40 Optimization of patient activity injected for thyroid scintigraphy in order to approach diagnostic reference level

Abstract

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.