Which attenuation coefficient to use in combined attenuation and scatter corrections for quantitative brain SPET?

We read with interest the recent paper by Shiga et al. published in the European Journal of Nuclear Medicine [1]. The authors report some interesting results concerning the effect of triple-energy window-based scatter correction on brain perfusion SPET using statistical para-metric mapping (SPM) analysis. The research performed is worthwhile and contributes significantly to our understanding of the effect of scatter correction; to the best of our knowledge, it is also the first time that SPM analysis has been used for this purpose instead of conventional qualitative and region of interest (ROI)-based quantitative evaluations [2]. However, we feel that certain relevant issues were not sufficiently addressed by the authors , and we would like to make some comments on this work. The variety of pertinent publications in this field emphasises the importance of methodological considerations. Unfortunately, there are a considerable number of references relating to the subject that were not cited in opinion, the reader would have gained a clearer picture of research performed in the field if these references had been cited and discussed. Firstly, the way in which the authors perform combined attenuation and scatter correction is not well elucidated in the Materials and methods section. There seems to be a misunderstanding regarding the choice of the linear attenuation coefficient to be applied during attenuation correction in brain SPET studies. It is well known that while attenuation decreases the number of photons which can be acquired from a source, scatter will add photons. Correction of simply the number of detected photons can be performed using lower values for the narrow-beam attenuation coefficient [5] (e.g., µ=0.10–0.12 cm –1 rather than µ=0.15 cm –1 for 99m Tc-labelled compounds), the most appropriate choice being dependent on the energy window setting. This empirical approach was often implemented in commercial systems in the 1990s and is considered to be an intrinsic scatter correction procedure [14]. A slightly lower value of the attenuation coefficient is used for the following reason. The full value of µ predicts how many photons will be removed from a single, narrow beam of radiation owing to the combined processes of absorption and scatter. It ignores the number of photons that can be scattered into the path from other directions. That is, it ignores the build-up caused by the broad-beam conditions of nuclear medicine imaging. Use of the actual narrow-beam value of µ without explicitly correcting for scatter will overcorrect for …