Phys. Med. Bioi., 1982, Vol. 27, No. zyxwvu 1, 25-36. Printed in Great Britain zyxw Proton computed tomography of human specimens K M Hanson, J N Bradbury, R A Koeppe, R J Macek, D R Machen, R Morgado, M A Paciotti, zyxwv S A Sandford and V W Steward? Los Alamos National Laboratory, Los Alamos, NM 87545, USA t University of Chicago, Chicago, Illinois, USA Received 6 May 1981, in final form 24 June 1981 Abstract. The experimental procedure and results of a comparative study of the imaging characteristics of proton and x-ray CT scans are presented. Scans of a human brain and heart are discussed. The proton produced images are found to be similar in information content while providing a decided dose advantage. 1. Introduction In previous work (Hanson et zyxwvu a1 1981) we demonstrated experimentally the dose advantage in computed tomography (CT) afforded by the proton energy-loss method compared to the standard x-ray absorption technique. With this dose advantage, it is feasible to achieve CT reconstructions with significantly better density sensitivity at a given dose by using protons instead of x-rays. Since in proton CT it isthe relative linear stopping power that is imaged instead of the linear attenuation coefficient as in x-ray CT, it might be expected that the two modalitieswould produce entirely different images of human anatomy. In this paperwe present the experimental procedure and some of the results of a study to compare the imaging characteristics of proton and x-ray CT scans of human organs. A comprehensive comparison of the specimen scans will be presented elsewhere. The apparatus is described in section 2 and the experimental method is explained in section 3. The results of the experiment and the comparison with x-ray CT scans are presented in section 4. Concluding remarks are made in section 5. 2. Apparatus The P3-West channel at the Clinton P Anderson Meson Physics Facility (LAMPF) produced protons of excellent energy stability and with a small spot size. Figure 1 shows the experimental layout. The proton beam was scanned across the phantom by a bending magnet positioned 5.2 m upstream from the water bath. A vacuumbox extended the channel vacuum upto a window just in front of the water bath to prevent multiple scattering degradation of the high quality beam spot. The beam possessed a waist near the entrance of the water bath with a horizontalwidth of about 1 mm FWHM and a divergence of 6 milliradians (FWHM). The vertical heightof the beam was limited by a slit to 1.0 cm. The beam energies in the experiment, 224 to 236 MeV, were restricted to a 0.4% FWHM energy spread. The proton beam was swept horizontally back and forth across the phantom by exciting the scan magnet with a triangular current waveform. The current did not 0031-9155/82/010025 zyxwvu + 12 $1.50 @ 1982 The Institute of Physics 25