Comparison to other modalities

Stress Echocardiography

Conventional indications for stress echocardiography (SE) are similar to those for a nuclear scan. This modality permits the assessment of ischaemia (the development of a new wall motion abnormality or worsening of an existing one at high doses of stress) and viability (low dose augmentation of function at a low dose in a previously impaired segment of myocardium).

The main stressors used in echocardiography are exercise (treadmill) and intravenous dobutamine. The former avoids the need for a cannula or drug infusion and is preferred in patients able to exercise. Acoustic windows must be identified shortly after the patient transfers from treadmill to couch since the effects of exercise wane within minutes.

For those that can't exercise, IV dobutamine is given in a stepwise protocol with additional atropine for patients who do not reach target heart rate. Stored loops are evaluated in a side-by-side quad screen layout showing images at baselines, low dose, high dose and peak stress. Image interpretation has been facilitated by scanner developments and the introduction of trans-pulmonary contrast, which opacifies the left ventricular cavity and improves endocardial border definition. Three-dimensional echocardiography offers the possibility of a single heart-beat entire volume acquisition during stress which may be most useful for patients with atrial fibrillation.

Cardiac Magnetic Resonance

Cardiac Magnetic Resonance Imaging (CMR) has a number of applications in coronary disease with elements borrowed from other modalities. The main drawbacks are its cost, limited availability and exclusion of certain patient groups (claustrophobics, implantable devices).

Beyond its excellent spatial resolution - which makes it the reference standard for volumetric and mass assessment - CMR’s main strength lies in tissue characterisation. Following its intravenous administration, gadolinium-chelate contrast enters the myocardium. In areas of healthy tissue it leaks out at a predictable rate. When the tissue is infarcted, the extracellular area is expanded and cellular integrity lost which encourages contrast retention. Following the administration of a presaturation pulse, delayed imaging defines normal myocardium as black and infarcted tissue as white.

This technique permits the degree of transmurality to be defined which is inversely related to that segment’s likelihood of recovery post-revascularisation. If early enhancement imaging is performed, cardiac thrombus can be identified. Earlier still, at the time of contrast administration, it is possible with the addition of adenosine to image myocardial perfusion in the same way as a nuclear scan with regional hypointensity reflecting stenosis of the responsible epicardial artery. Just as in stress echocardiography, dobutamine wall motion imaging can be performed with the additional benefit of improved endocardial border definition. This may be most valuable in patients with poor acoustic windows. Visualisation of the coronary arteries by CMR has been a long-held goal but remains in development.

Cardiac Computed Tomography

Cardiac Computed Tomography (CCT) is becoming an increasingly important modality in the non-invasive assessment of coronary disease. In its nascent form, coronary calcium could be measured which served as a surrogate for the total atherosclerotic plaque burden and hence the likelihood of future ischaemic events. Subsequent refinements have allowed for direct visualisation of the coronary tree with CT angiography.

Significant stenoses as well as plaque characteristics can be imaged, though sensitivity is lost in heavily calcified stretches of vessel. Although CT permits the adequacy of both angioplasty and surgical revascularisation to be assessed in patients with recurrent symptoms, it is most widely recommended for use in low risk chest pain patients where its very high negative predictive value becomes key.

Newer developments include the assessment of infarct and perfusion imaging. Concerns over CT's radiation continue unabated but efforts are being made to reduce exposure through changes to scan techniques. Although in the past it was difficult to acquire good images in patients with fast or erratic heart rates, the advent of single heart beat acquisitions should circumvent this problem.