Remy Klausz DIPLENG, Presenter: Employee, General Electric Company

Henri Souchay, Abstract Co-Author: Employee, General Electric Company

Demonstrate the properties of tomographic planes and question the concept of finite slice thickness in digital tomosynthesis.

The notion of slice thickness comes from computerized axial transverse tomography (CAT), where the thickness of slices parallel to the x-ray beam is a direct consequence of the shape and size of the x-ray beam. In contrast with CAT, the Digital Tomosynthesis (DT) reconstruction delivers images of tomographic planes perpendicular to the x-ray beam, and including neighborhood-related information as determined by the acquisition geometry and reconstruction method. The residual visibility of out-of plane structures is investigated and the corresponding Point Spread Function (PSF) computed. To measure the PSF, a special phantom has been designed, containing a small-diameter high-attenuation metallic wire, positioned at a small angle relative to the tomographic plane. Tomographic images of this phantom have been reconstructed from projections acquired on a Senographe Essential (GE Healthcare).

The image of the wire in a single tomographic plane image demonstrates the progressive blurring of out-of-plane structures with distance from the tomographic plane. They are convoluted with an aperture function, the width of which is proportional to the plane-to-plane distance, as predicted by  computation. We propose to replace slice thickness by the depth of focus, distance where the tomographic blurring equals the in-projection unsharpness.

Tomographic images represent a strict geometric plane. They contain in addition information from the neighbor planes, blurred proportionally to their distance to the tomographic plane. A simple method is proposed to demonstrate and quantify the tomographic depth of focus.

To avoid missing elements of the imaged object, the reconstruction pitch must be compatible with the depth-of-focus. In breast DT, care must be taken regarding microcalcifications and fibers.

Klausz, R, Souchay, H, Digital Tomosynthesis: Finite Thickness Slices or Tomographic Planes?.  Radiological Society of North America 2009 Scientific Assembly and Annual Meeting, November 29 - December 4, 2009 ,Chicago IL. http://archive.rsna.org/2009/8016830.html