The influence of backscatter material on Tc-99m and Tl-201 line source responses

SPECT projections are contaminated by scatter, resulting in reduced image c ontrast and quantitative errors. When tissue is present behind the source, some of the detected photons backscatter via this tissue. Particularly in d ual-isotope SPECT and in combined emission-transmission SPECT, backscatter constitutes a major part of the down-scatter contamination in lower-energy windows. In this paper, the effects of backscatter material were investigated. Plana r images of Tc-99m and Tl-201 line sources between varying numbers of Persp ex slabs were analysed using the photopeak windows and various scatter wind ows. In the Tc-99m photopeak window no significant change in total counts due to backscatter material was measured. In the Tl-201 photopeak window an incre ase of about 10% in total counts was observed. In the scatter windows an ev en more explicit influence of backscatter material was measured. For instan ce, at a forward depth of 10 cm, total counts of a Tc-99m source detected i n the 72 keV window eventually doubled with increasing backscatter material , compared with the situation without backscatter material. The backscatter contribution plateaued when more than 5-10 cm of scatter material was plac ed behind the source. In conclusion, backscatter should be taken into account, particularly in mo del-based down-scatter correction methods in dual-isotope SPECT and combine d emission-transmission SPECT.