We validated novel xSPECT and conventional F3D reconstruction using experimental data derived from phantoms and clinical data derived from patients. The experimental findings indicated that the image quality and quantitative accuracy of xSPECT exceeded those of F3D. We also found that xB can maintain high spatial resolution even at various numbers of iterations. The clinical study did not identify any significant differences in the CV and the SUV between xQ and xB, but xB further enhanced bone SPECT images in terms of spatial resolution.
Regardless of the reconstruction models, the impact of statistical noise theoretically increased with lower counts. In fact, suppressing noise caused the maximum radioactivity concentration in the highest TBR to almost reach the actual radioactive concentration (Fig. 2). In contrast, the average activity concentration approached the true value at lower TBR. This can be explained by the lack of a partial volume effect because the radioactivity concentrations inside and outside the ROI were the same when TBR and BG were equal (TBR = 1). On the other hand, the radioactive concentration decreases as a result of spillage from within a hot sphere to the background, which raises the TBR . Nevertheless, TBR 1 was slightly overestimated due to the radioactivity concentration being increased by statistical noise. The quantitative differences between F3D and xSPECT are considered to be statistical noise and a partial volume effect caused by low spatial resolution. All radioactive concentrations essentially converged within 48 iterations. The number of iterations recommended for the Siemens xSPECT is the same , but the mean radioactive concentrations for xQ and xB were similar at > 24 iterations. However, stable convergence was slightly delayed for F3D compared with xSPECT, which has been explained by the study that compared OSEM and OSCGM . The number of iterations is associated with a trade-off between signal and noise. Thus, we determined that 30 iterations were the most appropriate for xSPECT reconstruction in clinical practice.
The FWHM with xQ and F3D considerably improved and converged at ~ 15 and 20 mm, respectively, when the number of iterations increased. Consequently, image quality was better for xQ at the appropriate parameter compared with F3D. In contrast, the spatial resolution in xB rarely changed even when the number of iterations increased, and the actual size of 10 mm was almost achieved. The xB iterative operation can be weighted by zero or any other value according to the corresponding zone class in the divided pixel . Thus, we considered that not only bone class weighted by the optimal value, but also non-bone classes weighted by zero with a zonal map were responsible for the improved spatial resolution in the xB technology.
The xSPECT enhances SPECT images by applying the merit function to suppress noise caused by the fast convergence of OSCGM reconstruction. The Mighell-modified chi-squared gamma statistic algorithm is applied to the merit function for xSPECT reconstruction. Shinohara et al. found that Mighell-modified noise suppression was better than other image reconstructions based on chi-square statistics . Thus, the xSPECT with the Mighell merit function considerably suppressed noise compared with F3D algorithms at the same number of iterations. Furthermore, xB suppressed image noise more effectively than xQ. Okuda et al. showed that the noise suppression effect differed between xQ and xB depending on the CT value . In regions with low counts such as the background, the OSCGM based on the OSEM algorithm might lead to a contradictory effect of fast convergence. Background noise in xQ rapidly increased and appeared to differ from other images at 48 iterations (Fig. 4). An increasing CV with more iterations might have ramifications for lesion detectability. Therefore, we determined an appropriate number of iterations for clinical applications. The RC with xSPECT which has high spatial resolution, was better than that of F3D due to the suppressed partial volume effect. However, the high spatial resolution of xB was not directly associated with an RC improved by the partial volume effect. This can be interpreted as being independent of the iterative operation of each zone class based on CT data, and the quantitative xB image could be considered as a weighting effect of each tissue class that does not increase bone uptake.
Our clinical study showed that the quantitative indexes did not significantly differ (p > 0.05) between the xQ and xB algorithms, and in fact were equivalent. These findings were similar to the results of a previous phantom experiment . Consequently, the spatial resolution was better in xB than xQ images and thus image quality was high and quantitative accuracy was equivalent. However, quantitative variation caused by misalignments such as motion and respiratory error during clinical scanning is a concern. In particular, xB imaging might cause different behavior due to the unique zone map system. Therefore, misalignment between SPECT and CT due to respiratory errors such as the ribs and sternum should be considered when clinically applying xB.
The present study has several limitations. Reconstructed SPECT images were assessed using different cross-calibration methods. The CCF on quantitative SPECT images varied depending on the radioactive concentration . Thus, slight quantitative errors might arise between the F3D and xSPECT models. In addition, the body types of the 20 patients were essentially standard (average, 15.9 ± 2.8 MBq/kg). We could not take dependence on physique into consideration, and the impact of factors such as counts and scatter remains unclear. Further study is required to assess the relationship between body weight and the quality of images reconstructed using the xSPECT algorithm.