99mTechnetium-labeled Macroaggregated Albumin Study in HPS

99mTcMAA nuclear medicine scintigraphy scans may conclusively document the presence of right-to-left shunt. These scans employ a radioactive tracer attached to albumin particles, which range in size from 10-40 microns in diameter. Normally, particles of this size, when injected intravenously, embolize the precapillary pulmonary arterioles and do not pass into the systemic circulation. However, in patients with HPS, the 99mTc-MAA particles may pass directly through the dilated vessels to embolize systemic organs, such as the brain, spleen, or kidney. Such systemic tracer embolization is characteristic of right-to-left shunting, but can be seen with cardiac or extracardiac shunts. Therefore, exclusion of non-pulmonary causes of right-to-left shunting is required to firmly establish the diagnosis of HPS.


Coronal image from a 99mTcMAA study shows radioactive tracer uptake in the brain (arrow), confirming the systemic embolization of the injected radiopharmaceutical. While systemic embolization of tracer does not necessarily confirm that the right-to-left shunt is occurring through intravascular pulmonary dilations, in the setting of end-stage liver disease and hypoxemia, such systemic embolization is highly suggestive of HPS.



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