Pulse, ECG, and blood pressure are monitored throughout the imaging process. Initially, a non-contrast scan is performed for coronary artery calcium (CAC) scoring. This is followed by angiography using a bolus of iodinated contrast. At this time, multislice cardiac scanning is synchronized with an ECG (i.e., gating the scan) and images are obtained during diastole.
Two types of CT are capable of obtaining the necessary high-speed images. Electron-beam CT (EBCT) and helical or spiral CT scanning. Multidetector helical CT (MDCT) or multislice CT (MSCT) is an evolution of helical CT. MDCT is available in four-, eight-, 16-, 32-, or 64-slice. Interpretation of the imaging uses the American College of Cardiology nomenclature: no stenosis, insignificant stenosis (less than 50% of coronary luminal diameter), significant stenosis (more than 50%), and total occlusion.
click for large version
Figure 1. Noninvasive Visualization of Coronary Artery Anatomy by 64-slice Multislice Computed Tomography.
The presence of CAC has been shown in numerous studies to have prognostic significance for cardiac events and death. An elevated CAC score more accurately predicts coronary heart disease (CHD) risk in adults with an initial Framingham risk score between 10% and 19%. In addition, compared with a CAC score of zero, a score of more than 300 was predictive of CHD risk.7
Some studies suggest negative CAC scores alone are enough to risk-stratify patients presenting with chest pain, reporting sensitivities between 96% and 100% for the detection of significant cardiac events.8
Clinical studies of CT angiography typically consist of patients already selected to receive traditional invasive coronary angiography. Initial reports by Leschka, et al., showed sensitivity and specificity of 94% and 97%, respectively, with positive predictive values and negative predictive values of 87% and 99%.9
Additionally, a meta-analysis revealed multislice computed tomography (MSCT) for the diagnosis of CHD was highly sensitive (95% to 100%) for the detection of significant CHD. Specificity was 100% with the 64-slice MSCT and only slightly lower with the four- and 16-slice scanners (87%). A positive predictive value of 66% and negative predictive value of 98% for the presence of significant coronary stenosis have also been reported.10
Gallagher, et al., and previous studies comparing CT angiography with traditional invasive coronary angiography suggest further testing, such as stress nuclear imaging for any patient with stenosis more than 25% or CCS of greater than 100 and further recommends cardiac catheterization of any patient with greater than 70% stenosis by MSCT.
Projections demonstrate severe soft plaque of the LAD.
Figure 1. Noninvasive Visualization of Coronary Artery Anatomy by 64-slice Multislice Computed Tomography.
The utility of CT coronary angiography in the evaluation of patients presenting to the ED with chest pain was studied by Goldstein and peers.11 A randomized trial was conducted with patients admitted with chest pain or anginal equivalent, considered to be at low risk for CAD. Patients with negative cardiac enzymes and non-diagnostic ECG were assigned to either MSCCT or myocardial perfusion scan. CT angiography was adequate to diagnose or exclude CAD as the cause of chest pain in 75% of patients. The remaining 25% of patients had additional nuclear stress imaging. Average cost of the MDCT cohort was $1,586 compared with $1,872 for standard diagnostic evaluation. The time required for diagnosis was significantly shorter in the CT cohort: 3.4 hours, compared with 15 hours in the standard group.
An ongoing study at Beaumont Hospital in Royal Oak, Mich., is evaluating the length of stay and cost of care in patients presenting to the ED with chest pain. Patients are randomized to standard evaluation with serial enzymes, ECG, and nuclear perfusion testing or 64-slice CT angiography, plus coronary calcium score. Preliminary results presented at the 2006 American College of Cardiology Annual Scientific Session regarding the initial 200 patients showed a shorter length of stay and lower cost of care with similar patient outcomes in the group that underwent CT angiography.12