Virtual Imaging Case Studies

Elliot K. Fishman, MD

I. Airway (trachea & bronchial tree)
II. Colon
III. Vascular
IV. Bladder

One of the most impressive sets of post processed images that can result from combining volume data acquisition of CT with computer-assisted imaging is virtual imaging. Whether you are referring to imaging the airway (virtual bronchoscopy), the colon (virtual colonoscopy), the vascular tree (virtual angioscopy) or the bladder (virtual cystoscopy), the imaging technique are in many ways very similar. The computer is programmed to use the difference in Hounsfield unit value between structures for creating 3D images with virtual imaging (i.e. air in trachea vs. tracheal wall; air in colon vs. colonic wall) instead of the usual 3D projection that is a typical 3D view of the data is as if you were outside the dataset while with virtual imaging, the projection is as if you are within the organ or structure of interest. In fact, virtual type displays can be used in other structures including the sinuses, spiral cord or stomach based on the same principles.

One the most critical aspects of this virtual technology is measuring its true clinical value. Although, the idea of flying thru a colon or vessel has a certain ‘coolness’ value it has not been shown to be of specific clinical value in most cases. Additionally to date the technology to create fly-thru’s has been buggy at best and image creation display and manipulation has been very time consuming. Some new systems such as the Terarecon Aquarius workstation provide excellent fly-thru’s along a centerline with little user intervention. The user simply chooses the starting point and the system will fly from there. Interacting with the data as well as recording the procedure is seamless and the best implematation I have seen to date. Most of the cases in this section are from that system.

Study Technique:

Scans are acquired on a Siemens Sensation 4 scanner using .75-subsecond tube rotation. We ideally use the 1 mm detectors reconstructing 1.25 mm thick sections at 1.0 mm intervals during a single breathhold. The pitch used is 6-8 for each application there are certain specific protocols, which include:

Endovascular:

120 cc of Omnipaque 350 contrast is injection at 3ml/sec. Scan delay is usually 25 sec for arterial phase and 50 sec for venous phase studies. The need for single or dual phase imaging depends on the clinical application (aortic aneurysm, carotid stenosis etc). Images must be reconstructed with a standard reconstruction a lgorithm.

Virtual Colonoscopy:

The success of the study is dependent are a number of technical factors that begin with a adequate patient bowel prep (Fleet – 1 kit) and a quality CT exam which includes adequate air distention (2 liters or 50 puffs of the balloon) and scanning the patient supine and prone. The patient must be cooperative enough to hold the small rectal tube in place and the air during the study. IV contrast can be helpful but is not routinely used.

Virtual Cystoscopy:

The key to a detailed exam of the bladder is the distention of the bladder with contrast dripped in under gravity. We put 30 cc of Omnipaque –350 in a bag of 500 cc of saline and then drip this in thru a foley until the patient feels full. This gives homogeneous enhancement to the bladder fluid and provides a good interface for the 3D mapping. Alternatively, the bladder can be distended with air and the study performed like a virtual colonoscopy. Please note that using contrast in the bladder from a peripheral IV injection usually results in a poor study, as the bladder is typically not fully distended and the contrast layers, which causes significant artifact on the 3D.

Case Studies

CONCLUSION

Virtual imaging is becoming of increased interest as documented by this article. My advice is the same as that old TV commercial; "Try it, you’ll like it". (p.s. I can’t remember what the commercial was for.)