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Assessment of Scatter Compensation Strategies for ⁶⁷Ga SPECT Using Numerical Observers and Human LROC Studies

¹ Department of Nuclear Medicine, Hamilton Health Sciences Center, Hamilton, Ontario, Canada
² Division of Nuclear Medicine, Department of Radiology, University of Massachusetts Medical School, Worcester, Massachusetts
³ CPS Innovations, Knoxville, Tennessee
⁴ Department of Radiology, Johns Hopkins University, Baltimore, Maryland

⁶⁷Ga citrate is an oncologic SPECT imaging agent often used to diagnose or stage patients with non-Hodgkin’s lymphoma. As ⁶⁷Ga decay involves the emission of multiple-energy -rays, significant photon downscatter will be present within each photopeak energy window. We have previously shown that the inclusion of these scattered photons significantly degrades lesion detectability. The goal of this study was to investigate the extent to which this decrease in detectability can be reversed by applying scatter compensation strategies. Methods: We have compared 5 different scatter compensation methods to the case of no scatter compensation in iterative SPECT image reconstruction. The strategies consisted of (a) perfect scatter rejection, (b) ideal scatter compensation, (c) triple-energy window (TEW) scatter estimation, (d) effective scatter source estimation (ESSE), and (e) postreconstruction scatter subtraction. Reconstruction parameters used for each method were first optimized using a channelized Hotelling numerical observer. Strategies were then ranked in terms of lesion detectability using a human observer localization receiver operating characteristic (LROC) study. An additional comparison was made comparing the human LROC rankings with a recently developed channelized nonprewhitening (CNPW) LROC numerical observer. Results: Using the area-under-the-LROC-curve (A_LROC) as the assessment criterion, our results indicate that the TEW and ESSE scatter compensation methods are able to significantly improve lesion detectability over no compensation (A_LROC = 0.75 and 0.73 vs. 0.67, respectively). However, these compensations failed to achieve the same detectability as perfect scatter rejection (A_LROC = 0.84). Both ideal scatter compensation and postreconstruction scatter subtraction resulted in numerical increases in detection accuracy that were not statistically significant from no scatter compensation. Good agreement is seen between the CNPW observer and human LROC studies (Spearman rank order coefficient, r_s = 0.74), thus indicating that the LROC observer may be a good predictor of human observer performance in ⁶⁷Ga SPECT. Conclusion: Scatter compensation in ⁶⁷Ga SPECT imaging using techniques such as TEW or ESSE is able to improve lesion detectability compared with no scatter compensation. A recently developed numerical observer model appears to be a good predictor of human observer performance and may be used to perform imaging optimizations, thereby reducing the need for human LROC studies.

Key Words: ⁶⁷Ga SPECT • scatter compensation • localization receiver operating characteristic studies • numerical observer

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