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Data Acquisition

The single most important factor in the improvement of the performance of virtual colonoscopy has been the development and installation of CT scanners that allow large volumes of data to be scanned with very thin sections in a single breath-hold (3, 41, 42). This greatly reduces any distortions caused by any involuntary body movement while the images are being taken. The new CT scanners also allow multiple angle views to be generated from a single "snapshot." This helps the radiologist distinguish polyps from folds in the colon wall and residual fecal material (3).

At NYU, we use a state-of-the-art Siemens 4- or 16-row multi-detector CT scanner, which scans the entire colon within a 30-second breath-hold. CT images are reconstructed as 1.25 mm-thick sections with a 1 mm reconstruction interval. The examination is networked to a workstation

State-of-the-art Siemens 16-slice multislice
scanner used at NYU.

As mentioned above, less radiation can be used for virtual colonoscopy examinations than for many other procedures using CT (3, 44). This is possible because the contrasting densities of the colon wall and the gas that is pumped into it make them appear very differently on the images and makes the surface of the colon wall stand out more clearly with a lower dose of radiation. The newer CT scanners make the examination possible with an even lower dose of radiation. Moreover, most CT manufacturers are installing automated features on the scanners that allow the dose to be decreased when scanning relatively thinner areas of patient anatomy. In fact, as performed today, the effective dose that a patient receives from a virtual colonoscopy examination is lower than that from a conventional double-contrast barium enema (3, 42).

When performing virtual colonoscopy examinations, there is an opportunity to evaluate more than just the colon. Abnormalities outside the colon may also be detected (45). These lesions may be difficult to detect and importantly difficult to characterize since a low radiation dose is used. In addition, patients do not routinely receive IV or oral contrast material during a virtual colonoscopy examination. Despite this, radiologists interpreting virtual colonoscopy examinations could include a routine check for abnormalities outside the colon that may be revealed on the images.

(A) Incidental discovery of 5 cm kidney cancer (arrow).

(B) Two different patients with incomplete colonoscopies. The patient on the left had a left inguinal hernia (black arrow). The patient in the right shows a large fibroid uterus (white arrow).