Real-time multi-directional flow MRI using model-based reconstructions of undersampled radial FLASH - A feasibility study.

The purpose of this work was to develop an acquisition and reconstruction technique for two- and three-directional (2d and 3d) phase-contrast flow MRI in real time. A previous real-time MRI technique for one-directional (1d) through-plane flow was extended to 2d and 3d flow MRI by introducing in-plane flow sensitivity. The method employs highly undersampled radial FLASH sequences with sequential acquisitions of two or three flow-encoding datasets and one flow-compensated dataset. Echo times are minimized by merging the waveforms of flow-encoding and radial imaging gradients. For each velocity direction individually, model-based reconstructions by regularized nonlinear inversion jointly estimate an anatomical image, a set of coil sensitivities and a phase-contrast velocity map directly. The reconstructions take advantage of a dynamic phase reference obtained by interpolating consecutive flow-compensated acquisitions. Validations include pulsatile flow phantoms as well as in vivo studies of the human aorta at 3 T. The proposed method offers cross-sectional 2d and 3d flow MRI of the human aortic arch at 53 and 67 ms resolution, respectively, without ECG synchronization and during free breathing. The in-plane resolution was 1.5 × 1.5 mm2 and the slice thickness 6 mm. In conclusion, real-time multi-directional flow MRI offers new opportunities to study complex human blood flow without the risk of combining differential phase (i.e., velocity) information from multiple heartbeats as for ECG-gated data. The method would benefit from a further reduction of acquisition time and accelerated computing to allow for extended clinical trials.

Quantification of spinal cord compression using T1 mapping in patients with cervical spinal canal stenosis - Preliminary experience.

BACKGROUND: Degenerative changes of the cervical spinal column are the most common cause of spinal cord lesions in the elderly. Conventional clinical, electrophysiological and radiological diagnostics of spinal cord compression are often inconsistent. MATERIALS AND METHODS: The feasibility and diagnostic potential of a novel T1 mapping method at 0.5 mm resolution and 4 s acquisition time was evaluated in 14 patients with degenerative cervical spinal canal stenosis (SCS) and 6 healthy controls. T1 mapping was performed in axial sections of the stenosis as well as above and below. All subjects received standard T2-weighted MRI of the cervical spine (including SCS-grading 0-III), electrophysiological and clinical examinations. RESULTS: Patients revealed significantly decreased T1 relaxation times of the compressed spinal cord within the SCS (912 ±â€¯53 ms, mean ±â€¯standard deviation) in comparison to unaffected segments above (1027 ±â€¯39 ms, p < .001) and below (1056 ±â€¯93 ms, p < .001). There was no difference in mean T1 in unaffected segments in patients (p = .712) or between segments in controls (p = .443). Moreover, T1 values were significantly lower in grade II (881 ±â€¯46 ms, p = .005) than in grade I SCS (954 ±â€¯29 ms). Patients with central conduction deficit tended to have lower T1 values within the SCS than patients without (909 ±â€¯50 ms vs 968 ±â€¯7 ms, p = .069). CONCLUSION: Rapid high-resolution T1 mapping is a robust MRI method for quantifying spinal cord compression in patients with cervical SCS. It promises additional diagnostic insights and warrants more extended patient studies.

Carotid artery flow as determined by real-time phase-contrast flow MRI and neurovascular ultrasound: A comparative study of healthy subjects.

BACKGROUND: The assessment of carotid artery flow by neurovascular ultrasound (nvUS) can be complemented by real-time phase-contrast (RT-PC) flow MRI which apart from quantitative flow parameters offers velocity distributions across the entire vessel lumen. MATERIALS AND METHODS: The feasibility and diagnostic potential of RT-PC flow MRI was evaluated in 20 healthy volunteers in comparison to conventional nvUS. RT-PC flow MRI at 40 ms temporal resolution and 0.8 mm in-plane resolution resulted in velocity maps with low phase noise and high spatiotemporal accuracy by exploiting respective advances of a recent nonlinear inverse model-based reconstruction. Peak-systolic velocities (PSV), end-diastolic velocities (EDV), flow volumes and comprehensive velocity profiles were determined in the common, internal and external carotid artery on both sides. RESULTS: Flow characteristics such as pulsatility and individual abnormalities shown on nvUS could be reproduced and visualized in detail by RT-PC flow MRI. PSV to EDV differences revealed good agreement between both techniques, mean PSV and EDV were significantly lower and flow volumes were higher for MRI. CONCLUSION: Our findings suggest that RT-PC flow MRI adds to clinical diagnostics, e.g. by alterations of dynamic velocity distributions in patients with carotid stenosis. Lower PSV and EDV values than for nvUS mainly reflect the longer MRI acquisition time which attenuates short peak velocities, while higher flow volumes benefit from a proper assessment of the true vessel lumen.

An eigenvalue approach for the automatic scaling of unknowns in model-based reconstructions: Application to real-time phase-contrast flow MRI.

The purpose of this work is to develop an automatic method for the scaling of unknowns in model-based nonlinear inverse reconstructions and to evaluate its application to real-time phase-contrast (RT-PC) flow magnetic resonance imaging (MRI). Model-based MRI reconstructions of parametric maps which describe a physical or physiological function require the solution of a nonlinear inverse problem, because the list of unknowns in the extended MRI signal equation comprises multiple functional parameters and all coil sensitivity profiles. Iterative solutions therefore rely on an appropriate scaling of unknowns to numerically balance partial derivatives and regularization terms. The scaling of unknowns emerges as a self-adjoint and positive-definite matrix which is expressible by its maximal eigenvalue and solved by power iterations. The proposed method is applied to RT-PC flow MRI based on highly undersampled acquisitions. Experimental validations include numerical phantoms providing ground truth and a wide range of human studies in the ascending aorta, carotid arteries, deep veins during muscular exercise and cerebrospinal fluid during deep respiration. For RT-PC flow MRI, model-based reconstructions with automatic scaling not only offer velocity maps with high spatiotemporal acuity and much reduced phase noise, but also ensure fast convergence as well as accurate and precise velocities for all conditions tested, i.e. for different velocity ranges, vessel sizes and the simultaneous presence of signals with velocity aliasing. In summary, the proposed automatic scaling of unknowns in model-based MRI reconstructions yields quantitatively reliable velocities for RT-PC flow MRI in various experimental scenarios.

Rapid Diffusion-Weighted Magnetic Resonance Imaging of the Brain Without Susceptibility Artifacts: Single-Shot STEAM With Radial Undersampling and Iterative Reconstruction.

OBJECTIVE: The aim of this study was to develop a rapid diffusion-weighted (DW) magnetic resonance imaging (MRI) technique for whole-brain studies without susceptibility artifacts and measuring times below 3 minutes. MATERIALS AND METHODS: The proposed method combines a DW spin-echo module with a single-shot stimulated echo acquisition mode MRI sequence. Previous deficiencies in image quality due to limited signal-to-noise ratio are compensated for (1) by radial undersampling to enhance the flip angle and thus the signal strength of stimulated echoes; (2) by defining the image reconstruction as a nonlinear inverse problem, which is solved by the iteratively regularized Gauss-Newton method; and (3) by denoising with use of a modified nonlocal means filter. The method was implemented on a 3 T MRI system (64-channel head coil, 80 mT · m gradients) and evaluated for 10 healthy subjects and 2 patients with an ischemic lesion and epidermoid cyst, respectively. RESULTS: High-quality mean DW images of the entire brain were obtained by acquiring 1 non-DW image and 6 DW images with different diffusion directions at b = 1000 s · mm. The achievable resolution for a total measuring time of 84 seconds was 1.5 mm in plane with a section thickness of 4 mm (55 sections). A measuring time of 168 seconds allowed for an in-plane resolution of 1.25 mm and a section thickness of 3 mm (54 sections). Apparent diffusion coefficient values were in agreement with literature data. CONCLUSIONS: The proposed method for DW MRI offers immunity against susceptibility problems, high spatial resolution, adequate signal-to-noise ratio and clinically feasible scan times of less than 3 minutes for whole-brain studies. More extended clinical trials require accelerated computation and online reconstruction.

Model-based reconstruction for real-time phase-contrast flow MRI: Improved spatiotemporal accuracy.

PURPOSE: To develop a model-based reconstruction technique for real-time phase-contrast flow MRI with improved spatiotemporal accuracy in comparison to methods using phase differences of two separately reconstructed images with differential flow encodings. METHODS: The proposed method jointly computes a common image, a phase-contrast map, and a set of coil sensitivities from every pair of flow-compensated and flow-encoded datasets obtained by highly undersampled radial FLASH. Real-time acquisitions with five and seven radial spokes per image resulted in 25.6 and 35.7 ms measuring time per phase-contrast map, respectively. The signal model for phase-contrast flow MRI requires the solution of a nonlinear inverse problem, which is accomplished by an iteratively regularized Gauss-Newton method. Aspects of regularization and scaling are discussed. The model-based reconstruction was validated for a numerical and experimental flow phantom and applied to real-time phase-contrast MRI of the human aorta for 10 healthy subjects and 2 patients. RESULTS: Under all conditions, and compared with a previously developed real-time flow MRI method, the proposed method yields quantitatively accurate phase-contrast maps (i.e., flow velocities) with improved spatial acuity, reduced phase noise and reduced streaking artifacts. CONCLUSION: This novel model-based reconstruction technique may become a new tool for clinical flow MRI in real time. Magn Reson Med 77:1082-1093, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Advances in real-time phase-contrast flow MRI using asymmetric radial gradient echoes.

PURPOSE: To provide multidimensional velocity compensation for real-time phase-contrast flow MRI. METHODS: The proposed method introduces asymmetric gradient echoes for highly undersampled radial FLASH MRI with phase-sensitive image reconstruction by regularized nonlinear inversion (NLINV). Using an adapted gradient delay correction the resulting image quality was analyzed by simulations and experimentally validated at 3 Tesla. For real-time flow MRI the reduced gradient-echo timing allowed for the incorporation of velocity-compensating waveforms for all imaging gradients at even shorter repetition times. RESULTS: The results reveal a usable degree of 20% asymmetry. Real-time flow MRI with full velocity compensation eliminated signal void in a flow phantom, confirmed flow parameters in healthy subjects and demonstrated signal recovery and phase conservation in a patient with aortic valve insufficiency and stenosis. Exemplary protocols at 1.4-1.5 mm resolution and 6 mm slice thickness achieved total acquisition times of 33.3-35.7 ms for two images (7 spokes each) with and without flow-encoding gradient. CONCLUSION: Asymmetric gradient echoes were successfully implemented for highly undersampled radial trajectories. The resulting temporal gain offers full velocity compensation for real-time phase-contrast flow MRI which minimizes false-positive contributions from complex flow and further enhances the temporal resolution compared with acquisitions with symmetric echoes.

Peak flow velocities in the ascending aorta-real-time phase-contrast magnetic resonance imaging vs. cine magnetic resonance imaging and echocardiography.

This prospective study of eight healthy volunteers evaluates peak flow velocities (PFV) in the ascending aorta using real-time phase-contrast magnetic resonance imaging (MRI) in comparison to cine phase-contrast MRI and echocardiography. Flow measurements by echocardiography and cine phase-contrast MRI with breath-holding were performed according to clinical standards. Real-time phase-contrast MRI at 40 ms temporal resolution and 1.3 mm in-plane resolution was based on highly undersampled radial fast low-angle shot (FLASH) sequences with image reconstruction by regularized nonlinear inversion (NLINV). Evaluations focused on the determination of PFV. Linear regressions and Bland-Altman plots were used for comparisons of methods. When averaged across subjects, real-time phase-contrast MRI resulted in PFV of 120±20 cm s(-1) (mean ± SD) in comparison to 122±16 cm s(-1) for cine MRI and 124±20 cm s(-1) for echocardiography. The maximum deviations between real-time phase-contrast MRI and echocardiography ranged from -20 to +14 cm s(-1) (cine MRI: -10 to +12 cm s(-1)). Thus, in general, real-time phase-contrast MRI of cardiac outflow revealed quantitative agreement with cine MRI and echocardiography. The advantages of real-time MRI are measurements during free breathing and access to individual cardiac cycles.

Spatiotemporal phase unwrapping for real-time phase-contrast flow MRI.

PURPOSE: To develop and evaluate a practical phase unwrapping method for real-time phase-contrast flow MRI using temporal and spatial continuity. METHODS: Real-time phase-contrast MRI of through-plane flow was performed using highly undersampled radial FLASH with phase-sensitive reconstructions by regularized nonlinear inversion. Experiments involved flow in a phantom and the human aorta (10 healthy subjects) with and without phase wrapping for velocity encodings of 100 cm·s(-1) and 200 cm·s(-1) . Phase unwrapping was performed for each individual cardiac cycle and restricted to a region of interest automatically propagated to all time frames. The algorithm exploited temporal continuity in forward and backward direction for all pixels with a "continuous" representation of blood throughout the entire cardiac cycle (inner vessel lumen). Phase inconsistencies were corrected by a comparison with values from direct spatial neighbors. The latter approach was also applied to pixels exhibiting a discontinuous signal intensity time course due to movement-induced spatial displacements (peripheral vessel zone). RESULTS: Phantom and human flow MRI data were successfully unwrapped. When halving the velocity encoding, the velocity-to-noise ratio (VNR) increased by a factor of two. CONCLUSION: The proposed phase unwrapping method for real-time flow MRI allows for measurements with reduced velocity encoding and increased VNR.

The post-stimulation undershoot in BOLD fMRI of human brain is not caused by elevated cerebral blood volume.

Functional magnetic resonance imaging (fMRI) based on blood oxygenation level dependent (BOLD) contrast is the most widely used technique for imaging human brain function. However, the dynamic interplay of altered cerebral blood flow (CBF), cerebral blood volume (CBV), and oxidative metabolism (CMRO2) is not yet fully understood. One of the characteristics of the BOLD response is the post-stimulation undershoot, that is increased deoxyhemoglobin, which has been suggested to originate from a delayed recovery of elevated CBV or CMRO2 to baseline. To investigate the CBV contribution to the post-stimulation BOLD undershoot, we performed bolus-tracking experiments using a paramagnetic contrast agent in eight healthy subjects at 3 T. In an initial BOLD experiment without contrast agent, we determined the individual hemodynamic responsiveness. In two separate experiments, we then evaluated the relative CBV (rCBV) during visual stimulation and the post-stimulation undershoot, respectively. The results confirm a pronounced rCBV increase during stimulation (31.4+/-8.6%), but reveal no change in rCBV relative to baseline in the post-stimulation phase (0.7+/-7.2%). This finding renders a CBV contribution to the BOLD MRI undershoot unlikely and--in conjunction with a rapid post-stimulation return of CBF to baseline--supports the idea of a prolonged elevation of oxidative metabolism.