Stephen Edward Jones MD, PhD, Presenter: Nothing to Disclose

Ijad Madisch, Abstract Co-Author: Nothing to Disclose

Rajiv Gupta PhD, MD, Abstract Co-Author: Nothing to Disclose

Spatial resolution of a CT scanner is typically characterized by its modulation transfer function and slice selectivity profile. These methods are inherently anisotropic and fail to fully describe the capabilities of recently introduced volume CT scanners (VCT). A new method is required to describe the spatial resolution of VCT that is inherently isotropic and three-dimensional.

The principle is to image a large set of microspheres with decreasing diameters. The largest span multiple voxels while the smallest are contained within a voxel. Plots of the image profile vs bead diameter will asymptotically converge to the system’s delta-function response. From this the point spread function (PSF) and modulation transfer function (MTF) can be constructed. Four sets of glass beads with nominal sizes ranging from 750-500, 300-250, 210-150, and 100-50 microns were imaged. Multiple scattered beads were held fixed in a baked egg-white suspension providing a background of near vacuum. Images of each bead were summed to provide a precise mean image. From this the x,y, and z resolutions were computed from the full-width at half maximum (FWHM). Two scanners were characterized and compared with this method, a high-resolution flat-panel based VCT and a multidetector CT (Siemens Sensation-64).

VCT has a nominal, isotropic spatial resolution of 25 line-pairs/cm (200 x 200 x 200 micron voxel size). The Sensation-64 (S-64) has an in-plane resolution approaching 20 line-pairs/cm (approximately 250 microns), and a Z-resolution of 12.5 line-pairs/cm (slice thickness = 0.4 mm). To demonstrate anisotropy in resolution, S-64 was operated with 0.4 x 0.4 mm in-plane resolution and 0.6mm slice thickness. These values were clearly demonstrated by the new method. For VCT, the computed X, Y, and Z resolutions were within 10% of each other. The expected ratio was demonstrated between the in-plane and z-resolution of S-64. The method also illustrated resolution deterioration with distance from the center slice in VCT, and away from iso-center in S-64.

A new method is developed that describes the three dimensional resolution of a CT scanner and is inherently isotropic.

Jones, S, Madisch, I, Gupta, R, Spatial Resolution Anisotropy in CT: Beyond MTF and Slice Selectivity Profiles.  Radiological Society of North America 2005 Scientific Assembly and Annual Meeting, November 27 - December 2, 2005 ,Chicago IL. http://archive.rsna.org/2005/4415594.html