SPECT imaging of pediatric brain tumor with hexakis (methoxyisobutylisonitrile) technetium (I). Technetium-99m-Hexamibi [Hexakis (methoxyisobutylisonitrile) technetium (i)] was developed as a myocardial perfusion agent with biologic properties similar to those of thallium-201 (201TI). As 201TI has recently been observed to be of value for the diagnosis of brain tumors when used in conjunction with single-photon emission computed tomography (SPECT) imaging technology, the possibility that the biologic similarity of the two radiopharmaceuticals extended to their affinity for tumors was tested. A 5-yr-old female patient with a brain stem astrocytoma showed marked focal uptake of 99mTc-Hexamibi at the site of tumor recurrence as defined by biopsy and prior 201TI/SPECT study. Tumor-to-normal cortex radioactivity ratios for the 99mTc-Hexamibi/SPECT study were 132:1 and the spatial resolution of the 99mTc-Hexamibi images was high. This observation suggests that 99mTc-Hexamibi merits further study as a potential agent for SPECT imaging of human brain tumors. 