Acute splenic sequestration crisis in two adults with sickle cell disease: US, CT, and MR imaging findings. Acute splenic sequestration crisis (ASSC) is a rare complication in adults with sickle cell disease that is diagnosed clinically by means of sudden splenic enlargement and a rapid fall in hematocrit. Two cases of ASSC in adults with heterozygous sickle cell disease (sickle cell-thalassemia and sickle cell-hemoglobin C disease) were studied with use of duplex Doppler ultrasound (US), computed tomography (CT), and magnetic resonance (MR) imaging. In both cases, US showed patency of the splenic vein and multiple hypoechoic lesions on the periphery of an enlarged spleen that were of low attenuation on CT scans and hyperintense on both T1- and T2-weighted MR images. These findings were believed to be suggestive of subacute hemorrhage. This was confirmed pathologically in one case and suggested in the other by the presence of a low-signal-intensity ring, probably hemosiderin, surrounding one of the lesions. Also, the remainder of the spleen in both patients was of normal signal intensity, unlike the diminished signal intensity seen in patients with homozygous sickle cell disease. Further study is needed to determine the role of imaging in the diagnosis and treatment of ASSC. 