AN INHERENTLY STABLE SYSTEM FOR MEASURING TISSUE IODINE USING ITS K-ABSORPTION EDGE

¹ Research and Neurology Services, Wadsworth Hospital, Veterans Administration, Los Angeles, California; and the Department of Neurology, School of Medicine, University of California, Los Angeles, California.
² The Edward Mallinckrodt Institute of Radiology, Washington University, St. Louis, Missouri.
³ The University of Chicago and the Argonne Cancer Research Hospital, Chicago, Illinois,
⁴ Department of Neurology, School of Medicine, University of California, Los Angeles, California.

An experimental system for detection of small concentrations of iodine in tissue is described.

A collimated beam of lanthanum Ka1 and Ka2 x rays is passed through phantoms of iodine and water. The energies of these 2 x rays straddle the K-absorption edge of iodine and are differentially absorbed by the iodine phantom. After emerging from the phantom, the 2 x rays are separated by a stable iodine filter placed in the beam path with resulting photoelectric absorption of the Ka1 x rays on the high side of the K-edge of iodine. This results in production of isotropically propagated iodine fluorescence x rays which are counted by a surrounding NaI(Tl) crystal-photomultiplier detector. The Ka2 x rays are counted by a second, more distant detector. The ratio of the Ka2/Ka1 x rays is determined by simultaneously counting the scintillations produced in these 2 detectors. In the phantom, an iodine concentration of 1 mg/cm² produces a change in this ratio of about 3 per cent. The change in this ratio is about 1,000 times greater and in the opposite direction for iodine in the beam path relative to an equivalent amount of water.

The system is inherently stable because the original photon energies and the separation threshold are established by the electron shell energies of lanthanum and iodine.

Technical advantages and disadvantages of this system are discussed.

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