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RF exposure during MR exams induces heating of the exposed tissues. Measurement of tympanic membrane temperature with an infrared sensor has been used to estimate noninvasively the global RF-induced temperature rise. In this study, the tympanic membrane temperature was measured before and after having the volunteers lying for 15 minutes with earplugs with neither an RF nor a static magnetic field. Large physiological variations of temperature rise were observed across the volunteers after this dummy “exposure”, showing the low reliability of this metric to measure small temperature rises, that make it problematic for RF-induced temperature rise estimation.

PURPOSE An MR thermometry (MRT) method with motion and field fluctuation compensation is proposed to measure non-invasively sub-degree brain temperature variations occurring through radiofrequency (RF) power deposition during MR exams. METHODS MRT at 7T with a multi-slice echo planar imaging (EPI) sequence and concurrent field monitoring was first tested in vitro to assess accuracy in the presence of external field perturbations, an optical probe being used for ground truth. In vivo, this strategy was complemented by a motion compensation scheme based on a dictionary pre-scan, as reported in some previous work, and was adapted to the human brain. Precision reached with this scheme was assessed on eight volunteers with a 5 minute-long low specific absorption rate (SAR) scan. Finally, temperature rise in the brain was measured twice on the same volunteers and with the same strategy, this time by employing a 20-minutes scan at the maximum SAR delivered with a commercial volume head coil. RESULTS In vitro, the root mean square (RMS) error between optical probe and MRT measurements was 0.02°C with field sensor correction. In vivo, the low SAR scan returned a precision in temperature change measurement with field monitoring and motion compensation of 0.05°C. The 20-minutes maximum SAR scan returned a temperature rise throughout the inner-brain in the range of 0-0.2°C. Brain periphery remained too sensitive with respect to motion to lead to equally conclusive results. CONCLUSION Sub-degree temperature rise in the inner human brain was characterized experimentally throughout RF exposure. Potential applications include improvement of human thermal models and revision of safety margins.

PURPOSE: The SNR at the center of a spherical phantom of known electrical properties was measured in quasi-identical experimental conditions as a function of magnetic field strength between 3 T and 11.7 T.METHODS: The SNR was measured at the center of a spherical water saline phantom with a gradient recalled echo sequence. Measurements were performed at NeuroSpin at 3, 7, and 11.7 T. The phantom was then shipped to Maastricht University and then to the University of Minnesota for additional data points at 7, 9.4, and 10.5 T. Experiments were carried out with the exact same type of birdcage volume coil (except at 3 T, where a similar coil was used) to attempt at isolating the evolution of SNR with field strength alone. Phantom electrical properties were characterized over the corresponding frequency range.RESULTS: Electrical properties were found to barely vary over the frequency range. Removing the influence of the flip-angle excitation inhomogeneity was crucial, as expected. After such correction, measurements revealed a gain of SNR growing as B0 1.94 ± 0.16 compared with B0 2.13 according to ultimate intrinsic SNR theory.CONCLUSIONS: By using quasi-identical experimental setups (RF volume coil, phantom, electrical properties, and protocol), this work reports experimental data between 3 T and 11.7 T, enabling the comparison with SNR theories in which conductivity and permittivity can be assumed to be constant with respect to field strength. According to ultimate SNR theory, these results can be reasonably extrapolated to the performance of receive arrays with greater than about 32 elements for central SNR in the same spherical phantom.

Sub-degree brain temperature rise occurring through RF power deposition during the MR exam was measured with MR thermometry at 7T using a multi-slice EPI sequence and concurrent field monitoring. This sequence was first tested in vitro with field perturbations, an optical probe being used for ground truth measurements, and the accuracy of the method reached 0.02°C. In vivo, this methodology was complemented by a motion compensation step and precision reached 0.05°C. After 20-min of scanning at maximal SAR, temperature rise induced by RF power deposition throughout the inner brain reached 0-0.2°C, well below the values predicted by thermal models.

Summary of Main Findings: Analytical expressions for optimal tSNR and t-score for the mean coil combinations are described and compared to the commonly used covSoS coil combination. Findings suggest that optimal tSNR or t-score for the mean do not guarantee better BOLD signal detection in fMRI time series. Synopsis: For MRI with a multi-receiver RF coil, one image per coil element and per time frame is obtained. The final image is typically calculated from the root sum of squares (rSoS) combination across channels. While this combination approach is quasi-optimal for SNR, it is not necessarily optimal for temporal SNR (tSNR) of the time-series. We present two analytical and voxel-wise coil combination expressions reaching optimality in tSNR and t-score for the mean (TSM) respectively. Their BOLD sensitivity is compared to the gold standard covariance root sum of squares. Both improved tSNR and TSM but yielded weaker t-scores than covSoS.

Purpose: In parallel transmission (pTX), subject-tailored RF pulses allow achieving excellent flip angle (FA) accuracy but often require computationally extensive online optimisations, precise characterisation of the the static field (ΔB0) and the transmit RF field (B+1 ) distributions. This costs time and requires expertise from the MR user. Universal Pulses (UP) have been proposed to reduce this burden, yet, with a penalty in FA accuracy. This study introduces the concept of standardised universal pulses (SUP), where pulses are designed offline and adjusted to the subject through a fast online calibration scan. Methods: A SUP is designed offline using a so-called standardised database, wherein each B+1 map has been normalised to a reference transmit RF field distribution. When scanning a new subject, a 3-slice B+1 acquisition (scan time < 10 s) is performed and used to adjust the SUP to the subject through a linear transform. SUP performance was assessed at 7T with simulations by computing the FA-normalised root mean square error (FA-NRMSE) and the FA profile stability as measured by the average and coefficient of variation of the FA across 15 control subjects, along with in vivo experiments using an MP2RAGE sequence implementing the SUP variant for the FLASH readout. Results: Adjusted SUP improved the FA-NRMSE (8.8 % for UP versus 7.1 % for adjusted SUP). Experimentally, in vivo, this translated in an improved signal homogeneity and more accurate T1 quantification using MP2RAGE. Conclusion The proposed SUP approach improves excitation accuracy (FA-NRMSE) while preserving the same offline pulse design principle as offered by UPs.

Background A limited number of studies have used the intravoxel incoherent motion (IVIM) approach on bone marrow. In none of the previous studies were the effects of fat suppression on the IVIM parameters investigated. Purpose To measure the water diffusion coefficient and the perfusion fraction in vertebral bone marrow using IVIM with multishot, readout-segmented (RESOLVE) echo-planar imaging and to assess the effects of different fat suppression techniques on the measurement of the IVIM parameters. Study type Prospective. Population/subjects Six healthy volunteers (24.2 ± 4.3 years). Field strength/sequence 1.5T, RESOLVE. Assessment Four experiments were performed: 1) RESOLVE imaging without fat suppression, 2) with fat saturation (FS), 3) with spectral attenuated inversion recovery (SPAIR), and 4) with short-tau inversion recovery (STIR). The water diffusion coefficient D, pseudo-diffusion coefficient D*, and the perfusion fraction f were assessed in the vertebral bodies of the lumbar vertebrae. Statistical tests One-way repeated-measures analysis of variance (ANOVA) followed by Bonferroni's multiple comparison test. Results The RESOLVE IVIM protocol allowed for measurement of D, D*, and f in all volunteers. The signal of lipid protons affected the quantification of the IVIM diffusion coefficient: D = 0.24 ± 0.10 (×10-3 mm2 /s), no FS; D = 0.43 ± 0.07 (×10-3 mm2 /s), FS; D = 0.42 ± 0.07 (×10-3 mm2 /s), SPAIR; D = 0.35 ± 0.10 (×10-3 mm2 /s), STIR; and IVIM perfusion fraction f = 7.5 ± 1.9% no FS, f = 14.5 ± 5.4%, FS; f = 12.5 ± 2.6%, SPAIR; f = 18.1 ± 6.1%, STIR. No significant effect (P = 0.36) was found on the quantification of D*. Data conclusion An IVIM-MRI protocol using the RESOLVE sequence was implemented for measurements of vertebral bone marrow diffusion and the perfusion. The comparison between the protocols with and without fat suppression indicates that the lipid signal results in an underestimation of both D and f. Level of evidence Technical Efficacy Stage: 1 J. Magn. Reson. Imaging 2019;49:768-776.

International audience; Background: A limited number of studies have used the intravoxel incoherent motion (IVIM) approach on bone marrow. In none of the previous studies were the effects of fat suppression on the IVIM parameters investigated.Purpose: To measure the water diffusion coefficient and the perfusion fraction in vertebral bone marrow using IVIM with multishot, readout‐segmented (RESOLVE) echo‐planar imaging and to assess the effects of different fat suppression techniques on the measurement of the IVIM parameters.Study Type: Prospective.Population/Subjects: Six healthy volunteers (24.2 ± 4.3 years).Field Strength/Sequence: 1.5T, RESOLVE.Assessment: Four experiments were performed: 1) RESOLVE imaging without fat suppression, 2) with fat saturation (FS), 3) with spectral attenuated inversion recovery (SPAIR), and 4) with short‐tau inversion recovery (STIR). The water diffusion coefficient D, pseudo‐diffusion coefficient D*, and the perfusion fraction f were assessed in the vertebral bodies of the lumbar vertebrae.Statistical Tests: One‐way repeated‐measures analysis of variance (ANOVA) followed by Bonferroni's multiple comparison test.Results: The RESOLVE IVIM protocol allowed for measurement of D, D*, and f in all volunteers. The signal of lipid protons affected the quantification of the IVIM diffusion coefficient: D = 0.24 ± 0.10 (×10−3 mm2/s), no FS; D = 0.43 ± 0.07 (×10−3 mm2/s), FS; D = 0.42 ± 0.07 (×10−3 mm2/s), SPAIR; D = 0.35 ± 0.10 (×10−3 mm2/s), STIR; and IVIM perfusion fraction f = 7.5 ± 1.9% no FS, f = 14.5 ± 5.4%, FS; f = 12.5 ± 2.6%, SPAIR; f = 18.1 ± 6.1%, STIR. No significant effect (P = 0.36) was found on the quantification of D*.Data Conclusion: An IVIM‐MRI protocol using the RESOLVE sequence was implemented for measurements of vertebral bone marrow diffusion and the perfusion. The comparison between the protocols with and without fat suppression indicates that the lipid signal results in an underestimation of both D and f.

Purpose - To assess the feasibility of measuring the fat fraction, T1 and T2 * relaxation times of water and fat signals in vertebral bone marrow using breath-hold magnetic resonance imaging (MRI) gradient echo images of the spine. Materials and methods - MRI experiments were performed at 1.5T on eight healthy volunteers (35.1 ± 15.7 years, five men and three women) using two sagittal four-echo 3D gradient echo volumetric interpolated breath-hold examination (VIBE Dixon) sequences acquired at two different flip angles (5° and 15°). The water/fat decomposition was performed in the vertebral bodies of L1 to L5 by fitting the signal to a function that depends on the echo time and the flip angle to calculate the fat fraction (FF) and T1 and T2 * relaxation times of water and fat signals. Repeatability was assessed by scanning one volunteer six times. Results - The mean fat fraction over L1 to L5 was 33 ± 8%. The mean T1 and T2 * of water and fat signals were respectively T1w = 701 ± 151 msec, T2 *w = 13.7 ± 2.9 msec, T1f = 334 ± 113 msec, and T2 *f = 11.4 ± 2.7 msec. When considering each vertebra separately, the fat fraction increased from L1 to L5 and the T1w decreased from L1 to L5. The mean coefficients of variation obtained from the repeatability study were 8% (FF), 11% (T1w ), 17% (T1f ), 8% (T2 *w ), and 27% (T2 *f ). Conclusion - The method introduced in the current study allows for the measurement of the fat fraction and water and fat relaxation times, with a total acquisition time of less than 40 seconds. J. Magn. Reson. Imaging 2016;44:549-555.

International audience; Purpose - Bone marrow is found either as red bone marrow, which mainly contains haematopoietic cells, or yellow bone marrow, which mainly contains adipocytes. In adults, red bone marrow is principally located in the axial skeleton. A recent study has introduced a method to simultaneously estimate the fat fraction (FF), the T1 and T2* relaxation times of water (T1w, T2*w) and fat (T1f and T2*f) in the vertebral bone marrow. The aim of the current study was to measure FF, T1w, T1f, T2*w and T2*f in five sites of bone marrow, and to assess the presence of regional variations. Methods - MRI experiments were performed at 1.5T on five healthy volunteers (31.6±15.6years) using a prototype chemical-shift-encoded 3D multi-gradient-echo sequence (VIBE) acquired with two flip angles. Acquisitions were performed in the shoulders, lumbar spine and pelvis, with acquisition times of

International audience; Purpose - To assess the feasibility of measuring the fat fraction, T1 and T2 * relaxation times of water and fat signals in vertebral bone marrow using breath-hold magnetic resonance imaging (MRI) gradient echo images of the spine. Materials and methods - MRI experiments were performed at 1.5T on eight healthy volunteers (35.1 ± 15.7 years, five men and three women) using two sagittal four-echo 3D gradient echo volumetric interpolated breath-hold examination (VIBE Dixon) sequences acquired at two different flip angles (5° and 15°). The water/fat decomposition was performed in the vertebral bodies of L1 to L5 by fitting the signal to a function that depends on the echo time and the flip angle to calculate the fat fraction (FF) and T1 and T2 * relaxation times of water and fat signals. Repeatability was assessed by scanning one volunteer six times. Results - The mean fat fraction over L1 to L5 was 33 ± 8%. The mean T1 and T2 * of water and fat signals were respectively T1w = 701 ± 151 msec, T2 *w = 13.7 ± 2.9 msec, T1f = 334 ± 113 msec, and T2 *f = 11.4 ± 2.7 msec. When considering each vertebra separately, the fat fraction increased from L1 to L5 and the T1w decreased from L1 to L5. The mean coefficients of variation obtained from the repeatability study were 8% (FF), 11% (T1w ), 17% (T1f ), 8% (T2 *w ), and 27% (T2 *f ). Conclusion - The method introduced in the current study allows for the measurement of the fat fraction and water and fat relaxation times, with a total acquisition time of less than 40 seconds. J. Magn. Reson. Imaging 2016;44:549-555.

Purpose A MR thermometry (MRT) method with field monitoring is proposed to improve the measurement of small temperature variations induced in brain MRI exams. Methods MR thermometry experiments were performed at 7 Tesla with concurrent field monitoring and RF heating. Images were reconstructed with nominal k-space trajectories and with first-order spherical harmonics correction. Experiments were performed in vitro with deliberate field disturbances and on an anesthetized macaque in 2 different specific absorption rate regimes, that is, at 50% and 100% of the maximal specific absorption rate level allowed in the International Electrotechnical Commission normal mode of operation. Repeatability was assessed by running a second separate session on the same animal. Results Inclusion of magnetic field fluctuations in the reconstruction improved temperature measurement accuracy in vitro down to 0.02°C. Measurement precision in vivo was on the order of 0.15°C in areas little affected by motion. In the same region, temperature increase reached 0.5 to 0.8°C after 20 min of heating at 100% specific absorption rates and followed a rough factor of 2 with the 50% specific absorption rate scans. A horizontal temperature plateau, as predicted by Pennes bioheat model with thermal constants from the literature and constant blood temperature assumption, was not observed. Conclusion Inclusion of field fluctuations in image reconstruction was beneficial for the measurement of small temperature rises encountered in standard brain exams. More work is needed to correct for motion-induced field disturbances to extract reliable temperature maps.