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

Abstract

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 parametric 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 the paper (e.g. [3,4,5,6,7,8,9,10,11,12,13]). In our 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 narrowbeam value of µ without explicitly correcting for scatter will overcorrect for attenuation, and an image excessively hot in the centre will result. Although images may be cosmetically enhanced, improvements in image quantitation are only minimal with the reduced μ method of scatter correction. This is because this method assumes that scatter affects all locations in the image to the same extent. However, scatter is object and depth dependent, and a correction method which does not take this fact into accoun...

Establishing On-Site Reference Values for 123I-FP-CIT SPECT (DaTSCAN®) Using a Cohort of Individuals with Non-Degenerative Conditions

“…Uniform Chang attenuation correction was performed to compensate for photon attenuation, using the theoretical mean attenuation coefficient of soft tissues for 159 keV 123I photons (0.15 cm 2 ) and a triple-energy window method for scatter correction [30].…”

Attenuation compensation in cerebral 3D PET: effect of the attenuation map on absolute and relative quantitation

“…Similarly, the paradoxical effect of the complexly shaped skull base surrounding the lower half of the human brain in lowering the mean effective broad-beam attenuation coefficient is well understood in SPET imaging [5,15,16]. However, very few studies have been performed to characterise this effect using 3D PET data.…”

“…Theoretically, measured transmission images introduce the least bias. While the clinical relevance of non-uniform attenuation correction is well established in thoracic imaging, it is still the subject of heated debate in brain scanning, where in most cases clinical diagnosis is based on qualitative assessment of PET images [2,3,4,5,6]. In principle, brain PET studies simplify this task owing to the homogeneous characteristics of tissue components in the head region [4].…”

Experimental determination of the weighting factor for the energy window subtraction-based downscatter correction for I-123 in brain SPECT studies

“…Attenuation correction with Chang’s first-order correction[ 13 ] was applied with an empirical linear attenuation factor of 0.10 cm –1 for I-123 imaging without Compton scatter correction of the primary 159 keV photons. [ 11 14 – 16 ] The determination of the attenuation map was aided by an algorithm, which finds the most outward placed crossing of a manually set threshold (tuned at the edge) and the intensity for every projection angle in the sinogram [Figure 2a and b ].…”