Radiation dose-based comparison of PET and SPECT for preclinical bone imaging

This project aimed to compare the small-animal imaging capabilities of PET and SPECT. Typical comparison metrics such as spatial resolution and sensitivity do not indicate how well a modality will perform a specific imaging task. Because metastatic osteolytic and osteoblastic bone lesions can be imaged with both PET and SPECT using ¹⁸F-Fluoride (¹⁸F) and ^99mTc-methylene diphosphonate (^99mTc-MDP), respectively, we used imaging bone metastases as the basis of a task-based comparison. As most tumor progression and treatment studies are longitudinal, accumulated radiation dose to the subject is a concern as it can lead to alterations in the tumor being studied. Therefore, a task-based comparison of PET and SPECT using injected activities that yield equivalent absorbed radiation doses to bone was developed to allow for a realistic and relevant understanding of differences in image quality. Mice were injected with either MDA-MB-231 (osteolytic) or LuCaP (osteoblastic) cells. Mice were imaged the same day on both modalities after injection of approximately 120 MBq of ^99mTc-MDP for SPECT and approximately 9.25 MBq of ¹⁸F for PET acquisitions, with each radiotracer injection delivering an estimated 590 mGy to bone. Quantification in the osteolytic study proved challenging as larger lesions presented as cold spots, but were better visualized by SPECT in one case. Large osteoblastic tumors were easily visualized and quantified well in both modalities but one small lesion was not detectable in either. The results presented here give insight into the image quality obtainable from PET and SPECT when imaging bone metastases.