Nuclear imaging techniques are primarily used in ischaemic heart disease. Myocardial structure and function can be assessed by radionuclide-irnaging techniques. Thallium-20l imaging This is used to detect myocardial ischaemia and infarction. Thallium, which behaves lilee potassium, is talcen up by healthy myocardium. Ischaemia or infarction produces a nuclear image with a ‘cold’ spot. The isotope is usually administered during exercise and an image is talcen soon after the exercise. Three or four hours later the heart is scanned again to obtain a redistribution image. The disappearance of the cold spot on the redistribution image implies ischaemia provoked by exertion and reversed by rest, whereas a persistent cold spot indicates infarction.
Pyrophosphate labelled with technetium-99m concentrates in bone and acutely infarcted myocardium. The isotope should be injected intravenously between 1 and 5 days (best on the second or third day) following a myocardial infarction. Imaging a few hours later detects a ‘hot spot’ in the region of the infarction. Scans are difficult to interpret because calcified costal cartilage and breast tissue may both concentrate the isotope. On the other hand, infarction may not result in the expected hot spot because the isotope is prevented from reaching the infarct owing to complete occlusion of coronary vessels to the infarcted myocardium. False-positive scans may occur in unstable angina.
Two methods are used to obtain blood pool images:
1 A MUGA (multigated acquisition) or equilibrium image is obtained by intravenous injection of technetium- 99m which attaches to the patient’s own red cells in vivo and which is therefore retained in the vascular space. Over 200 heart beats are imaged. Comparison of the study with the ECG allows systolic and diastolic points of the cycle to be identified.
2 A first-pass study images the heart as a bolus of isotope makes a single pass through the circulation. These techniques are complementary, but both outline the cardiac chambers, particularly the left ventricle, by imaging the isotope within the central circulation during systole and diastole. The percentage of the left ventricular volume ejected with each systole (the ejection fraction) can be accurately measured, and any section of the left ventricular wall that contracts abnormally (a wall motion defect) can be visualized. Radionuclide studies may be performed at rest and during exercise to assess changes in cardiac function. A deterioration on exercise suggests coronary disease or myocardial abnormality.
Cardiac catheterization is the introduction of a thin radiopaque tube (catheter) into the circulation.
The right heart is catheterized by introducing the catheter into a peripheral vein and advancing it through the right atrium and ventricle into the pulmonary artery. The left heart is reached by way of a peripheral artery. The catheter is manipulated through the aortic valve into the left ventricle.
The pressures in the right heart chambers, left ventricle, aorta and pulmonary artery can be measured directly. An indirect measure of left atrial pressure can be obtained by ‘wedging’ a catheter into the distal pulmonary artery. In this position the pressure from the right ventricle is obstructed by the catheter and only the pulmonary venous and left atrial pressures are recorded. Pressure measurements are used to quantify stenoses or measure contractile function. During cardiac catheterization, blood samples may be withdrawn to measure the concentration of ischaemic metabolites, e.g. lactate, and the oxygen content. These estimations are used to gauge ischaemia, quantify intracardiac shunts, and measure cardiac output.
Contrast cine-angiograms are also taken during catheterization. Radio-opaque contrast material is injected into the cardiac chambers, arterial trunks or coronary arteries.shows a normal angiogram compared with angiograms showing an aneurysm and cardiomyopathy.
Digital subtraction angiography
This technique permits the injection of small volumes of radiocontrast agents during cardiac catheterization with the production of computer-analysed high-quality angiograms. Unfortunately, peripheral injection of contrast does not give adequate visualization of the coronary arteries, but aortic lesions can be visualized.
CT scanning is particularly useful for showing the size and shape of the cardiac chambers as well as the thoracic aorta and mediastinum.
Magnetic resonance imaging (MRI)
MRI is a non-invasive imaging technique which does not involve harmful radiation. A powerful magnetic field is used to line up the protons in the hydrogen atoms of the body, each of which can be thought of as a tiny magnet. A radio frequency emission distorts this line-up, but when the radio waves are turned off, the atoms return to their previous position and give off energy. This energy can be reconstituted as an image. MRI of the heart is complicated because the heart is a moving structure, but the technique is already finding clinical application for imaging vascular structures.
Synchronization with the ECG allows cardiac images in systole and diastole to be obtained.