Phase contrast (PC) magnetic resonance imaging with a three-dimensional, radially undersampled acquisition allows for the acquisition of high resolution angiograms and velocimetry in dramatically reduced scan times. However, such an acquisition is sensitive to blurring and artifacts from offresonance and trajectory errors. A dual-echo trajectory is proposed with a novel trajectory calibration from prescan data coupled with a multi-frequency reconstruction to correct for these errors. Comparisons of phantom data and in vivo results from volunteer, and patients with arteriovenous malformations patients are presented with and without these corrections and show significant improvement of image quality when both corrections are applied. The results demonstrate significantly improved visualization of vessels, allowing for highly accelerated PC acquisitions without sacrifice in image quality. Volumetric phase contrast (PC) MR imaging with velocity encoding in three spatial dimensions holds the potential to be a comprehensive vascular imaging method; providing both anatomical and quantitative velocity measurements, all without the use of a contrast agent. As a lumenographic imaging tool, it has been effectively used for the identification of aneurysms, arteriovenous malformations (1), and vascular stenoses (2) in the cerebrovascular system, great vessels, and renal arteries. Additional hemodynamic information can be obtained through postprocessing of the acquired anatomical and velocity data, providing either velocity visualization and/or quantitative hemodynamic analysis. Visualization of complex velocity fields can be performed by flow vectors, streamlines, and particle traces to visually identify pathologic flow patterns (3). Quantitative flow measurements can be accomplished retrospectively with oblique reformats, avoiding difficulties of prospectively targeted two-dimensional (2D) PC measurements. Hemodynamic measures such as wall sheer stress and relative pressure can be determined directly from the velocity data (4,5). However, despite the plethora of diagnostic measures available from 3D PC, its clinical use has been hindered by relatively lengthy imaging times and the occurrence of flow related artifacts.For 3D PC to become a viable clinical solution, the scan time for images of diagnostic resolution must be reduced. This has been achieved through protocol optimization for vascular territories with larger vessels (6), which usually still results in extended imaging times. Parallel imaging techniques (7,8) can be used in conjunction with optimized protocols, but generally only allow accelerations on the order of 2-4 and can lead to additional signal-to-noise ratio (SNR) degradation. In addition to these accelerated imaging approaches, non-Cartesian trajectories may be used for more efficient sampling schemes, accelerated imaging by undersampling, and the reduction of flow related artifacts.We have previously introduced Vastly undersampled Isotropic PRojection (VIPR) imaging (9), a 3D radial trajectory with angul...

Phase-contrast MR angiography with VIPR enables reliable measurements of TSPG in carotid and iliac lesions that are comparable to those obtained with endovascular pressure-sensing guidewires. However, further work to compensate for respiratory motion is required to extend this technique to the renal arteries.

Purpose:To prospectively compare a new three-dimensional (3D) radial phase-contrast magnetic resonance (MR) angiographic method with contrast material-enhanced MR angiography for anatomic assessment of the renal arteries. Materials and Methods:An institutional review board approved this prospective HIPAA-compliant study. Informed consent was obtained. Twenty-seven subjects (mean age, 52.6 years 6 20.5 [standard deviation]) were imaged with respiratory-gated phase-contrast vastly undersampled isotropic projection reconstruction (VIPR) prior to contrast-enhanced MR angiographic acquisition with a 3.0-T clinical system. The imaging duration for phase-contrast VIPR was 10 minutes and provided magnitude and complex difference ("angiographic") images with 3D volumetric (320 mm) coverage and isotropic high spatial resolution (1.25 mm 3 ). Quantitative analysis consisted of comparing vessel diameters between the two techniques. Qualitative assessment included evaluation of the phase-contrast VIPR and contrast-enhanced MR angiographic techniques for artifacts, noise, and image quality. Bland-Altman analysis was used for comparison of quantitative measurements, and the Wilcoxon signed rank test was used for comparison of qualitative scores. Results:Phase-contrast VIPR images were successfully acquired in all subjects. The vessel diameters measured with phasecontrast VIPR were slightly greater than those measured with contrast-enhanced MR angiography (mean bias = 0.09 mm). Differences in mean artifact, quality scores for the proximal renal arteries, and overall image quality scores between phase-contrast VIPR and contrast-enhanced MR angiographic techniques were not statistically signifi cant ( P = .31 and .29, .27 and .39, and .43 and .69 for readers 1 and 2, respectively). The quality scores for the segmental renal arteries were higher for phase-contrast VIPR than for contrast-enhanced MR angiography ( P , .05).Although the noise scores were higher with phase-contrast VIPR than with contrast-enhanced MR angiography and were statistically signifi cant ( P , .05), the presence of noise did not interfere with the ability to interpret the images. Conclusion:Isotropic, high-spatial-resolution, unenhanced MR angiography of the renal arteries is feasible with 3D radial undersampling.q RSNA, 2010 1

Purpose: To assess the feasibility and the quality of abdominal three-dimensional (3D) contrast enhanced MR angiograms acquired at 3.0 Tesla (T) using a new 2D-accelerated autocalibrating parallel reconstruction method for Cartesian sampling (2D-ARC). Materials and Methods:With institutional review board approval and written informed consent, a prospective trial in 6 normal healthy volunteers and 23 patients referred for evaluation of suspected renovascular disease was performed. The volunteers underwent abdominal MRA with and without 2D-ARC acceleration. Images were evaluated independently by two blinded vascular radiologists in randomized order. Vessel conspicuity was rated on a fivepoint scale. Evaluation for significant differences between the scores for each technique was performed using a Wilcoxon signed-rank test.Results: In the series of six volunteers, no statistical significance was found between the image quality scores for 2D-ARC accelerated and nonaccelerated exams. A high proportion of the 23 clinical 2D-ARC exams were graded as diagnostic (vessel conspicuity score !2; Reader 1, 96%; Reader 2, 100%) for overall image quality.Conclusion: Subjective image quality of 2D-ARC accelerated MRA was equivalent to the conventional MRA method. However, the 2D-ARC accelerated sequence provided a 3.5-fold increase in imaging volume, complete abdominal coverage, and a 30% reduction in voxel volume, all within the same acquisition time.

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Quantitative flow measurements with volumetric coverage and three directional flow encoding are technically feasible with magnetic resonance imaging yet prohibitively long in clinical settings. Data reconstruction from three dimensional angular undersampled MR acquisitions allows for dramatic reductions in scan time with tolerable imaging artifacts in many clinical applications. This approach provides high spatial resolution suitable for hemodynamic analysis in smaller vessels such as the renal artery, thereby providing additional crucial diagnostic information in a non invasive fashion. In an animal model, transstenotic pressure gradient measurements obtained with the novel acquisition scheme compared favorably with invasive intra arterial measurements (r=0.977; 95% CI: 0.931-0.998; p<0.001). In addition, human studies demonstrate the suitability of the technique for lumen measurements as an alternative for contrast enhanced MR Angiography and the associated risks with the use of an external contrast agent in certain patient populations.

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Purpose: To achieve three dimensional isotropic dynamic cardiac CT imaging with high temporal resolution for evaluation of cardiac function with a slowly rotating C‐arm system. In this work we propose an acquisition and image reconstruction framework which enables simultaneously high spatial resolution and high temporal resolution. Method and Materials: A recently introduced extension to compressed sensing in which a prior image is used as a constraint in the reconstruction has enabled this application. This new algorithm is referred to as Prior Image Constrained Compressed Sensing (PICCS). An in‐vivo animal experiment (e.g. a beagle model) was conducted using an interventional C‐arm system. The imaging protocol was as follows: contrast was injected, the contrast equilibrated, breathing was suspended for ∼14 seconds during which time 420 equally spaced projections were acquired. This data set was used to reconstruct a fully sampled blurred image volume using the conventional FDK algorithm (e.g. the prior image). Then the data set was retrospectively gated into 19 phases according to the recorded ECG signal (heart rate ∼ 95bpm) and images were reconstructed with the PICCS algorithm. Results: Cardiac MR was used as the gold standard due to its high temporal resolution. The same short‐axis slice was selected from the PICCS‐CT data set and the MR data set. Manual contouring on the peak systolic and peak diastolic frames was performed to assess the ejection fraction contribution from this single plane. The calculated ejection fractions with PICCS‐CT agreed well with the MR results. Conclusion: We have demonstrated the ability to use a slowly rotating interventional C‐arm system in order to make measurements of cardiac function. The new technique provides high isotropic spatial resolution (∼0.5 mm) along with high temporal resolution (∼ 33 ms). The evaluation of cardiac function demonstrated agreement with single slice cardiac MR.