli. This comprehensive metabolic imaging protocol will enable personalized, muscle specific OXPHOS measurements in both healthy aging and myriad other disease states impacting muscle mitochondria.

PurposeNuclear Overhauser effect magnetization transfer ratio (NOEMTR) is a technique used to investigate brain lipids and macromolecules in greater detail than other techniques and benefits from increased contrast at 7 T. However, this contrast can become degraded because of inhomogeneities present at ultra‐high field strengths. High‐permittivity dielectric pads (DP) have been used to correct for these inhomogeneities via displacement currents generating secondary magnetic fields. The purpose of this work is to demonstrate that dielectric pads can be used to mitigate inhomogeneities and improve NOEMTR contrast in the temporal lobes at 7 T.MethodsPartial 3D NOEMTR contrast images and whole brain field maps were acquired on a 7 T MRI across six healthy subjects. Calcium titanate DP, having a relative permittivity of 110, was placed next to the subject's head near the temporal lobes. Pad corrected NOEMTR images had a separate postprocessing linear correction applied.ResultsDP provided supplemental to the temporal lobes while also reducing the magnitude across the posterior and superior regions of the brain. This resulted in a statistically significant increase in NOEMTR contrast in substructures of the temporal lobes both with and without linear correction. The padding also produced a convergence in NOEMTR contrast toward approximately equal mean values.ConclusionNOEMTR images showed significant improvement in temporal lobe contrast when DP were used, which resulted from an increase in homogeneity across the entire brain slab. DP‐derived improvements in NOEMTR are expected to increase the robustness of the brain substructural measures both in healthy and pathological conditions.

Dielectric pads were used to improve the contrast of volumetric (3D) gluCEST via enhanced homogenization of B 1 fields. Effective placement of the pads required additional coverage of the face due to their physical size to achieve the saturated B 1 strength necessary to recover the gluCEST signal from the anterior portions of the brain. The origin of gluCEST's need for high B 1 is discussed, and examples of 3D gluCEST images acquired with and without the use of the dielectric pads are provided and repeatability of the 3D gluCEST in the presence of pad placement was determined.

BACKGROUND:The imaging of patients with implanted electrically-conductive devices via magnetic resonance imaging at ultra-high fields is hampered by uncertainties relating to the potential for inducing tissue heating adjacent to the implant due to coupling of energy from the incident electromagnetic field into the implant. Existing data in the peer-reviewed literature of comparisons across field strengths of tissue heating and its surrogate, the specific absorption rate (SAR), is scarce and contradictory, leading to further doubts pertaining to the safety of imaging patients with such devices. PURPOSE:The radiofrequency-induced SAR adjacent to orthopedic screws of varying length and at frequencies of 64 to 498 MHz was investigated via full-wave electromagnetic simulations, to provide an accurate comparison of SAR across MRI field strengths. METHODS:Dipole antennas were used for RF transmission to achieve a uniform electric field tangential to the screws located 120 mm above the antenna midpoints, embedded in a bone-mimicking material. The input power to the antennas was constrained to achieve the following targets without the screw present: (i) E = 100 V/m, (ii) B1 +  = 2 μT, and (iii) global-average-SAR = 3.2 W/kg. Simulations were performed with a spatial resolution of 0.2 mm in the volume surrounding the screws, resulting in 76-137 MCells, noting the maximum 1 g-averaged SAR value in each case. Simulations were repeated at 128 and 297 MHz for screws embedded in muscle tissue. RESULTS:The peak SAR, occurring at the resonant screw length, substantially increased as the frequency decreased when the input power to the dipole antenna was constrained to achieve constant electric field in background tissue at the screws' locations. A similar pattern was observed when constraining input power to achieve constant B1 + and global-average-SAR. The dielectric properties of the tissue in which the screws were embedded dominated the SAR comparisons between 297 and 128 MHz. CONCLUSIONS:The study design allowed for a direct comparison to be performed of SAR across frequencies and implant lengths without the confounding effect of variable incident electric field. Lower frequencies produced substantially larger SAR values for implants approaching the resonant length for the worst-case uniform incident electric field along the screws' length. The data may inform risk-benefit assessments for imaging patients with orthopedic implants at the new clinical field strength of 7 Tesla.

Purpose To monitor the metabolic turnover of β‐hydroxybutyrate (BHB) oxidation using 2 H‐MRS in conjunction with intravenous administration of 2 H labeled BHB. Methods Nine‐month‐old mice were infused with [3,4,4,4]‐ 2 H 4 ‐BHB (d 4 ‐BHB; 3.11 g/kg) through the tail vein using a bolus variable infusion rate for a period of 90 min. The labeling of downstream cerebral metabolites from the oxidative metabolism of d 4 ‐BHB was monitored using 2 H‐MRS spectra acquired with a home‐built 2 H surface coil on a 9.4T preclinical MR scanner with a temporal resolution of 6.25 min. An exponential model was fit to the BHB and glutamate/glutamine (Glx) turnover curves to determine rate constants of metabolite turnover and to aid in the visualization of metabolite time courses. Results Deuterium label was incorporated into Glx from BHB metabolism through the tricarboxylic acid (TCA) cycle, with an increase in the level of [4,4]‐ 2 H 2 ‐Glx (d 2 ‐Glx) over time and reaching a quasi‐steady state concentration of ∼0.6 ± 0.1 mM following 30 min of infusion. Complete oxidative metabolic breakdown of d 4 ‐BHB also resulted in the formation of semi‐heavy water (HDO), with a four‐fold (10.1 to ∼42.1 ± 7.3 mM) linear (R 2 = 0.998) increase in its concentration by the end of infusion. The rate constant of Glx turnover from d 4 ‐BHB metabolism was determined to be 0.034 ± 0.004 min −1 . Conclusion 2 H‐MRS can be used to monitor the cerebral metabolism of BHB with its deuterated form by measuring the downstream labeling of Glx. The integration of 2 H‐MRS with deuterated BHB substrate provides an alternative and clinically promising MRS tool to detect neurometabolic fluxes in healthy and disease conditions.

Nuclear Overhauser effect magnetization transfer ratio (NOEMTR) is a technique used to investigate brain lipids and macromolecules in greater detail than other techniques and benefits from increased contrast at 7 T. However, this contrast can become degraded because of B1+$$ {\mathrm{B}}_1^{+} $$ inhomogeneities present at ultra‐high field strengths. High‐permittivity dielectric pads (DP) have been used to correct for these inhomogeneities via displacement currents generating secondary magnetic fields. The purpose of this work is to demonstrate that dielectric pads can be used to mitigate B1+$$ {\mathrm{B}}_1^{+} $$ inhomogeneities and improve NOEMTR contrast in the temporal lobes at 7 T.

Nuclear Overhauser effect magnetization transfer ratio (NOEMTR ) is a technique used to investigate brain lipids and macromolecules in greater detail than other techniques and benefits from increased contrast at 7 T. However, this contrast can become degraded because of B1+$$ {\mathrm{B}}_1^{+} $$ inhomogeneities present at ultra-high field strengths. High-permittivity dielectric pads (DP) have been used to correct for these inhomogeneities via displacement currents generating secondary magnetic fields. The purpose of this work is to demonstrate that dielectric pads can be used to mitigate B1+$$ {\mathrm{B}}_1^{+} $$ inhomogeneities and improve NOEMTR contrast in the temporal lobes at 7 T.Partial 3D NOEMTR contrast images and whole brain B1+$$ {\mathrm{B}}_1^{+} $$ field maps were acquired on a 7 T MRI across six healthy subjects. Calcium titanate DP, having a relative permittivity of 110, was placed next to the subject's head near the temporal lobes. Pad corrected NOEMTR images had a separate postprocessing linear correction applied.DP provided supplemental B1+$$ {\mathrm{B}}_1^{+} $$ to the temporal lobes while also reducing the B1+$$ {\mathrm{B}}_1^{+} $$ magnitude across the posterior and superior regions of the brain. This resulted in a statistically significant increase in NOEMTR contrast in substructures of the temporal lobes both with and without linear correction. The padding also produced a convergence in NOEMTR contrast toward approximately equal mean values.NOEMTR images showed significant improvement in temporal lobe contrast when DP were used, which resulted from an increase in B1+$$ {\mathrm{B}}_1^{+} $$ homogeneity across the entire brain slab. DP-derived improvements in NOEMTR are expected to increase the robustness of the brain substructural measures both in healthy and pathological conditions.

Purpose: Nuclear Overhauser effect (NOE) is based on dipolar cross-relaxation mechanism that enables the indirect detection of aliphatic protons via the water proton signal. This work focuses on determining the reproducibility of NOE magnetization transfer ratio (NOEMTR) and isolated or relayed NOE (rNOE) contributions to the NOE MRI of the healthy human brain at 7 Tesla (T).Methods: We optimized the B-1(+) amplitude and length of the saturation pulse by acquiring NOE images with different B-1(+) values with multiple saturation lengths. Repeated NOE MRI measurements were made on five healthy volun-teers by using optimized saturation pulse parameters including correction of B-0 and B-1(+) inhomogeneities. To isolate the individual contributions from z-spectra, we have fit the NOE z-spectra using multiple Lorentzians and calculated the total contribution from each pool contributing to the overall NOEMTR contrast.Results: We found that a saturation amplitude of 0.72 mu T and a length of 3 s pro-vided the highest contrast. We found that the mean NOEMTR value in gray matter (GM) was 26%, and in white matter (WM) was 33.3% across the 3D slab of the brain. The mean rNOE contributions from GM and WM values were 8.9% and 9.6%, which were similar to 10% of the corresponding total NOEMTR signal. The inter-subject coefficient of variations (CoVs) of NOEMTR from GM and WM were 4.5% and 6.5%, respectively, whereas those -rNOE were 4.8% and 5.6%, respectively. The intrasubject CoVs of the NOE(MTR )range was 2.1%-4.2%, and rNOE range was 2.9%-10.5%.Conclusion: This work has demonstrated an excellent reproducibility of both inter-and intrasubject NOEMTR and rNOE metrics in healthy human brains at 7 T.

Nuclear Overhauser effect magnetization transfer ratio (NOEMTR) is a technique used to investigate brain lipids and macromolecules in greater detail than other techniques and benefits from increased contrast at 7 T. However, this contrast can become degraded because of B1+$$ {\mathrm{B}}_1^{+} $$ inhomogeneities present at ultra‐high field strengths. High‐permittivity dielectric pads (DP) have been used to correct for these inhomogeneities via displacement currents generating secondary magnetic fields. The purpose of this work is to demonstrate that dielectric pads can be used to mitigate B1+$$ {\mathrm{B}}_1^{+} $$ inhomogeneities and improve NOEMTR contrast in the temporal lobes at 7 T.

Purpose: Nuclear Overhauser effect magnetization transfer ratio (NOEMTR) is a technique used to investigate brain lipids and macromolecules in greater detail than other techniques and benefits from increased contrast at 7 T. However, this contrast can become degraded because of B+ (1) inhomogeneities present at ultra-high field strengths. High-permittivity dielectric pads (DP) have been used to correct for these inhomogeneities via displacement currents generating secondary magnetic fields. The purpose of thiswork is to demonstrate that dielectric pads can be used to mitigate B+ (1) inhomogeneities and improve NOEMTR contrast in the temporal lobes at 7 T. Methods: Partial 3D NOEMTR contrast images and whole brain B+ (1) field maps were acquired on a 7 T MRI across six healthy subjects. Calcium titanate DP, having a relative permittivity of 110, was placed next to the subject ' s head near the temporal lobes. Pad corrected NOEMTR images had a separate postprocessing linear correction applied. Results: DP provided supplemental B+ (1) to the temporal lobes while also reducing the B+ (1) magnitude across the posterior and superior regions of the brain. This resulted in a statistically significant increase in NOEMTR contrast in substructures of the temporal lobes both with andwithout linear correction. The padding also produced a convergence in NOEMTR contrast toward approximately equal mean values. Conclusion: NOEMTR images showed significant improvement in temporal lobe contrast when DP were used, which resulted from an increase in B+ (1) homogeneity across the entire brain slab. DP- derived improvements in NOEMTR are expected to increase the robustness of the brain substructural measures both in healthy and pathological conditions.

PURPOSE:Ultra-high field MR imaging lacks B1 + inhomogeneity due to shorter RF wavelengths used at higher field strengths compared to human anatomy. CEST techniques tend to be highly susceptible to B1 + inhomogeneities due to a high and uniform B1 + field being necessary to create the endogenous contrast. High-permittivity dielectric pads have seen increasing usage in MR imaging due to their ability to tailor the spatial distribution of the B1 + field produced. The purpose of this work is to demonstrate that dielectric materials can be used to improve glutamate weighted CEST (gluCEST) at 7T. THEORY AND METHODS:GluCEST images were acquired on a 7T system on six healthy volunteers. Aqueous calcium titanate pads, with a permittivity of approximately 110, were placed on either side in the subject's head near the temporal lobes. A post-processing correction algorithm was implemented in combination with dielectric padding to compare contrast improvement. Tissue segmentation was performed to assess the effect of dielectric pads on gray and white matter separately. RESULTS:GluCEST images demonstrated contrast enhancement in the lateral temporal lobe regions with dielectric pad placement. Tissue segmentation analysis showed an increase in correction effectiveness within the gray matter tissue compared to white matter tissue. Statistical testing suggested a significant difference in gluCEST contrast when pads were used and showed a difference in the gray matter tissue segment. CONCLUSION:The use of dielectric pads improved the B1 + field homogeneity and enhanced gluCEST contrast for all subjects when compared to data that did not incorporate padding.

PURPOSE A 7T magnetic resonance thermometry (MRT) technique was developed to validate the conversion factor between the system-measured transmitted radiofrequency (RF) power into a home-built RF wrist coil with the system-predicted SAR value. The conversion factor for a new RF coil developed for ultra high magnetic field MRI systems is used to ensure that regulatory limits on RF energy deposition in tissue, specifically the local 10g-averaged specific absorption rate (SAR10g ), are not exceeded. MRT can be used to validate this factor by ensuring that MRT-measured SAR values do not exceed those predicted by the system. METHODS A 14 cm diameter high-pass birdcage RF coil was built to image the wrist at 7T. A high spatial and temporal resolution dual-echo gradient echo MRT technique, incorporating quasi-simultaneous RF-induced heating and temperature change measurements using the proton resonance frequency method, was developed. The technique allowed for high temperature resolution measurements (~ ± 0.1°C) to be performed every 20 s over a 4 minute heating period, with high spatial resolution (2.56 mm3 voxel size) and avoiding phase discontinuities arising from severe magnetic susceptibility-induced B0 inhomogeneities. MRT was performed on a phantom made from polyvinylpyrrolidone to mimic the dielectric properties of muscle tissue at 297.2 MHz. Temperature changes measured with MRT and four fiber optic temperature sensors embedded in the phantom were compared. Electromagnetic simulations of the coil and phantom were developed and validated via comparison of simulated and measured B1 + maps in the phantom. The position of maximum SAR within the coil was determined from simulations, and MRT was performed within a wrist-sized piece of meat positioned at that SAR hotspot location. MRT-measured and system-predicted SAR values for the phantom and meat were compared. RESULTS Temperature change measurements from MRT matched closely to those from the fiber optic temperature sensors. The simulations were validated via close correlation between the simulated and MRT-measured B1 + and SAR maps. Using a coil conversion factor of 2 kg-1 , MRT-measured point-SAR values did not exceed the system predicted SAR10g in either the uniform phantom or in the piece of meat mimicking the wrist located at the SAR hotspot location. CONCLUSIONS A highly accurate MRT technique with high spatial and temporal resolution was developed. This technique can be used to ensure that system-predicted SAR values are not exceeded in practice, thereby providing independent validation of SAR levels delivered by a newly-built RF wrist coil. The MRT technique is readily generalizable to perform safety evaluations for other RF coils at 7T.

Motivation: Metallic implant compatibility at ultra-high field strengths (≥7T) continues to be often contraindicated as RF induced heating can result in surrounding tissue damage. Metasurface technology has been shown in the past to locally null the B1+ field, thereby providing a potential solution for shielding implants. Goal(s): To demonstrate a metasurface based method for shielding metallic implants to reduce RF heating. Approach: Eight implants were tested in a polyacrylic acid (PAA) phantom using a high-SAR sequence with and without a prototype metasurface. Results: On average the metasurface design reduced RF induced implant heating in the phantom by 41.6%. Impact: Patient undergoing invasive brain or trauma surgeries typically have passive metallic devices placed, making them unable to receive MRI scans at ultra-high field strengths due to RF induced heating. This work benefits this patient population by reducing RF heating.