Clinical implication of down-scatter in attenuation-corrected myocardial SPECT

Background. Interpretation of myocardial perfusion single photon emission c omputed tomography (SPECT) studies is hampered by attenuation artifacts. At tenuation correction methods with simultaneous emission and transmission ar e now commercially available. However, it has been observed in clinical pra ctice that attenuation correction without down-scatter correction in a 1-da y rest/stress myocardial perfusion protocol may lead to serious interpretat ion errors. Therefore the aim of this study was to study errors resulting f rom down-scatter under realistic conditions, thus providing a background fo r the assessment of further corrections. Methods and Results, Forty-six patients underwent myocardial perfusion scin tigraphy in a 1-day technetium 99m-tetrofosmin rest-stress SPECT protocol, with a moving Gd-153 line-source device for attenuation correction without down-scatter correction. Short-axis slices were quantified as inferior/ante rior, septal/lateral, and apical/remainder count ratios, The changes at res t (350 MBq) and exercise (900 MBq) induced by attenuation correction were s tudied, Attenuation correction gave differences in apparent perfusion betwe en rest and exercise not seen before correction. The gender differences in inferior-anterior ratio were greatly reduced after correction at rest but r emained at exercise. A torso phantom study indicated that these results wer e due to under-correction at exercise because of down-scatter. Conclusions. Down-scatter results in an underestimation of attenuation in s imultaneous emission and transmission, if not accurately accounted for. Par ticularly, a high-dose study compared with a low-dose study, as in the 1-da y protocol, might cause serious interpretation errors.