Prediction of aortoiliac stent graft length: comparison of a semiautomated computed tomography angiography method and calibrated aortography

A systematic review of protocols for the three-dimensional morphologic assessment of abdominal aortic aneurysms using computed tomographic angiography

The morphology of infrarenal abdominal aortic aneurysms (AAAs) directly influences the perioperative outcome and long-term durability of endovascular aneurysm repair. A variety of methods have been proposed for the characterization of AAA morphology using reconstructed three-dimensional (3D) computed tomography (CT) images. At present, there is lack of consensus as to which of these methods is most applicable to clinical practice or research. The purpose of this review was to evaluate existing protocols that used 3D CT images in the assessment of various aspects of AAA morphology. An electronic search was performed, from January 1996 to the end of October 2010, using the Embase and Medline databases. The literature review conformed to PRISMA statement standards. The literature search identified 604 articles, of which 31 studies met inclusion criteria. Only 15 of 31 studies objectively assessed reproducibility. Existing published protocols were insufficient to define a single evidence-based methodology for preoperative assessment of AAA morphology. Further development and expert consensus are required to establish a standardized and validated protocol to determine precisely how morphology relates to outcomes after endovascular aneurysm repair.

Two-dimensional versus three-dimensional CT angiography in analysis of anatomical suitability for stentgraft repair of abdominal aortic aneurysms

BACKGROUND

The morphological analysis prior to endovascular abdominal aneurysm repair (EVAR) plays an important role in long-term outcomes. Post-imaging analysis of computed tomographic angiography (CTA) by three-dimensional reconstruction with central lumen line detection (CLL 3D-CTA) enables measurements to be made in orthogonal slices. This might be more precise than equal post-imaging analysis in axial slices by two-dimensional computed tomographic angiography (2D-CTA).

PURPOSE

To evaluate the intra- and interobserver variability of CLL 3D-CTA and 2D-CTA post-imaging analysis methods and the agreement between them in pre-EVAR suitability analysis of patients with abdominal aortic aneurysm (AAA).

MATERIAL AND METHODS

Anonymized CTA data-sets from 70 patients with AAA were analyzed retrospectively. Length measurements included proximal and distal aortic neck lengths and total distance from the lower renal artery to the higher iliac bifurcation. Width measurements included proximal and distal neck diameters, maximum AAA diameter and common iliac diameters just above the iliac bifurcations. The measurements were performed in random order by two vascular surgeons, twice per method with 1-month interval between readings. In the CLL 3D-CTA method we used semi-automated CLL detection by software and manual measurements on CTA slices perpendicular to CLL. The equal measurements in 2D-CTA were performed manually on axial CTA slices using a DICOM viewer workstation. The intra- and interobserver variability, as well as the agreement between the two methods were assessed by Bland-Altman test and bivariate correlation analysis.

RESULTS

The intraobserver variability was significantly higher in 2D-CTA measurements for both readers. The interobserver variability was significant in 2D-CTA measurements of proximal neck dimensions while the agreement in CLL 3D-CTA analysis between the two readers was excellent in all studied parameters. The agreement between the two suitability analysis techniques was poor for both readers, especially in measurements of proximal neck's dimensions and in total aortoiliac length (p = 0.001).

CONCLUSION

In pre-EVAR morphological evaluation of AAAs the CLL-3D CTA post-imaging analysis has better intra- and interobserver correlation than 2D-CTA and might represent a useful tool for the proper selection of endograft's type and size.

Predicting changes in blood flow in patient-specific operative plans for treating aortoiliac occlusive disease

Traditionally, a surgeon will select a procedure for a particular patient on the basis of past experience with patients with a similar state of disease. The experience gained from this patient will be selectively used when treating the next patient with similar symptoms. This article describes a surgical planning system that was developed to enable a vascular surgeon to create and test alternative operative plans prior to surgery for a given patient. One-dimensional and three-dimensional hemodynamic (i.e., blood flow) simulations were performed for rest and exercise for operative plans for two aorto-femoral bypass patients and compared with actual postoperative data. The information obtained from one-dimensional (volume flow distribution and pressure losses) and three-dimensional (flow, pressure, and wall shear stress) hemodynamic simulations may be clinically relevant to vascular surgeons planning interventions.

Correlation between the Measurement of Transverse Diameter in the Proximal Neck on Computed Tomography and on Aortography before Endovascular Treatment of Infrarenal Aortic Aneurysm

The aim of this study was to determine the correlation between the measurement of transverse diameter of the proximal neck on computed tomographic angiography (CTA) and graduated catheter aortography in patients who are candidates for endovascular graft placement in order to replace, if both measurements are equivalent, aortography for CTA alone. Preoperative dual-slice CTA and graduated catheter aortography were performed in 35 consecutive patients with infrarenal aortic aneurysm within 10 days. Transverse proximal neck diameters were measured on a true axial section on CTA reconstructions and on aortographic images, always 6 mm distal from the most inferior main renal artery. Mean, median, and standard deviation were obtained and the measurements correlated for each patient using Pearson's correlation and linear regression analysis. A significant difference in proximal neck transverse diameter measurements was found between graduated catheter aortography and CTA in all cases. CTA values were a mean of 1.74 mm higher than aortography values. Pearson's correlation indicates a strong correlation between both techniques, and a regression equation determines the predictive value of aortography on the basis of CTA values. Estimation of the transverse diameter of the proximal neck on aortography on the basis of that obtained on CTA allows us to affirm that CTA could be used as the sole method for the preoperative selection of appropriate endograft size in patients with infrarenal aortic aneurysm.

Prediction of altered endograft path during endovascular abdominal aortic aneurysm repair with the Gore Excluder

OBJECTIVE

During endovascular abdominal aortic aneurysm (AAA) repair (EVAR), the rapid deployment of the Gore Excluder endograft may be associated with anatomic shortening of the endograft path. This shortened path may result in coverage of the hypogastric artery origin or overly conservative graft length selection that may lead to unnecessary extensions. We quantified the degree of path alteration with this endograft and developed an algorithm to predict it.

METHODS

Preoperative and postoperative three-dimensional (3D) computed tomographic (CT) scans were evaluated for 50 consecutive patients with Gore Excluder endografts by using 21 anatomic measurements and 6 calculated indices. Measurements were evaluated as if only 3D lumen centerline measurements were available, rather than complete 3D computer-aided measurement and "virtual graft" simulation. Tortuosity was quantitated from the renal artery to the hypogastric origin, using the difference between a straight line and the lumen centerline.

RESULTS

The endograft was deployed successfully in all cases. The graft end points were typically quite close to the preoperative plan: mean renal artery-to-graft distance was within 2.0 +/- .5 mm, and the limb end point-to-hypogastric origin differed by an average of only 1.8 +/- 1.6 mm. Although accurate in most cases, the actual graft path shortened 1 cm or more relative to the centerline in 11% of limbs. On univariate analysis, determinants of alteration of >1 cm in the graft deployment path were (1) aortoiliac tortuosity (renal-to-hypogastric artery, P < .002), (2) the degree of planned graft rotation (73% of cases altered >10 mm were in the rotated position, P < .05), and (3) the insertion side (73% of alterations >or=10 mm were ipsilateral to the main device, P < .05). On multivariate analysis, the renal-to-hypogastric artery tortuosity index (RHTI) was significant ( P < .004), and device type and rotation approached significance ( P < .08). We developed a classification scheme based on RHTI to predict the risk of alteration of the graft path >or=1 cm (low risk, 0%; medium risk, 10%; high risk, 25%) and an algorithm to predict the degree of alteration of the anatomy that reduced the number of cases shortening >or=1 cm to zero.

CONCLUSIONS

The graft deployment path will be altered significantly in a minority of cases with the Gore Excluder endograft, but this can cause hypogastric occlusion or other problems. Anatomic shortening is predictable from morphologic features such as tortuosity, graft insertion side, and rotation. We developed an algorithm based on a tortuosity index that quantitates the risk and degree of shortening associated with endograft deployment.

Creation of individual ideally shaped stents using multi-slice CT: in vitro results from the semi-automatic virtual stent (SAVS) designer

To plan stent-grafting for thoracic aortic aneurysm with complicated morphology, we created a virtual stent-grafting program [Semi Automatic Virtual Stent (SAVS) designer] using three-dimensional CT data. The usefulness of the SAVS designer was evaluated by measurement of transformed anatomical and straight stents. Curved model images (source, multi-planer reconstruction and volume rendering) were created, and a hollow virtual stent was produced by the SAVS designer. A straight Nitinol stent was transformed to match the curved configuration of the virtual stent. The accuracy of the anatomical stent was evaluated by experimental strain phantom studies in comparison with the straight stent. Mean separation length was 0 mm in the anatomical stent [22 mm outer diameter (OD)] and 5 mm in the straight stent (22 mm OD). The straight stent strain voltage was four times that of the anatomical stent at the stent end. The anatomical stent is useful because it fits the curved structure of the aorta and reduces the strain force compared to the straight stent. The SAVS designer can help to design and produce the anatomical stent.

Abdominal aortic aneurysms: virtual imaging and analysis through a remote web server

The study describes the application of a web-based software in the planning of the endovascular treatment of abdominal aortic aneurysms (AAA). The software has been developed in the framework of a 2-year research project called Aneurysm QUAntification Through an Internet Collaborative System (AQUATICS); it allows to manage remotely Virtual Reality Modeling Language (VRML) models of the abdominal aorta, derived from multirow computed tomography angiography (CTA) data sets, and to obtain measurements of diameters, angles and centerline lengths. To test the reliability of measurements, two radiologists performed a detailed analysis of multiple 3D models generated from a synthetic phantom, mimicking an AAA. The system was tested on 30 patients with AAA; CTA data sets were mailed and the time required for segmentation and measurement were collected for each case. The Bland-Altman plot analysis showed that the mean intra- and inter-observer differences in measures on phantoms were clinically acceptable. The mean time required for segmentation was 1 h (range 45-120 min). The mean time required for measurements on the web was 7 min (range 4-11 min). The AQUATICS web server may provide a rapid, standardized and accurate tool for the evaluation of AAA prior to the endovascular treatment.

Multidetector CT Angiography Versus Digital Subtraction Angiography for Aortoiliac Length Measurements Prior to Endovascular AAA Repair

PURPOSE

To assess observer variation between calibrated-catheter digital subtraction angiography (DSA) and software-enhanced multidetector computed tomography angiography (CTA) in measuring vessel length prior to endovascular aortic aneurysm repair (EVAR).

METHODS

Thirty patients (25 men; mean age 65 years, range 61-85) scheduled for EVAR underwent CTA in 4x2-mm collimation using advanced vessel analysis software. CTA measurements were performed twice by 2 blinded readers in random order with at least a 4-week interval between readings. Nine patients were found unsuitable for endovascular repair after the CTA, so DSA was performed in 21 patients for morphometric evaluation of the abdominal aorta and the iliac arteries. The following segments were measured: H1 (aneurysm neck), H2 (lower renal artery to distal aspect of the aneurysm), H3 (lower renal artery to aortic bifurcation), and H4a/H4b (lower renal artery to iliac bifurcations). Length measurements on DSA were made by (1) following the catheter path in the aortic lumen and (2) dividing tortuous vessel anatomy into segments and measuring each segment along an idealized centerline. Addition of the various segments allowed comparison with data obtained from CTA measurements.

RESULTS

CTA was performed with good intraobserver agreement for all length parameters except H3 in reader 2 (p<0.05). While good interobserver agreement was demonstrated for CTA over long aortoiliac distances (H4a, H4b), higher interobserver agreement was obtained with DSA for shorter segments (H1, H2). Considerable differences were observed between CTA and DSA for the lengths H2 and H4b.

CONCLUSIONS

CTA produces better intra and interobserver correlations in measuring vessel length than DSA. It has the potential to replace DSA as an imaging method before EVAR.

Decreased use of iliac extensions and reduced graft junctions with software-assisted centerline measurements in selection of endograft components for endovascular aneurysm repair

OBJECTIVE

The purpose of this study was to determine the impact of using computerized software-assisted centerline measurements for extensions and graft junctions during the selection of endograft components for modular aortic endografts in endovascular repair of abdominal aortic aneurysms.

METHODS

From April 1998 to December 2002, 289 modular aortic endografts were implanted at our institution. These included 248 grafts (prior to 2002, group 1) with components selected on the basis of manual caliper measurements from combined contrast computed tomography (CT) and marker-catheter arteriography data, and 41 grafts (2002, group 2) with components selected with the use of computerized software that allowed for centerline measurements on 3-dimensional reconstructions based on CT data. These 2 groups were compared for the number and type of extensions required per case. Seventeen other relevant variables were analyzed for their potential influence on selection of endograft components. These variables included age, gender, maximum aneurysm size, level of distal fixation, length and diameter at the fixation points, endograft manufacturer (make), and configuration. The significance of the observed differences was analyzed with a multivariate regression model, adjusting for potentially confounding preoperative measures.

RESULTS

Multivariate analysis demonstrated that the number of right iliac extensions, left iliac extensions, total extensions, and total graft junctions was significantly reduced by the use of computerized software-assisted centerline measurements (group 2) compared with caliper measurements (group 1), independent of all other 17 preoperative variables. Notably, the mean number of required right iliac extensions was double in group 1 versus group 2.

CONCLUSIONS

Centerline software-assisted measurements can significantly reduce the need for iliac extensions and, concomitantly, the number of required endograft junctions. On average, twice as many extensions were required for right iliac fixation when the manual caliper measurements were used compared with software-assisted measurements. These findings are highly relevant to issues of total endograft cost and long-term endograft integrity and focus attention on the tools that may need to be considered standards of care rather than optional for selection of endograft components.

Computed tomography scanning for endograft planning: evolving toward three-dimensional, single source imaging

One of the most fundamental differences between open surgical repair of aortic aneurysms and treatment by endograft exclusion is the dependency on imaging for all critically important aspects of the endovascular approach. Patient selection and preprocedure planning for device selection and sizing depend on imaging. Computed tomography (CT) scanning for endograft planning is capable of providing accurate and complete measurement data. There are problems with diameter, length, and angulation measurement when using conventional two-dimensional (2D) axial CT images that can be partly overcome by expert interpretation. However, when computerized 3D programs that include easy-to-use measurement tools process CT data, the problems of 2D CT scan data are relieved. In addition, virtual simulation of various endograft choices can be modeled within computerized renderings of patient-specific anatomy that allow testing of alternatives and provide an intuitive understanding during the planning phase.

Multidetector row computed tomography of the aorta and peripheral arteries

MDCT represents a significant advance on SDCT and the advantages it brings are particularly clear in CTA applications. It allows cost effective assessment of longer segments of vascular territories to be imaged with higher spatial, contrast, and temporal resolution. It has replaced SDCT as the new CT standard and its technical abilities are rapidly approaching the temporal resolution of EBCT. With 3-D post processing tools its main benefit is in imaging studies customized to the patient's pathology, with greater measurement accuracy and reliable longitudinal assessment. MDCTA is increasingly applied not to individual vascular territories but to a complete assessment dictated by the pathology, such as the entire aorta and branch vessels in aneurysm evaluation and the aorta and the coronary arteries in dissection cases or the vascular and non-vascular chest in acute chest pain. Greater accuracy of vessel assessment will lead to a refinement of interventional and surgical techniques for an individual patient and facilitates conservative management of conditions that can be reliably monitored non-invasively and the development of intervention criteria. MDCTA will continue to increase its major role in peripheral vascular evaluation in the future though its reliability in assessing small vessels below the knee and in the foot remains to be conclusively proven to obviate the need for diagnostic catheter angiography.