# Quantification of SPECT Concentric Ring Artifacts by Radiomics and Radial Features
## 1 Introduction  
Numerous factors influence the accuracy of single-photon-emission computed tomography (SPECT) reconstructions. In addition to patient motion issues (e.g., respiratory and cardiac), complications include photon attenuation and scatter, the variability of point spread function with distance from detectors, imperfect detector efficiency, and low counting statistics [1]. Over the years, considerable effort has been expended to address these issues through the application of physics to optimize the reconstruction process, togetherwith electric and mechanical engineering to improve SPECT system designs. However, data acquisition problems can occur abruptly, or as a result of the gradual electrical or mechanical deterioration of SPECT system components, which can compromise the reliability of acquired data [2] to correctly evaluate tumor progression/regression.  
For this reason, adherence to regularly scheduled standardized quality assurance (QA) procedures is important to identify problems with data acquisition before they can adversely affect clinical readings. These QA procedures include quarterly SPECT phantom reconstructions, which are typically assessed visually for contrast, spatial resolution, and uniformity of response [3].  
In the course of evaluating SPECT phantom transaxial reconstructions to assess the integrity of acquired image data, the appearance of conspicuous concentric ring artifacts often triggers the re-tuning of the detectors, as these artifacts may be due to inadequate uniformity corrections [4]. One cause of this can be the insufficient count density of the correcting flood field, the details of which have been studied extensively [5]. The choice of tomographic reconstruction filters affects the appearance of artifacts, and perceptibility of artifacts depends on their size and their position within the object [6].  
However, these evaluations are visual, not quantitative, and there are no published criteria for determining what constitutes an artifact that is sufficiently severe to necessitate electorre-tuning. When medical imaging physicists convene to read QA studies from other institutions, they begin with training data sets to calibrate their readings and to minimize the perceptual differences among them, and they read by consensus. However, factors such as brightness, contrast, and the screen resolution of the display monitor can influence visual impressions.  
To address these issues, algorithms have been developed to analyze standardized SPECT QA phantoms, both to report the results of spatial resolution and contrast results [7; 8], and to report the possibility of artifacts [9]. Some of these algorithms are based on texture analysis [7; 9]. Approaches to quantifying the severity of circular ring artifacts include annular sampling techniques [10] and the use of the Hough transform, the efficacy of which for detecting artifacts has been assessed using computerized Monte Carlo phantom simulations [11].  
Our study was conducted with two goals: (1) to determine how well medical physicists (who are experienced at performingthese evaluations) agree with one another about the presence and severity of concentric ring artifacts, and (2) to determine whether there are any quantitative image metrics that correspond to visual judgments regarding the significance of these artifacts. The use of a quantitative parameter could facilitate the decision as towhether corrective action needs to be undertaken.