The "amount of compression" in irreversible compression used to be determined by the
compression ratio, where the acceptable minimum is determined by the algorithm (typically
JPEG or
J2K) and the data type (body part and imaging method). Such a definition is easy to follow, and has been used by medical bodies in 2010 around the world. The image compression community has long used objective
quality metrics like
SSIM to measure the effects of compression. In the absence of good data regarding SSIM, the ESR review of 2010 concluded that it is still difficult to establish a criterion for whether a particular
irreversible compression scheme applied with particular parameters to a particular individual image, or category of images, avoids the introduction of some quantifiable risk of a diagnostic error for any particular diagnostic task. A 2020 study shows that
visual information fidelity (VIF),
feature similarity index (FSIM), and
noise quality metric (NQM) best reflect radiologist preferences out of ten metrics. It also mentions that the original version of SSIM works as poorly as a basic root-mean-square distance (RMSD) for this purpose, a result echoed by the 2017 study. The 4-G-r* modification is not tested in the study. ==References==