Yasuaki Shoji

Design and experimental validation of a quantitative myocardial /sup 201/Tl SPECT system

We have developed a quantitative SPECT system, and evaluated its potential for quantitative assessment of bio-physiological functions in the myocardium particularly with /sup 201/Tl. Our approach included development of a transmission system that provides accurate attenuation /spl mu/ maps, and implementation of ordered-subset EM reconstruction with transmission data based attenuation correction in addition to scatter correction using the transmission-dependent convolution subtraction (TDCS) technique. The transmission system was designed using Monte Carlo simulation to minimize the scatter in the transmission projection data while keeping loss of sensitivity minimal, and was attached to an opposing 2-head gamma camera fitted with parallel beam collimators. Observed /spl mu/ values agreed quantitatively well with the theoretical expected values in both phantoms and human thorax. Phantom experiments with /sup 201/Tl also demonstrated that, with both corrections for attenuation and scatter, observed images were directly proportional to the actual radioactivity distribution for various phantom geometries. Attenuation correction without scatter correction improved images in deep structure, but resulted in significant artifacts in the chest phantom in addition to dependency of observed radioactivity concentrations on the diameter of cylindrical phantoms. Absolute quantitation of bio-physiological functions, which is well established in PET, is shown to be feasible using SPECT, if both quantitative attenuation and scatter corrections are employed.

Requirement for radiation shields of transportation pipe for on line inhalation gases from compact cyclotron in positron emission tomography

In the unit housing of a compact cyclotron and positron emission CT (PET), positron emitting gas such as 15O, 11C, C15O2, C15O etc. is supplied from a cyclotron to a PET room through a transportation pipe with an appropriate shield to reduce positron annihilation radiation. Using lead or concrete shield blocks with various thicknesses, radiation leakage through the shield was measured by an ionization chamber type survey meter during continuous and constant supply of 15O gas of 1.85 GBq/min concentration which is the maximum dose for clinical use. The leakage radiation measured was 213.7, 56.0, 15.3, 5.0 muSv/week for lead shield with 1, 2, 3, and 4 cm thickness, respectively, and 193.3, 30.5 and 5.1 muSv/week for concrete shields with thickness of 10, 20, and 30 cm, respectively. The present study shows that to keep less than 300 muSv/week, which is the permissible dose rate of the boundary zone around the radiation controlled area by Japan Science and Technology Agency, it is required to use more than 8 mm thick lead shield or 7 cm thick concrete for continuous supply of 1.85 GBq/min 15O gas.