LAUSR

Introduction The aim of 99mTc thyroid scintigraphy using pinhole collimator is to differentiate thyroid nodules (hot/cold). The diagnostic reference level is 80 MBq [1] , which is 87% lower than the activity currently injected in clinical conditions. Following a statistical study on 50 patients, including 45 with euthyroidism, the current examination conditions were determined: 149 MBq ±  7.6 MBq injected activity, for a 10 ± 6 min waiting time after injection, at 7 ± 2 min acquisition time, and a total number of events of 127 ± 53 kcps. The objective is, through phantom experiments and in agreement with national recommendations, to lower the injected activity for a similar acquisition time while keeping a sufficient image quality. Methods A thyroid phantom with cold and hot nodules was used on Siemens Symbia S SPECT which have crystals 3/8 in. thick. Reference acquisition parameters for these experiments are: 128 × 128 matrix, zoom 2, stop at 250 Kcps on the detector, with a 4 mm aperture diameter ( d o ) pinhole collimator. Results Considering identical number of events detected, a 54% acquisition time reduction with do = 6 mm and 73% with do = 8 mm was observed. Contrast decreased by 9.6% (do = 6 mm) and 24% (do = 8 mm) for the cold nodule, 21% (do = 6 mm) and 34% (do = 8 mm) for the hot nodule. On the images, the nodules are more visible with do = 4 mm (Fig. 1.a) than for do = 6 mm (1.b) or do = 8 mm (1.c). This loss in spatial resolution can be partially compensated by using a 128 × 128 matrix to 256 × 256 one (1.d, do = 4 mm), without inducing additional time. Download : Download high-res image (92KB) Download : Download full-size image Conclusions Using 6 mm aperture for pinhole instead of 4 mm aperture initially used, enables to have an increase in signal and thus a significantly reduced acquisition time. The matrix modification in 256 × 256 helps to improve the spatial resolution. With these results and recommendations [2] , we have defined a new acquisition protocol: at least 20 min waiting time after injection (99mTc fixation time in the thyroid [3] ), using a 256 × 256 matrix, a zoom 2 and a 6 mm aperture pinhole. The acquisition is set at 130 kcps. A prospective study on 50 patients with 110 MBq injected activity would validate new protocol efficiency.