The electrophysiologist needs precise electromechanical investigation tools to quantify the electrical propagation and its impact through the heart muscle. Analysis of the timing of the myocardial contraction based on Fourier phase histograms allows mapping of the kinetics of the different ventricular segments from functional phase pictures and, by extrapolation, mapping of the electrical activity of the heart. Nuclear cardiology combines mechanical and electrical analysis in real time. In this chapter, we report on our experience of gated blood pool scintigraphy combined with phase analysis and myocardial innervation imaging in various diseases with electrical disorders.
Radionuclide functional imaging of the heart addresses multiple aspects of cardiac function.1 Although most applications of nuclear cardiology are focused on coronary artery disease, in which the combination of stress/rest myocardial perfusion and/or viability imaging plays a major clinical role, many other less common cardiac diseases may benefit from this noninvasive diagnostic approach.
Perfusion and viability imaging modalities are based on cellular uptake of a radioactive molecule such as thallium-201 (201Tl), which presents an active transport comparable to that of potassium cation (K+), or sestamibi or tetrofosmin labeled with technetium 99m (99mTc), which are larger cations whose cellular penetration is passive due to concentration gradient.
Imaging is usually performed in tomographic mode (SPECT) and is a two-step procedure that combines stress imaging (shortly after physical exercise or pharmacologic coronary vasodilation) with resting imaging after stress-injected tracer redistribution with time (thallium) or a second resting administration (technetium-labeled molecules). The first set of images describes coronary perfusion reserve at stress, whereas the second set reflects cellular viability.
In addition to single photon (gamma ray) methods, positron emission tomography (PET) may also be a choice for a comparable approach, but PET uses specific positron-emitting radiopharmaceuticals such as the generator-produced rubidium-82 for perfusion and fluorine 18–labeled fluorodeoxyglucose.
Myocardial sympathetic innervation imaging is based on the presynaptic uptake of catecholamines. The gamma-emitting tracer is meta-iodobenzyl guanidine (MIBG) labeled with iodine 123. Comparable positron-emitting tracers are used as well. Image acquisition can be planar or SPECT. Innervation defects appear as hypoactive areas, which have to be compared with perfusion defects using an adequate perfusion agent.
Imaging the mechanical contraction of the heart needs a dynamic acquisition of the periodic motion of the myocardium. Electrocardiographically (R wave) triggered acquisitions can be performed on myocardial perfusion SPECT images. Another option consists of blood pool labeling using an intravascular tracer such as albumin or red blood cells, both labeled with 99mTc. Gated blood pool pictures can be acquired in planar mode as well as in SPECT, but in any case, one single cardiac cycle is unable to provide a sufficient number of counts. Therefore, a high number of successive cardiac cycles (≥100) have to be superimposed for an adequate image (Fig. 9–1). Measurement of left ventricular ejection fraction is the most common variable obtained ...