Attenuation correction of small animal SPECT images acquired with 125I-iodorotenone

Iodine-125 is an inexpensive and widely available radioisotope that is used frequently in biological experiments. It is also possible to perform small animal imaging experiments with this isotope, although its low photon energy (27.5 keV) may lead to significant photon attenuation. We have developed a method to calibrate x-ray computed tomography (CT) image data in order to use microCT images to provide objective specific attenuation maps that are included in an iterative reconstruction algorithm to correct for photon attenuation. Phantom experiments with iodine-125 show that this method can compensate for the effects of photon attenuation. A uniform phantom (3.8 cm diameter) imaged without attenuation correction has a decrease in image intensity at its center of approximately 25%, but reconstruction with attenuation correction virtually eliminates the decreased image intensity in the center of the phantom. Using ¹²⁵I-iodorotenone, an experimental myocardial flow tracer, we demonstrate photon attenuation correction for iodine-125 imaging in a rat. The addition of attenuation correction improves the uniformity of the resulting perfusion images, better matching the results obtained with autoradiography.