This work addresses the fundamental limits imposed by the MRI process on the accuracy with which vessel diameters and cross-sectional areas can be derived from time-of-flight (TOF) and phase-contrast (PC) MR source images. By means of simulations and in vitro experiments, it is demonstrated that, even in the absence of flow-related artifacts, severe inaccuracies in the determination of diameters or cross-sectional areas may occur solely because of the physical process of the MR image acquisition. Resolution and intraluminal saturation have strong effects on the vessel appearance and thus on the diameter estimation error. It is shown that low resolution leads to diameter overestimation or even underestimation and that intraluminal saturation causes severe underestimation, even for relatively low flip angles. Velocity and velocity encoding do not have a major influence on lumen appearance in PC images. Accurate diameter estimations can be attained only if lumen diameters constitute at least three pixels for both TOF and PC acquisitions, provided that intraluminal saturation is suppressed or avoided. Additionally, since the constitution of TOF and PC images is dissimilar, lumina should be analyzed differently to obtain accurate diameters and cross-sectional areas.
This paper reports the development of dedicated catheters and real-time MR scan techniques for guiding vascular interventional procedures. By way of phantom experiments, it is shown that proper modification of the magnetic properties of catheters allows their conspicuous and consistent visualization in subsecond 2D gradient echo images and phase contrast angiograms. Dynamic scan times as low as 0.5 s could be achieved by exploiting the keyhole technique and purposeful postprocessing. The temporal resolution and spatial resolution of the resultant scan protocol shows promise for adequate tracking of catheter manipulation.
Contrast material-enhanced three-dimensional (3D) magnetic resonance (MR) angiography of the supraaortic arteries with randomly segmented central k-space ordering (ie, contrast-enhanced timing-robust angiography [CENTRA]) was performed in 16 patients. CENTRA enabled reliable depiction of the aortic arch up to the circle of Willis at high spatial resolution (true voxel size, 0.81 x 0.81 x 1.0 mm(3)). With CENTRA, the divergent demands of high spatial resolution, wide anatomic coverage, and arterial phase imaging have been reconciled. The random order of central k-space acquisition may minimize artifacts in contrast-enhanced 3D MR angiography caused by unstable contrast material opacification at the initiation of sampling.
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