Visualization by means of scintiscanning of organs that concentrate compounds labeled with gamma- or x-ray-emitting radioactive elements is now a well accepted clinical procedure. The apparatus most commonly used for this purpose is a scintiscanner, which is a scintillation counter fitted, in general, with a focusing collimator. The detector scans the area to be examined with a back-and-forth motion, and the information carried by the gamma- or x-ray photons is converted into a printed pattern. In a conventional scintiscanner the collimator excludes from the crystal all photons except those emitted from a small defined volume; from the standpoint of the examination, the radiation emitted by the rest of the organ is wasted. A system which would not suffer this inadequacy, namely, an instrument which would “look” at the entire organ continuously, would appear desirable. An apparatus embodying this feature has been developed by Dr. H. O. Anger of the University of California (1). This consists essentially of (a) a gamma-ray pinhole camera, (b) a detector composed of a large sodium iodide crystal fitted to an array of photomultiplier tubes, and (c) an electronic system permitting computation and reproduction of the location of a light signal in the crystal, in a homologous position on a cathode-ray tube. A similar result can be achieved with a conventional x-ray image amplifier. A full description of an image amplifier is outside the scope of this report. Briefly, it con- verts the energy of x-rays into light and multiplies the light photons thus obtained by a large factor. A typical light gain in such an amplifier is of the order of 3,000. The input screen of a conventional x-ray amplifier is thin, and its efficiency in stopping high-energy gamma rays emitted by most radioactive isotopes is low. This apparatus, however, can detect efficiently low-energy electromagnetic radiation such as that emitted by iodine 125, as suggested by Myers and Vanderleeden (2). Iodine 125 is a radioactive isotope of iodine which decays with a half-life of approximately sixty days, emitting x-rays with energies of the order of 30 kv (2). These x-rays can be readily detected by an x-ray amplifier, as they fall within the energy range for which this instrument was developed. Because of the low energy of the radiation emitted by iodine 125, this isotope is probably unsuitable for the visualization of deep-seated organs. On the other hand, it is adequate for the visualization of the thyroid, which lies near the skin surface. The apparatus used in this study consists of a honeycomb type collimator, an x-ray image amplifier, a fast lens system, and a Polaroid camera (Fig. 1). Several collimators were tested with holes of the order of 1.0 mm. in diameter, and of heights varying from 5 to 10 mm.