Efficiency of CT Urography Enhanced by 3D Techniques

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Researchers from Johns Hopkins University in Baltimore recently advised that 3D reconstructions of CT urography datasets are valuable materials that can help identify transitional cell carcinoma (TCC) in the upper urinary tracts that would otherwise go undetected by solely look at axial images.

“The ureters can be problematic to evaluate on CT, partly because of difficulties in obtaining adequate ureteral distention and pacification. Proper diagnosis hinges not only on appropriate interpretation of the axial images but also on the utilization of a 3D technique (volume rendering or maximum intensity projection) as an ancillary tool,” wrote Siva P. Raman, MD, and colleagues in the December issue of the American Journal of Roentgenology.

The authors noted that in addition to poor accession, the majority of ureteral TCCs are found in the distal third of the ureters, which is especially difficult to distend sufficiently.

To alleviate some of these challenges, Raman and colleagues wrote that their department at Johns Hopkins assesses CT urography datasets twice. First after a preliminary survey of source axial images, an independent second reader performs a 3D evaluation using two interactively created reconstructions: maximum-intensity projection (MIP) images and volume-rendered (VR) images.3d ct

For MIP, algorithms highlight and choose the highest-attenuation voxels along lines projected through the volumetric dataset. While regarded as a 3D technique, the MIP images are 2D representations of the source data, which can make determining a 3D relationship between structures difficult; however Raman and colleagues wrote that they found them to be quite helpful in assessing and studying the ureters.

According to the authors, VR uses a more complex algorithm to categorize particluar tissue types in each voxel before assigning a color and transparency level based on the percentage of each tissue type, and then rendering software produces an image. This technique can create “virtual endoscopic” images of collecting systems and ureters.

“At our institution, where two separate readers interpret the axial and 3D datasets, we have seen numerous TCCs initially missed, on a review of the axial images and multiplanar reformations but subsequently diagnosed on the 3D images,” wrote Raman and co-workers.

Moreover, in addition to helping to detect elusive carcinomas, the authors took note of several other benefits to 3D imaging techniques in CT urography, including:

Accentuating subtle narrowing – 3D reconstructions can make it easier to find subtle strictures or sites of narrowing by delineating both ureters in a single imaging plane.

Accentuating subtle differences in wall thickness and urothelial enhancement – Subtle wall thickening in small portions of the ureteral circumference can be easy to miss when not using a 3D technique.

Evaluating distal ureters – 3D reconstructions can be helpful in delineating true strictures or tumors in distal ureters.Identifying flat polyploid lesions – VR and MIP can help visualize lesions that would otherwise be difficult to identify on axial images.

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