Meetings » 2nd CTUG Meeting » A Novel Approach to Patient Dosimetry in CT

A Novel Approach to Patient Dosimetry in CT

Dr D McRobbie

Hammersmith Hospitals NHS Trust


For the best part of a decade, patient dosimetry for CT within the UK has relied upon the Monte Carlo methodology developed by the NRPB. Whilst this method has been refined and updated to include new CT scanners and represent new features such as spiral scanning, its reliance on approximations of scanner geometry and a particular mathematical anatomical model are limiting factors.

In an examination of organ data derived from the NRPB model, we came across the interesting observation than the organ doses from scattered calculated from Monte Carlo conform to a simple exponential relationship to the position of the irradiated slice. This led to develop a simple empirical model of patient dose, by considering a primary beam area and a scattered dose area, where the scattering attenuation factors can be determined empirically.

Data were acquired on a Siemens Plus 4 scanner using a Rando anthropomorphic phantom and linear attenuation coefficients for scatter determined for 5 and 10 mm slice width in cranial-caudal and caudo-cranial directions for a reference slice through the abdomen. Additionally, we investigated the in slice variation in primary beam and the radial variation of the scatter.

From this data we are able to produce a simple 1d model for dose deposition. In a further development we consider the examination dose to be a summation of primary and scattered dose profiles. Moreover we can extend this model to incorporate movement of the irradiated slice in spiral scanning thorough a convolution of the static dose profile and the couch movement. Fourier Transform theory allows us to simply estimate doses from arbitrary scans from the inverse transform of the product of transforms of the static dose profile and a scan length function. The convolution approach can also be applied to sequential scans if an appropriate discrete transform is used. Pilot data using this method compared the NRPB model will be presented.

Ref. NRPB R249 Survey of CT Practice in the UK: Part 2 Dosimetric Aspects 1991 Kiremidjian H, McRobbie D.W. & McBride A (1998) "A new Fourier-based dosimetry method for CT" Institute of Physics and Engineering in Medicine, Annual Conference, Brighton.

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