SPECT versus planar scintigraphy for quantification of splenic sequestration of 111In-labelled platelets

The splenic uptake of thrombocytes and spleen size were studied in 25 patients with idiopathic thrombocytopenic purpura (ITP) using two methods: anterior/posterior scintigraphy and single photon emission computed tomography (SPECT). Various factors (acquisition and reconstruction protocols) influencing the quality of 111In SPECT were studied. The splenic uptake, measured by SPECT, was found to be significantly higher in patients with a high level of autoantibodies in the blood than in patients without such antibodies (37 +/- 6%, n = 8 vs 22 +/- 3%, n = 11, P = 0.04, t-test). The correlation between the spleen SPECT volume and the geometric mean size in anterior and posterior images was 0.80 (P < 0.001). However, the spleen size calculated as geometric mean of anterior and posterior images differed by more than 50% from the SPECT volume in some patients. Based on these observations and on the results of phantom studies it is concluded that a reasonable estimate of the spleen:liver uptake ratio may be obtained using planar imaging, but to estimate the spleen volume and the absolute splenic uptake of platelets SPECT imaging is needed, in spite of the present technical limitations of SPECT.