Vascular visualization on CE MRA relies on the concentration of Gd-chelate contrast agent within the vascular bed during image acquisition [4-8]. Ideally, imaging is performed during peak enhancement of the target vessel when overlapping structures and background tissue are not enhanced. For arterial depiction, this means synchronizing imaging for the period of preferential arterial enhancement when arterial Gd concentration is high and no significant venous or background enhancement has occurred (Fig. 2).
For CE MRA at 1.5 Tesla, the T1 shortening effects of Gd-chelate contrast agents can be estimated by the equation [5,40]:
1/T1 = 1/1200 + R1 [Gd] where T1 is the T1 value of contrast-enhanced blood, R1 is the field-dependent T1 relaxivity of the gadolinium chelate, and [Gd] is the concentration of gadolinium chelate. In this equation 1200 (msec) represents the T1 of blood at 1.5 Tesla in the absence of Gd contrast agent. Thus, increases in Gd concentration will result in shorter T1. It is the shortening of blood T1 by the arrival of Gd that results in preferential vascular visualization (i.e. vascular image contrast) on CE MRA (Fig. 3).
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