Attenuation correction of (111)In planar images by use of dual energy, fundamental study by Monte Carlo simulation.

In this study, we devised and evaluated a method for attenuation correction of the hot spot in (111)In planar images. By use of the difference in transmittance between two energies (171 and 245 keV), the depth of the hot spot was calculated. Planar images of point sources in a numerical phantom (water) with depths from 0 to 20 cm at 2 cm intervals were prepared by Monte Carlo simulation. From the linear attenuation coefficient of the two energies and the 171/245 keV count ratio-depth relationship, the depth of the point source was calculated, and an attenuation correction was performed. A simulation was made under conditions taking into account both attenuation and scatter (A(+)S(+)) and attenuation alone (A(+)S(-)). The attenuation correction was evaluated with use of corrected and true counts obtained from homogeneous phantoms mimicking attenuation in soft tissue, bone, and the lungs, and heterogeneous phantoms prepared by combining them. In the A(+)S(+) condition, images were affected markedly by scattered photons in all phantoms at depths of 4-8 cm. The errors at depths of 10 cm or greater were within ±10 % in water and within ±6 % in soft tissue. However, the errors were about -30 % in bone and about +70 % in lung, indicating that scatter distributions different from those in water increased the errors. In the A(+)S(-) condition, the errors were within ±5 % in all homogeneous and heterogeneous phantoms, and satisfactory results were obtained. Precise attenuation correction of scatter-corrected planar images was confirmed to be possible with this method.