Cardiac MRI

Non-invasive techniques excluding echocardiography

The ability to image the heart using non-invasive techniques such as MRI, CT and radionuclides has evolved rapidly over recent years.

Nuclear imaging

These techniques use radiotracers which are extracted by normal myocardium. Examples include:

• thallium
• technetium (99mTc) sestamibi: a coordination complex of the radioisotope technetium-99m with the ligand methoxyisobutyl isonitrile (MIBI), used in 'MIBI' or cardiac Single Photon Emission Computed Tomography (SPECT) scans
• fluorodeoxyglucose (FDG): used in Positron Emission Tomography (PET) scans

The primary role of SPECT is to assess myocardial perfusion and myocardial viability. Two sets of images are usually acquired. First the myocardium at rest followed by images of the myocardium during stress (either exercise or following adenosine / dipyridamole). By comparing the rest with stress images any areas of ischaemia can classified as reversible or fixed (e.g. Following a myocardial infarction). Cardiac PET is predominately a research tool at the current time

MUGA

• Multi Gated Acquisition Scan, also known as radionuclide angiography
• radionuclide (technetium-99m) is injected intravenously
• the patient is placed under a gamma camera
• may be performed as a stress test
• can accurately measure left ventricular ejection fraction. Typically used before and after cardiotoxic drugs are used

Cardiac Computed Tomography (CT)

Cardiac CT is useful for assessing suspected ischaemic heart disease, using two main methods:

• calcium score: there is known to be a correlation between the amount of atherosclerotic plaque calcium and the risk of future ischaemic events. Cardiac CT can quantify the amount of calcium producing a 'calcium score'
• contrast enhanced CT: allows visualisation of the coronary artery lumen

If these two techniques are combined cardiac CT has a very high negative predictive value for ischaemic heart disease.