The rapid prototyping of anatomic models in pulmonary atresia

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Surgery for Congenital Heart Disease Ngan et al

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he rapid prototyping of anatomic models inulmonary atresia

lizabeth M. Ngan, MD, MSc,a Ivan M. Rebeyka, MD, FRCSC,b David B. Ross, MD, FRCSC,b

ohamed Hirji, MB ChB, FRCPC,a Johan F. Wolfaardt, BDS, MDent, PhD,c,e Rosemary Seelaus, MAMS,d,e

ndrew Grosvenor, MIMPT, LCGI, RDT,e and Michelle L. Noga, MD, FRCPCa

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From the Departments of Radiology andDiagnostic Imaging,a Surgery,b and Den-tistry,c and the Faculty of RehabilitationMedicine,d University of Alberta, Ed-monton, Alberta, Canada, and COMPRU,Misericordia Community Hospital, CaritasHealth Group,e Edmonton, Alberta, Can-ada.

Supported by a grant from the CaritasHealth Group. Financial support for theMedical Modeling Research Laboratorywas provided by the Government of Can-ada through Western Economic Diversifi-cation Canada.

Received for publication Sept 23, 2005;revisions received Dec 20, 2005; acceptedfor publication Feb 3, 2006.

Address for reprints: Elizabeth Ngan, MD,MSc, 2A2.41 Walter C. Mackenzie HealthSciences Centre, 8440-112 St, Edmonton,Alberta, Canada T6G 2B7 (E-mail: [email protected]).

J Thorac Cardiovasc Surg 2006;132:264-9

0022-5223/$32.00

Copyright © 2006 by The American Asso-ciation for Thoracic Surgery

rdoi:10.1016/j.jtcvs.2006.02.047

64 The Journal of Thoracic and Cardio

bjective: The goal of this study was to assess the utility and accuracy of solidnatomic models constructed with rapid prototyping technology for surgical plan-ing in patients with pulmonary atresia with ventricular septal defect and majorortopulmonary collateral arteries.

ethods: In 6 patients with pulmonary atresia with ventricular septal defect andajor aortopulmonary collateral arteries, anatomic models of the pulmonary vas-

ulature were rapid prototyped from computed tomographic angiographic data. Theurgeons used the models for preoperative and intraoperative planning. The models’ccuracy and utility were assessed with a postoperative questionnaire completed byhe surgeons. An independent cardiac radiologist also assessed each model forccuracy of major aortopulmonary collateral artery origin, course, and caliberelative to conventional angiography.

esults: Of the major aortopulmonary collateral arteries identified during surgerynd conventional angiography, 96% and 93%, respectively, were accurately repre-ented by the models. The surgeons found the models to be very useful in visual-zing the vascular anatomy.

onclusion: This study presents the novel vascular application of rapid prototypingo pediatric congenital heart disease. Anatomic models are an intuitive means ofommunicating complex imaging data, such as the pulmonary vascular tree, whichan be referenced intraoperatively.

ulmonary atresia with ventricular septal defect (VSD) and major aortopul-monary collateral arteries (MAPCAs) is an uncommon form of cyanoticcongenital heart disease in which the native pulmonary arteries are very

ypoplastic or absent. To provide blood supply to the lungs, MAPCAs form duringmbryogenesis, probably from the splanchnic vascular plexus.1 These MAPCAs, whichrequently have stenotic origins, arise from the aorta or one of its branches, and varyrom patient to patient in number, origin, size, and course.2,3 For any given segment ofung, the blood supply may come from the native pulmonary artery, MAPCAs, or both.2

The goal of corrective surgery is to repair the intracardiac defects, establish rightentricular to pulmonary arterial continuity, and connect the MAPCAs to theulmonary arterial system.4-7 This process may require a single operation (single-tage unifocalization and complete intracardiac repair) or multiple operations (se-uential unifocalization with subsequent intracardiac repair).4-7 One critical com-onent of this surgery is the identification and isolation of the MAPCAs, a processade difficult by the patient-specific variations in anatomy. Adequate unifocaliza-

ion of the MAPCAs is the key to successful surgery.In more complex cases, the preoperative workup includes conventional angiog-

aphy, computed tomographic (CT) angiography, or magnetic resonance (MR)

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ngiography to map out the pulmonary vascular supply.8-12

he precise 3-dimensional (3D) relationship of MAPCAs tohe aorta and native pulmonary arteries is significant andhould be rendered into a useful, intuitively graspable for-at that is easily referenced intraoperatively. Rapid proto-

yping provides a potential solution to this problem.Rapid prototyping is the fabrication of solid models from

omputer-generated virtual 3D surface models. The virtualodel is first broken down into thin slices or layers. A rapid

rototyping machine then builds the solid model layer uponayer, resulting in a physical replica of the virtual model.13 In

edical applications of this technology, the virtual modelsre typically constructed from CT or MR imaging data sets.

The most extensive medical applications of rapid proto-yping technology are in head and neck, oral, and crani-maxillofacial surgery.14-16 Models of the bony anatomy ofhe head and neck are used to help guide surgery and recon-truction. Relatively limited numbers of vascular applicationsf this technology have also been developed. These vascularpplications include models of heart valves,17-20 aortic an-urysms,21 intracranial aneurysms,22 carotid arteries,23 andmbryonic hearts.24,25

ethodsatient Selectionix consecutive patients, aged 6 months to 2 years 6 months at the

ime of surgery, with pulmonary atresia with VSD and MAPCAsresenting to the Stollery Children’s Hospital in Edmonton, Al-erta, Canada were studied. The operations took place betweenay 2004 and June 2005. Additional patients in whom the imag-

ng was not performed sufficiently in advance of the surgery tollow model construction were excluded from the study. Thistudy was approved by the Health Research Ethics Board at theniversity of Alberta Hospital. A parent or guardian of eachatient gave informed consent.

maging Studiesll patients underwent both conventional angiography and CT

ngiography of the aorta and pulmonary arterial vasculature. Be-ause of the complexity of this type of congenital anomaly, CTngiography was considered part of the standard preoperativevaluation. No additional CT angiography was performed for theurpose of this study. The CT angiograms were obtained with.625-mm or 1.25-mm thickness slices with 0.625-mm reconstruc-ion and a slice overlap of 50% on a 16 slice General Electric Lightpeed CT scanner (GE Medical Systems, Milwaukee, Wis). The

Abbreviations and Acronyms3D � 3-dimensionalCT � computed tomographyMAPCA � major aortopulmonary collateral arteryMR � magnetic resonanceVSD � ventricular septal defect

atients were under general anesthesia, and positive-pressure p

The Journal of Thoracic

reath-hold technique was used during image acquisition. Intrave-ous contrast was administered at a dose of 2 mL/kg over 10 to 15econds. The patients were 4 months 6 days to 2 years 4 monthsld at the time of CT angiography, with 5 of the 6 patients 6onths old or younger. The CT was performed at least 2 weeks

efore the operation, which allowed time for the model to be built.

odel Constructionhe models of the CT data sets were created with Mimics 8.11oftware (Materialise, Leuven, Belgium) and a rapid prototypingachine. In Mimics, a virtual 3D model was created of the pulmo-

ary arterial vasculature and aorta by a radiology resident and aediatric cardiac radiologist. Smoothing of the model was per-ormed with Magics 8.01 software (Materialise) or haptic model-ng software and hardware (FreeForm Modeling System, version.0 and PHANTOM Desktop Haptic Device; SensAble Technol-gies, Woburn, Mass). This virtual model was converted into aolid acrylic or plastic anatomic model with a rapid prototypingachine, either the Stratasys Prodigy Plus (Stratasys Inc, Edenrairie, Minn) or the InVision si2 3-D printer (3D Systems, Valencia,alif). The image manipulation and rapid prototyping were carriedut in the Medical Modeling Research Laboratory, COMPRU, Cari-as Health Group, Edmonton, Alberta, Canada. The models wereterilized and used within the operative field at the time of surgery.

To study the accuracy of this construction technique, severalest objects containing both flat and cylindric surfaces were imagednd then modeled in the same manner as the patient data. Thebjects and the models were then measured with digital calipersAbsolute Digimatic calipers; Mitutoyo, MTI Canada Ltd, Missis-auga, Ontario, Canada).

odel Evaluationhe assessment of the models was 2-fold. First, after each opera-

ion, the surgeons completed a short questionnaire to rate theodel’s overall usefulness and accuracy. The surgeons rated utility

nd accuracy on 3-point scales (very useful, somewhat useful, orot useful, and very accurate, minor inaccuracies, or major inac-uracies, respectively). MAPCAs identified at the time of surgeryere also quantified. Each MAPCA on the models was also ratedn a 3-point scale (accurate, minor inaccuracy, or inaccurate).

Second, an independent pediatric cardiac radiologist comparedhe models with the angiograms. The cardiac radiologist comparedhe number of MAPCAs on the model with the number demon-trated on the corresponding angiogram. The accuracy of therigin, course, and caliber of each MAPCA on the model was ratedn a 3-point scale (accurate, minor inaccuracy, or major inaccu-acy). Minor inaccuracy was defined as acceptable accuracy withminimal and clinically irrelevant discrepancy between the model

nd angiogram. Major inaccuracy was defined as unacceptableccuracy, potentially misleading to the surgeon and not reflectinghe angiographic findings. The models were therefore compared tohe absolute standard, surgery, and to the imaging standard, con-entional angiography.

The funding organizations, Caritas Health Group and the Gov-rnment of Canada through Western Economic Diversificationanada, had no role in the study design; in data collection, anal-sis, and interpretation; in writing this report; or in the decision to
ublish the results.
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esultsix patients were studied, with intraoperative results avail-ble in 5 cases. In case 4, the patient was referred to anotherenter for surgery because of an earlier available operatingate, so no intraoperative results are available. Figures 1 and 2re photographs of models with the accompanying angio-raphic images.

Figure 3 and Table 1 compare the numbers of MAPCAsdentified with each of the modalities: model based on CTngiography, conventional angiography, and intraoperativeesults. Of 25 MAPCAs identified at the time of surgery, 2496%) were accurately represented by the models. Of 30

APCAs identified on angiography, 28 (93%) were alsoccurately depicted on the models. One MAPCA was iden-ified during surgery and on the angiogram but not on theodel (case 3). This small (approximately 3 mm diameter)

essel was not identified as a MAPCA on the CT dataecause of its small caliber, its close proximity to the trueulmonary arteries, and table motion artifact affecting this

66 The Journal of Thoracic and Cardiovascular Surgery ● Augu

articular CT study. This case demonstrates the limitationsf CT angiography. Conversely, 1 MAPCA identified aturgery was found on the CT study and model but not thengiogram (case 2). This vessel was probably not identifiedy conventional angiography because of overlapping ves-els on the initial two projections obtained, so selectiveatheterization of this MAPCA was not pursued. In addi-ion, 1 MAPCA less than approximately 2 mm in diameterdentified on angiography was not identifiable even in ret-ospect from the CT angiographic data (case 4). This vesselay have become occluded between the time of the angiog-

aphy and the time of the CT (5 months 7 days), or itsaliber may have been below the threshold of CT. In thisase, intraoperative results are not available for confirma-ion of either interpretation. In this limited series, the mod-ls based on CT angiography rivaled conventional angiog-aphy in the identification of MAPCAs.

The surgeons’ assessments of the overall utility andccuracy of the models are presented in Figure 4. The

Figure 1. Comparison of angio-gram (A) with model (B) of tho-racic aorta and MAPCAs in pa-tient with pulmonary atresia,native left pulmonary artery,and MAPCAs (case 4).

Figure 2. Comparison of angio-gram (A) with model (B) of tho-racic aorta and MAPCAs in pa-tient with pulmonary atresiaand MAPCAs (case 1).

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odels were all deemed to be very useful, and most wereonsidered very accurate. The only model not receiving aery accurate rating was the model in which 1 of theAPCAs was not identified. Each MAPCA was also rated

y the surgeons for overall accuracy of position. Only thereviously mentioned unidentified MAPCA was not ratedy the surgeons as accurately representing the intraoperativendings. The subjective impressions of one of the surgeonsere that the models aided in surgical planning, allowed

apid isolation of the MAPCAs, and decreased operatingoom time.

The initial impression of one of the surgeons was that theodels were somewhat smaller than the actual blood ves-

els. To confirm the accuracy of our modeling techniques,everal cylindric test objects were scanned and modeled.

igure 3. Percentage of MAPCAs identified on angiograms anduring surgery included on models.

ABLE 1. Identification of MAPCAs in the model, on an-iography, and at the time of surgery

aseMAPCAs in

modelMAPCAs onangiogram

MAPCAs duringsurgery

1 4 4 42 4 3 43 5 6 64 5 6 NA5 5 5 56 6 6 6

Total 29 30 25

A, Not applicable.


he test models all had diameters slightly larger than theriginal objects, from 0.26 mm (2.0%) to 0.85 mm (13.2%)arger. In no case was the model’s diameter smaller than thatf the original. The apparent size discrepancy between thelood vessels and the model is thus unlikely to be due to theechnique used to build the model. Other possible explana-ions include the growth of the child in the interval betweenhe CT and the surgery (as long as 4 months 29 days) and theact that the model is of the vascular space within the bloodessel, not the external surface of the vessel as viewed byhe surgeon.

A total of 31 MAPCAs were identified on the models orn the angiograms. Figure 5 summarizes the findings of thendependent pediatric cardiac radiologist who compared eachf the models with the angiograms, rating the origin, course,nd caliber of each model MAPCA for accuracy. Of theAPCA origins, 93% were rated as having acceptable accu-

acy, and 97% of the MAPCAs received an acceptable accu-acy rating for course and caliber. The MAPCA in case 4ot identifiable on CT was rated as having unacceptableccuracy in all categories. The MAPCA in case 3 that wasot identified as a MAPCA but rather included on the models a branch of the right pulmonary artery received annaccurate origin rating. Only 2 MAPCAs were rated asaving minor inaccuracies. In both cases, the inaccuracyas in the caliber of a stenosis. Although it was not part of

he questionnaire completed by the cardiac radiologist, helso noted several cases in which the models were superioro the angiograms. The most obvious example of this is the

APCA that was not identified on the angiogram but wasccurately included on the model, matching the operativendings (case 2).

iscussionhis study provides an initial assessment of the applica-

ion of rapid prototyping technology to pediatric congen-

Figure 4. Surgeons’ ratings of model utility and accuracy.


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tal heart disease, specifically pulmonary atresia with VSDnd MAPCAs. Previous imaging studies of pulmonary atre-ia with VSD and MAPCAs focused on conventional im-ging modalities such as angiography, CT angiography,nd MR angiography.8-12 Rapid prototyping technologyakes imaging data a step beyond 2-dimensional computerisplays to solid anatomic models. This study adds to theimited number of reported soft tissue and vascular appli-ations of this technology.

echnical Considerationsodels of soft tissue structures are more difficult to con-

truct than models of bony structures because of the muchmaller variation in Hounsfield units, which is significant.bviously, contrast administration greatly helps in model-

ng vascular structures. In this particular application, carefulttention to anatomy is required to separate the pulmonaryrteries from the pulmonary veins. As a result, virtual modelonstruction cannot be automated and should be performedy a clinical specialist. One technical limitation of theodels is their fragility, particularly at stenoses. As a result,

he models must be handled with care. The techniquesresented in this article represent a starting point in opti-izing the model construction of congenital heart disease.

linical Implicationsf MAPCAs identified during surgery and conventional

ngiography, 96% and 93%, respectively, were accuratelyepresented by the models. These values are in line withrevious accuracy reports of CT angiography and MR an-iography of this condition.11,12 The accuracy of the models

igure 5. Cardiac radiologist’s ratings of model accuracy relativeo conventional angiography.

s limited by the data set on which they are based, CT e

68 The Journal of Thoracic and Cardiovascular Surgery ● Augu

ngiography. Two MAPCAs found either during surgery ory conventional angiography were not included on theodels. In both cases, when the CT data were retrospec-

ively assessed, it was either impossible or difficult to iden-ify the missed MAPCAs. Contributing factors to these

APCAs being missed were degraded images from tableotion artifact and probable occlusion of a small vessel in

he interval between the conventional angiogram and the CTngiogram. However, 1 MAPCA identified on the CT studynd so included on the model was missed at angiography.his demonstrates the complementary nature of CT angiog-

aphy with rapid prototyping and conventional angiographyn identifying as many MAPCAs as possible before surgery.ecause of the need for maximum accuracy of imaging, weelieve that CT angiography with rapid prototyping pro-ides useful supplemental information in addition to con-entional angiography. In this study, all MAPCAs weredentified either on the CT or the angiogram before surgery.

However, simple identification of vessels is not ade-uate. This study extends the utility of CT angiography byresenting the data in a more useful format, a solid anatomicodel showing the course, caliber, and origin of eachAPCA. Solid models communicate information in both

isual and tactile formats; however, it is difficult to quan-itatively evaluate the conveyance of information, particu-arly tactile information. The surgeons’ overall impressionf the ability of the anatomic models to communicate theecessary spatial information was quantified as usefulnessn this study. The surgeons rated all the models as veryseful for preoperative and intraoperative planning, allow-ng rapid isolation of MAPCAs during the surgery. Oneurgeon’s impression was that the models decreased oper-ting room time, although this study did not measure oper-ting room time as an outcome variable.

The surgeons had available to them a virtual model on aorkstation preoperatively and the solid model both preop-

ratively and intraoperatively. We view the models as anddition to rather than a replacement for a conventionalorkstation display. On the basis of this limited experience,

he 2-dimensional workstation screens did not convey thenformation as intuitively as the solid models.

tudy Limitationshe number of patients is one of the limiting factors in thistudy. In a larger study, variables such as operating room timend patient outcome could be examined. Because of themall number of patients, accounting for anatomic varia-ions and comorbidities that affect prognosis would be dif-cult. Because pulmonary atresia with VSD and MAPCAs

s a relatively rare condition, time is required to assess aarger number of patients. The creation of virtual modelsas relatively labor intensive and required expert knowl-

dge of the pulmonary vasculature. Automated virtual model

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onstruction was not possible with the current software usedMimics).

onclusionhis study represents a starting point in the investigation of

apid prototyping technology as it applies to complex pedi-tric congenital heart disease. The 3D anatomic modelsroduced were found to be useful in operative planning forulmonary atresia with MAPCAs. Further optimization ofhe imaging and virtual model construction techniques islanned. In addition, this technology has the potential to aidn the preoperative assessment of a broader range of condi-ions in congenital heart disease. The utility and applicationf rapid prototyping in other forms of congenital heartisease need to be assessed.

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The rapid prototyping of anatomic models in pulmonary atresia