Cardiac CT: New Hope for the Heart
Dr Angeline Poh, MBBS, MMed (Radiology), FRCR, Associate Consultant Department of Radiology, Changi General Hospital
End-stage coronary artery disease (CAD) is one of the leading causes of death in Singapore. Until recently, the only method of direct visualisation of the coronary arteries was with conventional catheter coronary angiography. Indeed, catheter angiography continues to remain the gold standard for the assessment of coronary artery stenosis. However, it is an invasive procedure and is associated with a small risk of complications. Up to two-thirds of catheter coronary angiography are performed for diagnostic purposes only. This has spurred the development of new non-invasive methods that can image the coronary arteries in patients who have equivocal signs and symptoms of coronary artery disease and in whom the risk of catheter angiography may not be justified.
Recent advancements in CT technology have enabled modern scanners to “freeze” the complex motion of the heart. It is now possible, in the span of a single breath-hold, to non-invasively obtain exquisite images of the heart and coronary arteries (Fig. 1-2). Thus, cardiac CT has the potential to revolutionize the prevention, diagnosis and treatment of CAD.
Cardiac CT comprises coronary CT angiography and coronary calcium scoring.
Coronary CT angiography (CCTA) is a useful diagnostic tool in the assessment of stable patients who have an intermediate pre-test probability of CAD and in whom an invasive procedure may not be ideal. The sensitivity and specificity of coronary CTA on a per-patient basis ranges from 90-95% and 83-86% respectively (Fig. 3). The greatest value of CCTA, however, is in its high negative predictive value. A patient with a normal study only has a 2-3% chance of having significant CAD. Moreover, CCTA is superior to all other non-invasive imaging modalities in the evaluation of structural coronary artery anomalies, such as an inter-arterial course of the right coronary artery (Fig. 4-5) and myocardial bridges, which have been known to be associated with sudden death in healthy individuals during exercise. CCTA has also been shown to be valuable in evaluating the position of existing coronary bypass grafts prior to cardiothoracic surgery and assessing in-stent stenosis.
The accuracy of
CCTA in evaluating the degree of
coronary arterial narrowing depends very much on the technical quality of the scan and the amount of calcified plaque present. Motion artifacts can occur in patients with very fast or irregular heart rates (such as
atrial fibrillation) and these can simulate or even obscure stenosis. In such patients, optimal heart rate control needs to be achieved with the administration of oral or IV beta-blockers. The presence of extensive calcified coronary plaque also precludes accurate assessment, as these cause “blooming artifacts” that tend to exaggerate the degree of narrowing. Obese patients are also challenging to scan as the images obtained in suchpatients tend to be “noisy” due to photon starvation. Thus, careful patient selection and meticulous technique are crucial in obtaining high-quality images that allow the most accurate evaluation.
Although CCTA is considered to be a safe procedure, like all CT studies that require intravenous iodinated contrast, it is associated with a small risk of contrast reaction (usually mild skin reactions in about 3-6% of patients). Patients are also exposed to radiation in the order of 8-13mSv, which is approximately the same or slightly more than that of a conventional coronary angiogram. As such, CCTA is not advisable in pregnant patients, in children or young patients. Most modern CT scanners however, have software that can reduce the radiation dosage by as much as 50%.
Coronary calcium scoring (CCS) is a screening tool that quantifies calcified plaque burden (Fig. 6). Together with other established risk factors such as age, gender and cholesterol levels, CCS can further assist in risk stratification and goal-directed management. Patients with a calcium score of zero are unlikely to have significant coronary artery disease and are at low-risk of developing a significant coronary event in the next 5 years. Conversely, patients with high calcium scores of greater than 500, have a high likelihood (>90%) of at least one significant coronary stenosis. Early studies evaluating the utility of following calcium scores of patients on statin therapy over time as a means of tracking treatment response have also shown promising results.
Cardiac CT is an ever-evolving modality with new applications being developed in tandom with advances in CT technology. The development of the dual-source CT scanner with improved the temporal resolution has allowed high-quality images of the coronary arteries to be obtained in patients with high heart rates without the use of beta-blockers. CT scanners that have 256 detectors and can image the heart in a single rotation are close to being approved for clinical use. It is possible in the future, for cardiac CT to be a fast, non-invasive “one-stop shop” for obtaining both functional and anatomical information from the heart, by correlating areas of decreased myocardial perfusion with segments of coronary arterial stenosis.
The Department of Radiology at CGH has two state-of-the-art, 64-multidetector CT scanners capable of performing cardiac CT. Studies can be scheduled by calling the Radiology department at 6850 4848 or via fax at 6260 2417.
References
- Raff GL, Gallagher MJ, O’Neill WW, Goldstein JA. Diagnostic accuracy of noslice spiral computed tomography. J Am Coll Cardiol. 2005; 46:552-7.
- 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; 114:1761-91.3.
- North 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. J Am Coll Radiol. 2006; 3:751-71.