Endovascular Aortic Reconstruction
Use of Preview software to plan and predict intervention.
To view the figures related to this article please refer to the print version of our April issue, page 34.
During the last 20 years, the development of catheter-based endovascular therapies for cardiovascular conditions has been paralleled by a revolution in the integration of computer technology with imaging modalities. The introduction of techniques to perform endovascular aortic construction in the early 1990s clarified the need for much more detailed anatomic imaging than had been the standard for open reconstruction. It was quickly recognized that accurate graft diameter and length sizing had a major impact on both acute and chronic outcomes. Tolerances in the 1 mm to 2 mm range were required to optimize graft performance for fixation and seal at attachment sites with respect to both diameter and length. The modalities used at the inception of endovascular aortic grafting, axial CT scans and contrast angiography, were both limited by their two-dimensional nature. Both of these techniques had significant artifacts because they could only depict anatomy with reference to an artificial frame of reference, rather than the frame of reference of the structure they were measuring. Endoleaks at attachment sites resulted from incorrect diameter measurements, incorrect neck length measurements, and spurious deployment from incorrect imaging alignment in the operating room. Graft length errors resulted in the occlusion of important tributaries or the failure to extend to the appropriate distal vasculature.
The recognition of these shortcomings fueled the development of software that utilized the raw data generated in a CT scan to provide the clinician with a three-dimensional (3-D) virtual anatomy of a specific patient. This effort originated at Dartmouth University and eventually resulted in the formation of a new company, Medical Media Systems (West Lebanon, NH). The product was called Preview. Although there were many other products that could produce 3-D images of reconstructed CT scans, Preview was specifically designed to segment the important elements of the cardiovascular structures as liquid blood, thrombus/soft plaque, and calcium so that they could be viewed separately or collectively. The most important element of the software was its metric capabilities. The clinician could measure diameters at right angles to the vasculature at any point along its tortuous path and measure lengths along the centerlines of vessels, and view the interaction of the anatomy with a “virtual graft” of a specified diameter and length to see if seal and fixation could be achieved. For these reasons, it was more of a “computer-assisted design” for planning endovascular therapies as opposed to an imaging tool. By providing the treating physician with a CD-ROM that could be used on a conventional desktop or laptop computer, the plan could be reviewed and reiterated with convenience and on multiple occasions. As an FDA-cleared measurement tool, it also fulfilled an important function of standardizing measurements from any facility with a CT scanner and served as a “core laboratory” for many clinical trials.
This article summarizes the key capabilities of this type of 3-D planning and virtual surgery for planning endovascular procedures. The primary areas of utility are:
• proximal neck diameter, length, approach, mural characteristics
• length measurements
• iliac diameter, tortuosity, vessel pathology/access
• deployment imaging optimization
• procedure follow-up and monitoring
PREVIEW SOFTWARE CAPABILITIES
Patient imaging is accomplished in a conventional CT scanner with intravenous contrast material with at least 3-mm thickness scans for data acquisition. In patients with renal insufficiency either no contrast or gadolinium can be used. The raw data are sent to a central facility at Medical Media Systems. A CD-ROM is returned to the physician and can be viewed on any computer. The physician can then view CT slices in six different planes: axial, aortic, aorto-right iliac, aorto-left iliac, coronal, and sagittal. All of the aortic views provide CT reconstruction orthogonal to the vessel at that location. A 3-D model is also visible. This model has separate elements of liquid blood (red), thrombus/soft plaque (yellow), calcium/stent (white), and vessel central line (green). Each of these elements can be made visible, transparent, or invisible independently of each other. Measurements can be made in a wide variety of ways. Lengths can be measured along the centerline of vessels, between user-designated marks, or along any line of user-designated marks in 3-D space. Angles can be measured between any set of three marks. User-designated marks can be made of any diameter and their interaction with the model is evident in both the CT slice and the 3-D model. Volumes of any user-defined vessel segment can be measured, as can the volume of all the elements, separately or together. A virtual graft can be defined by the user with diameter and length of proximal body and two limbs originating from a defined level. When viewed in the 3-D model, it quickly becomes evident where the graft will seal against the vessel wall and where it will not.
Proximal Neck Evaluation
The security of fixation and adequacy of seal at the proximal neck of an infrarenal aneurysm is probably the single most important factor in predicting the successful outcome of endovascular aneurysm repair. It is for this reason that precise sizing and accurate deployment are of paramount importance in the implantation of all endovascular aortic grafts. Axial CT scans and angiography have significant artifacts in given anatomic situations because they typically displace the lumen of the aorta in a plane oblique to the flow lumen and represent only a two-dimensional representation of a 3-D structure. The security of proximal fixation is also very dependent on accurate determination of mural characteristics (Figure 1).
Graft length is one of the most difficult aspects of endovascular graft design and implantation. Both conventional CT scanning and angiography with calibrated measurement catheters result in significant and variable artifacts that are a product of a given set of anatomic features. For this reason, the same techniques can both over- and underestimate appropriate graft lengths in different patients. The 3-D nature of Preview software and the virtual environment allows the measurement of any vessel length along the lumen centerline. This results in very accurate graft lengths and the avoidance of sizing at the time of implantation, and the inadvertent occlusion of distal vasculature (ie, internal iliac vessels). Even for modular grafts, the appropriate amount of overlap between adjoining graft members is important so as to prevent future disjunction. Preoperative measurements are vital to prevent the unexpected need for more length and the violation of modular overlap recommendations (Figure 2A through D).
The heterogeneity of iliac pathology and anatomy is the most complex and variable of all the elements involved in endovascular aortic grafting. Aneurysmal and occlusive elements are often present in the same patient and tortuosity is the rule rather than the exception. Severe calcification of the iliac vasculature represents one of the most difficult and complicating features of endovascular grafting because large catheters that require coaxial movement for deployment are often deformed beyond their ability to function by highly calcified vasculature. Access via the external iliac arteries also is very difficult to evaluate by conventional CT and angiography, and the overzealous attempt to push a delivery catheter through a small external iliac can result in rupture and uncontrolled hemorrhage (Figure 3A through C).
Deployment Imaging Optimization
The ability to precisely deploy an endograft in the infrarenal neck allows fixation in the best and least diseased infrarenal tissue. This generally means deploying the graft as close to the orifice of the lowest renal artery as possible. Obviously, errors at this stage of implantation result in either wasted aortic neck, and the resulting diminished fixation and potential for migration, or renal artery occlusion. During clinical trials for endovascular grafting, it was realized that angulation of the C-arm to bring the aortic neck into a perpendicular view was critical to accurate deployment. Preview allows precise angle measurements so that the C-arm can be precisely oriented to allow optimal proximal deployment (Figure 4).
Procedure Follow-Up and Monitoring
The ability to determine whether an implanted endovascular graft is performing its intended function of decompressing and isolating the aneurysmal aortic segment is one of the most difficult and debated aspects of this technique. Traditionally, transverse diameter measurements have been used to determine appropriate timing for surgical intervention. The same measurement is often used to determine shrinkage, stability, or increase in aneurysm size after endovascular therapy. Unfortunately, unidimensional measurements of a 3-D structure are relatively insensitive and many patients are subjected to prolonged and intensive follow-up protocols. The ability to measure volume of the aneurysmal segment allows a much earlier determination of whether the sac is diminishing or growing.
Migration has proven to be the most insidious and dangerous of all factors related to late endograft failure. Early identification of migration and its impact on limb patency (ie, kinking) or potential loss of fixation is critical to avoiding life-threatening endograft complications (ie, graft occlusion, aneurysm rupture) (Figure 5).
The advent of minimally invasive therapy for major vascular structures has clarified the necessity of far more detailed preoperative imaging and metric analysis. Preview represents an FDA-cleared measurement product for vascular structures that can render data from any facility in an identical and reproducible way. The ability of the surgeon to manipulate this 3-D environment on widely available computer platforms allows the performance of virtual surgery within that 3-D environment and the ability to recognize problems and potential failure modes before making an incision. It is invaluable in the postoperative period to determine the stability of the endograft and the early demonstration of aneurysm decompression. It avoids the necessity of preoperative arteriography, therefore saving the cost, risk, and discomfort of that invasive diagnostic study. For those reasons, the Centers for Medicare and Medicaid Services approved payment specifically for these services, making them available to all practitioners. The experience at Georgetown University with this method of pre- and postoperative assessment has led to fewer unexpected intraoperative events and fewer complications. In essence, it is to vascular assessment what GPS (Global Positioning Satellite) is to navigation!
David H. Deaton, MD, is Chief of Endovascular Surgery at Georgetown University Hospital in Washington, DC. He has served as a consultant to Medical Media Systems. Dr. Deaton may be reached at (202) 687-1265; firstname.lastname@example.org.
Richard Neville, MD, is Chief of Vascular Surgery at Georgetown University Hospital in Washington, DC. He does not hold a financial interest in any product or company mentioned herein. Dr. Neville may be reached at (202) 687-2255.