Contrast protocol assists 3D clarity in MR angiography
In MR angiography (MRA) of peripheral arteries, injecting contrast medium at a slow, balanced rate can alleviate contamination of veins in the calves while maintaining image quality, according to a multinational team of researchers from China and the United States. Their study, published this month in the American Journal of Roentgenology, suggests that it is possible to increase arterial visibility in the calf while signal-to-noise and contrast-to-noise ratios at higher levels are maintained.

A 46-year-old man with Fontaine stage III peripheral arterial occlusive disease on right side and Fontaine stage II disease on left. Injection protocol was 2 mL of contrast agent and 1 mL of saline solution at 1 mL/s. MR angiogram shows veins enhanced in calves. Arterial visibility score is 2 on right side and 3 on left. Venous contamination score is 3 on both sides. Calf contrast-to-noise ratio is 26.6 on right side and 27.8 on left. Decrease in signal intensity in femoral artery may be due to dielectric effect. Image and caption courtesy of the American Roentgen Ray Society.  
“An optimal vascular MRI examination should be performed after the contrast medium arrives in the target artery but before it dissipates into the veins,” the authors wrote. “In many patients, however, optimal imaging is not feasible because the MRI unit cannot match the rate of flow of the gadolinium bolus through the calves.”

A team from the Beijing Anzhen Hospital, Capital Medical University in China, and Johns Hopkins University School of Medicine in Baltimore sought a protocol that would balance venous contamination and arterial signal intensity in 3T MRA exams.

“Although there may be a difference between the times of bolus arrival and peak enhancement, which depends on different methods of measurement, it is clear that contrast medium travels faster than the MRI table,” they wrote. “The key point of our technique is to prolong the arterial to venous transition because short transit times result in venous signal intensity in the calf.”

They examined 80 consecutively registered patients with symptomatic peripheral arterial occlusive disease, who underwent a bolus-chase three-level MRA of the abdominal and lower-limb arteries. The patients were randomly and evenly split into two cohorts, Group A and Group B.

Both groups underwent their MRA on a 3T system (Signa Excite, GE Healthcare) using similar imaging parameters. However, contrast was administered at different levels and intervals for each group.

“In group A, the test bolus was 2 mL of contrast agent (Magnevist, Bayer Schering) administered at 2 mL/s followed by 20 mL of saline solution administered at 1 mL/s,” the authors wrote. “In group B, the test bolus was 2 mL of contrast agent administered at 1.2 mL/s followed by 20 mL of saline solution administered at 0.7 mL/s,” the authors wrote.

From the beginning of contrast injection, complete arterial phase image acquisition lasted approximately 100 seconds, according to the research team. Group A’s injection duration was 38 seconds, while Group B’s injection duration was 57 seconds. The scientists reported no statistical difference between the two groups in signal-to-noise ratio (SNR) or in contrast-to-noise (CNR) ratio.

All acquired images from the MRA exams were processed and reconstructed on a dedicated advanced visualization workstation (Advantage, GE) using the same software iteration (4.2). The researchers found that the venous contamination score was higher in Group A than in Group B and that the arterial visibility score was higher in Group B than in Group A.

“With a 3T system, use of a slow injection rate can alleviate venous contamination and increase arterial CNR in the calf without sacrificing SNR and CNR at the two upper levels,” they wrote. “As such, high magnetic field strength unconventionally contributes to the image quality of 3D bolus-chase peripheral MR angiography in a simple but technically valid approach.”