Abstract
1787
Objectives: It is essential to correct count losses due to dead time of gamma camera in dosimetric studies performed after therapeutic dose administration based on phantom studies. The purpose of this study was to find out the maximum administered activity of 131I at which there is no count loss in a modern gamma camera when filled in a phantom.
Methods: A cylindrical phantom made up of Lucite having diameter 20 cm and length 32 cm was filled with water. The phantom was capped after adding 155.44 mCi of 131I and mixed properly. The phantom was scanned every day to acquire anterior and posterior views at an interval of 24 hours for 68 days till the activity decayed to 0.48 mCi under a single head gamma camera with high energy collimator. The acquisition protocol (continuous mode at 15cm/minute scan speed, auto-contouring off, photopeak at 364 keV and energy window at 20%) was kept same to that used for dosimetric study in patients. To know the counts in phantom an ROI was drawn over it on anterior and posterior views and geometric mean was obtained for attenuation correction. For background correction a separate ROI was drawn outside the phantom area and it was corrected on pixel basis. The phantom and background ROI was stored and pasted on all the images of 68 days. The observed count in a phantom was compared with the extrapolated counts to determine the percentage count loss. A time-counts graph (log-linear) was also plotted to show the pattern of deviation between observed and expected counts. Results: There was no count loss upto 3.76 mCi but was observed above this activity. The loss percentage increased with the increase of activity in phantom. At maximum activity i.e. 155.44 mCi the loss was 46% (table 1). However, the graph of observed and expected counts showed almost linear pattern between 0.48 to 27.51 mCi (figure 1). In another study done by us in patients where patients were scanned under the same scanner at 2 h after therapy dose administration of upto 150 mCi keeping the same acquisition protocol, the count loss was not significant when extrapolated from the diagnostic activity (2 mCi) counts. Our study shows that count loss due to dead time in modern gamma cameras as calculated with phantoms differs significantly from in vivo results. In-vitro technique does not seem to simulate the in-vivo settings. This could be explained probably the activity that is administered to the patient gets biologically distributed and does not behave as a point source like in a phantom.
Conclusions: Iodine-131 filled in a phantom showed no counting loss upto 3.76 mCi.