3D techniques boost effectiveness of CT urography

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 - MIP_ureteral_lesion
52-year-old man with ureteral “pseudolesion” (arrow) on coronal maximum-intensity-projection image secondary to crossing vessel.
Source: (AJR 2013;201:1239-47)

3D reconstructions of CT urography datasets are invaluable tools that can help identify transitional cell carcinoma (TCC) in the upper urinary tracts that would otherwise be missed by reviewing only axial images, advised researchers from Johns Hopkins University in Baltimore.

“The ureters can be problematic to evaluate on CT, partly because of difficulties in obtaining adequate ureteral distention and opacification,” wrote Siva P. Raman, MD, and colleagues in the December issue of the American Journal of Roentgenology. “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.”

The authors noted that in addition to poor pacification, the majority of ureteral TCCs are located in the distal third of the ureters, which is particularly difficult to distend adequately.

To mitigate some of these challenges, Raman and colleagues wrote that their department at Johns Hopkins evaluates CT urography datasets twice. After an initial survey of source axial images, a separate second reader conducts a 3D evaluation using two interactively created reconstructions: maximum-intensity projection (MIP) images and volume-rendered (VR) images.

For MIP, algorithms select the highest-attenuation voxels along lines projected through the volumetric dataset, according to the authors. While considered a 3D technique, the MIP images are 2D representations of the source data—which can make determining a 3D relationship between structures difficult—but Raman and colleagues wrote that they found them to be helpful in evaluating the ureters.

VR uses a more complex algorithm to classify specific 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, according to the authors.

“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 et al.

In addition to helping ID hard-to-spot carcinomas, the authors noted 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.