Cardiology

Other Imaging Modalities

General description of procedure, equipment, technique

Cardiac computed tomography (CT) is typically subdivided into two major tests: calcium scoring (noncontrast) and CT angiography (contrast). Calcium scoring is typically used for asymptomatic persons for risk stratification and to decide upon further therapies, such as statins, or the intensity of therapy.

Calcium scoring is also used for triaging the symptomatic person, as a zero calcium score is associated with a very low likelihood of obstructive coronary artery disease (CAD). CT angiography is typically used to assess symptomatic persons to ascertain whether they have significant atherosclerosis and/or obstructive CAD.

Cardiac CT scanners typically must have at least 64 detectors and electrocardiographic gating capabilities. Both calcium scoring, and cardiac CT angiography have been validated as methods to assess persons with congestive heart failure. More recently, cardiac CT has been increasingly used for assessment of perfusion, in trans-catheter aortic valve replacement, congenital heart disease, and as a functional assessment by use of calculated fractional flow reserve (FFRct).

Indications and patient selection

Calcium scoring is most commonly used in the asymptomatic person for risk stratification. The test, if negative, confers a very low risk state for the patient, often leading to lifestyle modification and less common use of statins or other cardiac tests.

If positive, it is graded; so a higher score (volume or Agatston units, which is area times density) would be associated with more aggressive lifestyle modifications and typically increasing doses of statins, antiplatelets, and tighter blood pressure control.

According to guidelines, the best use of the calcium score in asymptomatic persons is in Framingham intermediate or low-intermediate risk (class IIa recommendation).

National guidelines also advocate for use in asymptomatic persons with diabetes, as some have severe plaque, requiring much more aggressive therapies, and others have no or minimal plaque. Persons with diabetes also have a Class IIa recommendation for use of calcium scoring for risk stratification.

Most commonly, the Framingham or other risk scoring algorithms are applied first to asymptomatic persons, and if they have a 6% to 20% subsequent 10-year risk, they are candidates for calcium scoring for more accurate risk assessment.

A zero score confers a very low risk of subsequent cardiovascular events and can be used to de-risk patients, allowing them to avoid more aggressive medications and testing in many cases. Up to 50% of the general population has a calcium score of zero.

According to the 2013 ACC/AHA guidelines “assessing CAC is likely to be the most useful of the current approaches to improving risk assessment among individuals found to be at intermediate risk after formal risk assessment”.

Calcium scoring is also advocated for persons with symptoms in both national and international guidelines. This focuses on the subsequent very low probability (often <1%) of finding obstructive disease in persons with calcium scores of zero.

Typically, candidates would be persons with low pretest probability of obstructive disease (1% to 29% likelihood). This very high negative predictive power for obstructive CAD has led to the use of calcium scoring to differentiate types of congestive heart failure with calcium scoring.

It has been demonstrated to be more accurate than echocardiography for differentiating ischemic from dilated cardiomyopathy, with persons with zero scores being highly likely to have dilated cardiomyopathy and no obstructive coronary artery disease, while those with significant calcifications are more likely to have an ischemic cardiomyopathy (Figure 1).

Figure 1.

Patient with large amount of coronary calcium, consistent with an ischemic etiology of heart failure.

CT angiography

CT angiography is most commonly advocated in the setting of symptoms, where there is a question as to whether the chest pain (or abnormal stress test) is related to obstructive CAD. It is recommended as a Class IIa recommendation (helpful), both by national and international guidelines, for two types of patients: the stable chest pain patient and in the setting of acute chest pain syndromes.

For the stable patient with low to intermediate likelihood of obstructive CAD, CT angiography will be most useful, with higher accuracy, sensitivity, and specificity for obstructive coronary artery disease than stress testing, with or without imaging. For the patient presenting with acute chest pain syndromes, it is advocated, again as a Class IIa recommendation, for persons with chest pain with normal first cardiac enzymes and no acute changes on electrocardiograms to rule out obstructive disease.

Thus, given the advanced abilities to visualize the myocardium and lumen with contrast enhanced studies, this test has garnered significant use in patients with congestive heart failure (CHF) of unclear etiology, and has now been incorporated into the guidelines for cardiac CT as an appropriate filter for angiography.

In addition to seeing the lumen, one can visualize the myocardium (to see scar tissue associated with infarction (Figure 2), and measure volumes and ejection fraction with high accuracy (Figure 3). Multiple studies have documented the ability to use cardiac CT for viability, similar to MR evaluations in this context.

Figure 2.

Arrow represents scar identified in this patient with congestive heart failure, found to have an anterior MI.

Figure 3.

Measures of volumes (right and left ventricle) including myocaridal mass and volumes on cardiac CT.

Current evaluations are studying the potential use of cardiac CT for perfusion imaging to evaluate for reversible causes of CHF.

New indications include use for perfusion imaging, most typically using adenosine, to detect stress induced ischemia with high concordance to invasive angiography and sestamibi. Fractional flow reserve (FFRct) estimation with CT is also now available with very high concordance to invasive FFR.

Contraindications

Calcium scoring has no contraindications, other than morbid obesity (weight >400 lb). CT angiography has several contraindications, the most important being renal dysfunction and serious contrast allergies, as the test requires iodinated contrast (similar to invasive angiography or other contrast CT scans).

Other general contraindications for the CT angiogram are a heart rate and rhythm that can be slowed to 50 to 75 beats per minute, and weight <350 lb (for diagnostic images).

Details of how the procedure is performed

Calcium scoring is quite easy for the patient and takes typically 5 minutes. The patient lies on the CT table, has three electrodes attached for electrocardiogram (ECG) monitoring and the test is done with a 10-second breath hold.

There is no intravenous or invasive portion of the test. The CT angiogram adds an intravenous injection of contrast, and often heart rate slowing (use of oral and/or intravenous beta-blockers or calcium channel blockers). There is no specific aftercare; the intravenous catheter is removed and the patient can leave the scanner.

Interpretation of results

The calcium score is just a number, representing severity of plaque volume, from zero to thousands. A zero score implies no advanced plaque present; the higher the score, the higher the subsequent risk of future cardiovascular events.

A score >100 implies a 10-fold risk of future events, and a score >1,000 represents a 20-fold+ risk of events over the next 3 to 7 years.

CT angiography is not scored, but rather a description, most similar to an invasive angiogram. The plaque and stenosis is described most often as: not present, <30% stenosis, 30% to 50% stenosis, 50% to 70% stenosis, or severe (>70%) stenosis.

Performance characteristics of the procedure (applies only to diagnostic procedures)

The sensitivity of calcium scoring for obstructive disease is 95%, the specificity is around 45%. Thus, a negative test (score of zero) is most commonly associated with a normal angiogram and no plaque present, with negative predictive values of >95%.

When calcified plaque is present, it can be obstructive or nonobstructive, leading to a lower specificity for obstructive disease. However, according the ACC/AHA guidelines and many histology studies, the sensitivity of calcium scoring for atherosclerosis (plaque in coronaries) is at least 95% for most patients (age >40 especially) and specificity is 99.9%, as calcified plaque represents disease of the intima (atherosclerosis) in almost all cases.

The advantage of the test is that it is a better risk stratifier than Framingham or other risk calculators, carotid intimal-media thickness testing, C reactive protein, and other biomarkers studied to date. The disadvantage is that it is associated with ionizing radiation, about 1 millisievert, which is lower than annual background radiation and just about the same as a screening mammogram.

CT angiography has a much higher sensitivity and specificity, (at least 90% for both in most studies) and a negative predictive value of 99% in most studies, when evaluating for obstructive coronary disease. It has been compared prospectively to nuclear testing with higher diagnostic accuracies, better prognosis of events, and lower radiation exposures.

The disadvantages of CT angiography are that, while the machines are widely available, the required expertise for reading and performing the test is not as widely available, and the test requires 1 to 3 times more radiation than a calcium score, but less radiation than nuclear imaging or invasive angiography. It also uses iodinated contrast, so dye allergies and renal function are important considerations.

It is a considerably lower cost than either stress nuclear testing or invasive angiography, and can sometimes supplant those other tests, especially in the setting of acute chest pain, where probability of disease is lower and can subsequently avert invasive coronary angiograms.

Alternative and/or additional procedures to consider

There are many choices for evaluating coronary artery disease, but less for atherosclerosis. An alternative to coronary calcium scanning may be a carotid intimal media thickness test (ultrasound); however, the power for predicting CHD is far lower than coronary calcification scanning and it is not recommended by the ACC/AHA 2013 guidelines for screening (class III).

Alternatives to CT angiography (to evaluate the presence and severity of coronary stenosis) may include: functional testing or invasive angiography. Functional testing is less accurate to identify significant coronary obstruction than CTA, but affords important information about exercise capacity and functional significance of lesions.

Invasive angiography is the reference standard, but is more expensive, invasive, and associated with a measurable risk to the patients. Nondiagnostic CT angiograms are most commonly followed up with functional imaging. CT angiograms are nondiagnostic approximately 5% of the time, with newer equipment (64+ detector scanners).

Complications and their management

The most important aspect of CT angiography is patient selection. Patients with abnormal renal function (most commonly used is GFR <45), contrast allergies, or irregular heart rhythms (i.e., atrial fibrillation), are poor candidates for CT angiography. Calcium scanning is not an issue with any of the above issues, as no contrast is used for the study.

What’s the Evidence?

Greenland, P, Alpert, JR, Beller, GA. "2010 ACCF/AHA guideline for assessment of cardiovascular risk in asymptomatic adults". J Am Coll Cardiol. vol. 56. 2010. pp. 50-103.

(The guidelines for screening asymptomatic persons, recommending Framingham Risk Assessment, and coronary calcium scanning)

Budoff, MJ, Achenbach, S, Blumenthal, RS. "Assessment of coronary artery disease by cardiac computed tomography: A scientific statement from the American Heart Association". Circulation. vol. 114. 2006. pp. 1761-91.

(The most current scientific statement related to cardiac imaging, recommending both calcium scoring for asymptomatic persons and CT angiography for symptomatic persons)

"Chest pain of recent onset, assessment and diagnosis of recent onset chest pain or discomfort of suspected cardiac origin". National Institute for Health and Clinical Excellence (NICE). 2010. http://www.nice.org.uk/guidance/cg95.

(United Kingdom guidelines for assessing persons with chest pain; advocating for calcium scoring and CT angiography as first line tests in those with low pretest probability of obstructive CAD)

Mark, DB, Berman, DS, Budoff, MJ. "ACCF/ACR/AHA/NASCI/SAIP/SCAI/SCCT 2010 expert consensus document on coronary computed tomographic angiography". J Am Coll Cardiol. vol. 55. 2010. pp. 2663-99.

(The most recent expert consensus document advocating the use of CT angiography for certain indications, especially chest pain and abnormal stress tests)

Hamm, CW, Bassand, JP, Agewall, S. "ESC guidelines for the management of acute coronary syndromes in patients presenting without persistent ST-segment elevation". Eur Heart J. vol. 32. 2011. pp. 2999-3054.

(The latest guidelines from the European Society of Cardiology, advocating for CT angiography studies in lieu of invasive angiography for low risk patients presenting with acute chest pain)

Budoff, MJ, Shavelle, DM, Lamont, DH. "Usefulness of electron beam computed tomography scanning for distinguishing ischemic from nonischemic cardiomyopathy". JACC. vol. 32. 1998. pp. 1173-8.

(A large diagnostic study demonstrating the high sensitivity of calcium scoring for obstructive disease in persons with congestive heart failure)

Hamirani, YS, Isma'eel, H, Zeb, I. "Multidetector-row computed tomography in the evaluation of heart failure". US Cardiology. vol. 1. 2010. pp. 34-40.

(A review article discussing the data available related to cardiac CT and congestive heart failure)

Le, T, Ko, JY, Kim, HT. "Comparison of echocardiography and electron beam tomography in differentiating the etiology of heart failure". Clin Card. vol. 23. 2000. pp. 417-20.

(A study comparing the diagnostic accuracy of calcium scanning and echocardiography in differentiating heart failure etiology)

Danciu, SC, Herrera, CJ, Stecy, PJ. "Usefulness of multislice computed tomographic coronary angiography to identify patients with abnormal myocardial perfusion stress in whom diagnostic catheterization may be safely avoided". Am J Cardiol. vol. 100. 2007. pp. 1605-8.

(A study demonstrating the clinical utility of CT angiography in persons with abnormal nuclear scans)

Taylor, AJ, Cerqueira, M, Hodgson, JM. "2010 Appropriateness use criteria for cardiac computed tomography". J Am Coll Cardiol. vol. 56. 2010. pp. 1-31.

(Appropriate use criteria for cardiovascular computed tomography)

Nørgaard, BL, Hjort, J, Gaur, S. "Clinical Use of Coronary CTA–Derived FFR for Decision-Making in Stable CAD". J Am Coll Cardiol Img. 2016.

(A study demonstrating the clinical evidence and accuracy of FFRct.)

Valenti, V, Hartaigh, BO, Heo, R. "A 15-Year Warranty Period for Asymptomatic Individuals Without Coronary Artery Calcium: A Prospective Follow-Up of 9,715 Individuals". J Am Coll Cardiol Img. vol. 8. 2015. pp. 900-9.

(A large and long study demonstrating the low risk of a CAC score of zero.)
You must be a registered member of Clinical Advisor to post a comment.
close

Next Article in Cardiology

Sign Up for Free e-newsletters