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Creation of an atlas of filter positions for fluence field modulated
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Fluence field modulated CT (FFMCT) and volume of interest (VOI) CT imaging
applications require adjustment of the profile of the x-ray fluence incident on a
patient as a function of view angle. Since current FFMCT prototypes can
theoretically take on an infinite number of configurations, measuring a
calibration data set for all possible positions would not be
feasible. The present work details a methodology for calculating an atlas of
configurations that will span all likely body regions, patient sizes, patient
positioning, and imaging modes. The hypothesis is that there exists a finite
number of unique modulator configurations that effectively span the infinite
number of possible fluence profiles with minimal loss in performance.
CT images of a head, shoulder, thorax, abdominal, wrist, and leg
anatomical slices were dilated and contracted to model small, medium, and large
sized patients. Additionally, the images were positioned from iso-center by
three different amounts. The modulator configurations required to compensate
for each image were computed assuming a FFMCT prototype, digital beam
attenuator, (DBA), was set to equalize the detector exposure.
Each atlas configuration should be different from the other atlas configurations.
The degree of difference was quantified using the sum of the absolute differences
in filter thickness between configurations. Using this metric, a set of unique
wedge configurations for which no two configurations have a metric value smaller
than some threshold can be constructed. Differences in the total number of
incident photons between the unconstrained filters and the atlas were
studied as a function of the number of atlas positions for each anatomical site
and size/off-centering combination.
By varying the threshold used in creating the atlas, it was found that roughly 322
atlas positions provided an incident number of photons within 20% of
using 19 440 unique filters (the number of atlas entries ranged from 7213 to 1).
Additionally, for VOI applications implemented with a single VOI region, the
number of required filter configurations was expressed in a simple closed form
The methodology proposed in this work will enable DBA-FFMCT and DBA-VOI
imaging in the clinic without the need for patient specific
air-scans to be performed. In addition, the methodology proposed here is directly
applicable to other modulator designs such as piecewise linear, TomoTherapy multi
leaf collimators, 2D fluid arrays, and inverse geometry CT.
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