Coronary CTA & Coronary Artery Calcium (CAC) Scoring

Although we have been performing coronary CT angiography (CTA) for several years, I recently attended an intense one-week course on coronary CTA in sunny and warm southern California. Unfortunately, I was inside 12 hours a day attending lectures, monitoring cases, and interpreting studies. On the bright side, this additional training was beneficial and fulfilled the requirements of the ACR and the ACC for level II coronary CTA certification.

Education and innovation are two core values at Parkside. Whether it is certification in coronary CTA, ACR accreditation, certification of additional qualification in Neuroradiology, or ARRS certification for our technologists,Parkside strives to prove our excellence in imaging and to reassure you and your patients of our dedication.

This Update will focus on coronary CTA including coronary calcium scoring. Other uses of cardiac CT, such as pulmonary vein mapping for radiofrequency ablation and assessment of congenital heart defects, are beyond the scope of this article. Parkside and I wish you and your family a happy Holiday Season.

William Okuno, MD
Medical Director, Parkside Diagnostic Imaging Centers

Click on any image below to see an enlargement of that image.

Figure 1. Curved reformat image of the proximal RCA.	Figure 2. Montage showing 3D volume rendered image and vessel analysis of a normal right coronary artery.
Figure 3: Curved reformat of a normal left anterior descending coronary artery.	Figure 4: Normal 3D volume rendered reconstruction showing normal left anterior descending (LAD) and first diagonal (1ST DIAG) coronary arteries. RV = right ventricle, LV = left ventricle, PA = pulmonary artery, AO = aorta.
Figure 5: 3D volume rendered reconstruction showing stenoses in the left anterior descending (LAD) and diagonal (DIAG) coronary arteries.	Figure 6: Curved reformat of the LAD shows the calcified stenoses.
Figure 7: Short axis view through the LAD shows the true severity of the stenosis.

All images are original CT scans performed at Parkside Imaging.

Coronary Artery Calcium (CAC) Scoring

As a stand alone examination, we have only two requirements for coronary artery calcium scoring: a physician order and a regular heart rate. Since contrast media are not administered, fasting, beta blockers, and renal function assessment are not required.

Coronary artery calcification is specific for coronary atherosclerosis. Although the likelihood of obstructive coronary artery disease (CAD) increases with the amount of CAC, the location of CAC is not directly correlated to the sites of significant angiographic stenoses. Tables exist to compare a patient’s total CAC score, and thus atherosclerotic burden, to age- and sex-matched cohorts.

The following are commonly accepted indications for CAC scoring:

1. Risk stratification in asymptomatic patients with an intermediate risk for coronary events based on traditional risk factor analysis. The coronary artery calcium score has independent and incremental prognostic value over the Framingham Risk Score (FRS) for the prediction of future coronary events(1). Since this incremental value is usually not enough to change the risk category and treatment of patients at low-risk or high-risk based on the FRS, CAC scoring is not indicated in these patients. However, about 40% of patients will have an intermediate FRS (10-20% 10-year risk) and the CAC score may change treatment by reclassifying these individuals into the low-risk or high-risk categories.

2. Patients with acute chest pain, no known coronary artery disease, negative initial cardiac enzyme studies, and normal or equivocal EKG. In this group of patients, a CAC score of zero effectively rules out acute myocardial infarction with a sensitivity and negative predictive value of 98-100%(2).

3. A filter before myocardial perfusion scintigraphy or coronary angiography in symptomatic patients. The sensitivity (85%) and specificity (75%) of CAC score for the detection of obstructive coronary artery disease is similar to exercise SPECT and stress echocardiography and the sensitivity is better than a stress treadmill study(3). Furthermore, when the CAC score is < 100 there is a very low probability (< 2%) of an abnormal nuclear stress test or a significant obstruction (>50% stenosis) on coronary angiography (< 1% probability)(3).

Imaging Centers

Coronary CTA
Our routine coronary CTA study is composed of two-parts and yields 3 types of studies. The first component is CAC scoring. Then IV contrast is administered and coronary CTA and cardiac functional analysis are performed.

Currently, the main goal of coronary CTA is to select patients for coronary angiography. A semi-quantitative stenosis grading system is utilized. Patients with no or mild (<30%) stenoses do not require further evaluation. In most cases, a moderate (<50%) stenosis does not require angiography. Coronary CTA showing moderate-severe (50-70%) or severe (>70%) stenosis should be confirmed with coronary angiography.

1. Coronary CTA study requirements and patient preparation
1. Patient should be NPO 6 hours prior to the study and have no caffeine for 12 hours prior to the study
2. A regular and slow heart rate is required
1. For optimal images, a regular heart rate less than 65 bpm is desired. If the patient has a heart rate less than 65, beta blocker is not required.
2. Beta blockers are given to all other patients without a contraindication (severe asthma, severe congestive heart failure, hypotension with SBP < 100 mmHg, or high grade AV conduction block). We recommend 50-100mg of oral metoprolol the night prior to the study and another 50-100mg 1 hour prior to the study. Patients with a pacemaker set at 70 or less should be beta blocked until pacemaker dependent.
3. No contraindication to iodinated contrast
1. Adequate renal function assessed in the preceding 3 months: Typically, a serum Cr < 1.5 mg/dl or GFR > 60 mL/min is required.
2. No known contrast allergy.
3. Diabetics taking metformin may take their medication on the morning of the study, but they should not take it for at least 48 hours after the study and have normal renal function prior to resuming it.
4. Physician order
2. Indications for coronary CTA
1. Symptomatic patients with an equivocal stress test. Coronary CTA has a high sensitivity and negative predictive value for significant obstructive coronary artery disease(3). Thus, a negative coronary CTA after an equivocal stress test eliminates the need for coronary angiography.
2. Filter for low risk patients prior to coronary angiography.
3. Known or suspected coronary artery anomalies. Coronary CTA is excellent for determining coronary artery anomalies(4).
4. Assessment of CABG patency and location. Coronary CTA has nearly 100% sensitivity for assessment of bypass graft patency(5). 3-D and 2-D reconstruction can show the relationship of these grafts to other mediastinal structures prior to a repeat CABG and can identify the grafts when the type of prior surgery is unknown. Of note, CTA has a limited role in assessing the native, often heavily calcified, coronary arteries.

References

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