Transmission scanning: a useful adjunct to conventional emission scanning for accurately keying isotope deposition to radiographic anatomy.

Abstract

In 1952 Mayneord (1) proposed the usefulness of a transmission counter for plotting outlines of organs. Later, he produced images of lead letters by recording the transmitted beam from a Tm170 (84 keV) source which was maintained in direct alignment with the aperture of a scintillation detector during scanning (2, 3). In 1963, Cameron and Sorenson reported use of Am241 (60 keV) and I125 (30 keV) sources to determine bone mineral content by measuring the change in intensity of a transmitted photon beam moved across a bone (4). No clinical application of transmission scanning for forming images of body structures was reported by these authors. We have explored transmission scanning as a means of improving the orientation of the radionuclide emission scan (5). Accurate evaluation of a radionuclide distribution in the body requires that the spatial relationships of emission scan data be oriented to the anatomy of the patient. Usually, data on the scan record are keyed to anatomy either by using reference marks to represent external features or by superimposing a roentgenogram of the part (6). These methods may introduce inaccuracy due to geometric distortions; also, if the patient moves during the scan, counting data and anatomic reference may no longer correspond. Transmission scanning can be employed to reduce these inaccuracies. During a conventional emission scan, a small radioactive source of either Am241 or I125 is made to move under the patient so as to follow the motion of the detector (Fig. 1). The photons from this source are collimated and directed through the patient to the detector. Pulse-height analysis is used to separate the emission and transmission counting data which are then recorded separately. As the scan progresses, the point-to-point variation of count rate from the transmitted beam depends on attenuation by anatomic structures. The transmission scan image that is reconstructed from these data is similar to a roentgenogram of the scanned part and can be oriented to the corresponding emission scan image with no geometric distortion. Any patient movement during the study is apparent in both records. Method 1. Scanning and Read-Out System: The scanning and read-out system used in this study is described in detail elsewhere (7–9). The patient is supported on a cantilever aluminum pan 0.6 em thick. A pair of opposed scintillation detectors, rnechanically coupled by a yoke, scan both sides of the body at the same time. A maximum field size measuring 35 cm wide and 115 cm long can be scanned on any plane parallel to the long axis of the body. The crystal of each detector is 5.1 cm thick and 7.6 cm in diameter. Focused collimators with either low-energy or highenergy designs are used, depending on the choice of radionuclide for the emission scan (10). During the study, counting and position data are recorded in binary code on perforated paper tape. Each character on the paper tape is made to represent counts integrated in a predetermined elapsed detector travel distance.