Target:non-target ratio

A high target:non­target ratio is critical. If this ratio is not high enough (5:1 minimum for planar imaging, about 2:1 for SPECT imaging), a non­diagnostic scan can result, making it difficult or impossible to distinguish pathology from background. For example, when performing a thyroid scan, ideally, all the radioactivity will be in the thyroid and nowhere else in the neck region. While liver uptake of the radioiodide would be undesirable dosimetrically, it would have no impact on the actual imaging process since it is not in the field of view.

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.

The result of a very low target:non­target ratio may be a non­diagnostic scan, resulting in an unnecessary radiation dose, a delay in the diagnosis and the necessity of repeating the procedure. These failures may be related to product quality problems that went unidentified because quality control was not performed; often, however, the patient himself is the source of a poor scan, due perhaps to poor renal function or to interfering medications.