X-ray computed tomography, including conventional, helical and electron-beam ("Ultrafast®") forms, provides cross-sectional images of the chest, including the heart and great vessels. In general, cardiac tomography (also called CT scan and coronary artery scanning) is useful to evaluate aortic disease (such as aortic dissection), cardiac masses and pericardial disease.

CT provides clinically relevant anatomic and functional information, is relatively noninvasive, and has very low short- and long-term risks (if the well-known potential hazards are avoided).

Computerized axial tomographic scan is also used to examine how the brain looks, functions and gets its blood supply. This test can outline the affected part of the brain and help define the problem a stroke creates.

Electron-Beam Computed Tomography (EBCT or EBT)

EBCT is an especially fast form of X-ray imaging technology. It's particularly useful to

• evaluate bypass graft patency, intra- and congenital cardiac lesions, and
  
  
• quantify right and left ventricular muscle mass, chamber volumes, and systolic and diastolic function (such as cardiac output and ejection fraction).

Electron-beam CT can also measure calcium deposits in the coronary arteries. The amount of calcium detected by EBCT is related to the amount of underlying coronary atherosclerosis. The coronary calcium score, derived from EBCT scans of the coronary arteries, is known to predict the occurrence of cardiac events, such as fatal and nonfatal heart attacks or the need for coronary bypass surgery or coronary (balloon) angioplasty over the next one or two years. A negative calcium score implies a very low risk for obstructing coronary lesions and has a high negative predictive value for coronary events.

The increased predictive value of EBCT of the coronary arteries relative to traditional risk factor assessment isn't yet completely defined at this time.

EBCT isn't a substitute for cardiac catheterization. EBCT measurement of coronary calcium is of no known value in patients who've already had a heart attack or undergone coronary bypass surgery or coronary angioplasty because their risk is known.

Cardiac Positron Emission Tomography (PET)

Positron emission tomography of the heart allows the study and quantification of various aspects of heart tissue function. Its use in research has provided novel observations in cardiac physiology and pathophysiology. PET combines

1. tomographic imaging with radionuclide tracers of blood flow metabolism and receptors and
2. tracer kinetic principles for noninvasively quantifying regional myocardial blood flow, substrate fluxes, biochemical reaction rates and neural control.

Clinical studies suggest an important role for PET in diagnosing patients, describing disease and developing treatment strategy. Two areas of clinical application have emerged:

• PET, which is noninvasive, is highly accurate for detecting, localizing and describing coronary artery disease that impairs blood flow to the myocardium (heart muscle).
• PET accurately identifies injured but viable myocardium (heart muscle), such as reversible ventricular dysfunction.

Technological improvements have occurred in PET scanners, cyclotron production of tracer labels and radiotracer synthesis. These have greatly enhanced the performance of cardiac PET studies, which now appear to be feasible. Cardiac PET studies can also be performed without an on-site cyclotron, using generator-produced isotopes such as rubidium-82 and/or tracers of metabolism produced off-site.

Digital Cardiac Angiography, Digital Subtraction Angiography (DCA or DSA)

This modified form of imaging records pictures by computer of the major blood vessels to the heart or brain. It lets a doctor know if there are any blockages, how severe they are, and what can be done about them. In this test, dye is injected into a vein in the arm, and an X-ray machine quickly takes a series of pictures of the chest or head and neck.

Magnetic Resonance Imaging (MRI)

Magnetic resonance imaging (MRI) is also called nuclear magnetic resonance (NMR) imaging. It uses powerful magnets to look inside the body. Computer-generated pictures can show the heart muscle, identify damage from a heart attack, diagnose certain congenital cardiovascular defects and evaluate disease of larger blood vessels such as the aorta. It can outline the affected part of the brain and help define the problems created by stroke. Unlike radiographic imaging methods,

• It's non-ionizing and has no known biological hazards.
• It can produce high-resolution images of the heart's chambers and large vessels without the need for contrast agents.
• It's intrinsically three-dimensional.
• It produces images of cardiovascular structures without interference from adjacent bone or air.
• It has high tissue contrast.

MRI is an acceptable technique for evaluating diseases of the aorta such as dissection, aneurysm and coarctation; diseases of the pericardium such as constrictive pericarditis or hematoma; congenital cardiac lesions before or after surgical repair; heart muscle diseases, including those affecting the right ventricle such as dysplasia; and cardiac masses such as intracardiac tumor or invasive lung malignancy.

Other proven but less-common applications of MRI include evaluation of cardiac chamber morphology such as ventricular mass; global or regional ventricular function; and valve regurgitation.

Other potential applications are now under active investigation, including evaluation of coronary artery anatomy and flow; evaluation of myocardial blood flow; assessment of myocardial viability with pharmacologic stress; and assessment of myocardial metabolism by spectroscopic techniques.

In summary, MRI provides clinically relevant anatomic and functional information noninvasively and with minimal risk, if the well-known contraindications (such as pacemakers) and potential hazards (such as attraction of metallic objects) are avoided.

Radionuclide Imaging or Radionuclide Angiography

(includes such tests as thallium test, MUGA scan or acute infarct scintigraphy)

These tests involve injecting radioactive substances called radionuclides into the bloodstream. Computer-generated pictures can then find them in the heart. These tests show how well the heart muscle is supplied with blood, how well the heart's chambers are working, or identify a part of the heart damaged by heart attack.

Radionuclide angiography can also be used as a nuclear brain scan. In it radioactive compounds are injected into a vein in the arm, and a machine similar to a Geiger counter creates a map showing their uptake into different parts of the head. The pictures show how the brain functions rather than its structure. This test can detect blocked blood vessels and areas where the brain is damaged.

Single Photon Emission Computed Tomography (SPECT)

SPECT of the heart is a well-established nuclear imaging technique. It involves taking a series of pictures around the chest after injecting a radioactive tracer into the blood. Then computer graphics are used to create images of slices through the heart. This technique has been applied to the heart for myocardial perfusion (blood flow) imaging with agents like thallium-201 and the technetium-based myocardial perfusion tracers. These agents are injected either at rest or with exercise or pharmacologic stress.

Cardiac SPECT was introduced for myocardial perfusion imaging to overcome some of the limitations of planar imaging and to improve the localization and quantification of perfusion defects. Cardiac SPECT has been shown to make it easier to detect and localize myocardial perfusion defects at rest and during stress. The ability of SPECT to localize coronary artery disease and assess the extent and severity of perfusion abnormalities is enhanced compared to planar imaging. As a result, SPECT imaging is now widely used in nuclear cardiology laboratories across the country.

Several large, published studies have demonstrated the quantitative methods of interpretation. When SPECT is used to image the technetium-based myocardial perfusion tracers, global and regional function of the ventricle can be obtained in addition to regional perfusion. SPECT imaging of the heart can also be used along with newer agents that evaluate metabolism, but these applications are at present investigational.

In summary, SPECT myocardial perfusion imaging is a well-established, clinically useful technique for diagnosing coronary artery disease and for managing patients with known coronary artery disease.

Related AHA Scientific Statements:
Angiography/Angioplasty
Imaging
Tomography, Electron Beam Computed