Kenta Miwa , Reo Nemoto, HirotsuguMasuko, TenshoYamao, Rinya Kobayashi, Noriaki Miyaji, Kosuke Inoue, Hiroya Onodera
Abstract
Although scatter correction improves SPECT image contrast and thus image quality, the effects of quantitation accuracy under various conditions remain unclear. The present study aimed to empirically define the conditions for the optimal scatter correction of quantitative bone SPECT/CT images. Scatter correction was performed by applying dual and triple energy windows (DEW and TEW) with different sub-energy window widths, and effective scatter source estimation (ESSE) to CT-based scatter correction. Scattered radiation was corrected on images acquired using a triple line source (TLSP) phantom and an uniform cylinder phantom. The TLSP consisted of a line source containing 74.0 MBq of 99mTc in the middle, and a background component containing air, water or a K2HPO4 solution with a density equivalent to that of bone. The sum of all pixels in air, water and the K2HPO4 solution was measured on SPECT images. Scatter fraction (SF) and normalized mean square error (NMSE) based on counts from the air background as a reference were then calculated to assess quantitative errors due to scatter correction. The uniform cylinder phantom contained the same K2HPO4 solution and 222.0 MBq of 99mTc. The coefficient of variation (CV) was calculated from the count profile of this phantom to assess the uniformity of SPECT images across scatter correction under various conditions. Both SF and NMSE in SPECT images of phantoms containing water in the background were lower at a TEW sub-window of 3% (TEW3%), than in other scatter corrections, whereas those in K2HPO4 were lower at a DEW sub-window of 20% (DEW20%). Larger DEW and smaller TEW sub-energy windows allowed more effective correction. The CV of the uniform cylinder phantom, DEW20%, was inferior to all other tested scatter corrections. The quantitative accuracy of bone SPECT images substantially differed according to the method of scatter correction. The optimal scatter correction for quantitative bone SPECT was DEW20% (k = 1), but at the cost of slightly decreased image uniformity.
Introduction
Bone scintigraphy using 99mTc-labeled phosphate compounds is the most prevalent means of detecting bone metastases of prostate and breast cancer [1]. Planar whole-body bone scintigraphy has high sensitivity, although the specificity is limited to characterizing bone metastases. Adding single photon emission computed tomography (SPECT) to planar acquisition has improved diagnostic confidence [2], and when combined with computed tomography (CT), bone SPECT/CT provides better specificity with more precise localization, and better contrast between hot and cold lesions [3].
Materials and methods
Data acquisition and image reconstruction
All imaging data were acquired using a Brightview XCT SPECT/CT system (Philips Healthcare, Cleveland, OH, USA) with a high-resolution flat panel x-ray detector (40 × 30 cm2) with low-dose cone-beam CT imaging for localization, and attenuation correction of images. The detector is mounted on the same gantry as the SPECT camera, which allows the acquisition of SPECT and cone-beam CT images. The SPECT images were acquired under a ± 10% main energy window at 140 keV with ⅜″ crystal thickness, a low-energy high-resolution collimator (LEHR), 128 × 128 matrix with 4.8-mm pixels, and 60 projections of 20 s/view over 360° in circular orbit continuous acquisition mode. The Brightview XCT SPECT/CT system allows the simultaneous setting of 16 energy windows. With a 360° rotation of the gantry, a 47-cm diameter transverse field of view (FOV) and a 14.4-cm axial length can be visualized along a patient during slow rotation (60 s per rotation) as a cone-beam CT image [30]. We reconstructed the SPECT images using the Philips Astonish (Philips Healthcare) 3D iterative method, with combinations of 15 subsets and 2 iterations, attenuation, scatter correction, and resolution recovery.
Results
Fig 3A and 3B shows the SF and NMSE, respectively, for each scatter correction. Both SF and NMSE in SPECT images of phantoms containing water in the background were lower at a TEW sub-window of 3% (TEW3%) than any other scatter correction, whereas those in the K2HPO4 solution were lower at a DEW sub-window of 20% (DEW20%). The SF and NMSE became smaller with larger DEW and smaller TEW sub-energy windows, and ESSE overcorrected the scatter radiation in the K2HPO4 solution. Table 2 shows the CV of the uniform cylinder phantom. The CV of ESSE was superior, whereas the CV of DEW20% was inferior to that of any other assessed method of scatter correction. We visually confirmed the ability of different scatter correction technologies in scatter radiation images after scatter correction (Fig 4). Scattered radiation images were obtained by subtracting the reference image with DEW20% (considered as the most accurate in bone equivalent solutions) from the SPECT images with scatter corrected under different conditions. The remaining scattered radiation was visually almost identical between DEW20% and ESSE, although the scattered radiation of ESSE might be overcorrected.
Discussion
Scatter correction for quantitative bone SPECT images has not yet been standardized. We evaluated various scatter correction methods based on EWSC and ESSE using phantoms. Our findings indicated that the accuracy of scatter correction for SPECT image quality and quantitation depends on the method applied, sub-energy window width and the background material that produces scattered radiation. We also found that the scatter correction for quantitative bone SPECT was optimal with a DEW20% sub-window (k = 1).
Conclusions
The quantitative accuracy of bone SPECT images considerably differed according to the method of scatter correction. The optimal scatter correction for quantitative bone SPECT was DEW20% (k = 1) according to our phantom study using water and K2HPO4 solution as scatter radiation components representing soft tissue or bone, respectively, but at the cost of slightly decreased image uniformity. The present findings provide useful information about how to confirm optimal scatter correction for quantitative evaluations of bone SPECT images. Compton scatter is object-dependent and spatially varying. Further studies should validate bone 99mTc SPECT images acquired from more complex geometry and realistic phantom, e.g. NEMA IEC body phantom, and actual patients using optimal scatter correction obtained from our phantom study.
Acknowledgments
For valuable contributions in the data collection, we would like to thank Mr. Yukinori Fukuhara and Mr. Shinichiro Terada from Tokyo Women’s Medical Hospital.
Citation: Miwa K, Nemoto R, Masuko H, Yamao T, Kobayashi R, Miyaji N, et al. (2022) Evaluation of quantitative accuracy among different scatter corrections for quantitative bone SPECT/CT imaging. PLoS ONE 17(6): e0269542. https://doi.org/10.1371/journal.pone.0269542
Editor: Aaron Specht, Harvard School of Public Health, UNITED STATES
Received: October 10, 2021; Accepted: May 23, 2022; Published: June 6, 2022
Copyright: © 2022 Miwa et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Data Availability: All relevant data are within the paper.
Funding: This work was supported in part by KAKENHI Grant-in-Aid for Young Scientists (B) (No. 16K19831) and from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), the Japanese Government. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0269542#abstract0
