SAN FRANCISCO — With computed tomography angiography, “patients literally go home with a Band-Aid and a bottle of water” after just 20 minutes, Matthew J. Budoff, M.D., remarked at a cardiovascular imaging conference sponsored by the American College of Cardiology.
With high sensitivity and specificity and images that rival the resolution obtainable with traditional coronary angiography from the catheterization lab, CT angiography will allow many more patients to avoid an invasive procedure, said Dr. Budoff of Harbor-UCLA Medical Center, Torrance, Calif.
After an injection of 80–100 mL of nonionic iodinated contrast solution, up to 4,000 two-dimensional images can be obtained within 20–30 seconds as the patient holds his or her breath. The entire procedure takes 20 minutes, and interpretation takes another 10 minutes.
Sophisticated workstations assemble the stack of 2D images into a three-dimensional reconstruction. Interpretations are made on the basis of the 3D reconstruction with reference to the 2D images.
Dr. Budoff started working with CT angiography in the mid-1990s. In those days it took 3 weeks of full-time computation to assemble a single 3D reconstruction. This same function takes just 30 seconds today.
And these workstations allow the cardiologist to rotate the heart image in three dimensions, to zoom in to interesting features, and to easily reference the original 2D data from any point of interest.
The initial studies of four-slice CT angiography revealed the limitations of this early technique. Only 30% of patients had all three major arteries available for analysis, and in detecting stenosis the sensitivity was just 58% with 76% specificity.
Now, as 16-slice and even 64-slice CT angiography become available, the sensitivity and specificity have improved considerably. Studies have calculated sensitivities as high as 97% and specificities as high as 94%.
Most important, the negative predictive value is 98%–100%. “The benefit of CT angio is that when the coronaries look normal, the coronaries are normal,” Dr. Budoff said.
The temporal resolution of the CT images is about 175 milliseconds, so reducing the heart rate to below 60 beats per minute is important for accuracy and interpretability. Most centers use 100 mg metoprolol 1 hour prior to the study and/or a 5-mg intravenous metoprolol push every 5 minutes until the patient achieves a slow heart rate.
A regular rhythm is also important. With multiple detectors obtaining images at specific parts of the heart cycle, the modality reaches an effective frame speed of 15 images per second. This is slower than the cath lab, but fast enough that the images are free of motion artifact.
CT angiography may be the best technique for imaging the results of bypass grafting as the anastomoses are clearly visible. Other clinical indications for CT angiography are in cases of equivocal results following stress testing; to evaluate patency post angioplasty, post stent, and post bypass surgery; in cases of congenital abnormalities and anomalous coronaries; before and after atrial fibrillation ablation; and before placing a biventricular pacer.
CT angiography is not without its disadvantages, however. It's not very good for visualizing vessels with diameters less than 1.5 mm. It is subject to artifacts from extensive calcification, stents, or extensive clips after bypass grafting. And it subjects patients to a relatively high dose of radiation—about 9.3–11.3 mSv, compared with 2.1–2.3 mSv for the cath lab and 0.1 mSv for a chest x-ray.
CT angiography reveales high-grade stenosis (dark area) in the mid-left anterior descending coronary artery. Courtesy Dr. Matthew J. Budoff