Activity quantification of planar gamma camera images

Determination of absolute activity of 99mTc and 123I in organs for the purpose of patient-specific dosimetry in nuclear medicine examinations and therapy is of great importance. In order to make accurate quantification of the organ content from planar gamma camera images of phantoms, several corrections has to be made. The accuracy of a number of correction methods and different combinations of these were evaluated in phantom studies of liver, kidneys and heart. Correction was made for attenuation by transmission measurements. Three different scatter correction methods were used; the Triple Energy Window method (TEW), the Double Energy Window method (DEW) and the Lower Energy Window method (LEW). For background correction three different methods were used; 1. Gates correction method (conventional correction method); 2. Kojima method (with organ thickness and depth at which the organ is placed taken into account); and 3. Bujis correction method (organ thickness and body thickness taken into account). Attenuation correction was made for all measurements, and each of the three scatter corrections were combined with each of the three background corrections to nd the most suitable set of correction methods. The accuracy of the estimations of organ activity is highly dependent of the correction methods used. Applying background correction in conjunction with scatter correction gave an underestimation ranging between 22% and 0.3% (for 99mTc and all organs) depending on the correction methods used. For 123I, these values ranged between an overestimation of 12% and 1.4%. For higher background activities, the deviation from the true activity values became higher, but reasonably accurate. The most consequent results when considering all organs and organ-to-background activities was found when applying the LEW scatter correction method combined with the Bujis background correction method