Paul FitzGerald, Presenter: Nothing to Disclose

William Robert Ross PhD, Abstract Co-Author: Nothing to Disclose

Samit Basu PhD, Abstract Co-Author: Nothing to Disclose

Michael A. Rumsey MS, Abstract Co-Author: Nothing to Disclose

John McLeod, Abstract Co-Author: Nothing to Disclose

Jeffrey W. Eberhard PhD, Abstract Co-Author: Nothing to Disclose

As CT systems with increasingly high resolution are developed, azimuthal blur becomes a significant limitation on in-plane resolution, even close to the center of the scan field of view (SFOV). Increased angular sampling, by reducing angular displacement during the x-ray exposure associated with each view, can mitigate this effect.This study presents a comparison between two nearly identical high-resolution CT system configurations, one using a continuous x-ray source, the other a pulsed source. The resulting in-plane spatial resolution is compared.

The systems tested are research prototypes developed at GE Global Research, and employ two flat-panel x-ray detectors (20cm square active area, 200µm pitch a-Si pixels, CsI scintillator) and rotating anode x-ray tubes with sub-millimeter focal spots. Geometric magnification is 1.5 at isocenter; SFOV is 33cm; Z coverage is 4.2cm per rotation.A custom phantom was used, containing thirty 25µm diameter tungsten wires, precisely aligned parallel to the system’s Z-axis, and spaced at 5mm intervals from the center of the SFOV to 14cm radius.Data were acquired using 500 to 4000 views. After cone-beam reconstruction, the in-plane Point Spread Function (PSF) of each wire was determined.

For all configurations, radial resolution was nearly constant over the SFOV, as expected.For the continuous exposure system using 1000 views, azimuthal resolution was 400µm (PSF FWHM) at the SFOV center, but degraded to 900µm at 14cm radius. Using 2000 views, azimuthal resolution was nearly constant over the SFOV.For the pulsed exposure system, using a 1000 views and a 25% duty cycle, the azimuthal resolution was approximately 300µm, and constant over the SFOV. The improved initial resolution vs. the continuous system can be attributed to a smaller focal spot.

In a CT system that employs very high-resolution detectors, resolution is high near the SFOV center, and radial resolution is constant over the SFOV. However, with continuous x-ray exposure, angular undersampling can substantially degrade azimuthal resolution. This resolution can be recovered by increasing the effective angular sampling through the use of a pulsed x-ray source.

W.R.R.,P.F.,S.B.,M.A.R.,J.M.,J.W.E.: All authors are employees of GE Global Research.

FitzGerald, P, Ross, W, Basu, S, Rumsey, M, McLeod, J, Eberhard, J, Reduction of Azimuthal Blur in Ultrahigh Resolution CT Using a Pulsed X-ray Source.  Radiological Society of North America 2004 Scientific Assembly and Annual Meeting, November 28 - December 3, 2004 ,Chicago IL. http://archive.rsna.org/2004/4403729.html