Amide proton transfer weighted imaging in pediatric neuro-oncology: initial experience.

Amide proton transfer weighted (APTw) imaging enables in vivo assessment of tissue-bound mobile proteins and peptides through the detection of chemical exchange saturation transfer. Promising applications of APTw imaging have been shown in adult brain tumors. As pediatric brain tumors differ from their adult counterparts, we investigate the radiological appearance of pediatric brain tumors on APTw imaging. APTw imaging was conducted at 3 T. APTw maps were calculated using magnetization transfer ratio asymmetry at 3.5 ppm. First, the repeatability of APTw imaging was assessed in a phantom and in five healthy volunteers by calculating the within-subject coefficient of variation (wCV). APTw images of pediatric brain tumor patients were analyzed retrospectively. APTw levels were compared between solid tumor tissue and normal-appearing white matter (NAWM) and between pediatric high-grade glioma (pHGG) and pediatric low-grade glioma (pLGG) using t-tests. APTw maps were repeatable in supratentorial and infratentorial brain regions (wCV ranged from 11% to 39%), except those from the pontine region (wCV between 39% and 50%). APTw images of 23 children with brain tumor were analyzed (mean age 12 years ± 5, 12 male). Significantly higher APTw values are present in tumor compared with NAWM for both pHGG and pLGG (p < 0.05). APTw values were higher in pLGG subtype pilocytic astrocytoma compared with other pLGG subtypes (p < 0.05). Non-invasive characterization of pediatric brain tumor biology with APTw imaging could aid the radiologist in clinical decision-making.

Indirect 1H-[13C] MRS of the human brain at 7 T using a 13C-birdcage coil and eight transmit-receive 1H-dipole antennas with a 32-channel 1H-receive array.

The neuronal tricarboxylic acid and glutamate/glutamine (Glu/Gln) cycles play important roles in brain function. These processes can be measured in vivo using dynamic 1H-[13C] MRS during administration of 13C-labeled glucose. Proton-observed carbon-edited (POCE) MRS enhances the signal-to-noise ratio (SNR) compared with direct 13C-MRS. Ultra-high field further boosts the SNR and increases spectral dispersion; however, even at 7 T, Glu and Gln 1H-resonances may overlap. Further gain can be obtained with selective POCE (selPOCE). Our aim was to create a setup for indirect dynamic 1H-[13C] MRS in the human brain at 7 T. A home-built non-shielded transmit-receive 13C-birdcage head coil with eight transmit-receive 1H-dipole antennas was used together with a 32-channel 1H-receive array. Electromagnetic simulations were carried out to ensure that acquisitions remained within local and global head SAR limits. POCE-MRS was performed using slice-selective excitation with semi-localization by adiabatic selective refocusing (sLASER) and stimulated echo acquisition mode (STEAM) localization, and selPOCE-MRS using STEAM. Sequences were tested in a phantom containing non-enriched Glu and Gln, and in three healthy volunteers during uniformly labeled 13C-glucose infusions. In one subject the voxel position was alternated between bi-frontal and bi-occipital placement within one session. [4-13C]Glu-H4 and [4-13C]Gln-H4 signals could be separately detected using both STEAM-POCE and STEAM-selPOCE in the phantom. In vivo, [4,5-13C]Glx could be detected using both sLASER-POCE and STEAM-POCE, with similar sensitivities, but [4,5-13C]Glu and [4,5-13C]Gln signals could not be completely resolved. STEAM-POCE was alternately performed bi-frontal and bi-occipital within a single session without repositioning of the subject, yielding similar results. With STEAM-selPOCE, [4,5-13C]Glu and [4,5-13C]Gln could be clearly separated. We have shown that with our setup indirect dynamic 1H-[13C] MRS at 7 T is feasible in different locations in the brain within one session, and by using STEAM-selPOCE it is possible to separate Glu from Gln in vivo while obtaining high quality spectra.

The use of variable delay multipulse chemical exchange saturation transfer for separately assessing different CEST pools in the human brain at 7T.

PURPOSE: Current challenges of in vivo CEST imaging include overlapping signals from different pools. The overlap arises from closely resonating pools and/or the broad magnetization transfer contrast (MTC) from macromolecules. This study aimed to evaluate the feasibility of variable delay multipulse (VDMP) CEST to separately assess solute pools with different chemical exchange rates in the human brain in vivo, while mitigating the MTC. METHODS: VDMP saturation buildup curves were simulated for amines, amides, and relayed nuclear Overhauser effect. VDMP data were acquired from glutamate and bovine serum albumin phantoms, and from six healthy volunteers at 7T. For the in vivo data, MTC removal was performed via a three-pool Lorentzian fitting. Different B1 amplitudes and mixing times were used to evaluate CEST pools with different exchange rates. RESULTS: The results show the importance of removing MTC when applying VDMP in vivo and the influence of B1 for distinguishing different pools. Finally, the optimal B1 and mixing times to effectively saturate slow- and fast-exchanging components are also reported. Slow-exchanging amides and rNOE components could be distinguished when using B1 = 1 µT and tmix = 10 ms and 40 ms, respectively. Fast-exchanging components reached the highest saturation when using a B1 = 2.8 µT and tmix = 0 ms. CONCLUSION: VDMP is a powerful CEST-editing tool, exploiting chemical exchange-rate differences. After MTC removal, it allows separate assessment of slow- and fast-exchanging solute pools in in vivo human brain.

No need to detune transmitters in 32-channel receiver arrays at 7 T.

Ultrahigh field magnetic resonance imaging facilitates high spatiotemporal resolution that benefits from increasing the number of receiver elements. Because high-density receiver arrays have a relatively small element size compared with the transmitter, a side effect is that such setups cause low flux coupling between the transmitter and receiver. Moreover, when transmitters are designed in a multitransmit configuration, their relative size is much smaller than the sample, reducing coupling to the sample and thereby potentially also the coupling to the receivers. Transmitters are traditionally detuned during reception. In this study, we investigate, for a 32-channel receiver head array at 7 T, if transmitter detuning of a quadrature birdcage or of an eight-channel transmit coil can be omitted without substantially sacrificing signal-to-noise ratio (SNR). The transmit elements are operated once with and once without detuning and, in the latter, the received signals are either merged with the array or excluded for image reconstruction. For each of the three measurements, SNR and 1/g-factor maps are investigated. The tuning of the quadrature and eight-channel transmit coils during signal reception introduced a 10.1% and 6.5% penalty in SNR, respectively, relative to the SNR received with detuned transmitters. When also incorporating the signal of the transmit coils, the SNR was regained to 98.5% or 101.4% for the quadrature and eight-channel coil, respectively, relative to the detuned transmitters, while the 1/g-factor maps improved slightly. For the 32-channel receive coil used the SNR penalty can become negligible when omitting detuning of the transmit coils. This not only simplifies transmit coil designs, potentially increasing their efficiency, but also enables the transmitters to be used as receivers in parallel to the receiver array, thus increasing parallel imaging performance.

Multiparametric Renal MRI: An Intrasubject Test-Retest Repeatability Study.

BACKGROUND: Renal multiparametric magnetic resonance imaging (MRI) is a promising tool for diagnosis, prognosis, and treatment monitoring in kidney disease. PURPOSE: To determine intrasubject test-retest repeatability of renal MRI measurements. STUDY TYPE: Prospective. POPULATION: Nineteen healthy subjects aged over 40 years. FIELD STRENGTH/SEQUENCES: T1 and T2 mapping, R2 * mapping or blood oxygenation level-dependent (BOLD) MRI, diffusion tensor imaging (DTI), and intravoxel incoherent motion (IVIM) diffusion-weighted imaging (DWI), 2D phase contrast, arterial spin labelling (ASL), dynamic contrast enhanced (DCE) MRI, and quantitative Dixon for fat quantification at 3T. ASSESSMENT: Subjects were scanned twice with ~1 week between visits. Total scan time was ~1 hour. Postprocessing included motion correction, semiautomated segmentation of cortex and medulla, and fitting of the appropriate signal model. STATISTICAL TEST: To assess the repeatability, a Bland-Altman analysis was performed and coefficients of variation (CoVs), repeatability coefficients, and intraclass correlation coefficients were calculated. RESULTS: CoVs for relaxometry (T1 , T2 , R2 */BOLD) were below 6.1%, with the lowest CoVs for T2 maps and highest for R2 */BOLD. CoVs for all diffusion analyses were below 7.2%, except for perfusion fraction (FP ), with CoVs ranging from 18-24%. The CoV for renal sinus fat volume and percentage were both around 9%. Perfusion measurements were most repeatable with ASL (cortical perfusion only) and 2D phase contrast with CoVs of 10% and 13%, respectively. DCE perfusion had a CoV of 16%, while single kidney glomerular filtration rate (GFR) had a CoV of 13%. Repeatability coefficients (RCs) ranged from 7.7-87% (lowest/highest values for medullary mean diffusivity and cortical FP , respectively) and intraclass correlation coefficients (ICCs) ranged from -0.01 to 0.98 (lowest/highest values for cortical FP and renal sinus fat volume, respectively). DATA CONCLUSION: CoVs of most MRI measures of renal function and structure (with the exception of FP and perfusion as measured by DCE) were below 13%, which is comparable to standard clinical tests in nephrology. LEVEL OF EVIDENCE: 2 TECHNICAL EFFICACY: Stage 1.

Volume increase in the dentate gyrus after electroconvulsive therapy in depressed patients as measured with 7T.

Electroconvulsive therapy (ECT) is the most effective treatment for depression, yet its working mechanism remains unclear. In the animal analog of ECT, neurogenesis in the dentate gyrus (DG) of the hippocampus is observed. In humans, volume increase of the hippocampus has been reported, but accurately measuring the volume of subfields is limited with common MRI protocols. If the volume increase of the hippocampus in humans is attributable to neurogenesis, it is expected to be exclusively present in the DG, whereas other processes (angiogenesis, synaptogenesis) also affect other subfields. Therefore, we acquired an optimized MRI scan at 7-tesla field strength allowing sensitive investigation of hippocampal subfields. A further increase in sensitivity of the within-subjects measurements is gained by automatic placement of the field of view. Patients receive two MRI scans: at baseline and after ten bilateral ECT sessions (corresponding to a 5-week interval). Matched controls are also scanned twice, with a similar 5-week interval. A total of 31 participants (23 patients, 8 controls) completed the study. A large and significant increase in DG volume was observed after ECT (M = 75.44 mm3, std error = 9.65, p < 0.001), while other hippocampal subfields were unaffected. We note that possible type II errors may be present due to the small sample size. In controls no changes in volume were found. Furthermore, an increase in DG volume was related to a decrease in depression scores, and baseline DG volume predicted clinical response. These findings suggest that the volume change of the DG is related to the antidepressant properties of ECT, and may reflect neurogenesis.

Validation of multiparametric MRI by histopathology after nephrectomy: a case study.

OBJECTIVES: Renal multiparametric MRI (mpMRI) is a promising tool to monitor renal allograft health to enable timely treatment of chronic allograft nephropathy. This study aims to validate mpMRI by whole-kidney histology following transplantectomy. MATERIALS AND METHODS: A patient with kidney transplant failure underwent mpMRI prior to transplantectomy. The mpMRI included blood oxygenation level-dependent (BOLD) MRI, T1 and T2 mapping, diffusion-weighted imaging (DWI), 2D phase contrast (2DPC) and arterial spin labeling (ASL). Parenchymal mpMRI measures were compared to normative values obtained in 19 healthy controls. Differences were expressed in standard deviations (SD) of normative values. The mpMRI measures were compared qualitatively to histology. RESULTS: The mpMRI showed a heterogeneous parenchyma consistent with extensive interstitial hemorrhage on histology. A global increase in T1 (+ 3.0 SD) and restricted diffusivity (- 3.6 SD) were consistent with inflammation and fibrosis. Decreased T2 (- 1.8 SD) indicated fibrosis or hemorrhage. ASL showed diminished cortical perfusion (- 2.9 SD) with patent proximal arteries. 2DPC revealed a 69% decrease in renal perfusion. Histological evaluation showed a dense inflammatory infiltrate and fibrotic changes, consistent with mpMRI results. Most interlobular arteries were obliterated while proximal arteries were patent, consistent with ASL findings. DISCUSSION: mpMRI findings correlated well with histology both globally as well as locally.

Decreased native renal T1 up to one week after gadobutrol administration in healthy volunteers.

BACKGROUND: Gadolinium-based contrast agents (GBCAs) are widely used in MRI, despite safety concerns regarding deposition in brain and other organs. In animal studies gadolinium was detected for weeks after administration in the kidneys, but this has not yet been demonstrated in humans. PURPOSE: To find evidence for the prolonged presence of gadobutrol in the kidneys in healthy volunteers. STUDY TYPE: Combined retrospective and prospective analysis of a repeatability study. POPULATION: Twenty-three healthy volunteers with normal renal function (12 women, age range 40-76 years), of whom 21 were used for analysis. FIELD STRENGTH/SEQUENCE: Inversion recovery-based T1 map at 3T. ASSESSMENT: T1 maps were obtained twice with a median interval of 7 (range: 4-16) days. The T1 difference (ΔT1 ) between both scans was compared between the gadolinium group (n = 16, 0.05 mmol/kg gadobutrol administered after T1 mapping during both scan sessions) and the control group (n = 5, no gadobutrol). T1 maps were analyzed separately for cortex and medulla. STATISTICAL TESTS: Mann-Whitney U-tests to detect differences in ΔT1 between groups and linear regression to relate time between scans and estimated glomerular filtration rate (eGFR) to ΔT1 . RESULTS: ΔT1 differed significantly between the gadolinium and control group: median ΔT1 cortex -98 vs. 7 msec (P < 0.001) and medulla -68 msec vs. 19 msec (P = 0.001), respectively. The bias corresponds to renal gadobutrol concentrations of 8 nmol/g tissue (cortex) and 4 nmol/g tissue (medulla), ie, ~2.4 µmol for both kidneys (0.05% of original dose). ΔT1 correlated in the gadolinium group with duration between acquisitions for both cortex (regression coefficient (ß) 16.5 msec/day, R2 0.50, P < 0.001) and medulla (ß 11.5 msec/day, R2 0.32, P < 0.001). Medullary ΔT1 correlated with eGFR (ß 1.13 msec/(ml/min) R2 0.25, P = 0.008). DATA CONCLUSION: We found evidence of delayed renal gadobutrol excretion after a single contrast agent administration in subjects with normal renal function. Even within this healthy population, elimination delay increased with decreasing kidney function. LEVEL OF EVIDENCE: 3 Technical Efficacy: Stage 3 J. Magn. Reson. Imaging 2020;52:622-631.

Whole-brain 3D FLAIR at 7T using direct signal control.

PURPOSE: Image quality obtained for brain imaging at 7T can be hampered by inhomogeneities in the static magnetic field, B0 , and the RF electromagnetic field, B1 . In imaging sequences such as fluid-attenuated inversion recovery (FLAIR), which is used to assess neurological disorders, these inhomogeneities cause spatial variations in signal that can reduce clinical efficacy. In this work, we aim to correct for signal inhomogeneities to ensure whole-brain coverage with 3D FLAIR at 7T. METHODS: The direct signal control (DSC) framework was used to optimize channel weightings applied to the 8 transmit channels used in this work on a pulse-by-pulse basis through the echo train in the FLAIR sequences. 3D FLAIR brain images were acquired on 5 different subjects and compared with imaging using a quadrature-like mode of the transmit array. Precomputed "universal" DSC solutions calculated from a separate set of 5 subjects were also explored. RESULTS: DSC consistently enabled improved imaging across all subjects, with no dropouts in signal seen over the entire brain volume, which contrasted with imaging in quadrature mode. Further, the universal DSC solutions also consistently improved imaging despite not being optimized specifically for the subject being imaged. CONCLUSION: 3D FLAIR brain imaging at 7T is substantially improved using DSC and is able to recover regions of low signal without increasing imaging time or interecho spacing.

Establishing upper limits on neuronal activity-evoked pH changes with APT-CEST MRI at 7 T.

PURPOSE: To detect neuronal activity-evoked pH changes by amide proton transfer-chemical exchange saturation transfer (APT-CEST) MRI at 7 T. METHODS: Three healthy subjects participated in the study. A low-power 3-dimensional APT-CEST sequence was optimized through the Bloch-McConnell equations. pH sensitivity of the sequence was estimated both in phantoms and in vivo. The feasibility of pH-functional MRI was tested in Bloch-McConnell-simulated data using the optimized sequence. In healthy subjects, the visual stimuli were used to evoke transient pH changes in the visual cortex, and a 3-dimensional APT-CEST volume was acquired at the pH-sensitive frequency offset of 3.5 ppm every 12.6 s. RESULTS: In theory, a three-component general linear model was capable of separating the effects of blood oxygenation level-dependent contrast and pH. The Bloch-McConnell equations indicated that a change in pH of 0.03 should be measurable at the experimentally determined temporal signal-to-noise ratio of 108. However, only a blood oxygenation level-dependent effect in the visual cortex could be discerned during the visual stimuli experiments performed in the healthy subjects. CONCLUSIONS: The results of this study suggest that if indeed there are any transient brain pH changes in response to visual stimuli, those are under 0.03 units pH change, which is extremely difficult to detect using the existent techniques. Magn Reson Med 80:126-136, 2018. © 2017 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.

Improving peak local SAR prediction in parallel transmit using in situ S-matrix measurements.

PURPOSE: Peak local specific absorption rate (pSAR10g) is an important parameter used to determine patient safety during radiofrequency transmission. pSAR10g predictions for parallel transmit MRI are affected by the level of coupling exhibited by a modeled array in the simulation environment. However, simulated array coupling is rarely equal to the physical array coupling. Accurately simulating the physical array coupling may improve the accuracy of predicted SAR levels. METHODS: The scattering parameter matrix (S-matrix) of a prototype 4-channel array was measured in situ using directional couplers installed on a 7T scanner. Agreement between the simulated and measured S-matrix was achieved by using network co-simulation with a modified cost function. B1+ maps acquired in a phantom were compared to B1+ distributions determined from simulations. RESULTS: The modified co-simulation technique forces the simulations to have S-matrices similar to the measured values. A comparison of realistically versus ideally simulated coupling conditions shows that ideally simulated coupling can result in large ( > 40%) error in pSAR10g predictions, even when the array is reasonably tuned. The simulated B1+ distributions match the measured B1+ distributions better when the coupling is accurately simulated. CONCLUSION: Considering the measured array coupling matrix in numerical simulations eliminates a potential confound in pSAR10g prediction. Magn Reson Med 77:2040-2047, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

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

PURPOSE: To compare two pulsed, volumetric chemical exchange saturation transfer (CEST) acquisition schemes: steady state (SS) and pseudosteady state (PS) for the same brain coverage, spatial/spectral resolution and scan time. METHODS: Both schemes were optimized for maximum sensitivity to amide proton transfer (APT) and nuclear Overhauser enhancement (NOE) effects through Bloch-McConnell simulations, and compared in terms of sensitivity to APT and NOE effects, and to transmit field inhomogeneity. Five consented healthy volunteers were scanned on a 7 Tesla Philips MR-system using the optimized protocols at three nominal B1 amplitudes: 1 µT, 2 µT, and 3 µT. RESULTS: Region of interest based analysis revealed that PS is more sensitive (P < 0.05) to APT and NOE effects compared with SS at low B1 amplitudes (0.7-1.0 µT). Also, both sequences have similar dependence on the transmit field inhomogeneity. For the optimum CEST presaturation parameters (1 µT and 2 µT for APT and NOE, respectively), NOE is less sensitive to the inhomogeneity effects (15% signal to noise ratio [SNR] change for a B1 dropout of 40%) compared with APT (35% SNR change for a B1 dropout of 40%). CONCLUSION: For the same brain coverage, spatial/spectral resolution and scan time, at low power levels PS is more sensitive to the slow chemical exchange-mediated processes compared with SS. Magn Reson Med 77:2280-2287, 2017. © 2016 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.

Optimal control design of turbo spin-echo sequences with applications to parallel-transmit systems.

PURPOSE: The design of turbo spin-echo sequences is modeled as a dynamic optimization problem which includes the case of inhomogeneous transmit radiofrequency fields. This problem is efficiently solved by optimal control techniques making it possible to design patient-specific sequences online. THEORY AND METHODS: The extended phase graph formalism is employed to model the signal evolution. The design problem is cast as an optimal control problem and an efficient numerical procedure for its solution is given. The numerical and experimental tests address standard multiecho sequences and pTx configurations. RESULTS: Standard, analytically derived flip angle trains are recovered by the numerical optimal control approach. New sequences are designed where constraints on radiofrequency total and peak power are included. In the case of parallel transmit application, the method is able to calculate the optimal echo train for two-dimensional and three-dimensional turbo spin echo sequences in the order of 10 s with a single central processing unit (CPU) implementation. The image contrast is maintained through the whole field of view despite inhomogeneities of the radiofrequency fields. CONCLUSION: The optimal control design sheds new light on the sequence design process and makes it possible to design sequences in an online, patient-specific fashion. Magn Reson Med 77:361-373, 2017. © 2016 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine.

Combining a reduced field of excitation with SENSE-based parallel imaging for maximum imaging efficiency.

PURPOSE: To show that a combination of parallel imaging using sensitivity encoding (SENSE) and inner volume imaging (IVI) combines the known benefits of both techniques. SENSE with a reduced field of excitation (rFOX) is termed rSENSE. THEORY AND METHODS: The noise level in SENSE reconstructions is reduced by removing voxels from the unfolding process that are rendered silent by using rFOX. The silent voxels need to be identified beforehand, this is done by using rFOX in the coil sensitivity maps. In vivo experiments were performed at 7 Tesla using a 32-channel receive coil. RESULTS: Good image quality could be obtained in vivo with rSENSE at acceleration factors that are higher than could be obtained using SENSE or IVI alone. With rSENSE we were also able to accelerate scans using an rFOX that was purposely designed to be imperfect or incompatible at all with IVI. CONCLUSION: rSENSE has been demonstrated in vivo with two-dimensionally selective radiofrequency pulses. Besides allowing additional scan acceleration, it offers a greater robustness and flexibility than IVI. The proposed method can be used with other field strengths, anatomies and other rFOX techniques. Magn Reson Med 78:88-96, 2017. © 2016 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution Non Commercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.

Amide proton transfer (APT) imaging of brain tumors at 7 T: The role of tissue water T1 -Relaxation properties.

PURPOSE: To provide insight into the effect of water T1 relaxation (T1wat ) on amide proton transfer (APT) contrast in tumors. Three different metrics of APT contrast-magnetization transfer ratio (MTRRex ), relaxation-compensated MTRRex (AREX), and traditional asymmetry (MTRasym )-were compared in normal and tumor tissues in a variety of intracranial tumors at 7 Tesla (T). METHODS: Six consented intracranial tumor patients were scanned using a low-power, three-dimensional (3D) APT imaging sequence. MTRRex and MTRasym were calculated in the region of 3 to 4 ppm. AREX was calculated by T1wat correction of MTRRex . Tumor tissue masks, which classify different tumor tissues, were drawn by an experienced neuroradiologist. ROI-averaged tumor tissue analysis was done for MTRRex , AREX, and MTRasym . RESULTS: MTRRex and MTRasym were slightly elevated in tumor-associated structures. Both metrics were positively correlated to T1wat . The correlation coefficient (R) was determined to be 0.88 (P < 0.05) and 0.92 (P << 0.05) for MTRRex and MTRasym , respectively. After T1wat correction (R = -0.21, P = 0.69), no difference between normal and tumor tissues was found for AREX. CONCLUSIONS: The strong correlation of MTRRex and MTRasym with T1wat and the absence thereof in AREX suggests that much of APT contrast in tumors for the low-power, 3D-acquisition scheme at 7 T originates from the inherent tissue water T1 -relaxation properties. Magn Reson Med 77:1525-1532, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

On the transmit field inhomogeneity correction of relaxation-compensated amide and NOE CEST effects at 7 T.

High field MRI is beneficial for chemical exchange saturation transfer (CEST) in terms of high SNR, CNR, and chemical shift dispersion. These advantages may, however, be counter-balanced by the increased transmit field inhomogeneity normally associated with high field MRI. The relatively high sensitivity of the CEST contrast to B1 inhomogeneity necessitates the development of correction methods, which is essential for the clinical translation of CEST. In this work, two B1 correction algorithms for the most studied CEST effects, amide-CEST and nuclear Overhauser enhancement (NOE), were analyzed. Both methods rely on fitting the multi-pool Bloch-McConnell equations to the densely sampled CEST spectra. In the first method, the correction is achieved by using a linear B1 correction of the calculated amide and NOE CEST effects. The second method uses the Bloch-McConnell fit parameters and the desired B1 amplitude to recalculate the CEST spectra, followed by the calculation of B1 -corrected amide and NOE CEST effects. Both algorithms were systematically studied in Bloch-McConnell equations and in human data, and compared with the earlier proposed ideal interpolation-based B1 correction method. In the low B1 regime of 0.15-0.50 µT (average power), a simple linear model was sufficient to mitigate B1 inhomogeneity effects on a par with the interpolation B1 correction, as demonstrated by a reduced correlation of the CEST contrast with B1 in both the simulations and the experiments.

Relation between brown adipose tissue and measures of obesity and metabolic dysfunction in patients with cardiovascular disease.

PURPOSE: To evaluate whether brown adipose tissue (BAT) is present in middle-aged patients with cardiovascular comorbidities and to quantify how BAT presence associates with obesity and metabolic dysfunction. MATERIALS AND METHODS: Supraclavicular and subcutaneous adipose tissue fat-signal-fraction (FF) was determined with 1.5T water-fat magnetic resonance imaging (MRI) in 50 patients with coronary artery disease, cerebrovascular disease, or peripheral artery disease. The association between BAT presence, as measured by a higher FF difference between supraclavicular and subcutaneous adipose tissue, and obesity and metabolic dysfunction was quantified using multivariable linear regression. RESULTS: Supraclavicular adipose tissue displays a lower FF of 82.6% (interquartile range [IQR] 78.8-84.3) compared to 90.2% (IQR 87.3-91.9) in subcutaneous white adipose tissue (WAT, P < 0.0001). BAT presence was associated with less obesity and metabolic dysfunction. For example, 1 SD lower waist circumference (11.7 cm), 1 SD lower triglycerides (1.0 mmol/L), and absence of metabolic syndrome and type 2 diabetes were associated with 1.1% (95% confidence interval [CI] 0.1; 2.0), 1.1% (95% CI 0.1; 2.0), 2.1% (95% CI 0.1; 4.1), and 4.1% (95% CI 0.1; 7.1) higher FF difference between supraclavicular adipose tissue and subcutaneous WAT, respectively. CONCLUSION: Supraclavicular adipose tissue has BAT characteristics in adult patients with clinical manifest cardiovascular disease and BAT presence is associated with less obesity and a more favorable metabolic profile. LEVEL OF EVIDENCE: 2 Technical Efficacy: Stage 2 J. MAGN. RESON. IMAGING 2017;46:497-504.

The BOLD cerebrovascular reactivity response to progressive hypercapnia in young and elderly.

Blood Oxygenation Level Dependent (BOLD) imaging in combination with vasoactive stimuli can be used to probe cerebrovascular reactivity (CVR). Characterizing the healthy, age-related changes in the BOLD-CVR response can provide a reference point from which to distinguish abnormal CVR from the otherwise normal effects of ageing. Using a computer controlled gas delivery system, we examine differences in BOLD-CVR response to progressive hypercapnia between 16 young (28±3years, 9 female) and 30 elderly subjects (66±4years, 13 female). Furthermore, we incorporate baseline T2* information to broaden our interpretation of the BOLD-CVR response. Significant age-related differences were observed. Grey matter CVR at 7mmHg above resting PetCO2 was lower amongst elderly (0.19±0.06%ΔBOLD/mmHg) as compared to young subjects (0.26±0.07%ΔBOLD/mmHg). White matter CVR at 7mmHg above baseline PetCO2 showed no significant difference between young (0.04±0.02%ΔBOLD/mmHg) and elderly subjects (0.05±0.03%ΔBOLD/mmHg). We saw no significant differences in the BOLD signal response to progressive hypercapnia between male and female subjects in either grey or white matter. The observed differences in the healthy BOLD-CVR response could be explained by age-related changes in vascular mechanical properties.

Local specific absorption rate in brain tumors at 7 tesla.

PURPOSE: MR safety at 7 Tesla relies on accurate numerical simulations of transmit electromagnetic fields to fully assess local specific absorption rate (SAR) safety. Numerical simulations for SAR safety are currently performed using models of healthy patients. These simulations might not be useful for estimating SAR in patients who have large lesions with potentially abnormal dielectric properties, e.g., brain tumors. THEORY AND METHODS: In this study, brain tumor patient models are constructed based on scans of four patients with high grade brain tumors. Dielectric properties for the modeled tumors are assigned based on electrical properties tomography data for the same patients. Simulations were performed to determine SAR. RESULTS: Local SAR increases in the tumors by as much as 30%. However, the location of the maximum 10-gram averaged SAR typically occurs outside of the tumor, and thus does not increase. In the worst case, if the tumor model is moved to the location of maximum electric field intensity, then we do observe an increase in the estimated peak 10-gram SAR directly related to the tumor. CONCLUSION: Peak local SAR estimation made on the results of a healthy patient model simulation may underestimate the true peak local SAR in a brain tumor patient.

Fast 3D isotropic imaging of the aortic vessel wall by application of 2D spatially selective excitation and a new way of inversion recovery for black blood imaging.

PURPOSE: Aortic vessel wall imaging requires large coverage and a high spatial resolution, which makes it prohibitively time-consuming for clinical use. This work explores the feasibility of imaging the descending aorta in acceptable scan time, using two-dimensional (2D) spatially selective excitation and a new way of inversion recovery for black blood imaging. METHODS: The excitation pattern and field of view in a 3D gradient echo sequence are reduced in two dimensions, following the aorta's anisotropic geometry. Black blood contrast is obtained by partially inverting the blood's magnetization in the heart at the start of the cardiac cycle. Imaging is delayed until the inverted blood has filled the desired part of the aorta. The flip angle and delay are determined such that the blood signal is nulled upon arrival in the aorta. RESULTS: Experiments on eight volunteers showed that the descending aortic vessel wall could be imaged over more than 15 cm at a maximal resolution of 1.5 × 1.5 × 1.5 mm(3) in less than 5 min minimal scan time. CONCLUSION: This feasibility study demonstrates that time-efficient isotropic imaging of the descending aorta is possible by using 2D spatially selective excitation for motion artifact reduction and a new way of inversion recovery for black blood imaging.