The improved resolution of CTA over the last several years as well as faster imaging with the new 16- and 64-slice scanners has even made evaluation of the coronary arteries possible. The use of cardiac ECG gating during the study to reconstruct multiple sets of imaging data in different phases of the cardiac cycle is helpful to be able to visualize all segments of the coronary arteries. In addition, beta-blockers can be given to lower the heart rate and increase the diastolic proportion of the cardiac cycle, thereby minimizing misregistration artifacts due to cardiac motion. Although many challenges remain with regard to imaging the coronary arteries noninvasively, newer techniques may allow for CTA to be the initial mode of evaluation for suspected coronary artery disease.
There are a few disadvantages of CTA compared with ultrasound and MRI. CTA uses ionizing radiation unlike ultrasound or MRI. In addition, the contrast media used for CTAs can be harmful to the kidneys in patients with renal insufficiency. However, even with these few disadvantages, CT angiography is an extremely valuable tool for noninvasive imaging of the vascular system. As CT technology continues to evolve, the applications for CTA will continue to expand.
Like magnetic resonance imaging, magnetic resonance angiography (MRA) uses magnetic fields and radio waves to produce detailed images of the vascular system. An advantage of MRA over CTA is that it does not use ionizing radiation. It is also useful in patients who cannot tolerate the risks of iodinated contrast agents used in CTA or conventional angiography. MRA is also somewhat less operator dependent than ultrasound.
Many different techniques are used in MRA to provide the best possible images of the vascular system. Because of the multiple sequences that are employed in any single study, MRA can inherently characterize soft tissue. In addition, with the multiple data sets that are acquired, 3-dimensional imaging is possible and the data can be reformatted at any obliquity.
MRA is currently being used commonly to image the carotid vessels, the aorta and its branches as well as the peripheral arterial system. Contrast-enhanced MRA is accurate in detecting lesions at the carotid bifurcation as well as intracranially and can provide important information when considering surgical or endovascular treatment of a carotid stenosis with regard to anatomy as well as tandem lesions.