Cardiac CT

Computed tomography of the heart or cardiac CT is routinely performed to gain knowledge about cardiac or coronary anatomy, to detect or diagnose coronary artery disease (CAD), to evaluate patency of coronary artery bypass grafts or implanted coronary stents or to evaluate volumetry and cardiac function (including ejection fraction). 

Indications

• noninvasive evaluation of coronary artery anomalies and other thoracic vessels
• symptomatic patients with low/moderate probability of coronary artery disease (CAD)
  • normal or uninterpretable/non-diagnostic ECG
  • normal or equivocal cardiac biomarkers
  • moderate risk non-acute symptomatic patients without known heart disease (may be able to exercise)
  • low risk non-acute symptomatic patients without known heart disease (if the patient cannot exercise or undergo stress test)
• evaluating the patency of a coronary artery bypass graft (CABG)
• new onset heart failure
  • no prior history of CAD, low/intermediate probability
  • decreased ejection fraction
• preoperative assessment of the coronary arteries before noncoronary cardiac surgery if intermediate risk of CAD
• discordant ECG and imaging results after stress imaging
• new or worsening symptoms with past normal stress imaging study
• preoperative assessment for transcatheter aortic valve implantation (TAVI/TAVR)

Its utility is uncertain in some situations:

• high probability coronary artery disease
  • including non-acute symptomatic patients without known heart disease
• acute chest pain of uncertain cause
• evaluation of coronary artery stents >3 mm

Contraindications

It is not indicated in some situations:

• if the patient is having an acute myocardial infarction (heart attack)
• screening of asymptomatic patients with low-to-intermediate risk of CAD
• evaluation of coronary artery stents <3 mm
• evaluation of asymptomatic patients post CABG (<5 years old) and post stent (<2 years old)

Coronary CT angiography protocols

Coronary CT angiography (cCTA) is performed as it contains data about coronary and cardiac anatomy. Due to recent innovations during the last two decades, new cCTA protocols allow for significant dose reductions with reported mean sub-milliSievert doses. However, each examination must be tailored to each patient depending on patient characteristics and clinical indication.

In contrast to other CT angiography examinations, cCTA usually requires a rather high flow rate for the contrast media injection, usually between 4-5 mL/sec through an antecubital vein. Beta-blockers and sublingual nitrates are usually administered before the examination to lower the heart rate, avoid arrhythmia and dilate the coronary arteries.

To allow for an improved image quality and dose reduction, cCTA is usually ECG-triggered to adapt the scan sequence to the patient's heartbeat.

There a number of different protocol based on patient presentation that can be broken into prospective and retrospective based:

• prospective
  • stable sinus rhythm and a heart rate of fewer than 60 beats a minute
    • prospective ECG-gated high-pitch dual-source protocol
  • stable sinus rhythm and a heart rate of fewer than 70 beats a minute
    • prospective ECG gated (step and shoot) protocol
• retrospective
  • atrial fibrillation/arrhythmia
    • retrospective ECG gated helical protocol

Further detail surrounding the benefits and pitfalls of these protocols is covered in our cardiac gating (CT) article.

Calcium scoring

When evaluating the heart for potential CAD, usually a nonenhanced calcium scoring sequence is first performed to assess for coronary artery calcification. This low-dose technique allows for a detection of calcifications of the coronary arteries. Although this technique does not give any information about potential hemodynamically relevant stenoses, an Agatston score can be calculated based on that data. The calculated Agatston score allows for an early risk stratification of patients with a high Agatston score (>160) have an increased risk for a major adverse cardiac event (MACE) ⁴.

Post-processing

Due to the usually tortuous anatomy of the coronary arteries, curved multiplanar reconstructions (MPR) or maximum intensity projections (MIP) are usually generated to allow for assessment of coronary lumina on a dedicated workstation. In these images, coronary stenoses can be evaluated and lumen reduction can be measured.

Increasingly, sophisticated algorithms and machine learning techniques enable the estimation of fractional flow reserve ⁶. 

Artifacts

Several artifacts can potentially occur which include ⁸

• motion artifact
  • cardiac motion
  • respiratory motion
  • gross patient motion
• partial volume averaging
• beam hardening
• metal or streak artifact
• quantum mottle
• slab or banding artifacts
• poor contrast enhancement
• partial scan artifact
• artifacts from overlapping structures

See also

• Coronary Artery Disease - Reporting and Data System (CAD-RADS)