Key Words: noninvasive angiography; CTA; CT angiography; computed tomographic angiography; multidetector computed tomography; MDCT
Cardiac computed tomographic angiography (CT angiography) is increasingly being used clinically to define the coronary arteries noninvasively. Given that patients may undergo other tests, with significant radiation during nuclear imaging and contrast plus radiation with invasive angiography, choices for additional imaging must be made with caution. The recently published cardiac CT appropriateness criteria and the 2006 Scientific Statement on Cardiac CT both focus on the most common applications of cardiac CT (calcium scoring, CT angiography in a symptomatic patient). While the current appropriateness criteria clearly focus on the more common applications (intermediate risk for obstructive disease, equivocal stress testing or stress imaging, congestive heart failure, emergency department applications including pulmonary embolism and acute coronary syndrome evaluations), they neither preclude nor endorse use after invasive angiography. Dr. Hecht makes a compelling argument to perform CT angiography for certain indications after invasive angiography. While this seems counter-intuitive, as the “reference standard” (invasive angiography) has a higher spatial and temporal resolution than cardiac CT, there is rationale for certain indications explained in the article. To be fair to the authors of the appropriateness criteria, two of the indications Dr. Hecht discusses are covered by the appropriateness criteria. Anomalous coronary artery assessment, prior to or after angiography, is covered. This actually received the highest score possible (9 out of 9) for being an appropriate indication. Both the origin and the course of the vessel can often be better visualized with invasive angiography. For one national insurer (at the time of this article writing), CT angiography after nondiagnostic invasive angiography of coronary anomalies represented the only approved indication. Another indication that is covered in the appropriateness criteria is “noninvasive coronary arterial mapping, including internal mammary artery prior to repeat cardiac surgical revascularization.” This received a score of 8 out of 9, which is considered an appropriate indication. In patients undergoing repeat bypass surgery, the proximity of preexisting internal mammary and vein grafts to the sternum and the risk of surgical transection cannot be easily determined by conventional angiography. Another benefit of cardiac CT in this setting is the assessment of aortic atherosclerotic burden (there is a nine-fold increased risk of perioperative stroke in patients with large amounts of atheromatous disease of the ascending aorta). The surgeon can change their surgical approach and consider a more arterial-based revascularization to minimize stroke risk. The plaque burden of the aorta is not visualized during invasive angiography and some centers perform transesophageal echocardiography for this application, but this incompletely visualizes the aorta and the arch. CT angiography also allows simultaneous visualization of the right internal mammary artery, so the surgeon can visualize the adequacy of this conduit for revascularization. This vessel is usually not injected during invasive angiography. But perhaps the most important and most common findings raised by the author relate to the inability to selectively cannulate a native vessel or graft. This most likely occurs much more frequently than is acknowledged in the literature. At our center, we have identified 9 cases in which the invasive angiogram report described the graft as closed, but subsequent noninvasive CT angiogram visualized a patent graft. When a cardiologist cannot find an origin, they often report it closed rather than a nondiagnostic angiogram. Our center receives a few referrals for failure to cannulate or find a graft. Probably a more appropriate test would be the CT angiogram prior to the invasive angiogram for bypass patients, creating a roadmap for the invasive coronary angiogram. One can see the origins of the grafts, the relations to the native coronary arteries and the course of each graft. If the grafts are all patent, the physician may decide to forgo the more expensive and invasive coronary angiogram. Interestingly, CTA after bypass surgery to assess the grafts (not as a presurgical procedure) only received an appropriateness score of 6 (4–6 out of 9 represents an indeterminate indication according to the guidelines). The last few indications suggested by Dr. Hecht may provide the greatest benefit of all CT applications, yet remain unmentioned in the appropriateness criteria. If invasive angiography is unsafe (or incomplete), such as severe ostial disease or occurrence of severe pressure damping limiting safe angiography, then a noninvasive test may provide a safer evaluation of the artery in question. CT angiography does not require direct cannulation of the vessel in question, but rather nonselective injection in a vein. This allows for visualization of the coronaries even, when severe arterial or aortic disease precludes safe visualization. While many people equate CT and MR as two noninvasive techniques for visualization of the coronary arteries, they are significantly different. The spatial resolution is inferior with MR angiography and the appropriateness criteria give MR angiography post bypass a score of 2 (inappropriate), while giving CT angiography either a 6 or 8, depending on the clinical situation. Ultimately, even in Dr. Hecht’s paper, 99% of patients were done pre-angiography (or in lieu of angiography), so these “after invasive angiogram” indications will remain a small, albeit important, category of “appropriate” uses for cardiac CT angiography. References 1. Hendel RC, Patel MR, Kramer CM, et al. ACCF/ACR/SCCT/SCMR-/ASNC/NASCI/SCAI/SIR 2006 Appropriateness criteria for cardiac computed tomography and cardiac magnetic resonance imaging: A report of the American College of Cardiology Foundation Quality Strategic Directions Committee Appropriateness Criteria Working Group, American College of Radiology, Society of Cardiovascular Computed Tomography, Society for Cardiovascular Magnetic Resonance, American Society of Nuclear Cardiology, North American Society for Cardiac Imaging, Society for Cardiovascular and Interventions, and Society of Interventional Radiology. J Am Coll Cardiol 2006;48:1475–1497. 2. Budoff MJ, Achenbach S, Blumenthal RS, et al. Assessment of coronary artery disease by cardiac computed tomography, A scientific statement from the American Heart Association Committee on Cardiovascular Imaging and Intervention, Council on Cardiovascular Radiology and Intervention, and Committee on Cardiac Imaging, Council on Clinical Cardiology. Circulation 2006;11:1761–1791. 3. Hecht H. Indications for multi-detector computed tomographic coronary angiography after catheter-based coronary angiography. J Invasive Cardiol 2008;20:XXX–XXX.