Accurate synchronization between the peak (gadolinium) and central k-space acquisition is essential for high image quality. “Best-guess” methods in which the operator estimates the contrast arrival time to the area of interest have been replaced by a test bolus to a region of interest (usually the mid aorta for peripheral MRA), automated software for bolus tracking, or semiautomated software that gives the operator a visual cue to begin acquisition. In the three-station bolus-chase technique, bolus timing is optimized for the first station (abdomen and pelvis), and subsequent imaging is performed as rapidly as possible to chase the flow of contrast agent along the peripheral vasculature. With this technique, optimal below-the-knee opacification can be challenging.
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A contrast bolus injected in an antecubital vein arrives in the common femoral artery in 24 ± 6 seconds, and in the popliteal artery after an additional 5 ± 2 seconds . In the calf, peak arterial and venous enhancement can be determined from time-resolved images that also show asymmetric arrival times between the legs . In case of asymmetric flow between the legs, the scanning delay time should be adjusted according to the symptomatic leg, usually the side with slower flow.
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