Intravascular technique shows clinical potential

Virtual endoscopy, or computed endoscopy, is a new method of visualizing CT, MRI, ultrasound, and angiography data. Computer processing of 3D imaging data sets results in displays that are similar or equivalent to those produced by standard endoscopic procedures. The method has attracted attention from various branches of medicine, resulting in a variety of clinical applications.

Reports have described how virtual endoscopy can be used to view hollow organs and joints, such as the knee.1-3 Virtual colonoscopy has been explored widely, and satisfactory results have been obtained when compared with double-contrast barium enema and conventional colonoscopy.4,5 Most studies to date have used volumetric CT data, with only a few dealing with MRI data.

Virtual intravascular endoscopy has also been reported as visualizing vascular structures, such as normal and abnormal anatomic arteries.6-13 Unlike virtual bronchoscopy and colonoscopy, generating these images depends on appropriate selection of threshold values and the degree of contrast enhancement in arterial branches.

Use of virtual intravascular endoscopy could avoid the need for invasive procedures, such as conventional angiography or intravascular ultrasound. The technique is currently awaiting clinical validation in most areas, though. Recognition of its potential value may encourage radiologists and/or physicians to assess the method in practice.

Virtual intravascular endoscopy has attracted less attention than virtual colonoscopy to date. Researchers have, however, applied the method to data sets from CT angiography, MR angiography, and ultrasound. Reported applications include cerebral vascular disease, thoracic aortic atherosclerosis, thoracic and abdominal aortic dissection and aneurysms, peripheral vascular disease, aortic stent-grafting, renal artery stenting, and evaluation of a variety of cardiovascular diseases. 6-13 While the diagnostic value of virtual intravascular endoscopy varies, the method does show the potential to benefit patient management in particular areas (see table).

VESSEL ANATOMY

Noninvasive imaging techniques are used in cases of cerebral vascular disease to identify vascular lesions and their relation to the aneurysm neck as precisely and safely as possible. Different imaging modalities, such as digital subtraction angiography, CTA, and MRA, may be combined to aid preoperative decision making. But it can still be difficult to predict, for instance, whether coil-based embolization of an aneurysm will occlude small neighboring arteries. This makes intraluminal evaluation especially important.

Virtual endoscopic images showing different views can demonstrate intravascular stenosis and determine whether the branches arise from the aneurysm or aneurysm neck. They can also delineate the aneurysmal lumen and parent blood vessel lumen.6 Several case reports have demonstrated that the technique can be used to visualize intravascular malformations from high-resolution CTA and MRA data sets and to provide superior definition of both the aneurysm neck and the morphology of saccular aneurysms when compared with images reformatted using maximum intensity projection (MIP).7,8

Standard diagnostic criteria for all imaging techniques used in aortic dissection include:

- detection of an intimal flap separating the aortic lumen into two channels;

- detection of true and false lumens;

- demonstration of displacement of intimal calcification; and

- detection of entry site.

It is also crucial to localize and define the extent of disease.

The relationship between the arterial orifice and the intimal flap has been depicted with 3D virtual intravascular endoscopy, although it was reportedly seen more clearly on 2D axial source images.9 Researchers found that the 3D imaging technique provided no additional information about aortic dissections when compared with 2D axial images. While virtual intravascular endoscopy is a useful tool in the evaluation of aortic dissection and allows better anatomic definition, findings should still be correlated with axial images and multiplanar views. Its diagnostic value is consequently limited in this application and influenced significantly by operator experience.

Atherosclerosis often appears as an irregularity or roughness to the inner wall of a blood vessel. Aortic wall roughness is best visualized by looking directly at the endoluminal surface. Effects of lighting, shading, and reflectivity of the virtual light source used in virtual intravascular endoscopy images provide additional contrast. This may help observers detect wall irregularities better than when using MIP techniques. Endoscopic views also allow close inspection of the lumen.

Researchers have applied virtual intravascular endoscopy to evaluate thoracic aortic atherosclerosis in patients with homozygous familial hypercholesterolemia and in a rabbit model of atherosclerosis.10,11 Their results show that the method can demonstrate luminal irregularities of the aorta's inner surface. The virtual endoscopy views additionally increase confidence in scoring wall roughness over "exoscopic views" (extraluminal views) alone. The ability of virtual intravascular endoscopy to depict nonstenotic wall irregularities associated with thoracic aortic atherosclerosis in patients with homozygous familial hypercholesterolemia may be important for assessing disease progression and treatment response.

Thoracic and abdominal aortic aneurysms can be visualized easily with 3D CTA and MRA. These techniques can provide 3D information on the aneurysmal lumen and its relationship to aortic branches (Figures 1 and 2). Virtual intravascular endoscopy may be helpful in detecting wall-bound thrombus, which has a distinct "cobblestone" appearance. One study has shown the method to have no role in most preoperative situations when compared with axial CT and 3D images.14 It could, however, prove valuable in planning endovascular repair or conventional surgery.12

Peripheral arterial disease is among the most common diseases in Western countries. DSA is still considered to be the diagnostic gold standard, although CTA offers the advantages of rapid image acquisition, wide availability of CT scanners, and a noninvasive examination. Initial clinical results for peripheral artery evaluation comparing DSA with multislice CTA look promising.15,16 Virtual intravascular endoscopy can provide additional information about the relationship between an aneurysm and its adjacent branches. The technique's diagnostic value in peripheral arterial disease, however, is limited when compared with CTA or MRA.

INTRALUMINAL DEVICES

Endovascular stent-grafting is a less invasive method of treating patients with thoracic and abdominal aortic aneurysms. This method has gained worldwide acceptance since its introduction into clinical trials in 1991.17 The success of endovascular repair depends significantly on medical imaging. The most commonly used methods for pre- and postoperative assessment involve 2D axial source images and 3D postprocessing methods.

Virtual intravascular endoscopy has been reported as being especially helpful in the evaluation of patients with abdominal aortic aneurysms treated with suprarenal stent-graft. This method shows the relationship of stent struts to renal and other visceral ostia clearly and in 3D (Figure 3).13,14 It can add additional information to that acquired by conventional 2D and other 3D images, such as the number and configuration of stent wires crossing the aortic ostia. This could help physicians evaluate the long-term effect of suprarenal stent-grafting.

Renal artery stenosis is traditionally diagnosed by selective angiography. This approach is now changing, as CTA and MRA show themselves to be accurate, noninvasive methods of evaluating aortoiliac and renal arteries.18,19 One group of investigators found that CTA-based virtual intravascular endoscopy added no additional information in the detection of accessory renal arteries.20 A separate group of researchers showed that virtual intravascular endoscopy could visualize the renal stent directly, and provide 3D images of the internal stent lumen relative to the renal ostium. Image quality and vascular delineation were both good, particularly for patent stents.21 Virtual intravascular endoscopy can complement CTA in the follow-up of renal stenting.

The heart and coronary arteries are promising targets for virtual endoscopic imaging. Although conventional angioscopy may contribute to assessments of atheromatous plaques and ischemic heart disease, it is invasive and unacceptable for some patients. Existing 3D postprocessing software packages permit virtual coronary angioscopic images to be reconstructed with good image quality.

Virtual intravascular endoscopy can visualize intracardiac structures of the cardiac chambers and aortic valves. The method can also evaluate irregularities, plaque formation, and calcified deposits on the inner lumen of the coronary artery. Researchers have reported that coronary virtual intravascular endoscopy can detect complex lesions with an irregular surface and calcifications accurately within the lumen.22 The use of this technique to obtain quantitative measurements could help determine stenosis or plaque volume and predict the prognosis of coronary artery disease.

As shown in the table, virtual intravascular endoscopy has potential applications in a number of areas compared with other imaging visualizations. Specifically, the future diagnostic value of virtual intravascular endoscopy lies in the following areas.

- Presurgical planning of aortic aneurysms. This will be particularly helpful for patients undergoing fenestrated stent grafting,23,24 which depends heavily on 3D CT imaging. Virtual intravascular endoscopy can measure intraluminal distance between the aortic ostia, which is essential for successful placement of fenestrated stent grafts.

- Follow-up of endovascular aneurysm repair. This application will involve correlating virtual intravascular endoscopy findings with resultant in-terference with blood flow or renal function. A related application is follow-up of endovascular repair, specifically focusing on interference with the configuration of the aortic ostium from the presence of endoluminal stents.

- Coronary artery disease. Virtual intravascular endoscopy will be especially useful in quantitative measurement of coronary plaque volume.

The diagnostic value of virtual intravascular endoscopy in these areas deserves to be validated in a large cohort study. Virtual intravascular endoscopy has been applied in a variety of vascular diseases. Clinical validation must now be performed. This should include an assessment of the additional information or diagnostic value that it provides for each application. The technique's usefulness will depend on acquisition parameters, postprocessing techniques, and clinical indications for examination.

References

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Dr. Sun is a radiologist and lecturer in the department of imaging and applied physics at Curtin University of Technology in Perth, Australia. Dr. Lin is an associate professor and director in the department of radiological technology, Central Taiwan University of Science and Technology, Taichung City, Taiwan.