SPECT/CT and PET/CT detect stenosis, ischemia in one exam

November 1, 2007

Noninvasive cardiac imaging plays an important role in diagnosis and management of patients with known or suspected coronary artery disease. Traditionally, the degree of coronary artery stenosis has been evaluated by catheter coronary angiography, and the hemodynamic significance of the stenosis evaluated by a functional test, such as rest and stress myocardial perfusion scintigraphy using SPECT or PET.

Noninvasive cardiac imaging plays an important role in diagnosis and management of patients with known or suspected coronary artery disease. Traditionally, the degree of coronary artery stenosis has been evaluated by catheter coronary angiography, and the hemodynamic significance of the stenosis evaluated by a functional test, such as rest and stress myocardial perfusion scintigraphy using SPECT or PET.

Advances in multislice CT technology allow the presence and extent of epicardial coronary artery stenosis to be imaged with a high degree of accuracy using 64-slice scanners. Hybrid SPECT/CT and PET/CT systems using 64-slice technology have been developed as well. With these hybrid imaging systems, both the degree of epicardial coronary artery stenosis and the downstream functional consequences of this narrowing (stress-induced ischemia/microvascular dysfunction) can be detected in a single imaging session. CT can also be used for attenuation correction.

Cardiac PET/CT and SPECT/CT technology allow evaluation in one imaging setting of calcium scoring, coronary artery anatomy with contrast-enhanced coronary CT angiography, rest/stress myocardial perfusion scintigraphy (MPS), and localization of the hypoperfused regions to specific coronary arteries with the help of fusion images of the coronary tree superimposed on a 3D map of perfusion (see figure on page 99).1,2

Quality assurance of hybrid PET or SPECT/CT imaging is comparable to that of dedicated SPECT or PET imaging and considers adequacy of counts, patient motion, image reconstruction artifacts, and adequacy of coregistration of CT transmission and emission images for accurate attenuation correction. MPS and coronary CTA images can also be obtained from different imaging systems at different times and the fusion images generated using software packages.

Noninvasive coronary CTA allows visualization of vessels greater than or equal to 2 mm in size and stenose of >50%. Detection, description, and characterization of plaque by CT in the early stages of coronary artery disease could play an important role in preventing both CAD progression and its complications. Coronary CTA allows noninvasive detection of atherosclerotic plaques, estimation of plaque burden, and characterization of plaques. Various plaque subtypes can be characterized using CT density measurements, which are in Hounsfield units.

The sensitivity and specificity of coronary CTA using 16- and 64-slice CT range from 87% to 94% and 97% to 100%, respectively, compared with invasive coronary angiography.3,4 In these studies, the number of nonevaluable segments ranged from zero to 12%. A multicenter trial using 16-slice CTA, however, reported limited positive predictive value compared with catheter angiography due to a large number of false-positive studies related to unevaluable segments.5 Quantitative estimates of stenosis severity from 64-slice CTA correlate only modestly with quantitative coronary angiography.6

There is a high rate of agreement in the literature regarding the high negative predictive value of a normal coronary CTA: 97% with 16-slice CT7-9 and 99% with 64-slice CT.10 Therefore, coronary CTA may be very useful to exclude CAD in a population of patients with equivocal clinical or other findings. The American Heart Association concluded that use of CTA is reasonable for the assessment of obstructive disease in symptomatic patients.11

For evaluation of patients with coronary artery bypass grafts (CABG), coronary CTA can provide a good road map of the coronaries and graft prior to invasive catheter coronary angiography.12 When planning a coronary CTA of a patient with saphenous venous grafts, the field-of-view should extend from the aortic arch to the base of the heart. When planning a coronary CTA of a patient with a left internal mammary (LIMA) graft, the field-of-view should extend from the thoracic inlet to the base of the heart.

Coronary CTA is valuable in assessment of the patency of bypass grafts, which are relatively motionless and can be imaged fairly easily. Sensitivity and specificity to determine the patency of venous and arterial grafts in the 90% range were reported early in the development of spiral CTA. One increasingly used indication for hybrid imaging occurs prior to repeat thoracotomy or reoperation for CABG surgery.

The rest/stress perfusion study provides excellent information about ischemic and viable tissue, while CT angiography identifies native and graft disease and relationship of the functioning grafts to the sternum for surgical planning.

Although the combined information is helpful for guiding surgical planning, coronary CTA is not a feasible substitute for invasive angiography before repeat CABG. Similarly, in patients with known or suspected congenital coronary anomaly or myocardial bridge, a coronary CT angiogram identifies the precise course of the coronary artery, and the exercise perfusion study can identify whether this anomaly is associated with exercise-induced ischemia.

Coronary CTA has limitations for evaluation of coronary stent patency because beam-hardening artifacts limit sensitivity for detection of early in-stent restenosis. Stent occlusion can be documented, however, by lack of visualization of contrast in the vessel distal to the stent.13-15 Factors that affect the evaluation of stents include not only the type of scanner used but also the size, type, and material of the stent.

ADVANTAGES AND LIMITATIONS

The advantages of coronary CTA compared with catheter coronary angiography include noninvasiveness, true 3D imaging, cost-effectiveness, better characterization of plaques (calcium deposits versus soft plaque), better delineation of ostial stenoses, and combined evaluation of coronaries, plaque morphology, valves, myocardial mass and function, and lungs and thoracic aorta.

Coronary CTA also has limitations:

  • limited visualization of distal, small size, and tortuous segments;

  • overestimation or underestimation of stenosis secondary to coronary arterial calcification;

  • no real-time assessment of flow through vessels;

  • no direct assessment of collateral vessels;

  • no intervention possible during the examination; and

  • persistent irregular heart rate, such as atrial fibrillation, will result in interscan discontinuities, which will prohibit evaluation of coronary artery stenoses.

Not all stenoses detected on coronary CTA are flow-limiting, and evaluation of functional impact on myocardial perfusion using scintigraphic technique is critical. Coronary CTA has a relatively low positive predictive value for perfusion defects (in the 30% range). Integrated PET or SPECT and CT imaging is highly complementary because of the combination of anatomic and functional data obtained.

Combined information obtained by stress myocardial perfusion along with CT can be very helpful in guiding patient management in a variety of scenarios. The vessel causing ischemia, for example, can be identified in patients with multivessel CAD, so that management can be directed to that vessel. In addition, invasive coronary angiography may be avoided in patients with microvascular dysfunction without epicardial disease.

The value of hybrid SPECT/CTA is illustrated in Figures 1 through 3. In this patient's case, the combination of anatomic and functional studies led to more intense medical therapy and avoided invasive coronary angiography because revascularization was not possible.

The additive value of coronary artery calcium and coronary CTA, particularly in patients with normal stress myocardial perfusion, has been described.16 Preliminary data suggest that as many as 50% of patients with normal stress perfusion PET may show extensive (non-flow-limiting) coronary atherosclerosis (both calcified and noncalcified plaques).17 Assessment of perfusion alone would miss this cohort of patients due to lack of stress-induced ischemia. The presence of coronary artery calcium can identify patients with stable CAD who can be treated with more aggressive risk factor modification.

Hybrid SPECT or PET/CT may also play a key role in molecular imaging to localize metabolic processes, using hot spot imaging such as active/vulnerable plaques, gene expression and therapy, postinfarct remodeling, angiogenesis, and stem cell transplantation.

The anatomic and functional information obtained with combined MPS/coronary CTA is complementary. Debate continues regarding the cost-effectiveness of the combined approach and whether the sequential approach of MPS and coronary CTA, with one test guiding the other, would be more cost-effective in specific clinical scenarios.

Dr. Delbeke is director of nuclear medicine and PET in the radiology and radiological sciences department at Vanderbilt University Medical Center in Nashville.

References

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