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Fusion Techniques in Advanced Endovascular Repair
The performance of increasingly complex endovascular procedures has prompted a need for more eloquent angiographic imaging at the time of the initial procedure The amount of ionizing radiation and contrast media is directly proportional to the complexity of the procedure.
To reduce radiation and contrast dose, the intraoperative use of fusion technologies during endovascular aneurysm repair (EVAR) and fenestrated/branched EVAR (f/b-EVAR) was introduced by the Cleveland Clinic group in 2010. The technique comprises a combination of preoperative computed tomography angiography (CTA) with conventional intraprocedural angiographic techniques.
The technology allows the merging of precise CT-based three-dimensional (3D) anatomic information with the two-dimensional (2D) dynamic functional imaging based on intraoperative fluoroscopy. Fusion imaging provides the complementary benefit of both imaging systems, achieving a reduction of the radiation exposure and contrast dose normally required in such procedures.
Indications
Fusion overlay imaging can be used for every endovascular procedure if a preoperative CTA or magnetic resonance angiography (MRA) scan has been obtained. The greatest advantage is when target vessels need to be cannulated. The use of fusion imaging is especially attractive for advanced endovascular aortic repair. It also helps to overcome challenging anatomies with angulated aortic necks, tortuous vessels, and accessory aortic branches. The application of the technique can simplify complex aortic procedures, decrease the duration of the interventional procedure, and lower radiation exposure.
Technique
Currently, the latest-generation hybrid operating room (OR) is equipped with advanced imaging tools such as fusion imaging. These are coupled with a workstation to load and process the DICOM (Digital Imaging and Communications in Medicine) data from a previous CT (or less frequently MR) scan. A 3D model of the vasculature is segmented and then merged with the live fluoroscopic image. Several imaging companies have developed radio-proprietary systems to combine the different technologies. Recently, Cyder Medica developed a new system able to integrate the required workstation for the fusion technology in every OR angiographic system, including mobile C -arm equipment.
Prerequisites to fusion imaging are (1) the ability to extrapolate vessel anatomy from the 3D DICOM dataset, (2) a precise registration system of the image modalities, and (3) the ability to select and show the operator the relevant aspect of the patient anatomy on the live image without impairing the image quality.
The technique is performed in four simple, automated and manually assisted steps: (1) segmentation, (2) planning, (3) registration, and (4) live image guidance.
Segmentation
At first, the DICOM dataset from the preoperative CT or MR scan is loaded into the workstation. The anatomic structure pertinent to the procedure is selected, limiting the view to the relevant vasculature. Most of the available segmentation engines work semiautomatically. With a point-and-click system, the vessels of interest are selected for further processing (e.g., abdominal aorta and renal, iliac, and hypogastric arteries for infrarenal EVAR) ( Fig. 20.1 ).
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