COMPUTER-FOCUSING FOR AREA SCANS

Abstract

Scintillation area scanning is a relatively new diag nostic method in clinical medicine and its use has progressed rapidly over the years. Because the method is used to visualize the spatial distribution of radioactivity in internal organs, one would like to be able to detect and display the smallest possible lesions. Many methods using a wide range of in struments and radiopharmaceuticals have been advo cated to increase the resolving power of the scanner. At the present time, however, the resolution of area scanners is not sufficiently sharp. A multichan nel focusing collimator consisting of a honeycomb of hexagonal holes is usually used, but even with this focusing collimator the region of response is broad and has a circular cross section at the focal distance. The sensitivity in scanning must also be increased, but at present this can be achieved only by sacrificing spatial resolution. In a previous paper (/) we reported that a correc tion method based on iterative approximation which had been used to correct distortion in beta and gamma spectra (2) could be used to extract true information from the observed data; with this method corrected profiles obtained with a wholebody linear scanner showed a more detailed struc ture than did the original. We felt that a similar correction method could be used in the image of area scans. At present a wide variety of analog techniques are used to record area-scanning data. These analog techniques, however, appear to offer less accurate recording and result in a loss of information. More over, they are not adaptable to computer analysis. To use digital-computer processing, it is necessary to use digital recording in which all original information in an unmodified form is collected and recorded as an array of actual numbers. The purpose of this paper is to show how digital information suitable for computer processing can be used and how more information can be obtained from computer-corrected area scans than from the original digital scan or conventional analog data presentation. METHODS The data-collection system consisted of a commer cial scanner with a Nal(Tl) crystal, 2 in. in diam eter and 2 in. thick, and a 19-hexagonal-hole honeycomb collimator. Pulses from a single-channel pulse-height analyser were fed into a 128-channel multichannel analyser used in the multiscaling mode. Because a bidimensional multiscaler was not avail able, the single-dimensional 128-channel multiscaler was used to present a numerical profile for each scan sweep. One-way scanning was done at a speed of 2.7 mm/sec, and 1-mm spacing was selected. The pre set counting time in each channel of the multiscaler was 0.38 sec. Consequently each channel corre sponded to the accumulation of counts from a length of 1 mm of scan sweep at the scanning speed se lected. The recording of the counts for each sweep was started just when the reference point of the de tector passed over the scan registration line which met the scan sweep direction at right angles in order to include precise positional information. After each scan sweep, the counts accumulated in each of the 128 channels were printed with a Hewlett Packard fast printer. During the printing the detector returned to the next starting point but spaced 1 mm perpen dicular to the sweep direction. Then the multiscaler started to accumulate counts for the next scan sweep. A channel number corresponded to the position of the detector in each scan sweep, and the number of the sweep corresponded to the position in the space direction. Thus to provide a two-dimensional array of numbers representing area scanning data