difference of SSIM and multissim?

I work on MRI images and now I assess different k-space trajectories on mri images.

I used ssim and nrmse to assess the quality of images. 1st off, I don't know what is the difference between ssim and multissim, as they give different values for one comparison?

2nd, as I increase k-space sampling density the ssim value starts to decrease from a point while I see the quality of image is high and it's similiar to main image.

Answer 1

[DGM]

The documentation describes the differences:

https://www.mathworks.com/help/images/ref/ssim.html

https://www.mathworks.com/help/images/ref/multissim.html

Without any quantification of "starts to decrease from a point" and "I see the quality of the image is high", all anyone can conclude is that the error is increasing, but you simply aren't noticing the increased error.