What are the indications, contraindications, typical scanning protocols, advantages, and disadvantages for coronary CTA versus magnetic resonance angiography (MRA)? Which patients can benefit from coronary CTA?
The largest group of referrals for coronary CTA is patients with atypical or low-risk chest pain. In these patients, a negative study prevents a chain of cardiac investigations such as stress tests and cardiac catheterization. These patients may present to the emergency department or may be scanned on an outpatient basis. Chest pain patients with a high risk of acute coronary syndrome are not typically evaluated by coronary CTA because the probability of requiring intervention is high enough to justify catheterization in many cases. Evaluation for aberrant coronary artery origin and course is also a good indication for coronary CTA, as is further evaluation of a patient with an equivocal stress test. In addition, evaluation of coronary bypass graft patency is a common indication. General contraindications for CTA are allergy to iodinated contrast media, renal insufficiency, and pregnancy; specific contraindications to coronary CTA would also include dysrhythmia and tachycardia, particularly if the patient is unable to receive beta-blockers because of a history of asthma or recent cocaine use.
The advantages of coronary CTA over coronary MRA include lower cost, ability to identify calcified versus noncalcified plaque, and higher spatial resolution which can be obtained in isotropic fashion and a much shorter scan time. The main disadvantages are the use of ionizing radiation and iodinated contrast material. A typical protocol for coronary CTA utilizes prospective ECG triggering during a dual-phase injection of 60 to 100 ml of iodinated contrast.
The most common indication for coronary MRA is for evaluation of coronary artery anomalies. The utility of coronary MRA in assessing coronary artery stenosis is limited. Although software techniques and hardware have substantially changed over the years, MRI systems have not been able to match results from the various generations of CT systems, except in cases with high coronary calcium score in which coronary CTA may be of limited use. On the other hand, coronary MRA has the advantage of no exposure to ionizing radiation or potentially nephrotoxic contrast agents. Contraindications to MRA include claustrophobia and presence of implanted medical devices such as pacemakers. Typically, coronary MRA is obtained during free breathing using respiratory navigator gating. Coronary MRA at 1.5 T is generally performed without gadolinium-based contrast material using a steady state free precession technique, while 3T protocols typically use a nonbalanced gradient echo technique after contrast administration.