The prevalence of a false-positive myocardial perfusion stress SPET test in a skinny patient, induced by projection truncation.

During the last decade, technical developments in myocardial perfusion single photon emission tomography (SPET) imaging systems have significantly improved the accuracy of diagnosing coronary artery disease. Nevertheless, the patient's position and/or the acquisition protocol can affect the studies' quality, possibly leading to misdiagnoses. In HJNM and in other journals the importance of proper positioning of the heart of the patient to be examined by myocardial perfusion SPET stress/rest testing, has been emphasized. According to our knowledge, only three cases of truncation artifact during SPET myocardial perfusion imaging acquired with original SPET cameras, related to improper positioning in very thin patients, have been reported. In all cases, patients were examined according to a single day stress/rest technetium-99m-sestamibi protocol, using a dual 90 degree detector system, equipped with high resolution, parallel-hole collimators. However, several published manuscripts have underlined the significance of appropriate patients' positioning in myocardial perfusion scintigraphy using dedicated, cadmium-zinc-telluride (CZT) or small field-of-view cardiac SPET systems. A typical case is that of a 47 years old man (height 187cm, weight 67kg), heavy smoker, with atypical chest pain. He exercised very well according to the Bruce protocol, achieving 95% of maximal age-predicted heart-rate and a technetium-99m-tetrofosmin ((99m)Tc-TF) myocardial perfusion imaging with 370MBq of (99m)Tc-TF followed with a dual head camera (Infinia GE, USA), equipped with low-energy, high-resolution, parallel-hole collimators at 90° (L-mode configuration). Projection images were obtained from 45° RAO to 45° LPO position, in step and shoot mode (60 projections, 30sec per projection; matrix 64×64 and zoom 1.3). Auto body contour was not used. Unprocessed raw data, showed neither patient motion nor significant extracardiac activity that could result in false positive defects on myocardial perfusion stress images. However, truncation at the apex of the heart was observed. In detail, truncation of activity of apical portion of the heart from frame 45-60 (detector 1) and frames 1-5 (detector 2) was noticed. Processed stress images demonstrated a severe defect in the apex and the apical part of the anteroseptal wall. Moreover, less intense defects were observed in the inferior and septal walls. All acquisition parameters were double checked and a possible error regarding the "zoom" was ruled out. Hence, it became evident that the aforementioned artifact has originated from an eccentric patient's position and thus some heart projections were missed. A second stress acquisition was performed after repositioning the patient with emphasis on positioning the heart at the center of the field of view. As a result, improvement of the above mentioned defects, mainly in the apex and the apical anteroseptal wall. In the literature, a number of recent studies have mentioned the effect of the truncation artifact even with newly equipped gamma cameras, emphasizing the importance of the heart being in the field of view throughout the acquisition procedure. Few of them used parallel-hole collimation. In conclusion, it is suggested that in cases of very thin patients it is often necessary to avoid truncation artefacts by correctly positioning the patient's heart.