Attenuation coefficients for Tc-99m photons in water-filled phantoms, determined with a gamma camera: Variation with energy window

Values of the linear attenuation coefficient, μ, near 0.12 cm/sup -1/ used in attenuation compensation of Tc-99m ECT scans of uniform cylindrical phantoms give better results than use of 0.15 cm/sup -1/. The latter value is theoretical, and does not take into account recoil photons from Compton scattering, but continues to be used in new SPECT software development and other quantitative imaging situations. To investigate the influence of Compton scattering on the effective linear attenuation coefficient, a gamma camera and a multichannel pulse height analyzer were used to determine values of μ for photons in water as a function of energy window, using the entire camera field as a region of interest. Two cylindrical water-filled phantom, circular (22 cm O.D.) and elliptical (22.5 x 30.5 cm O.D.) were used with point sources of Tc-99m at depths up to 21 cm. Energy spectrum data were integrated over: top half of photopeak, 10%, 20% and 30% centered windows and bottom half of photopeak. Almost all attenuation plots were exponential, with highest values of μ (0.122 to 0.128 cm/sup -1/ at top half of photopeak, and with μ decreasing as the energy threshold was lowered (0.117-0.116, 0.108-0.106, 0.105-0.097, and 0.100-0.091 cm/sup -1/ for windows noted above, for circular and elliptical (short axis) phantoms, respectively). This variation with energy window suggests that there is within the ''photopeak'' a distribution of scattered photons the magnitude of which increases with decreasing energy, and accounts for improved attenuation corrections with values of μ less than the theoretical 0.15 cm/sup -1/