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Comparison of ^113mIn and Other Short-Lived Agents for Cerebral Scanning

State University of New York, Upstate Medical Center, Syracuse, New York

Correspondence: For reprints contact: Robert E. O'Mara, Div. of Nuclear Medicine and Radiobiology, Upstate Medical Center, State University of New York, Syracuse, N.Y., 13210.

ABSTRACT

^113mIn-DTPA is a satisfactory short-lived agent for brain-imaging applications. The degree of accuracy in the diagnosis of cerebral lesions in this small clinical series was comparable to ^99mTc-pertechnetate. ^113mIn-DTPA appeared to offer certain advantages over the latter agent including: (1) more rapid clearance from the bloodstream; (2) superior tumor-to-brain concentration ratios in both transplanted ependymoblastomas in mice and in human tumors; (3) absence of visible concentration in the choroid plexus and salivary glands; and (4) more economy for continuous use because of the much longer physical half-life of the parent nuclide ¹¹³Sn. Thus in remote geographical areas one may be relieved of the dependence on weekly shipments of a generator system.

These features, however, are offset by certain characteristics which are inferior to those of ^99mTc: (1) the higher energy of the gamma emission tends to produce somewhat inferior images to ^99mTc in resolution and, because collimators with thicker septa must be used, the sensitivity of detection of ^113mIn is lower for the same activity of ^99mTc; (2) the fraction of gamma emission undergoing internal conversion is higher than for ^99mTc, and this contributes to the radiation dose; (3) the radiation dose to the "critical organ," the bladder, is higher than for ^99mTc; (4) the agent is not as satisfactory as ^99mTc for imaging studies with the Anger camera because of somewhat poorer resolution and fall-off in detector efficiency with the higher gamma energy; and (5) repeat studies cannot be performed in the face of unsatisfactory examinations because of the rapid blood and tissue clearance combined with a relatively short physical half-life.

Compared with ⁶⁸Ga, the only other radioactive cation available from a generator system, ^113mIn appears to be much superior for imaging applications except for coincidence counting systems. Furthermore, the radiation dose levels are much lower than for equivalent activities of ⁶⁸Ga.

As a radioactive cation ^113mIn may find important applications in the future in various compounds which cannot be labeled with anionic ^99mTc.