# Quantification of SPECT Concentric Ring Artifacts by Radiomics and Radial Features
## 4 Discussion
### Limitations  
To maximize spatial resolution, patient data are usually acquired with contouring instead of with circular orbits. Our acquisitions were performed with contouring, but considering that the phantom we used was a simple cylinder, these orbits were likely close to being circular. It has been shown that non-uniform planar flood field corrections create circular artifacts in SPECT transaxial reconstructions for orbits with fixed radius, but more complicated shapes when elliptical orbits are used [5].  
With respect to thereconstruction process, in this study, the SPECT phantom images were reconstructed by means of FBP. This reconstruction was the one originally specified by the American College of Radiology (ACR), and the filter that ACR now recommends produces reconstructions that are quite similar. Moreover, the NEMA NU-1 2018 publication states that: \"The prescribed image evaluation steps for reconstructed resolution, which dictate filtered back-projection, were chosen as appropriate to the standard Gamma camera design. This technique may not be appropriate for novel SPECT systems that employ advanced reconstruction algorithms and where filtered back-projection is not the preferred or appropriate reconstruction technique.\" Since our SPECT systems (Table 1) have a median age of 15 years, we decided to standardize image reconstruction by using FBP reconstruction for all SPECT systemsindependent toy system age.Further investigations could be useful to determine whether our findings are affected by using OSEM reconstruction.  
Our study was limited to documenting the degree to which medical physicists agree with one another in their perception of the presence and severity of artifacts, not over-whether there were problems in the actual clinical studies. By linking \(R_{Contrast}\) and \(R_{KMSC}\) to their visual impressions, we have provided a means to connect quantified imaging parameters computed from standardized phantom scans to determine whether an artifact would have been judged to be sufficiently serious to require service on a SPECT system. We must, however, point out that the whole analysis conducted to identify the most informative features was really proof of principle due to the small sample size. The goal was to assess whether radial or radiomic features were able to discriminate between the presence or absence of ring artifacts. Alarger dataset and a combination of feature selection techniques would undoubtedly provide more robust results.  
Nonetheless, the results concerning the most discriminative features serve as a guideline for acting on the results of phantom scans. There are, in fact, innumerable variables involved in forming a final clinical diagnosis.  
Our analyses of radiomic features and radial parameters were performed only in the uniform sections, not in the sphere or rod transaxial sections. The inter-rater and intra-rater agreement measures were lowest for the rod sections, indicating that it was more difficult to agree on the presence and severity of artifacts in those sections than in the sphere and uniform phantom sections. However, we found that the concentric ring artifacts were detected by both readers throughout the individual phantom sections. The fact that some radiomic features agree well with visual impressions in uniform sections suggests that further work is warranted to enable texture analysis applications to non-uniform phantom areas containing spheres and rods, where artifacts also appear.  
The values of textural radiomic features can be affected by acquisition and reconstruction parameters, as reported for CT and PET imaging [28; 29; 30], and these influences require further investigation for SPECT imaging.