SCATTER AND ATTENUATION CORRECTION FOR IN-111 BASED ON ENERGY-SPECTRUM FITTING

A combined scatter and attenuation correction that does not require a transmission scan is proposed for In-111 imaging. Estimates of the uns cattered intensity at both 171 and 245 keV are obtained by fitting the observed energy spectrum at each pixel or region of interest using th e measured scatter-free spectrum and a simple model for scatter. The s catter model for the 171 keV peak accounts for scatter contributed by both the 171 and 245 keV emissions. After correcting for scatter, the attenuation is estimated from the observed ratio of photopeak intensit ies using the known difference in attenuation at the two emission ener gies and a model based on a point source in water. Accurate scatter co rrection is a prerequisite for the success of this method because scat ter from the higher energy emission will otherwise contaminate the low er photopeak. This differential attenuation method (DAM) of estimating attenuation is demonstrated and calibrated using a series of point so urce measurements with a wedge-shaped attenuator. The observed absolut e and differential attenuation are in good agreement with the narrow-b eam linear attenuation coefficients for water. Estimates of precision suggest a depth resolution of 1.0-2.5 cm for realistic count densities over the clinically relevant depth range (0-25 cm). The accuracy of D AM in a more realistic attenuation environment is assessed using a hot sphere inside the anthropomorphic data spectrum torso phantom viewed from several angles (with differing attenuation). Finally, the potenti al of DAM for SPECT attenuation correction was investigated by compute r simulation using the SIMSET Monte Carlo software. Preliminary result s based on measured planar data and simulated SPECT data indicate that DAM can improve the quality and quantitative accuracy of In-111 image s. In one SPECT simulation study, the average error in tumor to soft-t issue ratios was reduced from 32% for uncorrected data to 8% for data corrected with DAM. However, the technique is susceptible to significa nt noise amplification and can cause substantial streak artifacts in l ow-count SPECT studies if sufficient smoothing of the depth estimates is not performed. (C) 1996 American Association of Physicists in Medic ine.