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In internal emitter therapy. an accurate description of the absorbed dose distribution is necessary not only to establish an administered dose-response relationship. but also to avoid critical organ toxicity. In this context. quantitative scintigraphic imaging represents an important approach to extracting patient pharmacokinetic information. The most commonly employed imaging method for quantification of radioactivity in vivo uses 180degrees opposed planar images conjugate views technique) in combination with transmission data trough the subject and a system calibration factor.

This paper proposes a new approach to convert source region count rate into absolute activity, Measuring planar sources in air according to different geometries has in fact allowed LIS to calculate a calibration factor curve as a function Of source-collimator distance for each fixed threshold Value tested (ranging 10%-50%) to define ROIs. This method has proved to be successful for two reasons. On one hand, it permitted to subsequently derive the calibration factor C for conjugate views with the only knowledge of heads distance: nor prefixed geometric configurations neither reference sources are required. On the other hand. it permitted an accurate quantification even if the source is clearly visible only in one view.

Corrections for source thickness. attenuation, background arid scattering are easily integrated with a similar approach. In our analysis we tested step by step all these corrections on known sources in different situations: the total experiment test set consisted of 53 x 2 sources images, The error estimation was within 9% for single views and within 5% for double conjugate views.