Proton computed tomography applied to small biomedical samples

The computed tomography (CT) using the high-energy proton beam has the potential advantage of superior density resolution in the soft tissue analysis as compared to the conventional X-ray CT.We have developed the "first generation" type proton CT system using a 25-MeV proton beam to study the density distributions in small biomedical samples (<6 mm ϕ).The specimens were fixed in a cylindrical sample holder that was immersed in a water bath. The sample holder was scanned in front of the external semimicro beam of protons with a width of 0.06 mm at full width of half maximum (FWHM). The residual energy of the penetrated proton beam was analyzed with a solid state detector (SSD). The obtained integral density projection views were reconstructed into the density image by the filtered back projection method using the Shepp-Logan filter function.The spatial resolution of 0.1 mm and the density resolution of 0.1% were obtained by our system.The technique was applied to the eyes of small animals to study the changes in the density distributions of lenses, which develop with lens opacification in cataracts.