Samuel Robert Mazin MS, Presenter: Nothing to Disclose

Taly Gilat Schmidt MS, Abstract Co-Author: Nothing to Disclose

Norbert Joseph Pelc ScD, Abstract Co-Author: Nothing to Disclose

To examine feasible collimator designs for a multiple-eye inverse-geometry CT (IGCT) system that employs a large scanned source array. The system can acquire large FOV volumetric data in a single rotation with negligible cone-beam artifacts. For high dose efficiency, each source spot must be collimated to effectively transmit only photons directed toward the multiple, separated, detector arrays.

The source array is opposite three small detector arrays at 17º separation, each of which must be illuminated by all source spots. An ideal collimator would allow detectable rays to pass through while rejecting rays that are unusable. A 4 cm thick brass (60% Cu, 40% Zn, 8.5 g/cc) collimator was simulated. Source spots were simulated in both transverse (perpendicular to rotation axis) and axial views. Once holes were defined, total effective collimator material distances were measured for rays (detectable and undetectable) spanning 120 degrees from each source spot, and transmission profiles were calculated for each source spot assuming 80 keV monoenergetic photons. A leakage factor was calculated as (T-D)/D, where T and D are the total and detectable radiation intensities exiting the collimator, respectively. This was done for a range of spacings between source spots.

Because each source spot requires multiple diverging holes, the ideal collimator is composed of discrete layers of collimator material (with holes), parallel to the source plane and separated by gaps. For a 2.5 mm transverse spacing of source spots, the leakage factor was 16.9%. When the transverse source spot spacing was increased to 3 mm, the leakage factor dropped to 2.1%. Very little is gained with respect to the leakage factor beyond 3 mm transverse spacing. In the axial direction, source spots need not see the entire length of the detector; 50% illumination and 2.5 mm axial source spot spacing produced a leakage factor of 1.6%. Leakage would be lower with a denser collimator material.

Dose efficient collimation of a multiple-eye IGCT system is feasible. Collimator leakage is largely a factor of source spot spacing. Spatial resolution and sampling can be maintained through other design choices.

N.J.P.,S.R.M.,T.G.: Equipment provided by NexRay, Inc.N.J.P.,S.R.M.,T.G.: Grants from GE Medical Systems

Mazin, S, Gilat Schmidt, T, Pelc, N, Collimator for a Multiple-Eye Inverse-Geometry Volumetric CT System.  Radiological Society of North America 2004 Scientific Assembly and Annual Meeting, November 28 - December 3, 2004 ,Chicago IL. http://archive.rsna.org/2004/4412393.html