Special Session on Abdominal, Brain, Lumbar Spine CAD

Abstract

Keywords Computer-aided diagnosis Á Abdominal imaging Á CT colonography Á Electronic cleansing Purpose Fecal tagging in CT colonography (CTC) is a means of 'marking' fecal materials (stool and fluid) in the colon by use of a radiopaque oral contrast agent, such as barium or iodine. Electronic cleansing (EC) of the colon, is a promising technique for removing the tagged fecal materials in CTC images to 'virtually' cleanse the colon [1], [2]. A current problem with EC is that the majority of the existing EC methods are designed to remove only tagged fluid resulting from cathartic bowel preparation. Thus, the current EC schemes remain severely limited in removing solid or semi-solid stool that is the typical fecal residue in non-cathartic CTC, and they tend to generate severe cleansing artifacts that impair the diagnostic usefulness of the electronically cleansed non-cathartic CTC images. The artifacts of pseudo-soft-tissue structure and false fistula caused by partial volume effect and the incomplete cleansing caused by inhomogeneous tagging are two major artifacts in EC for non-cathartic CTC images. For the CTC examination imaged by a dual energy CT scanner (DE-CTC), the capability of material differentiation provides us a promising solution to identify the partial volume effect and inhomogeneous tagging in residual fecal materials.In this study, we developed a novel EC method for non-cathartic DE-CTC, denoted dual-energy EC (DE-EC), to effectively remove the residual fecal materials in non-cathartic CTC images while largely reducing the major cleansing artifacts, and validated its performance in a phantom study. MethodsThe EC artifacts -pseudo soft-tissue structures and false fistula, is caused by the partial volume effect (PVE) at the boundary between the air lumen and tagged regions, called the air-tagging boundary (ATboundary), and at a thin soft-tissue structure that is sandwiched between the air lumen and the tagged region, called the air-tissue-tagging layer (ATT-layer). EC requires the subtraction of the tagged regions and their associated AT-boundaries. Both the AT-boundary and the ATT-layer are formed by the PVE, i.e., a mixture of multiple materials at one voxel.Let each material k be modeled by a material-specific normal distribution P(A k ) parameterized by A k = {l k , P k }, where l k is the mean vector and P k is the variance matrix. Suppose that the attenuation value of a voxel can be decomposed into a linear combination of two different materials, i.e. l = a 1 9l 1 + a 2 9l 2 , where l 1 , l 2 are the mean attenuations of two materials inside the voxel, and a 1 , a 2 are the fractions of these two materials. In DE-CTC images, there are four types of materials of interest: air, fat, soft tissue, and tagged feces. Thus, there are six types of mixture combination: (air, fat/soft-tissue), (air, tagging), (fat/soft-tissue, tagging). Because the materials within each voxel are unknown, we need to calculate an estimate for the most significant materials within each voxel.Given a location x, the material...