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Cerebrospinal fluid (CSF) has historically been considered to function as a sink for brain‐derived waste disposal. Recent work suggested that CSF interacts even more intensely with brain tissue than previously recognized, through perivascular spaces that penetrate the brain. Cardiac pulsations, vasomotion, and respiration have been suggested to drive CSF flow in these perivascular spaces, thereby enhancing waste clearance. However, the intrinsic role of CSF production in relation to its distribution volume (turnover) is not an explicit component of recent concepts on brain clearance. Here, we review the work on CSF turnover and volume, focusing on preclinical evidence. Herein, we highlight the use of MRI in establishing CSF‐related parameters. We describe the impact of sleep, effect of anesthesia, aging, and hypertension on CSF turnover, and how this relates to brain clearance. Evaluation of the available evidence suggests that CSF turnover is a major determinant in brain clearance. In addition, we propose that several putative drivers of brain clearance, but also conditions associated with impaired clearance, such as aging, may actually relate to altered CSF turnover. [ABSTRACT FROM AUTHOR]

Static quantitative magnetic resonance imaging (MRI) provides readouts of structural changes in diseased muscle, but current approaches lack the ability to fully explain the loss of contractile function. Muscle contractile function can be assessed using various techniques including phase‐contrast MRI (PC‐MRI), where strain rates are quantified. However, current two‐dimensional implementations are limited in capturing the complex motion of contracting muscle in the context of its three‐dimensional (3D) fiber architecture. The MR acquisitions (chemical shift‐encoded water–fat separation scan, spin echo‐echoplanar imaging with diffusion weighting, and two time‐resolved 3D PC‐MRI) wereperformed at 3 T. PC‐MRI acquisitions and performed with and without load at 7.5% of the maximum voluntary dorsiflexion contraction force. Acquisitions (3 T, chemical shift‐encoded water–fat separation scan, spin echo‐echo planar imaging with diffusion weighting, and two time‐resolved 3D PC‐MRI) were performed with and without load at 7.5% of the maximum voluntary dorsiflexion contraction force. Strain rates and diffusion tensors were calculated and combined to obtain strain rates along and perpendicular to the muscle fibers in seven lower leg muscles during the dynamic dorsi‐/plantarflexion movement cycle. To evaluate strain rates along the proximodistal muscle axis, muscles were divided into five equal segments. t‐tests were used to test if cyclic strain rate patterns (amplitude > 0) were present along and perpendicular to the muscle fibers. The effects of proximal‐distal location and load were evaluated using repeated measures ANOVAs. Cyclic temporal strain rate patterns along and perpendicular to the fiber were found in all muscles involved in dorsi‐/plantarflexion movement (p < 0.0017). Strain rates along and perpendicular to the fiber were heterogeneously distributed over the length of most muscles (p < 0.003). Additional loading reduced strain rates of the extensor digitorum longus and gastrocnemius lateralis muscle (p < 0.001). In conclusion, the lower leg muscles involved in cyclic dorsi‐/plantarflexion exercise showed cyclic fiber strain rate patterns with amplitudes that varied between muscles and between the proximodistal segments within the majority of muscles. [ABSTRACT FROM AUTHOR]

Purpose: Aortic motion has direct impact on the mechanical stresses acting on the aorta. In aortic disease, increased stiffness of the aorta may lead to decreased aortic motion over time, which could be a predictor for aortic dissection or rupture. This study investigates the reproducibility of obtaining 3D displacement and diameter maps quantified using accelerated 3D cine MRI at 3 T. Methods: A noncontrast‐enhanced, free‐breathing 3D cine sequence based on balanced SSFP and pseudo‐spiral undersampling with high spatial isotropic resolution was developed (spatial/temporal resolution [1.6 mm]3/67 ms). The thoracic aorta of 14 healthy volunteers was prospectively scanned three times at 3 T: twice on the same day and a third time 2 weeks later. Aortic displacement was calculated using iterative closest point nonrigid registration of manual segmentations of the 3D aorta at end‐systole and mid‐diastole. Interexamination and interobserver regional analysis of mean displacement for five regions of interest was performed using Bland–Altman analysis. Additionally, a complementary voxel‐by‐voxel analysis was done, allowing a more local inspection of the method. Results: No significant differences were found in mean and maximum displacement for any of the regions of interest for the interexamination and interobserver analysis. The maximum displacement measured in the lower half of the ascending aorta was 11.0 ± 3.4 mm (range: 3.0–17.5 mm) for the first scan. The smallest detectable change in mean displacement in the lower half of the ascending aorta was 3 mm. Conclusion: Detailed 3D cine balanced SSFP at 3 T allows for reproducible quantification of systolic–diastolic mean aortic displacement within acceptable limits. [ABSTRACT FROM AUTHOR]

Purpose: To obtain homogeneous signal throughout the human liver at 7 T. Flip angle (FA) shimming in 7T whole‐liver imaging was performed through parallel‐transmit kT‐point pulses based on subject‐specific multichannel absolute B1+$$ {\mathrm{B}}_1^{+} $$ maps from Fourier phase‐encoded dual refocusing echo acquisition mode (PE‐DREAM). Methods: The optimal number of Fourier phase‐encoding steps for PE‐DREAM B1+$$ {\mathrm{B}}_1^{+} $$ mapping was determined for a 7T eight‐channel parallel‐transmission system. FA shimming experiments were performed in the liver of 7 healthy subjects with varying body mass index. In these subjects, first B0$$ {\mathrm{B}}_0 $$ shimming and Fourier PE‐DREAM B1+$$ {\mathrm{B}}_1^{+} $$ mapping were performed. Subsequently, three small‐flip‐angle 3D gradient‐echo scans were acquired, comparing a circularly polarized (CP) mode, a phase shim, and a kT‐point pulse. Resulting homogeneity was assessed and compared with estimated FA maps and distributions. Results: Fourier PE‐DREAM with 13 phase‐encoding steps resulted in a good tradeoff between B1+$$ {\mathrm{B}}_1^{+} $$ accuracy and scan time. Lower coefficient of variation values (average [min‐max] across subjects) of the estimated FA in the volume of interest were observed using kT‐points (7.4 [6.6%–8.0%]), compared with phase shimming (18.8 [12.9%–23.4%], p < 0.001) and CP (43.2 [39.4%–47.1%], p < 0.001). kT‐points delivered whole‐liver images with the nominal FA and the highest degree of homogeneity. CP and phase shimming resulted in either inaccurate or imprecise FA distributions. Here, locations having suboptimal FA in the estimated FA maps corresponded to liver areas suffering from inconsistent signal intensity and T1‐weighting in the gradient‐echo scans. Conclusion: Homogeneous whole‐liver 3D gradient‐echo acquisitions at 7 T can be obtained with eight‐channel kT‐point pulses calculated based on subject‐specific multichannel absolute Fourier PE‐DREAM B1+$$ {\mathrm{B}}_1^{+} $$ maps. [ABSTRACT FROM AUTHOR]