[Basic studies on radionuclide computed tomography using a rotating chair (author's transl)].

Radionuclide Computed Tomography (RCT) was studied from the technical standpoint of view. In this study, a gamma camera (Ohio Nuclear sigma 410S) and a rotating chair designed by one of us were used. The computer used was Scintipac 1200 (32 kW memories and 2.4 MB X 2 disk memories). A cylindrical phantom having a diameter of 20 cm was also designed by us into which various-sized tubes could be inserted for resolution study. The phantom was set on the chair, the center of which was 20 cm off from the surface of the detector. The chair was rotated manually 10 degrees, and finally 36 digital images in the form of 64 X 64 elements were obtained, covering an entire circumference. RCT images were displayed in the form of 128 X 128 elements on a X-ray film through a Microdot Imager. At first, the phantom was filled with 99mTc solution and the uniformity of the RCT image on it was checked using several formulae for count rate correction to find out which one of the formulae was best fitted. For the reconstruction of the RCT image, "filtered back projection" was used. Then, we found that, as far as out phantom study was concerned, simple geometrical mean on the data from the two opposing directions was found the best for the count rate corrections, which was exclusively used thereafter. The fluctuation on the uniform source was found to be approximately 15%. For the resolution study, hot tubes having diameters of 1.0, 2.0, 3.0, and 4.0 cm and cold tubes of 1.5, 2.4, 3.5, and 4.7 cm were inserted into the phantom. As for the hot tubes, all the tubes were depicted on the RCT image whereas 2.4 cm was the smallest depicted for the cold tubes. FWHM was checked with a fine line source in the phantom and was found to be 2.0 cm regardless to its depth. The Alderson liver phantom was also used to detect defects in the air and it was found both balls having diameters of 2.7 and 3.7 cm could be depicted on the RCT image. Now we are evaluating clinical usefulness of this technique on the liver. The results will be published in the near future.