Three-D CTA views assist endovascular aneurysm repair

Conventional surgical repair is considered the gold standard for the treatment of patients with an abdominal aortic aneurysm. Surgery is not only invasive, however; it also carries the risk of perioperative and postoperative complications. Endovascular repair is a less invasive option that has been shown to be an effective alternative.^1-3

The success of endovascular repair, unlike conventional surgery, is highly dependent on radiological imaging. Spiral CT angiography is the preferred modality for preoperative planning and postoperative follow-up.^4,5 Conventional 2D axial CTA images can be complemented by a series of 3D reconstructions that allow clinicians to evaluate the relationship among the aneurysm, stent-grafts, and arterial branches.^6-8 These reconstructed visualizations may include multiplanar reformation, curved multiplanar reconstruction, shaded surface display, maximum intensity projection, volume rendering, and virtual endoscopy. Some techniques will be more useful than others.

Multiplanar/curved reformation. Multiplanar reformation allows users to view image data in the acquisition plane, or any orthogonal plane, quickly and easily. Planes can be sliced away interactively in the three orthogonal orientations to reveal interior sections of the cube (volume data). This capability enhances viewers' understanding of the complex anatomy of an aortic aneurysm and its relationship to an endovascular stent-graft. Most aortic aneurysms are tortuous and angulated to some extent, and conventional MPR cannot always show the anatomical information required for assessment.

Curved multiplanar reformation (CVR) will generate any arbitrary plane interactively through a volume, and can be used to reformat a series or the entire volume. CVR is especially useful when assessing patients with tortuous or angulated aneurysms, because images along the centerline of the abdominal aorta can be generated. The relationship between suprarenal stent struts and renal arteries can also be appreciated (Figure 1).

Shaded surface display. Shaded surface display represents the surface of a structure within a volume data set. This view relies on simple thresholding and can be generated quickly. SSD offers superior speed and flexibility in terms of image rendering for the visualization of stent-grafts and arterial branches.Because not all information in the volume data is used, however, visualization is inherently limited (Figure 2). Voxels representing mixed tissue interfaces cannot be accurately classified. It is also difficult to differentiate high-density calcifications from stent wires. SSD provides little additional information when compared to 2D axial images and consequently has little role to play in the assessment of endovascular repair.

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• Maximum intensity projection. Independence from threshold selection makes maximum intensity projection the preferred method of image visualization in vascular imaging (Figure 3). The MIP algorithm selects voxels with the highest attenuation along lines projected through the volume data set. Calcifications, contrast-enhanced arteries, stent wires, and bones are visualized easily. The ability of MIP images to show endoleaks and stent-graft migration has made them useful for follow-up of endovascular repair.⁹

MIP volume data must be edited to avoid overlap between different structures, but manual editing is a time-consuming process. Postprocessing is feasible in clinical practice, however only if the editing process is automatic or semiautomatic. Various software packages available for 3D postprocessing make the generation of MIP images much faster and more efficient.

• Volume rendering. Volume rendering uses all of the information from volume data to demonstrate structures such as calcifications, aortic arterial branches, and stent-grafts. Color and a degree of opacity/transparency and shading are assigned to each particular structure, making them easy to identify and differentiate from one another. Metal stent wires, arterial branches, and bones can all be displayed in one image (Figure 4).

Volume rendering has been accepted as the favored 3D visualization tool for endovascular aneurysm repair because it improves visualization of complex anatomy and 3D relationships between arterial branches and stent-grafts.

• Virtual endoscopy. Virtual endoscopy differs from the other visualization techniques because it offers intraluminal information, such as intraluminal stent wires and their relation to the aortic artery ostia. The degree of encroachment to the aortic artery ostia, mainly due to interference of the renal artery ostium by stent wires, can be assessed accurately as well (Figure 5). It has been reported that virtual endoscopy enhances understanding of the effect of endovascular repair on aortic branches, especially in patients with AAA treated with suprarenal stent-grafts.^6-12

CLINICAL CHOICES

CT angiography can be performed more efficiently with multislice systems because of their faster scanning speed and higher spatial and temporal resolution.^13-15 Near-isotropic volume data can be obtained with 64-slice CTA, improving the quality of 3D postprocessed images significantly.

Previous studies have reported the diagnostic value of 3D CT visualizations in patients with AAA following endovascular repair when compared with 2D images.^6-12 Three-D MSCT postprocessing has become a routine protocol in our clinical practice that has been used as an effective alternative to conventional angiography in many areas. Multislice CTA has replaced conventional angiography, for example, in pre- and postoperative assessment of endovascular repair of AAA.

Accurate selection of the appropriate visualization method is of paramount importance. This should ensure that additional information to that delivered by 2D images is generated, that unnecessary examinations are avoided, and that staff workload is reduced.

MIP and volume-rendered visualizations are superior to 2D CTA images when evaluating stent-grafts in relation to aortic branches following endovascular repair. This is especially true when assessing stent-graft migration and the patency of arterial branches. Volume rendering requires minimal or no editing of bone to view the vasculature. MIP, on the other hand, requires substantial editing in most cases.

An additional issue with MIP is that the presence of other highly attenuating voxels may obscure the vasculature and make it difficult to view 3D relationships among the structures on display.¹⁶ Volume rendering always depicts soft tissues and 3D relationships accurately.

SSD has a limited role to play in the assessment of stent-grafts, whereas CVR can be helpful in patients with angulated or tortuous aneurysms. Because the long-term effect of aortic stent-grafting is unclear, additional information provided by 3D visualizations is important for vascular surgeons to assess the outcome of endovascular AAA repairs.^8,9,12 We have found virtual endoscopy to be particularly useful for visualizing encroachment to aortic artery ostia by stent wires.

FUTURE DIRECTIONS

As the use of multislice CTA for AAA endovascular repair de¬velops, a number of areas will require further investigation. First and foremost, all MSCT examinations are associated with a high radiation dose. CTA scanning protocols will need to be optimized to minimize the radiation dose delivered to patients.¹⁷

The huge amount of data generated by MSCT imaging also puts a substantial strain on any image analysis and archiving network. Changes to computer hardware, software, and interfaces are required to ensure rapid postprocessing of 3D images before and after any endovascular AAA procedures.

Three-D visualizations could additionally play a role in fenestrated stent-grafting, a modified technique used to treat patients with unfavorable aneurysm necks.^18,19 Fenestrated devices are more complicated than conventional endovascular repairs, and the distance between visceral vessels and the correct location of the visceral ostia from the aortic circumference must be calculated accurately. Virtual endoscopy may be of value because of its ability to reveal the intraluminal relationship between the aortic ostia. But this suggestion will need to be validated clinically.

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 director of the department of radiological technology at Central Taiwan University of Science and Technology in Taichung City.