Cardiovascular nuclear imaging

Scintigraphic imaging procedures record the spatial or temporal distribution of radioactive isotopes within the body using a gamma camera for the detection of radiation.

Image acquisition can be performed in the planar or tomographic mode. In planar imaging, projection images of the tracer distribution are recorded from selected angles to image the heart from different views. Planar imaging is a simple technique that can be performed with a mobile gamma camera and thus is available at the bedside. In tomographic or single-photon-emission CT (SPECT) imaging, the gamma camera rotates around the patient to record a sequence of 30 to 60 planar projection images. CT is then used to reconstruct and display tomographic transaxial images of the heart. ( CD.Figure.2)

CD Figure 2. Short-axis images of thallium-201 myocardial perfusion SPECT scintigraphy obtained in a patient with a 94 per cent area stenosis of the proximal left anterior descending artery (see Fig. cd1). Images are displayed starting from the apex (image 1) and continuing to the base of the heart (image 16) with the following orientation: septal E left, anterior E top, lateral E right, and inferior E bottom. Stress images (panel A) show a large perfusion defect extending into the anteroseptal, anterior, anterolateral, and apical segments. Tracer distribution is almost normal in the redistribution images (panel B) obtained at rest 4 h later.

ECG-gated image acquisition can be used with both planar and SPECT imaging to show the distribution of radioactivity at selected time periods within the cardiac cycle (e.g. at end-systole or end-diastole).

Short-axis images of thallium-201 myocardial perfusion SPECT scintigraphy obtained in a patient with a 94 per cent area stenosis of the proximal left anterior descending artery (see Fig. cd1). Images are displayed starting from the apex (image 1) and continuing to the base of the heart (image 16) with the following orientation: septal E left, anterior E top, lateral E right, and inferior E bottom. Stress images (panel A) show a large perfusion defect extending into the anteroseptal, anterior, anterolateral, and apical segments. Tracer distribution is almost normal in the redistribution images (panel B) obtained at rest 4 h later.

The main applications of cardiovascular nuclear imaging include the assessment of regional myocardial perfusion by perfusion scintigraphy, the measurement of left and right ventricular function by multigated radionuclide ventriculography, the detection of acute myocardial necrosis by infarct avid imaging, and the delineation of myocardial viability and cardiac metabolism by positron emission tomography (ZarelandWickers 1993).

Myocardial perfusion scintigraphy

Myocardial perfusion scintigraphy provides images of the relative distribution of regional myocardial perfusion. After perfusion tracers have been injected intravenously, the tracer is extracted by normal myocardial tissue in relation to myocardial perfusion. Thus the myocardial activity of the tracer substance reflects relative myocardial perfusion. However, myocardial tracer concentration depends not only on perfusion but also on the presence of viable tissue that is able to extract and retain the tracer. Tracer will not be retained in necrotic or fibrotic areas, and perfusion scintigraphy will be abnormal.

At rest, coronary blood flow in a stenotic artery remains unaltered until more than 80 to 90 per cent of the luminal area is obstructed. Thus, in the absence of a prior myocardial infarction, resting perfusion scintigraphy may be normal even though severe coronary disease is present. This situation changes when myocardial perfusion is increased by either stress techniques (e.g. treadmill testing or dobutamine infusion) or pharmacological vasodilatation (e.g. dipyridamole or adenosine). Under hyperemic conditions, perfusion in normal myocardium will increase two- to fivefold while little or no change of flow will occur distal to a stenosis of 50 per cent or more of diameter. The flow inhomogeneity between unobstructed and stenotic vessels will be reflected in a difference of relative tracer uptake on myocardial scintigrams. Thus, in myocardial scintigraphy, stress imaging is employed to assess the functional significance of coronary stenoses, while rest scintigrams are used to depict the extent of myocardial damage and delineate myocardial viability.

Radionuclide ventriculography

In radionuclide ventriculography, red blood cells are labeled and blood pool imaging is performed using a gamma camera. Right and left ventricular contractile function can be quantified by first-pass or equilibrium imaging. Both methods use ECG-gated image acquisition to measure the amount of radioactivity within each cardiac chamber at different time periods during the cardiac cycle. The amount of radioactivity is proportional to the ventricular volume, so that ventricular ejection fractions can be calculated from end-systolic and end-diastolic count rates independent of geometric assumptions. Gated images can be displayed on the display screen as a continuous loop in video mode, giving the visual impression of a beating heart. Regional cardiac wall motion can be assessed visually. Quantitative image analysis can be used to calculate global right and left ventricular ejection fractions, regional wall motion and synchronicity of contraction, and indices of diastolic function such as peak filling rate or time to peak filling rate.

Infarct avid imaging

Localization and extent of myocardial necrosis can be characterized by nuclear medicine imaging using tracers with an affinity for acutely necrotic myocardial tissue which accumulate in such areas over a period of hours following tracer injection. Scintigraphic images display an increased count activity in locations of recent myocardial damage.

Positron emission tomography

Positron emission tomography (PET) is a nuclear imaging technique that records the distribution of positron-emitting isotopes in the body. PET tracer substances can be synthesized that are chemically identical with naturally occurring compounds such as water, acetate, or ammonia. Therefore PET imaging can be used to study metabolic pathways directly rather than by tracing the fate of foreign substances. PET imaging permits quantitation of myocardial perfusion (in ml/min/g) and metabolic pathways (in pmol/g/min). Most PET isotopes have short half-lives in the range of minutes so that an on-site cyclotron is necessary for isotope production. As PET imaging puts considerable demands on financial and technical resources, it is currently available at specialized centers only.

Cardiac applications of PET imaging include the quantitative measurement of myocardial perfusion and assessment of myocardial glucose utilization. In particular, the presence of maintained glucose metabolism in dysfunctional myocardial tissue is considered to be the most established diagnostic method to identify residual myocardial viability (CD Figure.. . . 3).

CD Figure 3. Short-axis PET images of myocardial perfusion (top row) and glucose utilization (bottom row) of a patient with triple-vessel coronary disease and severe left ventricular dysfunction. Images of myocardial perfusion and glucose metabolism were obtained with 13N-ammonia and 1sF-deoxyglucose respectively as PET tracer substances. Images are displayed from left to right starting from the base and continuing to the apex with the following image orientation: septal E left, anterior E top, lateral E right, and inferior E bottom. In contrast with myocardial perfusion, which is abnormal in a large portion of the left ventricle, glucose utilization is maintained in almost all segments including the right ventricular wall. The pattern of maintained glucose utilization in hypoperfused myocardial tissue is referred to as a flow—metabolism mismatch and suggests myocardial viability indicating potentially reversible dysfunction (Courtesy of UCLA PET Center.)

Short-axis PET images of myocardial perfusion (top row) and glucose utilization (bottom row) of a patient with triple-vessel coronary disease and severe left ventricular dysfunction. Images of myocardial perfusion and glucose metabolism were obtained with 13N-ammonia and 1sF-deoxyglucose respectively as PET tracer substances. Images are displayed from left to right starting from the base and continuing to the apex with the following image orientation: septal E left, anterior E top, lateral E right, and inferior E bottom. In contrast with myocardial perfusion, which is abnormal in a large portion of the left ventricle, glucose utilization is maintained in almost all segments including the right ventricular wall. The pattern of maintained glucose utilization in hypoperfused myocardial tissue is referred to as a flow-metabolism mismatch and suggests myocardial viability indicating potentially reversible dysfunction (Courtesy of UCLA PET Center.)

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