Room: Track 1
Purpose: To develop and validate a method which determines equilibrium dose using a single CTDI phantom.
Methods: Dose profiles were obtained on a GE Discovery CT 16-slice scanner using a thimble ion chamber. These profiles were measured in three configurations, for a variety of scan parameters: in-air, single CTDI phantom, and equilibrium dose (Deq) phantom. The Deq phantom was composed of three CTDI phantoms placed end-to-end. In all three configurations the ion chamber was positioned in the mid-line and central axis of these phantoms, and the phantoms aligned to the central axis of the scanner. For each permutation of scan parameters, the following steps were applied in order to model the primary x-rays and scatter kernel. A model of the primary x-ray profile was created by fitting a Gaussian to the in-air profiles. Equilibrium phantom measurements, yielding 450-mm long dose profiles, were used to model the Deq profile. This model is composed of a two Gaussian function, respectively representing the primary and phantom scattered x-rays. The primary x-ray model was then subtracted from the Deq profile model, creating its scatter-dose profile. By deconvolution of the scatter-dose profile with the primary x-ray model, a scatter kernel was calculated. Finally, a dose profile of length L can be obtained by scatter kernel and in-air profile scan length L convolution. To validate this procedure, profiles were taken in a 32cm CTDI phantom and the resulting doses were compared to these calculations.
Results: Preliminary results show good agreement with measured data. Primary, scattered and total dose models fit the measured data well. The primary beam does not need to fit a Gaussian, but worked well in obtaining the scatter kernel.
Conclusion: This new method allows users to obtain equilibrium dose profiles without the burden of an unusually large and heavy phantom.