Current
output dose measurement in CT is based on (CTDI). The conventional methodology
of CT dosimetric performance characterization is not appropriate to modern CT
scanners with helical scanning modes, dose modulation, array detectors and
multiple slice planes or cone-beam irradiation geometries. AAPM TG 111 report
recognizes the shortfall of the CTDI methods and recommends a new technique
that more accurately characterizes the dose profile from modern CT scanners,
which utilizes a short conventional ion chamber rather than a pencil chamber.
We developed and characterize a in-house phantom design using a three separate
anatomical regions of clinical scan sequences (Head, chest and abdomen), and
determined the equilibrium dose in our dose equilibrium phantom, measured if
the attenuation of the beam is the equal to that of CTDI Perspex phantom and
compare CTDI dose estimations using a standard pencil chamber to the dose
equilibrium phantom measurements. This methodology allows measurements of the
accumulated dose for any clinical scan length and allowing measurement of the
equilibrium dose. Using the new methodology, we determined that the CTDI approach can underestimate the dose by 25% to
35% and all of our dose values from the water phantom and farmer chamber were
independently verified with TLD measurements.
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