# Prospects of Structural Similarity Index for Medical Image Analysis
## 4 SSIM in Medical Imaging
### Current Status of SSIM Research in Medical Imaging
#### 4.6.2 Reducing Metal Artifact  
A metal artifact can exist due to the presence of high-density objects, i.e., dental fillings and surgical clips. In CT imaging, the presence of metal artifacts causes issue of complications in imaging the region of interest (ROI) and may reduce the dose calculation accuracy. In order to suppress the metal artifact, a robust metal artifact reduction (MAR) algorithm is vital to be established. Therefore, in 2019, [PERSON] et al. tried to develop a MAR algorithm based on the tilted CT scan images. Their idea began from the understanding of the normal CT scan position may produce metal artifact contaminated images, and the tilted CT scan can complement the original CT scan position by offering reconstructed images with less metal artifact. Additionally, to obtain the less metal artifact images, they implemented modified SSIM by neglecting the luminance factor and only considering the contrast and structure components in the calculation. They claimed that the luminance factor containing intensity in the areas with the metal artifact between CT images and the superposition map is different completely, whereas the edge information is fairly identical. Thus, the correlation map calculated using the original SSIM may not accurately predict the metal artifact similarity score [99].  
The algorithm steps are described in Algorithm 1 for MAR with SSIM.  
```
Input: Reconstructed original CT and tilted CT images Output: Reconstructed image with the smallest SSIM
1. for reconstructed images of original CT and tilted CT do
2. Subtract original CT and tilted CT images
3. Obtain the artifact map
4. Modified SSIM calculation with 5\(\times\)5 windows
5. Obtain the original CT and tilted CT correlation maps
6. Compare original CT and tilted CT correlation maps that contain SSIM score
7. if original CT SSIM < tilted CT SSIM do
8. Choose the intensity of the original CT image as the best reconstruction image
9. Otherwise do
10. Choose the intensity of the tilted CT image as the best reconstruction image
11. End if
12. End for
```  
**Algorithm 1** MAR with SSIM  
To show the superiority of their proposed MAR, they compared the proposed method with well-known linear interpolation metal artifact reduction (LI-MAR), normalized metal artifact reduction (NMAR), and refined metal artifact reduction (RMAR). They obtained an exceptional result when using MAR with SSIM by presenting a mean absolute percentage error (MAPE) of 8.52%, whereas the original CT scan, tilted CT scan, LI-MAR, NMAR, and RMAR exhibited 62.98%, 9.27%, 19.97%, 14.12%, and 11.79%, respectively, in the simulation study. Moreover, in the experiment study, they extended the proposed method by combining it with RMAR and generated two new methods, namely tilted CT-based MAR (T-MAR) and augmented tilted CT-based MAR (AT-MAR). Particularly when employing AT-MAR, their results indicated that the metal artifact suppression algorithm can reduce the existence of streaks and bright/dark band artifacts near metallic objects by demonstrating MAPE of 10.12%, whereas original CT scan, tilted CT scan, T-MAR, LI-MAR, NMAR, and RMAR had MAPE of 92.12%, 60.62%, 46.37%, 45.84%, 34.62%, and 29.93%, respectively.