# Quantification of SPECT Concentric Ring Artifacts by Radiomics and Radial Features
## 4 Discussion  
As with any type of device used to collect data that contribute to forming a clinical diagnosis, it is important to assess the performance characteristics of rotating SPECT systems. SPECT equipment testing includes the acquisition and analysis of three-dimensional phantoms to gauge system tomographic spatial resolution, image contrast, and uniformity of response; it is recommended to be performed as part of acceptance testing [24], and thereafter on a quarterly basis [3]. The failure to recognize concentric ring artifacts can contribute to the misinterpretation of nuclear cardiology studies [25], and non-cardiac nuclear medicine studies [5]. Determining whether particular image feature is visible is not trivial. One of the goals of our investigation was to document the extent to which two independent medical imaging physicists agreed on the appearance and severity of artifacts. The respective thresholds of visible detection in a 34 \(\times\) 10\({}^{6}\) count phantom study and in a clinical study likely differ. Observer experience is a factor, and evaluations, even by experienced physicists, might differ. Visibility indexes, such as those dependent on feature contrast and feature dimensions, can help in this regard [26], but ultimately such indices require calibration with scores assigned by human observers [27]. We found that readers agreed more frequently when observing all of the transaxial sections rather than viewing only the summary images.Our readers had \"good agreement\" whendetecting artifacts in uniform and sphere sections but only \"fair agreement\" about rods sections. The \(\kappa\) values of reader agreement and reading reproducibility were higher for the uniform sections and for the sphere sections than for the rod sections. This may have been due to the fact that there was less distraction in the uniform sections in perceiving ring artifact patterns compared to the rod sectors, for which the rod patterns were superimposed on count variations within and between rod images of different sizes. Besides deciding whether a concentric ring artifact is visible, deciding whether an artifact is sufficiently severe to cause potential clinical reading problems is another matter, because this requires removing a camera from clinical service for long enough to perform thorough re-tuning of the detectors. The process of camera re-tuning may be fairly unobtrusive, as in routine detector tuning or flood calibration by a technologist of 100-120 \(\times\) 10\({}^{6}\) counts; or it can be more time-consuming, involving a full calibration of energy, linearity, and uniformity corrections by a field service engineer. Consequently, the use ofa quantitative basis for triggering the decision to retune detectors would offer useful information for technologists performing quarterly SPECT quality assurance scans. While the results of quality assurance phantom scans are usually assessed visually, recent progress has been reported on quantifying scanner performance automatically [7; 8; 9; 11]. Among the parameters that are evaluated in SPECT phantom scans, non-uniformity is perhaps the most challenging. [PERSON] et al. [6] found that the choice of reconstruction filters has a pronounced effect on the manifestation of planar flood field non-uniformityand on the formation of tomographic concentric ring artifacts, and that the ability of observers to perceive such artifacts depends on the size and location of these artifacts; smaller artifacts closer to the center-of-rotation were less perceptible than larger ones further from the center [6]. This is consistent with a reader tending to give more credence to a pattern that is perceived to be repeated over more pixels than over fewer pixels, as there would be a tendency to attributemore weight to a similar pattern confirmed in multiple locations, even when these deviations from neighboring counts are small.