Tc-99m attenuation coefficients in water-filled phantoms determined with gamma cameras

Quantitative imaging with gamma cameras requires compensation for attenuation of source photons. Some methods of compensation make use of a constant or average estimated attenuation coefficient μ. A value for μ of 0.15 cm-1 for 140.5-keV photons in water or tissue is commonly used. This value, however, neglects scattered photons which are detected within the energy window in gamma camera imaging. Values for μ of 0.12 cm-1 used in attenuation compensation of Tc-99m single-photon emission computed tomography scans of uniform cylindrical sources have been shown to give improved results compared with use of μ = 0.15 cm-1. In this study, gamma cameras and a multichannel pulse-height analyzer were used to determine effective values of μ for photons in water as a function of energy window. Two cylindrical water-filled phantoms, circular and elliptical, were used with a point source of Tc-99m at depths up to 18 cm. Energy data were integrated over the top half of the photopeak, and over 10%, 20%, and 30% windows centered on the photopeak. Attenuation curves were exponential for all photopeak windows with values of μ of 0.12- +- 0.014 cm-1 for all windows up to 20% and 0.1 cm-1 for a 30% window. This study suggests that a value of μ of 0.11--0.12 cm-1 is, in fact, appropriate for use in attenuation compensations where an average is required