Mechanisms of flow‐induced signal loss in MR angiography

Abstract

Mechanisms of signal loss in magnetic resonance angiography were studied with a stenotic flow phantom. The results indicate that while signal loss induced by mean fluid motions is localized about the stenosis, the fluctuating component of fluid motion induces signal loss over a much larger region, primarily distal to the stenosis. For both motion components, use of gradient moment nulling (GMN) above first order was found to be an ineffective means of reducing signal loss. In contrast, shortened gradient durations were found to reduce signal loss substantially. However, though a zeroth-order gradient is generally of the shortest duration, use of a slightly longer, first-order gradient was found to be the most robust means of reducing signal loss.

Magnetic Resonance Angiography

“…34 This phenomenon is encountered within the decelerating jet distal to a critical lesion. The tendency to overestimate stenosis severity is greatly reduced if one interprets the study from source images or reformations rather than from calculated projections.…”

Improved Visualization of Intracranial Vessels by Gradient Moment Nulling in Hybrid of Opposite-contrast Magnetic Resonance Angiography (HOP MRA)

“…7 Spins in a voxel at bifurcation or bending portions of vessels are likely to have greater components of incoherent motion. 8,9 In the ‰ow dephasing gradient, b-values at zero frequency (k＝0) for 2-and 3-axis GMN sequences were almost the same and below 0.01 s/mm 2 , whereas those at the maximum frequency (k＝128) were 0.05 and 0.1 s/mm 2 . In the present study, we examined the images obtained when we applied GMN to 2 and 3 axes in phantom and clinical studies.…”

“…As a result, the ‰ow void seemed to be more prominent in the sequence with GMN applied to 3 axes than to 2 axes. 8,9 The calculation of b-value showed that dispersion in the phase encoding on the 3-axis GMN sequence was twice that on the 2-axis GMN sequence. Therefore, the signal magnitude of such voxels on the 3-axis GMN was reduced to a greater extent than on the 2-axis GMN sequence.…”

Magnetic Resonance Signal Loss in Turbulent Shear Flow

“…The effect of turbulence on MR images were in vestigated by Evans et al [12], Urchuk et al [13] and Oshinski et al [14] It was found that the decrease of signal intensity due to turbulence was negligible in fully developed pipe flow except at a very high Rey nolds number [12]; however, a dark region distal to a stenosis was observed for the Reynolds number as low as 600. Note that flow compensations in both sliceselective and readout directions were used in the ex periments.…”