Stress single-photon emission computed tomographic myocardial perfusion imaging (SPECT) performed with Technetium (Tc)-99m–labeled radiopharmaceuticals is widely used for diagnosing coronary artery disease (CAD) and stratifying patients for cardiac risk (1). The number of SPECT studies performed annually is likely to increase further in consideration of an aging population and the increasing number of concerned individuals. However, this reality is juxtaposed to an escalating emphasis on 1) cost containment, 2) improvement of laboratory efficiency, and 3) reduction of radiation exposure from medical imaging (2,3).
1 - Radiation exposure and Ultrafast (UF) cameras using cadmium zinc telluride (CZT) detectors
A systematically-lead revision of recent literature, on the subject of strategies to minimise radiation exposure originating from SPECT, PET, CT, and coronary angiography was performed. It revealed a need for the determination of a selection of protocols both for individual patients as well as standard laboratory operating procedures using an “As Low As Reasonably Achievable” approach. Weighing of the dosimetry of cardiac imaging protocols in use would be a first step toward the implementation of a test selection strategy to minimise overall risk to patients without detracting from high quality diagnostic information.
In an effort to meet these challenges, new dedicated ultrafast (UF) cameras, using pin-hole collimation design and multiple cadmium zinc telluride (CZT) crystal arrays, have been developed (4-7). The most promising of these new technologies is the CZT detector, which directly converts gamma radiation into an electronic pulse thereby eliminating the need for scintillating crystal and photomultiplier tubes. The CZT detector offers substantially better energy and spatial resolution than the NaI detector. Thanks to its compact design, new detector configurations enable multiple independent detectors to be positioned around the patient. With multiple detectors focused only on the heart, image quality is increased with added sensitivity for determination of activity in the heart. Patients enjoy shorter imaging times, reduced by a factor of 5 or greater -only 2-5 minutes are required for test to take place- and radiation exposure is also reduced because smaller administered doses of radioisotope are needed.
2 - UF SPECT compared to standard SPECT
Previously published studies focused on the reduction of imaging time with good agreement when compared to standard (S) SPECT approaches (6-9). In a recent study (10) UF SPECT was compared to S SPECT using coronary angiography as gold standard. Even though the data were obtained from a small group of patients, results were encouraging. Per-patient analysis for UF-SPECT showed a non-significant trend towards higher diagnostic accuracy, while the per-vessel analysis offered higher accuracy in detecting obstructive CAD in the LCx and RCA coronary arteries. The improved per-vessel detection of CAD corresponded with a significant improvement in the delineation of multivessel CAD (Figure 1). These published data support the general subjective impression that UFC systems provide not only comparable, but better and more accurate SPECT images (11). These findings extend the results of prior studies comparing UF- and S SPECT with respect to detection of obstructive CAD, as defined angiographically. The characteristics of UFC systems should result in better diagnostic sensitivity and specificity.
Possible explanations for these findings are two-fold. One relates to the higher spatial resolution of UF-SPECT and consequently to the better identification of smaller and less severe defects within individual coronary territories. A second reason would be that the increased sensitivity with UF-SPECT may have reduced attenuation artifacts, especially in the LCx and RCA territories. Indeed, the identification of multivessel coronary artery disease by UF SPECT was highly improved mostly through this mechanism.
Moreover, UF SPECT could be used for acquiring cardiac images using a lower injected dose. A pilot clinical study was performed to assess the feasibility of a new low dose stress-rest single-day fast protocol using a UF SPECT for the evaluation of CAD (12). Preliminary results showed that despite the use of a significantly lower dose than the previously validated one (6-10), the UF SPECT images maintained good accuracy in the evaluation of myocardial perfusion, with a significant reduction in imaging time that lies in improved patient comfort and fewer motion artifact (6-10) and with a radiation dose < 7 mSv in almost all patients referred for evaluation of CAD (Figure 2). These preliminary data open new perspectives in the use of UF SPECT in ischemic patients.
3 - Cost and benefit in cardiovascular imaging
Several recent studies indicate the necessity of calculating the cost of all applied procedures in order to optimise the allocation of limited resources. The possibility of using this new type of protocol with comparable results in detecting CAD versus standard SPECT can reduce costs significantly. In fact, immediate costs can be reduced by approximately 30% because of the reduction of the injected tracer and acquisition times. Long-term costs due to radiation are also significantly reduced as well as societal costs relating to the possible environmental impact.
Figure 1: Standard (S) SPECT and ultrafast (UF) SPECT
Comparison of sensitivity and specificity of S SPECT and ultrafast UF-SPECT. UF SPECT showed better sensitivity and specificity in the left circumflex and
right coronary artery territories. They were similar in the left anterior descending coronary artery. Modified by Gimelli et al, ref 10.
Figure 2: Stress/rest UF SPECT
Stress/rest UF SPECT in a patient with left anterior descending coronary artery proximal stenosis.
The possibility of using a low-dose radiation protocol with UF SPECT could change the impact of myocardial perfusion imaging in the evaluation of patients suspected of CAD. Combined to a reduction in costs, the significant reduction in a patient’s exposure to radiation without losing accuracy in detecting ischemia represents the main innovation of this new technology and could help to ascertain it as a new strategy in nuclear cardiology.