Comparison of maximum likelihood and conventional PET scatter scaling methods for ¹⁸F‐FDG and ⁶⁸Ga‐DOTATATE PET/CT

Abstract

Purpose We aim to quantify differences between a new maximum likelihood (ML) background scaling (MLBS) algorithm and two conventional scatter scaling methods for clinical PET/CT. A common source of reduced image quantification with conventional scatter corrections is attributed to erroneous scaling of the initial scatter estimate to match acquired scattered events in the sinogram. MLBS may have performance advantages over conventional methods by using all available data intersecting the subject. Methods A retrospective analysis was performed on subjects injected with 18F‐FDG (N = 71) and 68Ga‐DOTATATE (N = 11) and imaged using time‐of‐flight (TOF) PET/CT. The scatter distribution was estimated with single scatter simulation approaches. Conventional scaling algorithms included (a) tail fitted background scaling (TFBS), which scales the scatter to “tails” outside the emission support, and (b) absolute scatter correction (ABS), which utilizes the simulated scatter distribution with no scaling applied. MLBS consisted of an alternating iterative reconstruction with a TOF‐based ML activity image update allowing negative values (NEG‐ML) and nested loop ML scatter scaling estimation. Scatter corrections were compared using reconstructed images as follows: (a) normalized relative difference images were generated and used for voxel‐wise analysis, (b) liver and suspected lesion ROIs were drawn to compute mean SUVs, and (c) a qualitative analysis of overall diagnostic image quality, impact of artifacts, and lesion conspicuity was performed. Absolute quantification and normalized relative differences were also assessed with an 18F‐FDG phantom study. Results For human subjects 18F‐FDG data, Bland‐Altman plots demonstrated that the largest normalized voxel‐wise differences were observed close to the lower limit (SUV = 1.0). MLBS reconstructions trended towards higher scatter fractions compared to TFBS and ABS images, with median voxel differences across all subjects for TFBS‐MLBS measured at 1.7% and 7.6% for 18F‐FDG and 68Ga‐DOTATATE, respectively. For mean SUV analysis, there was a high degree of correlation between the scatter corrections. For 18F‐FDG, ABS scatter correction reconstructions trended towards higher liver mean SUVs relative to MLBS. The qualitative image analysis revealed no significant differences between TFBS and MLBS image reconstructions. For a uniformly filled relatively large 37 cm diameter phantom, MLBS produced the lowest bias in absolute quantification, while normalized voxel‐wise differences showed a trend in scatter correction performance consistent with the human subjects study. Conclusions For 18F‐FDG, MLBS is at least a valid substitute to TFBS, providing reconstructed image performance comparable to TFBS in most subjects but exhibiting quantitative differences in cases where TFBS is typically prone to inaccuracies (e.g., due to patient motion and CT‐based attenuation map truncation). Particularly for low contrast regions, quantification differs for ABS compared to MLBS and TFBS, and caution shoul...

Validation of image-derived input function using a long axial field of view PET/CT scanner for two different tracers

“…Our study showed a significant lower IDIF accuracy for the absolute scatter correction method compared to the relative scatter correction set as default. Our results are consistent with a previous study showing an underestimation of scatter using the absolute scatter correction method [ 29 ]. However, the absolute scatter correction method improved the precision of IDIF in our study.…”

“…The relative method is the default technique implemented in clinical PET imaging because it compensates for multiple scatter and for scatter contributions, scaling the activity located outside the axial FOV. The higher contrast between normophysiological uptakes for [ 18 F]DPA-714 than for [ 18 F]FDG probably explains why differences were greater for the CF IDIF between the use of relative and absolute scatter correction for [ 18 F]DPA-714 compared to [ 18 F]FDG data, which is consistent with two previous studies [ 29 , 36 ].…”

“…Secondly, we showed that contrasts in tracer uptake among organs neighboring the aorta are much higher for [ 18 F]DPA-714 than for [ 18 F]FDG, due to lower count statistics in the blood pool for [ 18 F]DPA-714 but also due to the clearly higher liver, lung, myocardium and spleen uptakes for [ 18 F]DPA-714. These higher contrasts lead to bias from the scanners scatter correction algorithm [ 29 ]. As reported previously, the accuracy is lower as the tail regions have comparatively low counts using the relative scatter correction method [ 30 ].…”

Supplemental Transmission Aided Attenuation Correction for Quantitative Cardiac PET

“…Finally, we used absolute scatter estimation for correcting counts from the patient. The method underestimates scatter from out of FOV [ 26 ], which may bias -map estimation at edge planes; increasing PET bias of extracardiac disease.…”

“…To estimate scatter originating from the patient alone, in (13) and in (19) , we utilized absolute scatter estimation [ 26 ]. In this approach, the current EM image reconstruction and attenuation image were fed into a 2D implementation of the SSS.…”