Bolus Timing

In order to get maximum arterial Gd concentration during the center of k-space, perfect bolus timing is essential. There are several reliable and precise techniques available for determining contrast travel time from IV site to renal arteries in order to correctly coordinate the initiation of bolus injection with initiation of scanning. The most widely used techniques involve a test bolus to measure the contrast travel time, using an automatic pulse sequence that monitors signal in the aorta and then initiates imaging after contrast is detected arriving in the aorta (Fluoroscopic triggering, Bolustrak, Carebolus or MR SmartPrep). An alternative procedure that reduces the need for accurate bolus timing is time-resolved imaging. It is useful to ask the manufacturer of your equipment which technique works best. Time-resolved MRA with a temporal resolution of about 7 seconds per 3D acquisition and a single dose of contrast material has been reported to detect unilateral renal artery stenosis with sensitivity of 75% and specificity of 96% [14]. Time-resolved MRA may have less spatial resolution but it is particularly useful in high flow lesions such as arteriove-nous fistulae or renal vascular malformations. More sophisticated time-resolved techniques under development with oversampling of the center of k-space and sliding window reconstruction such as TRICKS (time-resolved imaging of contrast kinetics) potentially offer high temporal resolution without compromising spatial resolution.

Fig. 7a, b. CE renal MRA with single dose (0.1 mmol/kg) Gd-BOPTA (a) versus double dose (0.2 mmol/kg) of Gd-DTPA (b). Equivalent diagnostic information of renal artery stenosis (arrowin a) is obtained at half the dose [Images courtesy of Dr. G. Schneider]

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