3 The technology

3 The technology

3.1 MPS involves the intravenous injection of small amounts of a radioactive tracer to evaluate perfusion of living cardiac muscle via the coronary arteries after stress and at rest. After injection, the tracer is taken up by cardiac muscle cells, and its distribution within the myocardium is imaged using a gamma camera. Three tracers are commercially available in the UK: thallium-201 thallous chloride, technetium-99m 2-methoxy-isobutyl-isonitrile, and technetium-99m 1,2-bis(bis[2-ethoxyethyl]phosphino)ethane. MPS is a non-invasive procedure which provides more detailed information about coronary function than sECG and CA. Cardiovascular stress can be induced by exercise as in sECG, but is most commonly induced by pharmacological agents.

3.2 MPS was originally developed as a planar imaging technique, but SPECT is the clinical standard in current practice. In SPECT, the camera rotates around the patient for 10–20 minutes and the raw data are processed to obtain tomographic images of the myocardium. The stress and rest images are normally separated by 3–4 hours. The total patient contact time for stress induction, injection and image acquisition is approximately 60 minutes.

3.3 Homogeneous uptake of tracer throughout the myocardium indicates the absence of clinically significant infarction or coronary stenosis. A defect in the stress images that normalises in the rest images usually corresponds to a significant coronary stenosis. A defect in both stress and rest images indicates an area with loss of viable myocardium, such as after MI.

3.4 Two technical improvements to SPECT were also considered in this appraisal. Attenuation-corrected SPECT compensates for the fact that many emitted photons never reach the detector as a result of interactions with body tissues. ECG-gated SPECT is synchronised with the patient's ECG, thereby minimising artefacts caused by cardiac motion. Also, left ventricular ejection fraction can be measured at rest with ECG-gated SPECT.

3.5 The complication rates for SPECT are no different from those of sECG, and are usually related to exercise or pharmacological stimulation given as part of the stress component in the procedure, with an associated mortality of around 0.01% and a morbidity of around 0.02%. The radiation exposure from SPECT is similar to the exposure from uncomplicated CA.

3.6 The cost of a SPECT scan is estimated to be around £265, whereas the costs for sECG and CA are £104 and £1103, respectively (2002 NHS reference costs).