Technical note: computed tomography imaging with a radiotherapy simulator.

An inexpensive system for obtaining cross-sectional information and accurate body outlines of patients destined for radiotherapy, using a radiotherapy simulator without any major modifications, has been investigated. The image intensifier of the simulator was moved laterally and a narrow fan beam of X-rays passed through the phantom onto the intensifier. Several television (TV) lines of the video signal from the TV camera were digitized by a frame grabber and stored for reconstruction. Multiple projections were acquired by rotating the gantry of the simulator. The field of view was enlarged by increasing the offset distance of the image intensifier and taking two sets of projections. Reconstruction was carried out by using the convolution and back-projection method. The gradient between pixel values in the reconstructed images was used to detect the outlines of structures in the images. The accuracy of outline detection was evaluated with images of a Rando phantom. The outlines of the images were compared with the actual outlines of the phantom. The spatial resolution of the simulator computed tomography (CT) was measured to be 4.05 mm. Large inhomogeneities could be clearly seen. The average difference between the measured and the actual outlines was 3.0 mm with a maximum difference of 10.0 mm at sharp curves in the outline. The simulator CT provides an inexpensive, alternative method of obtaining body outlines and does not require any modifications to the simulator. Data acquisition, processing and display can be performed on a personal computer with image processing facilities.