AbstractID: 6725 Title: An ¹⁹²Ir CT Scanner for High-Dose-Rate
Brachytherapy
For brachytherapy (BT) treatments computed tomography (CT) scans provide
much more assurance than planar radiographs that the tumor volume is being adequately
treated and nearby critical structures are being spared. We are developing a fourth
generation CT scanner consisting of a ring of 96 8-channel photodiode scintillator
(CdWO 4 ) detectors. The
192
Ir BT source from a commercial high-dose-rate treatment
unit provides the photons required to form an image.
The detectors used in the prototype scanner are linear over 2.6 orders of
magnitude and are quantum noise limited for incident γ - ray intensities associated with
43 cm of attenuating Plexiglas. A single pair of lead collimators collimate both the source
and the detector to give a scanner inner bore diameter of 50 cm and a radiation profile of
width 2.7 cm in the axial direction.
Image quality was assessed by computer modeling the scanner, assuming a 7.5 Ci
192
Ir source, 768 detectors of width 0.275 cm and 864 source positions (source length =
0.36 cm). This allowed us to examine the effect of varying detector spacing, source size
and number of source positions. The width of the point spread function is 0.22 cm. The
standard deviation in CT number at the centre of a 25 cm diameter Plexiglas phantom is
23 HU. The CTDI for a 100 second scan (2.4 cm slice spacing) is 0.9 cGy. The high
contrast resolution and noise characteristics are adequate for visualizing metal objects
and contrast filled organs. Preliminary images clearly show streak free visualization of
BT needles and bone.