This definition appears rarely
For some procedures, e. g. , bone imaging, there are two target:nontarget ratios that must be considered. It is important to see bones against soft tissue, so the bone:soft tissue ratio must be acceptably high. It is also important to be able to identify a metastatic lesion on bone tissue, so the ratio of tumor:bone must also be high. These ratios are multiplicative; if the tumor:bone ratio is 5:1 and the bone:soft tissue ratio is also 5:1, then the tumor:soft tissue is 25:1.