A Hoffman brain phantom lesion study with the transformable HOTPET camera

The High-resolution Oncologic Transformable PET (HOTPET) camera has intrinsic spatial resolution of better than 3 mm and can operate in different modes including the whole-body mode and brain mode. In the whole-body mode, HOTPET has a transverse field-of-view (FOV) of 60 cm and an axial FOV of 13 cm. In the brain mode it has a transverse FOV of 39 cm and an axial FOV of 21 cm. The purpose of this study was to compare the performance of the two modes of HOTPET for studying small brain lesions (< 10 mm) to measure the effective gain in lesion detection by using a dedicated brain PET with higher sensitivity. We compared the two modes of camera by scanning the Hoffman brain phantom with three small lesion inserts for several lesion activity contrasts. The inner diameters of the lesion were in the range of 3.95 mm to 7.86 mm. The ratio of the activity concentration in the lesions to the activity in the background (gray matter regions) ranged from 1 to 20. Lesions were compared visually and also by measuring the contrast recovery and by studying their profiles. Visual inspection of the images showed that the 7.86-mm lesion with lesion to background activity ratio of 2 or higher was observable in both modes of the HOTPET. The 4.95-mm lesion was observable with lesion to background activity ratio of 4 (5.5) or higher in HOTPET brain (whole-body) mode. The 3.95-mm lesion was observable with lesion to background activity ratio of 8 (11) or higher in the brain (whole-body) mode. This study demonstrated the advantage of the brain mode of the HOTPET camera for detection of small lesions (< 5 mm) compared to its whole-body mode and also compared to a simulated whole-body camera with crystals twice the average size of HOTPET's crystals.