Spin Dephasing on 3D Phase Contrast

On 3D phase contrast (3D PC) MRA with flow encoding in all three axes, image intensity corresponds to how fast the blood is flowing. Flowing

Echo For Renal Artery Stenosis

Fig. 20a, b. Hyper-concentration of urine (arrowin b) due to ipsilateral severe renal artery stenosis {arrowin a)

Fig. 20a, b. Hyper-concentration of urine (arrowin b) due to ipsilateral severe renal artery stenosis {arrowin a)

blood is bright while stationary tissues are dark. With mild stenoses, flow accelerates and creates a blooming effect, making the stenosis appear less severe (Fig. 19); when a stenosis reaches critical severity (> 70% narrowing), flow accelerated through the tight stenosis becomes disorganized, separated, swirling and turbulent. This chaotic, accelerated flow occurs when there is a pressure gradient [33]. Flow jets created by pressure gradients also dephase and destroy MR phase coherence causing loss of MR signal (Fig. 19). This dephasing is especially prominent on 3D PC MRA due to the relatively long echo times and the motion of protons during the application of flow-encoding gradients. On 3D PC images, the underestimation of mild stenoses and overestimation of severe stenoses (with pressure gradients) was formerly considered a disadvantage of flow sensitive MRA. However, by facilitating differentiation of unimportant mild stenoses from hemodynamically significant stenoses it can also be considered an advantage. An in-vitro study [33] shows that the degree of this spin dephasing is directly correlated with the trans-stenotic pressure gradient. This raises the exciting possibility of using the information on spin dephasing to estimate pressure gradients. More dephasing indicates a more significant stenosis. Combining 3D CE MRA information with 3D PC MRA offers more accurate grading of renal artery stenosis and facilitates clinical decision-making (Table 3).

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Essentials of Human Physiology

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