Description: Purpose 3D time-resolved (4D) phase contrast MRI can be used to study muscle contraction. However, 3D coverage with sufficient spatiotemporal resolution can only be achieved by interleaved acquisitions during many repetitions of the motion task, resulting in long scan times. The aim of this study was to develop a compressed sensing accelerated 4D phase contrast MRI technique for quantification of velocities and strain rate of the muscles in the lower leg during active plantarflexion/dorsiflexion. Methods Nine healthy volunteers were scanned during active dorsiflexion/plantarflexion task. For each volunteer, we acquired a reference scan, as well as 4 different accelerated scans (k-space undersampling factors: 3.14X, 4.09X, 4.89X, and 6.41X) obtained using Cartesian Poisson disk undersampling schemes. The data was reconstructed using a compressed sensing pipeline. For each scan, velocity and strain rate values were quantified in the gastrocnemius lateralis, gastrocnemius medialis, tibialis anterior, and soleus. Results No significant differences in velocity values were observed as a function acceleration factor in the investigated muscles. The strain rate calculation resulted in one positive (s + ) and one negative (s − ) eigenvalue, whereas the third eigenvalue (s 3 ) was consistently 0 for all the acquisitions. No significant differences were observed for the strain rate eigenvalues as a function of acceleration factor. Conclusions Data undersampling combined with compressed sensing reconstruction allowed obtainment of time-resolved phase contrast acquisitions with 3D coverage and quantitative information comparable to the reference scan. The 3D sensitivity of the method can help in understanding the connection between muscle architecture and muscle function in future studies.

Description: Purpose To develop a rapid, non-CPMG high-resolution volumetric imaging approach, exhibiting a speed and in-plane resilience to field inhomogeneities comparable to RARE/turbo-spin-echo (TSE) while endowed with unique downsampling characteristics. Methods A multi-scan extension of cross-term spatiotemporal encoding (xSPEN) is introduced and analyzed. The method simultaneously yields k y / k z data containing low and high frequency components, as well as transposed, low-resolution z / y images. This dual k -/spatial-domain information is captured by a multi-scan procedure that phase-encodes k y while simultaneously slice-selecting z . A reconstruction scheme converting this information into high resolution 3D images with fully multiplexed volumetric coverage is introduced and exemplified. Results Phase-encoded xSPEN was tested by human brain imaging at sub-mm resolutions. The method exceeded 2D TSE's sensitivity by factors of ≈3–4, while providing similar resolution and SNR as 3D TSE in ≈50% acquisition times. The method's contrast is dominated by T 2 and is free from “bright-fat” effects associated to spin-echo trains. Further acceleration is enabled by the method's downsampling abilities. Tradeoffs between encoding time, number of measurements, spatial resolution, SNR, and artifact levels are also laid out. Conclusion A new MRI strategy is introduced delivering high in- and through-plane resolutions while enjoying full Fourier multiplexing, leading to fast acquisitions with high SNR.

Description: Purpose Many brain diseases are associated with an alteration in blood-brain barrier (BBB) and its permeability. Current methods using contrast agent are primarily sensitive to major leakage of BBB to macromolecules, but may not detect subtle changes in BBB permeability. The present study aims to develop a novel non-contrast MRI technique for the assessment of BBB permeability to water. Methods The central principle is that by measuring arterially labeled blood spins that are drained into cerebral veins, water extraction fraction ( E ) and permeability-surface-area product ( PS ) of BBB can be determined. Four studies were performed. We first demonstrated the proof-of-principle using conventional ASL with very long post-labeling delays (PLD). Next, a new sequence, dubbed water-extraction-with-phase-contrast-arterial-spin-tagging (WEPCAST), and its Look-Locker (LL) version were developed. Finally, we demonstrated that the sensitivity of the technique can be significantly enhanced by acquiring the data under mild hypercapnia. Results By combining a strong background suppression with long PLDs (2500–4500 ms), ASL spins were reliably detected in the superior sagittal sinus (SSS), demonstrating the feasibility of measuring this signal. The WEPCAST sequence eliminated partial voluming effects of tissue perfusion and allowed quantitative estimation of E  = 95.5 ± 1.1% and PS  = 188.9 ± 13.4 mL/100 g/min, which were in good agreement with literature reports. LL-WEPCAST sequence shortened the scan time from 19 min to 5 min while providing results consistent with multiple single-PLD acquisitions. Mild hypercapnia increased SNR by 78 ± 25% without causing a discomfort in participants. Conclusion A new non-contrast technique for the assessment of global BBB permeability was developed, which may have important clinical applications.

Description: Purpose We introduce the quantitative, continuous marker cartilage cavity that quantifies cartilage lesions by the total lesion volume. The aim was to quantify small lesions as well as large, full-depth lesions. Methods We included 315 knees from the Center for Clinical and Basic Research (CCBR), 972 knees from the Osteoarthritis Initiative (OAI), and 791 knees from the Prevention of OA in Overweight Females (PROOF) cohorts. In a subset, we digitally inserted artificial lesions. Each knee MRI was segmented using the knee imaging quantification (KIQ) framework. We quantified cartilage mean thickness and cavity from high-resolution cartilage thickness maps. Finally, we quantified lesion volume by the gradient peak method (GPM). Results Scan–rescan precision for cartilage cavity was 7.1%/3.0%. The cartilage cavity accuracy on the artificial lesions was determined as linear correlation at 0.88 with an average 8% under-estimation of lesion volume. Cavity and degree of radiographic osteoarthritis (ROA) correlated for all compartments (Spearman's rho between 0.14–0.56, P  〈 0.001). Cavity had modest correlations to whole-organ magnetic resonance imaging score (WORMS) cartilage lesion scores but strong correlations with Boston-Leeds osteoarthritis knee score (BLOKS)/MRI osteoarthritis knee score (MOAKS) scores in most compartments (rho between 0.08–0.65, P  〈 0.001). Cavity correlated with WOMAC pain for all tibio-femoral compartments in OAI (rho between 0.19–0.25, P  〈 0.001) and most compartments in PROOF. Comparing with the GPM estimate, cavity was more precise, more accurate, and correlated stronger with ROA, lesion scores, and pain levels. Conclusion The strong correlations with ROA, radiologist lesion scores, and pain demonstrated that cavity captured OA and lesion features. Thereby, it may be appropriate for quantification of cartilage surface irregularity.

Description: Purpose The purpose of this study was to investigate the feasibility of in vivo 13 C-〉 1 H hyperpolarization transfer, which has significant potential advantages for detecting the distribution and metabolism of hyperpolarized 13 C probes in a clinical MRI scanner. Methods A standalone pulsed 13 C RF transmit channel was developed for operation in conjunction with the standard 1 H channel of a clinical 3T MRI scanner. Pulse sequences for 13 C power calibration and polarization transfer were programmed on the external hardware and integrated with a customized water-suppressed 1 H MRS acquisition running in parallel on the scanner. The newly developed RF system was tested in both phantom and in vivo polarization transfer experiments in 1 J CH -coupled systems: phantom experiments in thermally polarized and hyperpolarized [2- 13 C]glycerol, and 1 H detection of [2- 13 C]lactate generated from hyperpolarized [2- 13 C]pyruvate in rat liver in vivo. Results Operation of the custom pulsed 13 C RF channel resulted in effective 13 C-〉 1 H hyperpolarization transfer, as confirmed by the characteristic antiphase appearance of 1 H-detected, 1 J CH -coupled doublets. In conjunction with a pulse sequence providing 190-fold water suppression in vivo, 1 H detection of hyperpolarized [2- 13 C]lactate generated in vivo was achieved in a rat liver slice. Conclusion The results show clear feasibility for effective 13 C-〉 1 H hyperpolarization transfer in a clinical MRI scanner with customized heteronuclear RF system.

Description: Purpose Dixon-based fat suppression has recently gained interest for dynamic contrast-enhanced MRI, but multi-echo techniques require longer scan times and reduce temporal resolution compared to single-echo alternatives without fat suppression. The purpose of this work is to demonstrate accelerated single-echo Dixon imaging with high spatial and temporal resolution. Theory and Methods Real-valued water and fat images can be obtained from a single measurement if the shared initial phase and that due to are assumed known a priori. An expression for simultaneous sensitivity encoding (SENSE) unfolding and fat-water separation is derived for the general undersampling case, and simplified under the special case of uniform Cartesian undersampling. In vivo experiments were performed in extremities and brain with SENSE acceleration factors of up to R  = 8. Results Single-echo Dixon reconstruction of highly undersampled data was successfully demonstrated. Dynamic contrast-enhanced water and fat images provided high spatial and temporal resolution dynamic images with image update times shorter than previous single-echo Dixon work. Conclusion Time-resolved contrast-enhanced MRI with single-echo Dixon fat suppression shows high image quality, improved vessel delineation, and reduced sensitivity to motion when compared to time-subtraction methods.

Description: Purpose To improve 2D noncontrast-enhanced MRA by using a helical time-of-flight (TOF) acquisition technique and a slice-super-resolution reconstruction. Methods The TOF technique is combined with a helical trajectory with golden-angle–based radial projection reordering. A continuous spatial shift in slice direction is realized by adjusting the frequency of the excitation pulse between the individual projections. The limited resolution along the shift direction is improved by a deconvolution with simulated slice profile. The helical TOF (hTOF) was compared in vivo with a conventional 2D and 3D TOF. Results Results from in vivo experiments on the carotid show that the visual resolution in slice direction can be improved by using hTOF and the slice-super-resolution reconstruction. The vessels appear up to 1.5 times sharper and can be better separated from each other. Compared to 2D TOF images, the stair step artifacts are strongly reduced in reformatted hTOF images, whereas measurement time is decreased by at least 35%. Compared to 3D TOF, the hTOF offers a higher blood-to-background contrast, better visualization of smaller vessels, and reduced measurement time. Conclusion The hTOF benefits from a 2D acquisition and a 3D reconstruction, which makes it a promising technique for the noncontrast-enhanced imaging of the carotid.

Description: Purpose To demonstrate a computationally efficient and theoretically artifact-free method to calculate static field (B 0 ) inhomogeneity in a volume of interest induced by an arbitrary voxelated susceptibility distribution. Methods Our method computes B 0 by circular convolution between a zero-filled susceptibility matrix and a shifted, voxel-integrated dipolar field kernel on a grid of size N S +N T – 1 in each dimension, where N S and N T are the sizes of the susceptibility source and B 0 target grids, respectively. The computational resource requirement is independent of source-target separation. The method, called generalized susceptibility voxel convolution, is demonstrated on three susceptibility models: an ellipsoid, MR-compatible screws, and a dynamic human heartbeat model. Results B 0 in an ellipsoid calculated by generalized susceptibility voxel convolution matched an analytical solution nearly exactly. The method also calculated screw-induced B 0 in agreement with experimental data. Dynamic simulation demonstrated its computational efficiency for repeated B 0 calculations on time-varying susceptibility. On the contrary, conventional and alias-subtracted k-space-discretized Fourier convolution methods showed nonnegligible aliasing and Gibbs ringing artifacts in the tested models. Conclusion Generalized susceptibility voxel convolution can be a fast and reliable way to compute susceptibility-induced B 0 when the susceptibility source is not colocated with the B 0 target volume of interest, as in modeling B 0 variations from motion and foreign objects.

Description: Purpose Deranged metabolism is now recognized as a key causal factor in a variety of heart diseases, and is being studied extensively. However, invasive methods may alter metabolism, and conventional imaging techniques measure tracer uptake but not downstream metabolism. These challenges may be overcome by hyperpolarized MR, a noninvasive technique currently crossing the threshold into human trials. The aim of this study was to image metabolic changes in the heart in response to endogastric glucose bolus and to acute hypertension. Methods Five postprandial pigs were scanned with hyperpolarized [1- 13 C]pyruvate cardiac MR at baseline, after oral glucose bolus, and after infusion of angiotensin-II. Results No effect of glucose bolus was seen using hyperpolarized [1- 13 C]pyruvate MR despite changes in circulating substrates. During angiotensin-II infusion, blood pressure increased 179% ( P  = 0.008) and ejection fraction decreased from 54 ± 2% to 47 ± 6% ( P  = 0.03) The hemodynamic changes were accompanied by increases in the hyperpolarized [1- 13 C]pyruvate MR derived ratios of lactate/alanine (from 0.58 ± 0.13 to 0.78 ± 0.06, P  = 0.03) and bicarbonate/alanine (from 0.55 ± 0.12 to 0.91 ± 0.14, P  = 0.007). Conclusion Glucose loading did not alter cardiac metabolism, but during acute hypertensive stress, cardiac aerobic, carbohydrate metabolism, and pyruvate-lactate exchange was altered. Hyperpolarized MR allows noninvasive evaluation of acute changes in cardiac metabolism. However, hemodynamics must be taken into account when interpreting the results.