Cardiac Computed Tomography

Procedures

Image Acquisition for Coronary Artery Calcification Score

A CAC score is obtained using CCT through 3-mm axial slices focused on the cardiac region. The images are synchronized with the electrocardiogram (ECG) R-R interval, typically captured during late or mid-diastole using prospective ECG gating. No heart rate-lowering medication or intravenous cannulation is needed for CAC scoring. Significant calcification is defined as at least 1 mm of hyperattenuation in the coronary artery wall with either 3 or more pixels or greater than 130 Hounsfield units (HU).[3]

The extent of CAC is quantified using the Agatston score, which is calculated by multiplying the area of CAC by its density. The density factor depends on the calcium deposits' density in HU: 1 for 130 to 199 HU, 2 for 200 to 299 HU, 3 for 300 to 399 HU, and 4 for 400 and above HU. These definitions are used to quantify CAC and coronary plaque burden. The CAC score helps stratify patients; a score of 0 indicates no identifiable disease, 1 to 99 indicates mild disease, 100 to 399 signifies moderate disease, and a score greater than 400 implies severe CAD.

Preparation for Coronary Computed Tomography Angiogram

A 64-slice scanner or better is required. The ideal heart rate is at least 60 beats per minute. Intravenous access with an 18-gauge cannula should be established, preferably in the antecubital vein. Proper placement of ECG leads must be ensured. A contrast injection rate of 5 to 7 mL/s is used. A typical biphasic injection protocol involves 80 mL of contrast at a rate of 5 mL/s, followed by 40 mL of saline at the same rate. Prospective gating is preferred, allowing radiation exposure during a prespecified phase of the cardiac cycle, typically lasting 75% of the cardiac cycle.

Indications

CAC scoring may be used for the following indications:

• To help decide about statin therapy in patients with intermediate risk for atherosclerotic cardiovascular disease (ASCVD) when uncertainty exists regarding the need for statin use.
• To serve as a reliable prognostication tool for risk assessment in asymptomatic patients. CAC scoring outperforms other measures, such as high-sensitivity C-reactive protein, carotid intima-media thickness, and Framingham Risk Score. A CAC score of 0 indicates a low-risk profile, irrespective of the presence or absence of traditional risk factors, such as smoking, dyslipidemia, and hypertension.
• To reclassify the Framingham Risk Score.[4]

The following are the indications of CCT and CCTA, as per the Society of Cardiovascular Computed Tomography (SCCT) 2021 Expert Consensus Document:[5]

• Evaluation of patients with stable CAD with typical or atypical symptoms or angina-equivalent symptoms
• Evaluation of patients with stable CAD and a history of prior coronary revascularization, including percutaneous coronary intervention and coronary artery bypass grafting
• Functional assessment of intermediate-grade coronary artery stenosis using fractional flow reserve
• Assessment of CAD in miscellaneous conditions such as valvular heart disease, nonischemic cardiomyopathies, and congenital anomalies
• Assessment of chronic allograft vasculopathy
• Scar assessment in patients who have contraindications to cardiac magnetic resonance imaging (MRI)
• Risk assessment in asymptomatic patients using CAC scoring
• Risk assessment in special groups, including those with diabetes and a family history of CAD
• Accurate evaluation of periprosthetic windows, allowing optimal viewing to rule out a perivalvular thrombus or pannus [6][7]
• Assessment of intracardiac tumors, thrombi, and pericardial diseases
• Assessment of the great thoracic vessels

Normal and Critical Findings

Normal and critical findings include the following (see Image. Computed Tomography Coronary Angiogram):

• Normal plaque attenuation: calcium deposits of less than 130 HU
• Abnormal plaque attenuation:
  • Factor 1: calcium deposits of 130 to 199 HU
  • Factor 2: calcium deposits of 200 to 299 HU
  • Factor 3: calcium deposits of 300 to 399 HU
  • Factor 4: calcium deposits of 400 to 499 HU

The CAC score scale is:

• 0: No evidence of CAD
• 1 to 10: Minimal evidence of CAD
• 11 to 100: Mild evidence of CAD
• 101 to 400: Moderate evidence of CAD
• Greater than 400: Severe evidence of CAD [8]

Interfering Factors

The following factors can interfere with image acquisition and interpretation:

• Arrhythmia or heart rate of more than 70 bpm
• Severe asthma, contraindicating β-blocker intake
• Hypotension or phosphodiesterase inhibitor use, contraindicating nitrate administration
• Agitation or claustrophobia
• Failure to follow breath-holding instructions
• Serum creatinine greater than 1.8 mg/dL (in the context of CCTA)
• Acute decompensated heart failure

Patient Safety and Education

Patients are placed supine on the examination table. Electrodes are positioned on the right and left clavicles, as well as below the costal margin. These electrodes enable ECG gating, which enhances image accuracy by synchronizing imaging with the cardiac cycle. An ideal slower heart rate supports this synchronization, which is why preprocedural β-blocker administration is recommended.

The patient will receive instructions to minimize gross body movements and to hold their breath for a few seconds at a time. Failure to comply with these restrictions may impair the adequate delineation of cardiac structures. Additionally, severe agitation or claustrophobia can prevent a patient from undergoing CCT.

Clinical Significance

Cardiac imaging is a robust yet essential part of cardiovascular medicine. The development of magnetic resonance imaging (MRI), plain chest radiography, echocardiography, and CCT provides clinicians with many tools to evaluate cardiac pathology. Choosing the optimal modality has sometimes proven difficult, as all methods possess specific limitations and indications but share many overlapping characteristics.[9] Specifically, cardiac MRI and CCT or CCTA have developed into modalities of choice when small coronary and cardiac structures must be visualized, likely due to their unrivaled spatial resolution. The clinical uses of CCT are explained below.

Predicting Coronary Artery Disease-Related Events

CAC has been linked to atherosclerosis for decades. CCT provides clinicians with a faster, less cumbersome, more patient-safe method for risk stratification of asymptomatic high-risk patients for CAD. Computer software then evaluates the CAC volume and density, providing data points for prognostic tests, most notably the Agatston method, as discussed above.[10] A CAC score calculated per the Agatston method may be used in conjunction with an already approved stratification tool or alone as a future coronary heart disease (CHD) predictor. 

Risk stratification tools that may be used in conjunction with CAC scoring include:

• The Multiethnic Study of Atherosclerosis CAC (MESA CAC) has demonstrated the efficacy of CAC scores by incorporating individual CAC values into the study's demographic database. Including the CAC score in the cardiovascular event prediction model developed by MESA established this parameter as a dominant predictor of CHD and all-cause ASCVD outcomes.[11]
• The CAC Data and Reporting System (CAC-DRS) has been shown to provide better prognostic stratification regarding CHD, cardiovascular disease, and all-cause mortality than the Agatston reporting system alone. This risk stratification considers the CAC score as well as the number of coronary vessels involved.[12]
• Using the CAC score as a reclassification tool in the context of the Framingham Risk Score has shown benefits for intermediate-risk patients. Roughly 60% of atherosclerotic coronary events occur in low-to-intermediate-risk patients, according to the Framingham risk assessment. The addition of a patient's CAC score substantially increases the accuracy of this stratification.[13]

CAC risk stratification provides clinicians with data to support starting an asymptomatic patient on aspirin or statin across all 3 discussed methods.

CAC scoring may also be used as an independent predictor of future CAD-related events. According to the American College of Cardiology Foundation/American Heart Association (ACCF/AHA) consensus report from 2007, the following relative risk (RR) ratios for CAD-related events were calculated based on isolated CAC scores in asymptomatic patients:

• CAC scores of 100 to 400 have an RR of 4.3
• CAC scores of 401 to 999 have an RR of 7.2
• CAC scores greater than 1000 have an RR of 10.8 [14]

Predicting Coronary Artery Disease Degree of Stenosis

A recent study demonstrated the CAC score to accurately predict the degree of coronary arterial stenosis via histological evaluation of postmortem coronary arteries. This study showed CAC scoring to be an effective tool in determining a patient's degree of coronary arterial stenosis.[15]