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A method of calculating the activity of 99 Tc m -MAG3 to be administered to children of different ages has been developed and evaluated. The suggested administered activity schedule is only valid for estimation of split renal function. The activity required to obtain the same count rate over the kidneys for all ages was calculated as a fraction of the activity administered to an adult by using a biokinetic model and taking attenuation effects into account. The activity schedule is based on the age of the child and was tested using renograms from patients of different ages. Statistical noise was added to the smoothed renograms simulating an injected activity corresponding to 45 MBq for an adult. The precision in the determination of split renal function calculated with four different methods was determined for 500 simulated renograms. The precision was approximately the same for all ages, but varied with the method used. The activity to be administered to a very small child is 90% of the adult activity, decreases to less than 50% between 2 and 5 years of age, and then slowly increases to 100% as the child grows to adulthood.

SUMMARY Kidney scintigraphy using 99Tcm-dimercaptosuccinic acid (DMSA) is a common gamma camera investigation in paediatric patients. The present study deals with methods of calculating the activity to be administered to children. Our aim is to find a method of calculation which gives the same count density in the gamma camera images for all ages. Four different methods, based on body weight, height, body surface area and a theoretical model, are compared. The count density in the images of the kidneys was studied retrospectively in 85 patients aged between 3 weeks and 19.5 years. Fourteen young adults aged 22.0–31.9 years were also studied. Two of the calculation methods, based on body surface area and the theoretical model, showed no apparent age dependency (i.e. a constant count density was observed for all ages). The variation between individuals was similar with both methods, but only the body surface area method gave a count density comparable to that for the young adults. As the method based on body surface area is also easy to implement, this method is recommended.

Four methods of calculating the activity of 99Tcm-hexamethylpropyleneamine oxime to be administered in cerebral blood flow investigations of children were evaluated in patient studies. Three of the methods were based on the size of the child. We also constructed a theoretical dosage model based on physiological data and attenuation effects. The aim of the study was to find a dosage calculation method that gave the same image quality for children of all ages as well as for adults. The results showed that the dosage method based on body weight is the only one of the four methods that does not exhibit an age-dependent variation in image quality and therefore this method is recommended.