[Effect of reconstruction arc in myocardial SPECT imaging--comparison between 180 degrees and 360 degrees arc--]

A series of thallium-201 (201Tl) and technetium-99m (99mTc) myocardial perfusion images with or without perfusion defects were acquired by 360 degrees arc using a triple-head camera. The vertical long, horizontal long, and short axis images of 201Tl and 99mTc with 360 degrees, 180 degrees, and opposite 180 degrees were identically reconstructed and reoriented without attenuation correction. In order to detect the detailed differences between 360 degrees (30 degrees - 390 degrees) and 180 degrees (30 degrees - 210 degrees) images, the opposite 180 degrees (210 degrees - 30 degrees) images were utilized. Opposite 180 degrees images revealed the contribution of those to the 360 degrees images. In the normal perfusion images, the CV (coefficient of variation) of the counts in 360 degrees images with 201Tl by the segments was larger than 180 degrees images. On the contrary, the CV of the counts in 360 degrees images with 99mTc was smaller than 180 degrees images. This difference, caused by the difference of photon energy, were detected mainly as the extent of counts decreasing in the deep segments in the opposite 180 degrees images. Lesion contrasts of the opposite 180 degrees images were lower than those of 180 degrees images due to distance related collimator response (the detector response of the collimated gamma camera varies with source-to-detector distance) and scatter. Therefore, lesion contrasts were more decreased in the 360 degrees images compared with those of 180 degrees images, especially in the segments of lateral and anterior walls. The differences in lesion contrasts between 201Tl and 99mTc images were mainly in the segments of inferior and posterior walls (p < 0.05). Lesion contrast was dependent on the location of the perfusion defects, reconstruction arc, and photon energy.