Activity Quantification from Planar Images

Planar imaging was for a long time the most-used technique for image-based activity quantification. Owing to its simplicity and speed, the possibility to acquire dynamic image sequences, and to generate whole-body images at a reasonable acquisition time, it is still frequently used in nuclear-medicine imaging. Today, planar imaging is most commonly used for diagnostic examinations with qualitative or semi-quantitative evaluation but is also performed for patients receiving radionuclide therapy. Activity quantification may then be applied to estimate the absorbed doses delivered to organs and tumours. It is well recognized that planar images suffer from the superposition of counts from activity located at different depths in the patient and that generally quantitative SPECT is superior for activity quantification. However, the techniques for planar-based activity quantification preceded and, in parts, formed the basis for the subsequent development of quantification methods from tomographic SPECT images, and these techniques still carry value in terms of understanding the process with which planar image projections are formed. Notably, also when acquisition is made in SPECT mode, the raw image data consist of a set of planar projections. Moreover, planar-based activity quantification is still used, for example, to estimate the total-body absorbed dose, and in combination with SPECT-based activity quantification in so-called hybrid planar/SPECT methods.