Objective: Gamma cameras contain energy discriminators that allow only those photons within a specified energy range to be recorded. A spontaneous shift in peak of 1 head of a dual-head gamma camera may cause artifacts. We present our experience with the incidental occurrence of off-peak status in 1 head of a dual-head gamma camera that resulted in subsequent artifacts and poor-quality images.
Methods: Four patients had been scheduled to undergo imaging on a newly installed dual-head gamma camera on the same morning. The first patient underwent (201)Tl-chloride anterior and posterior total-body imaging to check for metastatic thyroid cancer, and the images were of adequate quality for interpretation. The next 2 patients underwent dual-isotope rest (201)Tl-chloride and gated dipyridamole (99m)Tc-tetrofosmin myocardial SPECT. The rest (201)Tl myocardial SPECT images of both patients showed normal perfusion in the left ventricular wall, but the dipyridamole (99m)Tc SPECT images showed virtual absence of perfusion in the apical, anterior, and lateral walls. These findings might suggest myocardial ischemia. In addition, 1 of 2 patients' cardiac gated SPECT findings led to inaccurate left ventricular ejection fractions. Technologists did not become aware of the artifacts until the fourth patient underwent total-body bone scanning, which showed faint activity with loss of contrast in the entire anterior body. One of the camera heads was then found to have off-peak status. Thus, we abandoned use of 1 detector of the dual-head gamma camera and repeated the anterior bone scanning of the 4th patient and the dipyridamole SPECT of 1 of the 2 cardiac patients.
Results: Gated cardiac SPECT abnormalities resulting from off-peak status were difficult to identify and included abnor-mal left ventricular wall perfusion and an inaccurate left ventricular ejection fraction. It was determined later that the off-peak status was caused by malfunction of a photomultiplier tube.
Conclusion: Degraded planar images resulting from the off-peak status of a camera head are easily identified. The presence of the same pattern of abnormalities in 2 consecutive myocardial SPECT examinations should alert technologists to the possibility that the abnormalities are, in fact, artifacts.