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IDEA Résumé de rapport

Project ID: FIKR-CT-2001-00164
Financé au titre de: FP5-EAECTP C
Pays: Austria

Mathematical calibration of a whole body counter

The Application of MIRD phantoms in whole body counting in the scanning geometry.

To introduce computer supported calibration techniques the measurements of bottle phantoms filled with a Ho-166m and Co-60 solution on a whole body counter in scan geometry have been compared to computer simulations. For the simulation the Monte Carlo n-particle simulation code MCNP has been used to calculate the peak efficiency of the experiment. On the other hand side uncertainty factors of both, the measurement and the simulation, have been calculated to find out which method is more accurate.

For the comparison 5 different bottle phantoms with different weight and height factors have been used. These phantoms have been simulated by using simple mathematical descriptions of the bottles. The scanning detector geometry has been approximated by calculating the peak efficiencies at 7 fixed points of the detectors and by calculating the average of these points. The differences between the results of the simulation and measurement were less than 8%.

The most important factors influencing the standard deviation of the simulation are listed below. Here it should be explained how those values have been calculated. All uncertainties are stated as one standard deviation:

-The most obvious factors are the statistical uncertainty factors of the measurement and the simulation due to a finite amount of events.
-Due to the production process the calibration sources of the bottle phantoms also have a specific standard deviation of 3 %.
-Different kinds of experiments have been performed to analyse the standard deviation due to geometrical reasons. The sum of all these geometrical factors results in an over all uncertainty of 2.0 %.
-As mentioned above the integration over the efficiencies at every point is performed by averaging over 7 points the standard deviation amended to 2 %.
-The simulation itself uses optimised models for the simulated physical effects, which can be summed to an uncertainty of 2 % and is called the simulation uncertainty.

It can be seen that the calibration by measurement and calibration by simulation have comparable accuracies of 4.1% and 3.8% respectively. So the simulation can be used to calibrate the peak efficiency of a whole body counter. This gives the opportunity to use more anthropomorphic phantoms.

In this study MIRD (Medical internal radiation dose) phantoms have been used to calculate uncertainty factors due to weight and height of the subject and due to inhomogeneous distributions in the human body. For whole body counters in the scan geometry the uncertainty due to height differences to the standard height of 170cm is less then 5%, but the uncertainties due to weight deviations to the standard weight of 70kg can be up to 25%. Inhomogeneous activity distributions influence the result by up to 25 % for whole body counters in the scanning geometries. These high influencing factors have been seen for low photo energies short time after intake or a long time after intake of Am-241 and Ra-226 respectively.

Informations connexes


Helmut FISCHER, (Scientist)
Tél.: +43-505-502542
Fax: +43-505-502502