Angina module 5: advanced cardiac imaging

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Introduction

Guidance from the National Institute for Health and Clinical Excellence (NICE) covers the investigation of chest pain of recent onset.1 In people without confirmed coronary artery disease (CAD), in whom stable angina cannot be diagnosed or excluded based on clinical assessment alone, it recommends that we estimate the likelihood of CAD based on age, the type of chest pain, and the presence or absence of risk factors (table 1). We can then determine whether an individual is at low or high risk.

NICE advises on the most appropriate form of diagnostic tests to apply. If people have features of typical angina based on clinical assessment and their estimated likelihood of CAD is greater than 90% (table 1), further diagnostic investigation is unnecessary. These should be managed for angina.

Table 1. Percentage of people estimated to have coronary artery disease according to typicality of symptoms, age, sex and risk factors

Findings from the clinical assessment and the resting 12-lead electrocardiogram (ECG) are taken into account when making the estimate, and further diagnostic testing is advised as follows:

  • If the estimated likelihood of CAD is 10–29%, offer computed tomography (CT) calcium scoring as the first-line diagnostic investigation
  • If the estimated likelihood of CAD is 30–60%, offer functional imaging as the first-line diagnostic investigation e.g. myocardial perfusion scintigraphy (MPS), magnetic resonance imaging (MRI), stress echocardiography
  • If the estimated likelihood of CAD is 61–90%, offer invasive coronary angiography as the first-line diagnostic investigation if appropriate
Figure 1. Non-contrast coronary calcium CT imaging showing no calcium deposits with coronary calcium score of zero

This module describes these imaging techniques in more detail.  NICE does not recommend the use of exercise ECG to diagnose or exclude stable angina for people without known CAD.

Patients with chest pain and a low (<30%) probability of angiographically significant CAD

Calcium and coronary artery imaging by computed tomography

In the assessment of patients with symptoms of obstructive CAD, coronary calcium imaging with CT is indicated after functional tests are either not possible or inconclusive. Current NICE guidelines,1 however, recommend coronary calcium imaging as a first-line test in symptomatic patients with a low (<30%) pre-test probability of CAD. Calcium deposits within the coronary arterial wall correlate with the presence of atherosclerosis and hence represent a highly sensitive marker for CAD.

Figure 2. Significant coronary calcification in the left coronary artery. The coronary calcium score was estimated at 485 for the entire left coronary system (square). No calcium was identified in the right coronary artery

Coronary calcification can be quantified using non-enhanced CT imaging (figure 1). Calcium causes hyperattenuation of X-rays; pixels with a density above a pre-defined threshold of 130 Hounsfield Units (HU) are assumed to represent calcium deposits (figure 2). Areas within the coronary arteries with >130HU are selected and added up to estimate the individual’s coronary calcium score (CCS) or Agatston Score.

Coronary calcification not only reveals the presence of atherosclerosis but also predicts a worse outcome. It is possible to risk-stratify patients according to the total amount of calcium present in their coronary arteries. Hence, patients with a CCS of zero are deemed at very low risk of cardiac events (<1% risk of myocardial infarction [MI] or cardiac death/year; figure 1) while patients with CCS ≥1 have a higher risk. As the calcium score increases so does the risk of future adverse events (figure 3).2

Patients with CCS ≥1 are considered to have CAD and hence further assessment is recommended. NICE advocates the use of CT angiography (CTA) as second-line test in patients with CCS ≥1 but ≤400. CTA allows visualisation of the coronary vasculature and detection of coronary luminal narrowing with the use of contrast-enhanced imaging (see below). Patients with CCS >400 should be considered for functional imaging (discussed later) or invasive X-ray coronary angiography.

Figure 3. Risk stratification for adverse cardiac events according to the presence and extent of coronary calcification

CTA offers the best resolution to date and hence it is the preferred imaging technique for the non-invasive assessment of the coronary arteries. CTA is performed by injecting an intravenous iodine-based contrast agent to enhance visualisation of the coronary arterial wall. Most studies are currently performed using multi-detector CT systems.

Image acquisition is synchronised to the patient’s ECG and images obtained during a pre-selected phase of the cardiac cycle (prospective gating) or throughout the cardiac cycle and then retrospectively aligned to the ECG tracing (retrospective gating). The former method reduces radiation exposure. The acquired data is reconstructed into a stack of transaxial slices that can be processed further using different post-processing methods (figure 4).

Figure 4. Panel A shows three-dimensional volume-rendered images from a normal CT angiography study in a 44-year-old man with atypical chest pain; panel B shows curved multiplanar reformat images from the same study showing unobstructed coronary arteries