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. 2013 Winter;18(1):48–52.

A novel measurement technique for the design of fenestrated stent grafts: Comparison with three-dimensional aorta models

Ji Hoon You 1, Sung-Gwon Kang 2,✉, Bong Man Kim 2
PMCID: PMC3716489  PMID: 24294038

Abstract

BACKGROUND:

Stent graft placement is an acceptable treatment option for aortic disease, particularly for abdominal aortic aneurysm. At present, the use of stent grafts is expanding beyond current indications for use. Fenestrated stent grafts are used in patients with abdominal aortic aneurysms whose aortic anatomy is unsuitable for repair using standard devices. The success of fenestrated stent graft placement is largely dependent on planning, including obtaining measurements and designing the stent.

OBJECTIVE:

To demonstrate a measurement technique that may be used for the design of fenestrated stent grafts to repair endovascular aneurysms, and to compare these measurements, obtained using archived two-dimensional patient data, with measurements obtained using a three-dimensional (3-D) computer-assisted design model.

METHODS:

Fenestrated stent grafts were designed and fabricated based on computed tomographic angiography images. 3-D models were constructed using modelling software and rapid prototyping technology incorporated with fused deposition modelling. The stent grafts were trunk-type, with four holes for the visceral branches (celiac axis, superior mesenteric artery, right renal artery and left renal artery). Computed tomography scans of 10 patients with abdominal aortic aneurysms were reviewed. Axial, multiplanar reconstruction and curved multiplanar reconstruction images were used to measure 11 parameters. Sizing of the fenestrated aortic stent grafts was performed independently by an experienced interventional radiologist, and the results were compared with the same measurements calculated using the 3-D aorta model (generated using Materialise Interactive Medical Image Control System software [Materialise NV, Belgium]). Data were reported as the mean of the measurements. Measurements were evaluated using Bland-Altman analysis and concordance correlation coefficients (CCCs).

RESULTS:

A total of 10 fenestrated stent grafts were fabricated. The proximal landing section above the celiac axis (one point of the wall being defined as the standard point) was 3 cm, and the distal flared section was 3 cm below the lowest renal artery. Ten computer-assisted design aorta models were successfully constructed. Measurements of the aortic diameter showed high agreement between those obtained using the archived patient computer system stent graft and those obtained using the 3-D aorta model. The CCC for variability was 0.9974. The distance from the standard point to the branch vessels also demonstrated good agreement. The CCC for variability was 0.9999.

DISCUSSION:

A direct measurement technique using a standard point was simple to perform and was easily applied to the fabrication process. Preparation time will likely be shortened and the versatility of stent grafts will be improved using this method. It will be possible to produce standardized fenestrated stent grafts once patients’ measurements are recorded and analyzed.

CONCLUSION:

A fenestrated stent graft design technique using measurements of distance from a standard point generally showed a high level of agreement with a 3-D aorta model.

Keywords: Abdominal aortic aneurysm, Fenestration, Stent graft

Since Parodi (1) reported the first stent graft placement in the human aorta and Volodos (2) performed an experimental study involving stent grafts, their use has become an accepted, valuable and safe option for the treatment of aortic diseases such as thoracic aortic aneurysm, thoracic aortic dissection and abdominal aortic aneurysm (3–5). This endovascular technique is associated with reduced morbidity and mortality (5–10). Technical improvements in stent graft production have made this treatment feasible, and the indications are diverse. However, the limitation of their use in endovascular repair (EVAR) and thoracic EVAR (TEVAR) has been anatomical suitability (11–13). To successfully place a stent graft device in the aortic lesion, the proximal and distal landing zone should be sufficiently long and angulation in this area should be <60° (13,14). The landing zone is a particularly important anatomical requirement in TEVAR and EVAR for two reasons: the stent graft procedure is technically feasible only when the landing zone is longer than 1.5 cm; and the landing zone anatomy is very important for clinical success (15). The most common and significant complication of TEVAR and EVAR is endoleak (16–26). When the length of the landing zone is short, the risk of endoleak increases. Fenestrated stent grafts have been introduced as a treatment option for patients with aortic aneurysms whose neck is anatomically unsuitable for TEVAR and EVAR using standard devices (27–31).

The purpose of the present study was to establish a technical method for designing fenestrated stent grafts for EVAR and to compare this method with three-dimensional (3-D) modelling using computer-assisted design (CAD).

METHODS

Image acquisition

Digital Imaging and Communications in Medicine (DICOM) files were obtained from 10 patients with abdominal aortic aneurysms. Computed tomography angiography (CTA) was performed according to the following protocol: 150 mL contrast media (Visipaque, GE Healthcare, USA) was injected into an antecubital vein at a rate of 3 mL/s to 5 mL/s. Automatic bolus tracking software was used to optimize arterial opacification (Spiral Auto System, Phillips, Nethelands). The slice thickness was 0.75 cm and the gap between slices was 0.3 cm. DICOM files were transferred using Osyrix MD (Pixmeo, Switzerland) to create a reconstructed image. Materialise Interactive Medical Image Control System (MIMICS) software version 14 (Materialise NV, USA) was used to create a 3-D CAD model of the aortic lesion from the CTA images.

Planning using the reconstructed image

Multiplanar reconstruction (MPR), volume rendering and curved MPR (cMPR) were used to design the fenestrated stent grafts. One point on the orifice of the celiac axis was used as the standard position. Diameters of the aorta and branch vessels were measured at 1 cm above the celiac axis for the aortic diameter and 0.5 cm from the aortic wall for each branch vessel, respectively (Figure 1). The diameter was defined as the maximum intima-to-intima distance at a vertical plane to the longitudinal axis. The shortest distance between the celiac axis and each branch vessel (superior mesenteric artery [SMA], right renal artery [RRA] and left renal artery [LRA]) was measured vertically and horizontally. Transverse distance was measured from the central vertical grid line directly to each branch vessel in the axial image (Figure 2A) and was calculated as the distance from the central vertical line to the SMA, RRA and LRA ± the distance from the central vertical line to the celiac axis. The vertical distance between the branch vessels was measured in the cMPR image (Figure 2B).

Figure 1).

Figure 1)

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Schematic drawing of measurements including diameters and horizontal and vertical distances between the celiac axis (CA) and branch vessels. Lt Left; LRA Left renal artery; RRA Right renal artery; Rt Right; SMA Superior mesenteric artery

Figure 2).

Figure 2)

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A Sample measurement of the transverse distance of the left renal artery on an axial image. The transverse distance was measured from the central grid line directly to the left renal artery. B Sample measurement of the vertical distance of the left renal artery from the celiac axis on a curved multiplanar reconstruction image

Fenestrated stent graft design

Trunk-type fenestrated stent grafts were designed according to the measurements and calculated distances. First, a fenestrated hall was made for the celiac axis 2.5 cm from the margin. From this first hall, the other three fenestrations were made based on the measured and calculated distances. On a paper template, vertical and transverse lines were drawn at the margin of the first fenestrated hall. Three transverse lines were drawn from the line at the lower margin of the first fenestrated hall at the measured distances. This paper template was attached to a round jig with a diameter 120% larger than the measured aortic diameter. Three points were marked on the template from the line that was drawn at both lateral margins of the first fenestrated hall at the calculated distance using a divider. Three halls (for the SMA, RRA and LRA) were made at the lateral lower corner at which two vertical and transverse lines intersected. Fenestrated halls were marked on Dacron fabric (S&G Biotech, Korea) and punched, and stents were sutured outside of this fabric (Figure 3).

Figure 3).

Figure 3)

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A The adjustments of measurements to the jig. B The formation of the hall for the branch vessels. C The completed fenestrated stent graft

The length of the proximal landing zone was 2.5 cm. When the trunk diameter was <26 mm in diameter, a 3 cm flared distal portion was produced to allow technical feasibility of branch placement. The stent graft diameter was 120% of the measured diameter at the proximal landing zone.

3-D model

DICOM data were converted to CAD data using the MIMICS program and the same parameters were measured. MIMICS is a modelling software that provides an interface between reconstructed tomographic angiography (TA), computed tomography (CT) or magnetic resonance image (MRI) slices and rapid prototyping machines by allowing the export of files in the stereolithography (STL) format, which is the industry standard for rapid prototyping using the STL+ model within MIMICS (32). The reconstruction process in MIMICS requires the selection of a proper threshold, which delineates all pixels in the entire data set within the specified range. This process assigns a certain colour to a region of interest (in this case, the diseased aorta).

The software refers to this combination of pixels as a mask and the 3-D model is generated based on this mask (Figure 4A) (33). After applying suitable thresholds, a region-growing function was used to retain all contiguous elements and a smoothing operation was performed on the 3-D object to obtain the final CAD model (Figure 3A). After exporting the STL files for the CAD models, the 3-D model of the aorta was produced using rapid prototyping technology incorporated with a fused deposition modelling (FDM) process. The FDM Maxum prototyping system (Stratasys Inc, USA) was used to process the FDM (Figure 3B). FDM Maxum has its own software (Insight, StratasysInc, USA) that can read only STL formatted files and automatically slices and generates necessary support structures as well as the material extrusion paths. On successful uploading of the STL files, FDM Maxum runs were performed to optimize the deposition sequencing of the model in the fabrication chamber. The process proceeds layer by layer, and creates an acrylonitrile-butadiene-styrene (ABS) resin model. Construction of the optimized FDM of the ABS aortic model took approximately 10 h to complete. Layers of ABS (178 μm thick) were deposited to produce the model (Figure 4B). This thickness represents the highest spatial resolution of the system (7).

Figure 4).

Figure 4)

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A Reconstructed three-dimensional model produced using the computer-assisted design program. B Actual printed three-dimensional model

Data analysis

Measurements were evaluated using Bland-Altman analysis (34) and concordance correlation coefficients (CCCs) (35). MedCalc statistical software version 10.0.0 (MedCalc Software, Belgium) was used to perform statistical analyses. The measurement error was calculated as the mean SD of the difference in measurement of the four measurements per target vessel (orientation and distance) and was averaged for all patients. The comparison with the measurements of the 3-D aorta model was expressed as the mean (± SD) difference in measurement and 95% CI. Orientation was represented by the transverse and vertical distances between a standard point (one point of the celiac orifice) and the other three branch vessels (SMA, RRA and LRA, respectively) in mm.

RESULTS

Image data acquisition

DICOM data sets of 10 patients were used in the present study. The quality of the CT images was considered to be satisfactory for postprocessing (MPR, cMPR and volume rendering) in all patients.

Stent design using reconstructed images

Four diameters, three vertical distances and three transverse distances were measured in 10 patients. Measurement data including SDs are summarized in Table 1. The diameters of the celiac axis, SMA, RRA and LRA were 6.70 mm, 8.43 mm, 5.92 mm and 6.31 mm, respectively.

TABLE 1.

Measurements of diameters and transverse and vertical distances (n=10)

PACS CAD
CA 6.70±1.39 6.39±1.34
SMA 8.43±1.84 7.93±1.29
RRA 5.29±2.19 5.78±1.87
LRA 6.31±2.21 5.40±1.69
CA-SMA (T) 4.02±3.41 2.87±2.56
CA-RRA (T) 8.59±4.74 8.21±4.77
CA-LRA (T) 15.62±4.57 14.64±4.73
CA-SMA (V) 18.59±9.07 18.46±8.57
CA-RRA (V) 32.23±18.23 33.9±21.83
CA-LRA (V) 33.58±16.46 33.48±16.30
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Data presented as mean ± SD. CA Celiac axis; CAD Computer-assisted design; LRA Left renal artery; PACS Picture archiving and communication system; RRA Right renal artery; SMA Superior mesenteric artery; T Transverse; V Vertical

Fenestrated stent graft fabrication

Ten fenestrated stent grafts were fabricated based on the measurements. Four holes were made in the fabric and the fenestrated fabric was attached to the stent skeleton. The diameter of the manufactured stent graft was 24 mm in two grafts, 26 mm in four grafts, 28 mm in two grafts and 30 mm in one graft. In six stent grafts (diameter 24 mm and 26 mm) the distal 3 cm segment was flared to 4 mm.

3-D model

Ten sets of DICOM CTA data were successfully converted to CAD data and processed to produce 3-D models. All distances were measured using the MIMICS program. Measurement data, including SDs, are shown in Table 1. The diameters of the celiac axis, SMA, RRA and LRA were 6.39 mm, 7.93 mm, 5.78 mm and 5.40 mm, respectively.

Data analysis

In Bland-Altman plots of variability (95% limits of agreement) of the two data sets, the bias was −0.1927 mm and the 95% limits of agreement were −3.9344 mm and 3.5490 mm for absolute difference (measurements in PACS versus 3-D aorta model).

The CCC for variability was 0.9974 (95% CI 0.9904 to 0.9993). The bias in transverse and vertical distances between the standard point and branch vessel was −0.06455 mm and the 95% limits of agreement were −1.1418 mm and 1.0127 mm for the absolute difference (measurements in PACS versus 3-D aorta model). The CCC for variability was 0.9999 (95% CI 0.9997 to 1.0000). Gross comparison and direct measurements between the fabricated stent grafts and the printed 3-D models were well matched.

DISCUSSION

Since Anderson et al (36) first described EVAR for pararenal and suprarenal aneurysm with fenestrated stent grafts in 1999, it has become a treatment option for complex aneurysms unsuitable for standard infrarenal placement of stent graft due to the lack of an adequate landing zone. However, the use of fenestrated stent grafts is technically demanding and time-consuming because measurements and design need to be performed using CTA data.

Accuracy is essential in the design and manufacturing of fenestrated stent grafts. The use of 3-D work stations have facilitated this task; however, concerns regarding intra- and interobserver variability persist. Recently, this has been demonstrated to be a significant problem in a small percentage of patients. Standardization of the terminology relating to juxtarenal aneurysm is currently lacking and may improve the communication of results and act as a guide in treatment. In our study, 10 patients with unsuitable anatomy were enrolled, and fenestrated stent grafts were fabricated based on the measurements. MPR and cMPR images can now be processed in workstations without difficulty. This novel measurement technique uses MPR and cMPR images, which are familiar to operators, and minimizes interobserver variability. Our results showed that the two data sets were identical, with a CCC of 0.999.

To improve the versatility of fenestrated stent grafts, the measurement technique needs to be simple, standardized and have low interobserver variability. This new measurement technique uses vertical and transverse distances to minimize interobserver measurement error. Transverse distance can be measured from the vertical centre line to the nearest outer wall of each branch vessel. The actual distance from the celiac axis to each branch vessel was calculated using the distance from the central line. For example, the distance of the RRA can be calculated using the following equation:

Distance of RRA=shortest distance of RRA from centralvertical line±shortest distance of outer wall of celiac axisfrom central vertical line

The concept of central repositories of ready-made, off-the-shelf fenestrated stent graft designs is gaining interest and appears to be a potentially realistic prospect in the near future (37). This may make fenestrated stent graft technology available to more patients, particularly those with emergency presentation, and would also reduce cost. With solutions to increase applicability on the horizon, it would appear logical for appropriately trained endovascular specialists in regional centres to be able to size and design fenestrated stent grafts (38). This would not only decrease the time to fabrication, but would also allow for proper fenestrated stent grafts to be chosen ‘off the shelf’ in the emergency setting and for those who are at risk of rupture and cannot wait the current six to eight weeks required for customization. A direct measurement technique using standard points was simple to perform and was also easily applied to fabrication. We believe the preparation time will be shortened with this method. Once patients’ measurements are collected and analyzed, standardization of fenestrated stent graft may be possible and the versatility of stent graft devices will be improved.

CONCLUSION

A new measurement technique for the design of fenestrated stent grafts to treat complex abdominal aortic aneurysms was simple to perform and showed high accuracy compared with a 3-D aorta model produced using the CAD technique. Clinical application of this fenestrated design for evaluating technical feasibility is warranted.

Footnotes

FUNDING: Supported by grant number 02-2007-027 from the Seoul National University Bundang Hospital research fund.

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