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Medicine Meets Virtual. Reality 16 J.D. Westwood et al. (Eds.) IOS Press, 2008 © 2008 The authors. All rights reserved.
The Use of Stereolithographic Hand Held Models for Evaluation of Congenital Anomalies of the Great Arteries Mark VRANICAR , William GREGORY , William I. DOUGLAS , Peter D I SESSA , and Thomas G. DI SESSA Department of Pediatrics, University of Kentucky, Lexington, Kentucky Department of Engineering, University of Kentucky Department of Surgery, University of Texas-Houston 3, l
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Abstract. Imaging anomalies of the great vessels has traditionally been accomplished using conventional biplane modalities as well as three-dimensional (3D) video displays. Our aim was to review the use of stereolithography to create 3D models to assess coarctation of the aorta and vascular rings. Twelve patients had high-resolution CT scans to evaluate anomalies of the great arteries (coarctation: 9, vascular ring: 3). Ages were 19 days to 29 years and weights were 3.3 to 139 kg. Digital dicom data from each scan were converted by a commercially available software package into a 3D digital image. The area of interest was selected and the image was exported to a 3D stereolithographic printer to create a 3D model. The models were then evaluated and the results compared to catheterization and surgical findings. All models accurately displayed the pathology investigated. All 3 of the vascular ring models correlated with surgical findings (double arch: 2, pulmonary sling: 1). Models of aortic coarctation allowed clear depictions of discrete narrowing as well as arch hypoplasia and tortuosity. Stereolithography can create realistic 3D models that accurately display aortic pathology and add important additional information, which may have implications regarding surgical and transcatheter interventions and may also be useful teaching tools for parents and students.
Keywords. Aortic coarctation, vascular ring, imaging
Introduction Anomalies of the great arteries consist of complex three dimensional (3D) structures that, by definition, are not situated in the usual anatomic planes. Such malformations include coarctation of the aorta and complex vascular rings. Imaging of these anomalies has, in the past, been performed using biplane modalities [1-6], as well as, 3D video displays [7]. These techniques are adequate for making a diagnosis in the majority of cases; however specific details are often missing in order for the surgeon or interventional cardiologist to devise a specific treatment plan.
Corresponding Author: Mark Vranicar, M.D., University of Kentucky Medical Center, Pediatric Cardiology, M N 470, Lexington, KY 40536; E-mail:
[email protected]. 1
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Stereolithography can be used to create life-sized 3D anatomical models from data obtained using standard imaging techniques [8]. This technology has been employed in planning reconstructive maxillo-facial surgery [9], in manufacturing 3D biodegradable scaffolds for bone growth [10], as well as in scaffold fabrication for tissue engineering of heart valves [11]. Our aim was to review our experience using stereolithographic 3D modeling in assessing anomalies of the great arteries.
1. Methods This is a retrospective review of all patients with anomalies of the great arteries who had a high resolution contrast CT scan at the University of Kentucky Medical Center between April 2003 and September 2005. The number of CT scan slices varied from 85 to 1044. The digital data obtained from these CT scans were then converted into a 3D image file using Amira software (Mercury Computer Systems, Chelmsford, Massachusetts). These image files were then edited to exclude non-vascular structures and to include only the areas of interest, that is, the aortic arch and adjacent blood vessels. The data were fed into a stereolithographic (STL) file for editing. Magics RP rapid prototyping software (Materialise, Ann Arbor, M I [Belgium]) was used to convert the data into a solid model file. These data were then saved into a final STL file. The STL file was transferred to 3D Lightyear file preparation software (3D Systems, Valencia, California) to create a support structure and slice the file into cross-sections. The file was then exported to a stereolithographic laser printer (3D Systems, Valencia, California). In this process a liquid photopolymer (Somos, New Castle, Rhode Island) is converted into the solid 3D model layer by layer. This process may take 6 to 12 hours depending on the size of the model. Once the laser process is complete the model is rinsed with isopropyl alcohol and cured in an ultraviolet oven (3D Systems, Valencia, California). The 3D models were then evaluated and the results were compared to catheterization and/or surgical findings. The CT scans used to create these models were previously interpreted using standard 2D axial slice imaging as well as reconstructed images allowing viewing of coronal, sagittal, as well as orthogonal slices. The protocol was reviewed and approved by the University of Kentucky Institutional Review Board. Subject's informed consent was not required since this was a retrospective review.
2. Results There were 12 patients that had high resolution CT scans for the evaluation of aortic arch anomalies. Their ages ranged from 19 days to 29 years (mean 11.7, median 8.0 years). Their weights ranges from 3.3 to 139 kg (mean 49.6, median 25.5 kg). Diagnoses were vascular ring in 3, native coarctation in 4, and recurrent coarctation in 5. The models allowed visualization of the entire aortic arch and its branches and the main and branch pulmonary arteries in a true-to-life three-dimensional fashion. The models were then simply held and rotated in order to help visualize abnormal branching patterns and narrowed areas. This allowed for an accurate display of the pathology in all of our subjects (Table).
Table, Results of diagnostic studies and catheterization and surgical findings No.
Age
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.6 mos 3.3 kcI Vascular ring due to double Vascular ring due to double arch with atretic left arch arch with atretic left arch Vascular ring due to 6.2 6.0 Vascular ring due to pulmonary sling pulmonary sling 59 Vascular ring due to double 14.5 Cannot exclude right arch with aberrent left subclavian arch with atretic left arch 8.2 Severe coarctation with Severe coarctation with 1.5 abnormal aortic arch tortuous aortic arch 67 22.7 Pseudo-coarctation should Discrete coarctation be considered Severe coarctation 171 63.2 Marked coarctation Coarctation 321 84.5 Coarctation 125 28.2 Coarctation with aortic Coarctation with tortuous aorta kinking 184 79.5 No definitive coarctation Long segment coarctation Stented coarctation 184 75.5 Stented coarctation 216 139 Subtle naiTowing, however Coarctation due to hypoplastic transverse arch no coarctation Stented coarctation, with 358 70.9 Stented coarctation, no proximal aortic narrowing areas of stenosis
2 3 4 5 6 7 8 9 10 11 12
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C T Diagnosis
Model Diagnosis.
Cath/Surgery Findings Vascular ring due to double arch with atretic left arch Vascular ring due to pulmonary sling Vascular ring due to double arch with atretic left arch Severe coarctation with unusual transverse aorta Discrete coarctation Coarctation Coarctation Coarctation with tortuous aorta Coarctation Stented coarctation Coarctation Stented coarctation, with proximal aortic narrowing
A l l 3 of the vascular ring models correlated with surgical findings. In subjects with a double aortic arch the more dominant arch could easily be distinguished from the less dominant arch simply by its larger size (figure 1). The models made it easier to differentiate these from other forms of vascular ring, such as right arch with aberrant left subclavian artery since the smaller, left arch could be seen branching into the left carotid and subclavian arteries. In addition, the proximal and distal segments of the atretic section of the left arch were easily distinguishable. This technique was particulary advantageous in depicting the three dimensional spatial relationship of the pulmonary artery sling (figure 2).
Figure 1. Model of a 4 year-old with a double aortic arch (subject 3). Front view (A) shows a dominant right aortic arch (RAA) and a smaller left aortic arch ( L A A ) with 2 vessels arising off of each arch. Above view (Bj shows how the two arches form the vascular ring through which the trachea and esophagus pass.
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Figure 2. Model from a 6 month old with a pulmonary sling (subject 2). The view from the front (A) shows the main pulmonary artery (MPA), but the origin o f the left pulmonary artery (LPA) is not well seen. Viewed from above (B) the LPA can be seen originating anomalously from the right pulmonary artery and is compressed (arrow) as it passes behind the trachea.
In subjects with coarctation, all of the models clearly showed the areas of coarctation regardless of the type or severity of lesion. In subjects 5, 6, and 7 a discrete coarctation was easily seen, however the models also made it easy to visualize the distance from adjacent vessels, as well as any post-stenotic dilation and the size of the proximal and distal aorta. Being able to rotate the models in 3-dimensional space made it possible to visualize each aortic segment simultaneously and made it easy to identify hypoplastic segments and long segment narrowings (subjects 9 and 11), which were not always apparent using standard CT viewing techniques.
Figure 3.10 year-old (subject 8) with recoarctation of the aorta. The transverse arch (TA) is tortuous and there is a discrete coarctation (arrow) just distal to the left subclavian artery (LSCA). PA = pulmonary artery.
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Figure 4. 1 month-old (subject 4) with an unusual tortuous transverse aorta (TA) and severe discrete coarctation (arrow) proximal to the origin of the left subclavian artery. PA = pulmonary artery.
The models were also useful in identifying more unusual forms of coarctation. A coarctation of the proximal aortic arch, in subject 12, was easily identified on the model and verified by catheterization. In subject 8 the model showed severe arch tortuosity that prohibited stent placement (figure 3) and in subject 4 the model clearly delineated a markedly abnormal tortuous aorta with severe coarctation (figure 4).
3. Discussion Stereolithography has been used in a variety of medical applications [8-11]. We have extended its use by demonstrating its ability to create realistic 3D models of complex aortic arch anomalies. Moreover, it has been shown herein that these models amplified and facilitated visualization of information seen by traditional CT viewing techniques in the diagnosis of coarctation of the aorta and vascular ring. In the patients with markedly abnormal anatomy such as arch tortuosity and hypoplasia the 3D models added valuable insight into the complex nature of the malformation. In these situations the models can be used by interventional cardiologists and cardiothoracic surgeons to help in planning catheter or surgical treatment options. When surgery is required, the models may eliminate the need for cardiac catheterization and angiography, long considered the gold standard for cardiac imaging of complex cardiac lesions including aortic arch anomalies. One limitation of this study was the small number of patients, which precluded statistical analysis, and another limitation was its retrospective method. In most cases, the 3D model was created after the patient underwent cardiac catheterization and/or surgery. Further studies are needed to look at how the evaluation of patients with the
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use of these models affects their treatment plan. Future technical developments may include the application of this technique to MRI imaging and 3D echocardiography.
4. Conclusion Stereolithographic hand held models accurately displayed complex anomalies of the great arteries. The development of a 3D model of an aortic arch anomaly provides the surgeon and interventional cardiologist with a unique realistic 3D aspect of the complex anatomy in question. Moreover, these models have potential utility for educating patients and their families as to the precise problems that are occurring and the planned treatments. Furthermore, these models can be used as teaching devices for medical students and residents in the future.
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Figure 1. Model from a 4 year-old with a vascular ring due to a double aortic arch. The view from the front (A) shows a dominant right aortic arch (RAA) and a smaller left aortic arch (LAA) with 2 vessels arising off of each arch. The view from above (B) shows how the two arches forming a ring through which the trachea and esophagus pass.
Figure 2. Anterior view of a model from a 10 year-old with recoarctation of the aorta. The transverse arch (TA) is tortuous and there is a discrete coarctation (arrow) just distal to the left subclavian artery (LSCA). PA = pulmonary artery.
