Many of us who have chosen the field of Cardiology enjoy it because there are so many ways in which to obtain tangible, objective evidence about the heart's structure and function. This makes the task of reaching a diagnosis easier, and allows more precision in tailoring our therapy for each patient.
The techniques for Cardiac imaging fall basically into two categories: invasive and non-invasive. "Invasive" implies that the body is being entered in some way, in this case with one or more fine, flexible catheters, which are inserted into an artery or vein and reach the heart via the aorta or the vena cava. Examples of invasive heart tests include a right heart catheterization (or Swan-Ganz catheter), left ventriculography, aortography, and coronary angiography.
"Non-invasive" implies that the test is performed without entering the body, although this does not exclude the use of intravenously administered pharmacologic or imaging agents.
Invasive tests:
Right heart catheterization is performed primarily for assessment of patients with signs of heart failure or valvular disease, and for patients with congenital heart abnormalities. The Swan-Ganz catheter can be safely passed from the femoral vein through the inferior vena cava and then through the right heart: the right atrium, right ventricle, and pulmonary artery to the pulmonary capillary wedge position. There, pressures in all right heart chambers can be measured and oxygen saturation readings can be obtained. Pressure measurements allow for quantification of pulmonary hypertension and for detection of abnormalities such as cardiac tamponade, constrictive pericarditis, and right-sided valvular abnormalities. Comparison of the pulmonary capillary wedge pressure (PCW) with the left ventricular end-diastolic pressure obtained during simultaneous left heart catheterization allows for quantitation of mitral stenosis. In addition, detection of abnormalities in the PCW tracing can help in the diagnosis of acute mitral regurgitation and allow measurement of the left atrial pressure, and, therefore, the LVEDP (this of course assumes that there is no mitral stenosis present).
Oxygen saturation measurements allow for detection and quantitation of intracardiac shunts (such as those seen in ASD or VSD) and allow determination of the cardiac output through the Fick method. Alternatively, thermodilution techniques can allow determination of cardiac output without measuring the patient's oxygen consumption.
Left heart catheterization may include both ventriculography (visualization of left ventricular systolic function by injection of dye into the left ventricle) and aortic root angiography (injection of dye into the ascending aorta), as well as its most common use: coronary angiography.
Ventriculography allows for assessment of regional left ventricular wall motion, calculation of the left ventricular ejection fraction, and estimation of the severity of mitral regurgitation (by its direct visualization). In addition, left ventricular aneurysm formation can be detected, and sometimes apical left ventricular thrombus can be seen. Aortic root angiography allows for estimation of the size of the ascending aorta, assesses for proximal aortic dissection, and allows direct visualization and quantitation of aortic insufficiency.
Coronary angiography is the "gold standard" for visualization of coronary anatomy, and allows detailed inspection of all branches of the patient's coronary tree. Stenoses can thereby be quantitated, and the complexity of coronary lesions, sometimes including presence of intracoronary thrombus, can be seen directly. In recent years, adjunctive methods to enhance the inspection of coronary lesions has included intracoronary ultrasound and even direct visualization using a miniature coronary "angioscope", a fiber-optic probe. For technical reasons, however, coronary angiography remains far and away the most utilized method for examination of the coronaries.
Non-invasive tests:
Echocardiography utilizes an ultrasound probe to visualize the cardiac structures either through the chest wall ("transthoracic") or from behind the heart, by placing a probe through the mouth into the esophagus ("transesophageal").
Transthoracic echocardiography is a commonly obtained test since it is essentially risk-free and allows for excellent visualization of most intra-cardiac structures including all four valves and all four chambers, as well as the pericardium and ascending aorta. In addition, when coupled with Doppler techniques, it is very effective at estimating severity of valvular stenosis and regurgitation. Specifically, valves can be inspected for thickening, calcification, prolapse and ruptured chordae tendineae. Chambers such as the left ventricle can be measured in millimeters both to obtain cavity diameters during the cardiac cycle, and to measure wall thickness (i.e. to quantitate left ventricular hypertrophy).
Transesophageal echocardiography is more "invasive", since a probe must be placed into the esophagus with the patient under sedation, but allows for incredibly detailed views of the cardiac structures, and is perhaps the single best technique for evaluation of prosthetic heart valves, ruptured chordae tendineae, valvular vegetations, and atrial myxomas or thrombi.
Echocardiography can be coupled with either exercise or, more commonly, with pharmacological stimulation of the heart (e.g. with infusion of dobutamine, a beta agonist) to assess for regional systolic wall motion abnormalities during stress, which would indicate coronary ischemia.
Nuclear imaging tests are performed by injection of one of several nuclear isotopes which, depending on the substance they are bound to, can be taken up into either myocardial cells or red blood cells.
Most commonly, isotopes are used to image the myocardium in order to detect ischemia elicited during exercise. Usually, a patient is exercised to maximum capacity on a treadmill, and, at peak exertion, is injected intravenously with an isotope of Thallium. This tracer then is taken up into those myocardial cells which are being normally perfused with blood, and is taken up to a lesser extent in those cells which are not normally perfused. In addition, those cells which are not viable (i.e. dead due to a prior myocardial infarction) do not take up the Thallium. Following injection, images are immediately taken in several views to obtain an "image" of myocardial perfusion during exercise. The same patient then returns for a repeat image of the heart several hours later. This "delayed" image reflects uptake of Thallium into all viable heart cells, whether or not perfusion is abnormal during exercise.
By comparison of the Thallium images taken with exercise ("stress" images) to those taken at rest ("delayed" images), an assessment of the degree and location of myocardial ischemia can be made. If a "defect" (lack of uptake) is seen in one region during exercise, which then disappears or redistributes at rest, this is indicative of ischemia in this region. If a defect is present in one region both during exercise and at rest, then this is indicative of infarction in this region. This type of nuclear imaging test allows for both quantitation and localization of areas of ischemia and necrosis, and also allows for a physiological evaluation of the patient during exercise.
Finally, nuclear tests utilizing labeled red blood cells can be analyzed and "gated" with the cardiac cycle in order to create a "gated loop" image which shows the left ventricular cavity as it moves during a recreated single cardiac cycle. This allows for visualization of regional wall motion abnormalities and calculation of ejection fraction, and is especially useful in patients in whom adequate echocardiographic images cannot be obtained for technical reasons (i.e. hyperexpanded chest wall due to COPD). In addition, estimation of valvular regurgitation can be made by comparison of left ventricular and right ventricular stroke volumes.