Three-Dimensional Computer Read-out of Radioisotope Scan Data
William J. MacIntyre, James H. Christie, Gretchen S. Curtis · Radiology · 1968
Since the deposition of radioactivity within an organ is a volume distribution, a three-dimensional portrayal would most closely approximate the spatial relationship of the radioactive material. The relative magnitude of deposition is usually shown on a conventional two-dimensional scan by variation in the number of dots per unit area or by the density of a photographic exposure. Many technics have been devised to make the variation of intensity more readily perceptible. Such devices include multiple cut-offs (3), photographic accentuation (6), and color scanning (10). Another system devised to obtain the configuration of varying deposition is that of contour line construction or isodensity curves. This has been accomplished by rescanning of a photographic negative (2), by television playback (5), and by computer technic (1, 11). An actual three-dimensional representation of liver scans was reported in 1960 by a method using a composite of juxtaposed profiles one-half inch apart (8). These profiles were recorded on clear acetate sheets placed between half-inch Lucite plates and were transilluminated. While the profiles were tedious to record and somewhat difficult to visualize and display, the method was found useful in several clinical situations. In selected cases the transparent view was transferred by a medical artist to an opaque representation (3), as shown in Figure 1. In a similar manner solid figures have been constructed from counting rate profiles or histograms and have been represented by solid rods (2) or layers (7). The purpose of the present report is to show how similar, three-dimensional models may be constructed by a computer with a method permitting views from all sections. Method Spatial information of all scans was arranged in a square matrix of 1,600 elements. Data for each scan were obtained by reading out the density from a large spot data-averaged photoscan (4, 9) at increments of 1∕16 to 1∕4 inch. It is necessary, of course, when digital information is obtained by this indirect method, that linearity of optional density and counts recorded be established (4). In the bounding program used, each element is compared to the mean of the eight surrounding elements. If the value is less than the mean minus the standard deviation it is replaced by the latter value. Similarly any value exceeding the mean plus one standard deviation is replaced by that value. Transition from one element to another, therefore, may not exceed a value of one standard deviation. Following this operation the numbers in each element are averaged in submatrices of nine elements with the center element weighted by a factor of two. The final values are rounded off to an arbitrary number of levels, twenty being sufficient for the distribution portrayed at this time.