Image Separation Radioisotope Scanning

Abstract

Discrimination of an image from its background in radioisotope scanning depends on many factors. Photoscanning (1, 2) is a useful means of increasing contrast. Discrimination also depends on how well the image contours may be separated from others in the background, and improvement should be possible, in some instances, by separation of images of radioactivity according to their depths in the body. We have sought to accomplish this by exploring the application of principles of stereoradiography and body-section radiography to radioisotope scanning. Rectilinear Scanning The rectilinear scanning pattern is most commonly employed; that is, an area of the body is scanned in a sequence of straight parallel strips (Fig. 1). With rectilinear scanning, images from all levels in the body are superimposed on the recording. As a consequence of this overlapping, the contours of a tumor image may be lost in a confusion of patterns resulting from overlying or underlying radioactivity. The characteristics of ordinary rectilinear scanning were demonstrated with a laboratory scanner to furnish a base line against which the results of other scanning modes could be compared. The instrument was assembled from the mechanical drive of a milling machine and equipped with an oscilloscope photorecording device. A shielded scintillation detector with a NaI(Tl) crystal 3 inches in diameter and 2 inches in thickness was used. The 31-hole focused collimator had a 3-inch focal length and provided approximately 1/4-inch radius of view at the focus. Pulse-height analysis was employed in counting to exclude all but the 364-kv photopeak pulses from I131. Scanning speed was 0.40 cm. per second and the line spacing was 0.5 cm. A Masonite phantom was constructed, approximately the size of a human head (Fig. 2). Twelve radioactive cylinders were distributed in three different planes within the phantom. Each cylinder measured 2 cm. in width and height, and each contained 5 microcuries of I131. For rectilinear scanning, the mechanical drive moved the detector back and forth across the entire top area of the phantom. At the same time, the spot of a cathode ray oscilloscope was moved back and forth in a rectilinear raster, synchronized with the position of the scanning detector by a potentiometer circuit geared to the traversing mechanism. The oscilloscope beam brightness was electronically modulated by the count rate. An oscilloscope camera with open shutter recorded the scanning pattern on film by photographic integration from the screen. The rectilinear scanning pattern which resulted is shown in Figure 3. In this recording, the sources on all planes are superimposed without depth discrimination. Some images are hidden by those above and the pattern of radioactivity on any particular plane cannot be distinguished from that on any other plane.

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