COMPUTED TOMOGRAPHIC ANGIOGRAPHY
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Computed tomographic angiography (CTA) is a 3D image reconstructed from multiple slices of tomographic images of a particular body part (e.g., brain, chest, blood vessels, abdomen, pelvis, joints). CTA of the heart includes the coronary arteries. Because CT studies are created by computer processing, the images can be seen in multiple planes, and ventricles, atria, veins, and arteries can be easily delineated. Pulmonary CTA can reveal emboli in both right and left pulmonary arteries and some subdivisions, as well as in the main pulmonary artery (see Plate 3-19).
CARDIAC CYCLE AND CALCIUM
CONTRAST STUDIES
High
resolution and high speed allow imaging of the coronary arteries as well as all
chambers of the heart and the great vessels. When imaging the coronary
arteries, ionizing radiation is usually confined to dias- tole, since the
majority of blood flow in the coronary arteries occurs during that time in the
cardiac cycle (see Plate 3-18). Coronary artery calcium
deposits can be quantitated. CTA is especially useful in identifying the course
of anomalous coronary arteries (between, posterior/anterior to great vessels).
The
radiation dose using CT coronary angiography initially once was quite high, 10
to 15 millisieverts (mSv) per study. The dose is now considerably reduced,
particularly when imaging the coronary arteries, which can be imaged in
diastole, thus decreasing the amount of radiation exposure as much as 1 to −3
mSv. However, many studies still require greater exposure. Importantly,
patients’ exposure to radiation has greatly increased over in the last several
years; 49% results from CT-related ionizing radiation, although CT makes up
only 12 % of medical radiation procedures. All health care personnel should be
aware of patient exposure to radiation.
Because
CTA studies require administration of intravenous contrast agents, similar to
catheter-based coronary angiography, patient risks include allergic reaction
and contrast nephropathy.
As
systems improve, many new clinical CTA investigations will become possible.
Computed tomography can help to assess the coronary artery calcium score and
evaluate the cardiac chambers and valves, congenital heart disease, aortic and
pulmonary disease, and extracardiac structures and abnormalities (see Plate
3-19). Techniques are now being developed to assess not only the anatomic
pathology of the coronary arteries but also the physiopathology of a specific
lesion that may need a revascularization procedure (i.e., PCI or coronary
artery bypass surgery). Preliminary results are encouraging compared with
catheter-based coronary angiography and fractional flow reserve, the current
reference standard for the physiologic significance of a specific coronary
artery stenosis.
