Contrast Administration

CE MRA has traditionally been performed using one of the commercially available extracellular Gd-chelate contrast agents. The rate of injection affects the peak Gd concentration and thus the achievable arterial SNR. Generally, a faster injection rate will result in higher arterial SNR, but shorter bolus duration and earlier venous enhancement. Injection rates of 2 mL/sec are ideal for most CE MRA applications, with little benefit shown for higher rates [63]. Slower injection rates result in lower overall Gd concentrations, but extend the plateau phase of arterial enhancement and delay venous enhancement. Slower injection rates (e.g. 0.8-1.0 mL/sec [31, 32]) have generally been used for multi-station CE MRA in order to prolong the arterial phase and delay venous enhancement, both of which are desirable to ensure adequate arterial signal for multi-station coverage. Recently, biphasic (e.g. 0.8 mL/sec followed by 0.4 mL/sec (Fig. 17); 1.5 mL/sec followed by 0.5 mL/sec [38] or 2.0 mL/sec followed by 1.2 mL/sec [39]) injection protocols have become popular for multi-station peripheral bolus chase CE MRA examinations. The higher initial injection rate provides a greater early Gd concentration for improved visualization of the abdominal aorta, which may be enlarged by aneurysm and require more Gd for proper visualization. Biphasic injections may also be useful for imaging patients requiring assessment of regional venous structures, such as patients undergoing pre-operative planning for renal cell carcinoma removal. In these patients, the arterial supply as well as possible venous invasion by the renal carcinoma are important findings, especially given the current trend towards lapro-scopic or other less invasive interventions. The use of a biphasic injection for CE MRA helps provide proper arterial depiction and adequate venous signal for proper determination of potential venous invasion by the renal cell carcinoma. Similar techniques may be useful for assessment of the portal vein or central venous system.

For most CE MRA examinations, a dose of 0.150.2 mmol/kg (typically 20 - 30 mL) is sufficient. If timing is good, lower doses (0.1 mmol/kg) have also been found to be adequate [22]. In general, the use of a larger contrast agent dose has the benefit of prolonging the arterial phase and providing the operator with an additional buffer to compensate for errors in timing. For novice practitioners, it is suggested that they begin with an 0.2 mmol/kg dose, and then reduce the dose based on their mastery of timing for CE MRA. Unfortunately, aside from increased cost, larger contrast agent doses have the unwanted effect of increasing the likelihood of venous contamination and diminishing operator options should additional CE MRA or repeat acquisitions be desired during the same exam period. Hany et al [62] have estimated that a dose of at least 0.12 mmol/kg of Gd-chelate contrast agent is required for diagnostic CE MRA. This dose is very close to the "single dose" (single 20 mL vial) suggested by Rofsky et al [29] for CE MRA.

More recently, a new gadolinium contrast agent (gadobenate dimeglumine, Gd-BOPTA, Multi-Hance; Bracco Imaging SpA, Milan, Italy [64-67]) has become available in Europe and numerous other countries around the world. Compared with the gadolinium chelates traditionally used for CE MRA, this agent has improved T1 relaxivity due to weak interactions of the Gd-BOPTA chelate with serum proteins such as albumin. Knopp et al [64] have shown that the signal intensity increase with Gd-BOPTA is as much as 50% higher than with conventional gadolinium contrast agents at the same dose and injection rate making this agent particularly suitable for CE MRA. The relative benefits of Gd-BOPTA are more apparent in smaller vessels, which are better visualized using Gd-BOP-TA than with traditional extracellular contrast agents [66, 67].

One final consideration for contrast administration is that of saline flush [68]. In practice, a large saline flush (at least 30 mL) should be used for all CE MRA examinations. A large flush will ensure that the entire contrast dose is administered beyond the tubing and that the bolus will travel through the peripheral veins into the right heart, ensuring sufficient Gd concentrations are delivered to the more distal arteries. The use of a large flush increases the slope of the enhancement

Fig. 17a-c. Multi-station 3D CE MRA (a, coronal large field of view MIP of overlapping three station exam) in a patient with severe peripheral vascular occlusive disease. Occlusions of both superficial femoral arteries (arrows) are clearly seen on the 3D MRA of the second station (b, coronal MIP). On this multi-station exam, distal run-off vessels were also well visualized (c, oblique MIP). Contrast was administered using a biphasic injection scheme whereby the initial half of the contrast dose was injected at 0.8 mL/sec and the remaining dose at 0.4 mL/sec curve, increases the duration of the arterial phase of the bolus (up to 50% [68]), and delays significant venous enhancement, all of which are preferable for arterial CE MRA.

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