Predictive values of spinal cord diffusion magnetic resonance imaging to characterize outcomes after contusion injury.

OBJECTIVES: To explore filtered diffusion-weighted imaging (fDWI), in comparison with conventional magnetic resonance imaging (MRI) and diffusion tensor imaging (DTI), as a predictor for long-term locomotor and urodynamic (UD) outcomes in Yucatan minipig model of spinal cord injury (SCI). Additionally, electrical conductivity of neural tissue using D-waves above and below the injury was measured to assess correlations between fDWI and D-waves data. METHODS: Eleven minipigs with contusion SCI at T8-T10 level underwent MRI at 3T 4 h. post-SCI. Parameters extracted from region of interest analysis included Daxial from fDWI at injury site, fractional anisotropy and radial diffusivity from DTI above the injury site along with measures of edema length and cord width at injury site from T2 -weighted images. Locomotor recovery was assessed pre- and weekly post-SCI through porcine thoracic injury behavior scale (PTIBS) and UD were performed pre- and at 12 weeks of SCI. D-waves latency and amplitude differences were recorded before and immediately after SCI. RESULTS: Two groups of pigs were found based on the PTIBS at week 12 (p < 0.0001) post-SCI and were labeled "poor" and "good" recovery. D-waves amplitude decreased below injury and increased above injury. UD outcomes pre/post SCI changed significantly. Conventional MRI metrics from T2 -weighted images were significantly correlated with diffusion MRI metrics. Daxial at injury epicenter was diminished by over 50% shortly after SCI, and it differentiated between good and poor locomotor recovery and UD outcomes. INTERPRETATION: Similar to small animal studies, fDWI from acute imaging after SCI is a promising predictor for functional outcomes in large animals.

Dual-Venc acquisition for 4D flow MRI in aortic stenosis with spiral readouts.

BACKGROUND: Single Venc 4D flow MRI with Cartesian readout is hampered by poor velocity resolution and noise when imaging during diastole. Dual Venc acquisitions typically require the acquisition of two distinct datasets, which leads to longer scan times. PURPOSE/HYPOTHESIS: To design and develop a 4D Spiral Dual Venc sequence. The sequence allows for separate systolic and diastolic Venc s as part of a single acquisition with a prescribed switch time. The implemented sequence was hypothesized to be comparable to Cartesian 4D flow, but with increased velocity resolution in the diastolic phase and with better scan efficiency and reduced noise. STUDY TYPE: Prospective. POPULATION: The studied populations were two phantoms-a straight pipe with a stenotic narrowing and a phantom of the aortic arch which included a calcific polymeric valve-under both steady and pulsatile flows, six healthy volunteers, and eight patients with severe aortic stenosis (AS). FIELD STRENGTH/SEQUENCE: 1.5T, Dual Venc 4D flow with spiral readouts. ASSESSMENT: Data from the proposed sequence were compared with data from 4D Cartesian Dual Venc and Single Venc acquisitions. Noise was assessed from the acquired velocity data with the pump turned off and by varying Venc . Steady acquisitions were compared to the proximal slice of the lowest Single Venc acquisition. STATISTICAL TESTS: Steady flows were compared using relative-root-mean-squared-error (RRMSE). For in vivo flows and pulsatile in vitro flows, net flow for corresponding timepoints were compared with the Pearson correlation test (P < 0.01). RESULTS: For steady flows, RRMSEs for Single Venc s ranged from 17.6% to 19.4%, and 9.6% to 16.5% for Dual Venc s. The net flow correlation coefficient for the aortic arch phantom was 0.975, and 0.995 for the stenotic phantom. Normal volunteer and patient comparisons yielded a correlation of 0.970 and 0.952, respectively. in vitro and in vivo pulsatile flow waveforms closely matched. DATA CONCLUSION: The Dual Venc offers improved noise properties and velocity resolution, while the spiral trajectory offers a scan efficient acquisition with short echo time yielding reduced flow artifacts. LEVEL OF EVIDENCE: 2 Technical Efficacy Stage: 1 J. Magn. Reson. Imaging 2020;52:117-128.

4D Dual-Venc Spiral Flow.

Dual-Venc flow acquisition sequences perform flow imaging with differing Vencs. The technique can be used to improve velocity to noise ratio and image quality for diastolic flow velocities as part of a single scan. In this paper, Dual-Venc was used in conjunction with spiral read-out trajectories, offering a faster coverage of k-space. The results illustrate that 4D Dual Venc Spiral Flow behaves similarly to 4D Dual-Venc Cartesian Flow but with the benefit of faster acquisition time and lower echo time (TE).

4D spiral imaging of flows in stenotic phantoms and subjects with aortic stenosis.

PURPOSE: The utility of four-dimensional (4D) spiral flow in imaging of stenotic flows in both phantoms and human subjects with aortic stenosis is investigated. METHODS: The method performs 4D flow acquisitions through a stack of interleaved spiral k-space readouts. Relative to conventional 4D flow, which performs Cartesian readout, the method has reduced echo time. Thus, reduced flow artifacts are observed when imaging high-speed stenotic flows. Four-dimensional spiral flow also provides significant savings in scan times relative to conventional 4D flow. RESULTS: In vitro experiments were performed under both steady and pulsatile flows in a phantom model of severe stenosis (one inch diameter at the inlet, with 87% area reduction at the throat of the stenosis) while imaging a 6-cm axial extent of the phantom, which included the Gaussian-shaped stenotic narrowing. In all cases, gradient strength and slew rate for standard clinical acquisitions, and identical field of view and resolution were used. For low steady flow rates, quantitative and qualitative results showed a similar level of accuracy between 4D spiral flow (echo time [TE] = 2 ms, scan time = 40 s) and conventional 4D flow (TE = 3.6 ms, scan time = 1:01 min). However, in the case of high steady flow rates, 4D spiral flow (TE = 1.57 ms, scan time = 38 s) showed better visualization and accuracy as compared to conventional 4D flow (TE = 3.2 ms, scan time = 51 s). At low pulsatile flow rates, a good agreement was observed between 4D spiral flow (TE = 2 ms, scan time = 10:26 min) and conventional 4D flow (TE = 3.6 ms, scan time = 14:20 min). However, in the case of high flow-rate pulsatile flows, 4D spiral flow (TE = 1.57 ms, scan time = 10:26 min) demonstrated better visualization as compared to conventional 4D flow (TE = 3.2 ms, scan time = 14:20 min). The feasibility of 4D spiral flow was also investigated in five normal volunteers and four subjects with mild-to-moderate aortic stenosis. The approach achieved TE = 1.68 ms and scan time = 3:44 min. The conventional sequence achieved TE = 2.9 ms and scan time = 5:23 min. In subjects with aortic stenosis, we also compared both MRI methods with Doppler ultrasound (US) in the measurement of peak velocity, time to peak systolic velocity, and eject time. Bland-Altman analysis revealed that, when comparing peak velocities, the discrepancy between Doppler US and 4D spiral flow was significantly less than the discrepancy between Doppler and 4D Cartesian flow (2.75 cm/s vs. 10.25 cm/s), whereas the two MR methods were comparable (-5.75 s vs. -6 s) for time to peak. However, for the estimation of eject time, relative to Doppler US, the discrepancy for 4D conventional flow was smaller than that of 4D spiral flow (-16.25 s vs. -20 s). CONCLUSION: Relative to conventional 4D flow, 4D spiral flow achieves substantial reductions in both the TE and scan times; therefore, utility for it should be sought in a variety of in vivo and complex flow imaging applications.

Noninvasive 3D pressure calculation from PC-MRI via non-iterative harmonics-based orthogonal projection: constant flow experiment.

Use of phase-contrast (PC) MRI in assessment of hemodynamics has significant clinical importance. In this paper we develop a novel approach to determination of hemodynamic pressures. 3D gradients of pressure obtained from Navier-Stokes equation are expanded into a series of orthogonal basis functions, and are subsequently projected onto an integrable subspace. Before the projection step however, a scheme is devised to eliminate the discontinuity at the vessel and image boundaries. In terms of the computation time, the proposed approach significantly improves on previous iterative methods for pressure calculations. The method has been validated using computational fluid dynamic simulations and in-vitro MRI studies of stenotic flows.

A biventricular multimodal (MRI/ultrasound) cardiac phantom.

A cardiac phantom can be of crucial importance in the development and validation of ultrasound and cardiac magnetic resonance (MR) imaging and image analysis methods. A biventricular multimodal cardiac phantom has been manufactured in-house that can simulate normal and pathologic hearts with different degrees of infarction. The two-chamber structure can simulate the asymmetric left ventricular motion. Poly Vinyl Alcohol (PVA) is utilized as the basic material since it can simulate the shape, elasticity, and MR and ultrasound properties of the heart. The cardiac shape is simulated using a two-chamber acrylic mold. An additional pathologic heart phantom has been built to simulate aneurysm and infarction. Segmental dyskinesis is modeled based on three inclusions of different shapes and different degrees of elasticity. The cardiac elasticity is adjusted based on freeze-thaw cycles of the PVA cryogel for normal and scarred regions.

Rapid flow quantification in iliac arteries with spiral phase-contrast MRI.

Phase contrast MRI is a powerful tool for blood flow quantification. Conventional cartesian phase contrast sequences require lengthy acquisition on the order of several minutes. Spiral acquisition phase-contrast (PC) MRI is capable of reducing the TR and TE in order to minimize flow dependent artifacts and total imaging time. Despite this, in general, spiral phase contrast sequences suffer from off-resonance artifacts and inconsistent data artifacts. In this work, we show that short interleaved spiral readout trajectories have the capability to obtain high spatio-temporal resolution flow images in the common iliac artery distal to the aortoiliac bifurcation with little or no artifacts and with significant savings in image acquisition time over the Cartesian trajectory. To verify the accuracy, we compare our results with a Conventional cartesian trajectory.