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A Transmission-Dependent Method for Scatter Correction in SPECT

Department of Nuclear Medicine, Royal Prince Alfred Hospital, Sydney, Australia

Correspondence: For correspondence or reprints contact: Steven Meikle, Department of Nuclear Medicine, Royal Prince Alfred Hospital, Missenden Road, Camperdown, NSW 2050, Australia.

ABSTRACT

A method of scatter compensation has been developed that incorporates planar transmission measurements in the estimation of photopeak scatter in SPECT. Methods: The scatter distribution is first estimated by convolving the plenar projections with a monoexponential scatter function. The number of scattered events that subsequently reach the detector as a proportion of total events (i.e., scatter fraction) is then determined for each point in the projections based on narrow-beam transmission values, obtained using an external source. The assumptions of the method were tested using ^99mTc and ²⁰¹Tl and point and line sources. The quantitative and qualitative impact of transmission-dependent scatter correction was assessed in realistic phantom experiments simulating blood-pool, lung and myocardial perfusion studies. Results: The method accurately predicts the scatter distribution from ^99mTc and ²⁰¹Tl line sources in a phantom with variable density. Reconstructed counts are artificially enhanced in regions of high tissue density when scattered events are not removed from the projections prior to attenuation correction. Using convolution-subtraction with a constant scatter fraction (k = 0.4), scatter is underestimated in the heart and overestimated in the lungs, whereas transmission-dependent scatter correction enables activity to be quantified with 95% accuracy in heart and lung regions. Conclusion: We conclude that incorporating transmission data enables accurate scatter compensation in objects with nonuniform density.

Key Words: Compton scattering • transmission tomography • attenuation correction • quantitative SPECT

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