Objective: The saturation phenomenon that limits vessel visibility in unenhanced time-of-flight MR angiograms can be overcome by the injection of MR contrast agents that shorten the T1 of the blood. Unlike the situation with unenhanced MR angiographic acquisitions, in which the MR parameters must be carefully adjusted to reduce saturation effects, optimization procedures for contrast-enhanced MR angiography focus on the acquired shortening of the T1 of the blood and the time evolution of the T1 over the entire measurement. To improve the quality of contrast-enhanced time-of-flight MR angiographic acquisitions in the brain, we compared different acquisition techniques that exploit shortening of the T1 of the blood.
Subjects and methods: High-resolution MR angiographic examinations were done with a 1-T MR system for 12 healthy volunteers. Doses of 0.1 and 0.2 mmol of gadopentetate dimeglumine per kilogram of body weight were injected on two separate days to allow for identical timing of the comparative studies after contrast material injection. The effects of dose of contrast material, MR parameters (TR and flip angle), method of injecting the contrast material (monophasic, biphasic, or continuous slow injection), and K-space acquisition schemes (regular schemes versus collecting the central K-space lines near the beginning or the end of the acquisition) were studied.
Results: Higher doses provided better MR angiograms. The flip angle and the TR had to be adjusted on the basis of the Ernst equation and therefore depended on an estimation of the averaged T1 value over the entire measurement. Comparison of the different K-space acquisition schemes confirmed that the evolution of the T1 of the blood over the entire measurement time is important, especially for longer measurements. There was no noticeable difference between the MR angiograms acquired with monophasic or biphasic injections. These injection schemes provided more detailed MR angiograms than did continuous slow injection over the entire measurement.
Conclusion: Our results show that the quality of contrast-enhanced MR angiograms can be remarkably increased when acquisition techniques that exploit the short T1 of the blood are used.