Investigating pinhole SPECT for wrist imaging

The wrist consists of eight small carpal bones, which may be as small as 1 cm³. This makes the wrist one of the most complex joints to image. Initial X-rays miss 35% of fractures. Planar nuclear medicine scans are available and add functional information over the anatomical X-ray image, but they lack in specificity. SPECT may increase specificity, but is limited by poor resolution. We hypothesize that by using pinhole collimators we can improve resolution, while maintaining sensitivity in the desired field of view (FOV). To measure sensitivity the counts obtained for each collimator were recorded. A 5 cm radius sphere was used as a volume of interest (VOI) for comparing the sensitivities between collimators. The Micro Deluxe phantom and a wrist phantom developed on site were used to measure and compare the resolution of a fanbeam collimator with a 3, 4, and 6 mm pinhole collimator at 6.5 and 7.5 cm radii of rotations. The same two phantoms were used to measure the resolution at different count levels to determine the relationship between resolution and sensitivity of the pinhole collimators. The sensitivity measured in the wrist VOI was shown to be slightly (30%) higher in the fanbeam collimator compared to the 3 mm pinhole. However, the 4 mm and 6 mm pinholes had an increase of 20% and 140% respectfully over the fanbeam collimator. We observed that increasing the sensitivity with a larger pinhole size does not improve resolution significantly. The best images were obtained using the smallest pinhole size (3 mm). With the 3 mm pinhole collimator at clinical count levels an average resolution of 4 mm was measured at by taking the FWHM of the derivative on the cube edge. Images of the Micro Deluxe Phantom at similar count levels confirmed this measurement.