Three-Dimensional Printing of Mitral Valve Models Using Echocardiographic Data: Does it Add to The Knowledge of Cardiology Fellow Physicians in Training?

Background: High delity three-dimensional Mitral valve models (3D MVM) printed from echocardiography are currently being used in preparation for surgical repair. We hypothesize that printed 3DMVM could have relevance to cardiologists in training by improving their understanding of normal anatomy and pathology. Methods: Fifteen fellow physicians in pediatrics and adult cardiology training were recruited. 3D echocardiography video clips of 6 mitral valves (one normal and ve pathological) were displayed and the fellows were asked to name the prolapsing segments in each. Following that, still images of 3D MVMs corresponding to the same MVs seen in the clip were presented on a screen and the participating physicians were asked to answer a comprehensive questionnaire aimed at assessing whether the 3D model has improved their understanding of valvular anatomy. Finally, a printed 3D MVM of each of the previously projected valves was handed out, and the same questionnaire was re-administered to identify any further improvement in the participants’ perception of the anatomy. Results: The correct diagnosis using the echocardiography video clip of the Mitral valve was attained by 45% of the study participants. Both pediatric and adult trainees, regardless of the year of training demonstrated improved understanding of the anatomy of MV’s after observing the latter’s model image, with an even further improvement in that understanding after they had seen and physically examined the printed model. Conclusion: Printed 3D MVM has a benecial impact on the understanding of MV anatomy and pathology compared to 3D-Echocardiography (3DE) images in cardiology trainees.


Introduction
Since its initial introduction in the 1980s, Three-dimensional (3D) printing of radiographic based imaging has gained momentum as a clinical tool in the medical eld over the last few years. From an initial use for industrial work [1], 3D printing rst made its debut in the eld of clinical care in the 1990s with the manufacture of the rst cranial bone model [2]. Its use has become widespread in various medical elds from bio-printing tissue organs [3] extending currently to patient-speci c implants, prostheses, and replicated anatomical models for education and surgical planning [4,5]. Three-dimensional models of hearts with several congenital defects are currently available [2]. The diagnostic and educational values of such models as physical tools for training, especially in the setting of pre-operative planning, are gaining interest.
Although the clinical use of 3D Echocardiography has been around for more than two decades, its value in generating 3D models of cardiac structures has not been thoroughly investigated. Echocardiographic 3D data sets of a mitral valve [ Fig. 1], obtained via the transesophageal window, have been shown to have enough spatial and temporal resolution for clinical use. However, this remains suboptimal for 3D printing. Hence, obtaining 3D prints from echocardiography data has not been fully perfected and is lagging other radiographic based images.
Many commercially available 3D rendering software are currently available. The rendered dynamic 3DE based images are an excellent adjunct to the conventional two-dimensional echocardiography for surgical repair. In addition to the rendered 3D echocardiographic image, a 3D geometrical Model [ Fig. 2] can be generated based on an echocardiographic image. The model can be used to perform several measurements that are crucial for the understanding of the structures of the MV and the planning of surgical intervention.
Projecting a 3D structure on a at screen inevitably affects the viewer's depth perception. Unlike 3DE images, the MVM when printed in 3D, provides excellent depth while conserving details.
We hypothesize that tangible printed 3D models of MV [ Fig. 3] could objectively improve the understanding, of the MV geometry and depth perception, for cardiologists in-training, further enhancing their understanding of normal valve anatomy and pathology.

Methods
This study is a cross sectional questionnaire-based educational study administered to cardiology physicians in training, between June 2017 and March 2018, at the American University of Beirut-Medical Center (AUBMC). The study was approved by the research Institutional Review Board of AUBMC.
Cardiology fellows consented to be part of the study, and emphasis was placed on the fact that participation is voluntary and that they can withdraw from the study at any point in time if they chose to do so. Moreover, all data collected including videos, clinical models, and fellows' details (except for their year of training and subspecialty) were anonymous. Con dentiality was maintained throughout the study. Sixteen cardiology fellows at different levels of training (eleven adult cardiology and ve pediatric cardiology) were recruited and completed the study.
Imaging of the MV and the acquisition of 3D data sets was completed using a (t7-1) transesophageal probe and (iE-33) echocardiography platforms (Philips, Eindhoven Netherlands). Rendered 3D images of the mitral valve were generated using QLAB software v.8 (Philips, Eindhoven Netherlands) and were saved as anonymous video clips, (Audio Video Interleave format, Microsoft, Washington, USA).
Native 3D data sets were imported into commercially available software (Image Arena, TomTec GmbH. Munich, Germany), where another, plug in semi-automated specialized software (MVA package 2.1) analyzes and tracks the mitral valve and generates a geometrical model of the valve to include the annulus, lea ets and coaptation line. Those models were saved in JPEG (Joint Photographic Experts Group) format and exported as stereolithographic les (stl).
Files of the models were printed, on a commercial Ultimaker 2 + R 3D printer (Ultimaker B.V. Watermolenweg, Netherlands). The delity of the resulting models was highly conserved, as they resembled the original echocardiographic model pictures.
A total of six (One normal and ve pathological) valves were rendered in 3D and saved as video clips. For each 3D rendered MV, a 3D model was generated, saved as still image and printed in 3D. Pathological To test our hypothesis, a PowerPoint (Microsoft incorporation-Washington, USA) based questionnaire was prepared. The questionnaire was speci cally designed for this project to elicit whether the 3D printed model improved participant perception and knowledge about the MV [ Fig. 4].
Movie clips of 3D echo of the 6 MVs were initially projected to the study participants, after which they were asked to name the pathological segment(s) of the MV lea ets, if any existed. Following that, pictures of the 3D generated models of the same valves were sequentially projected on a at screen. Participants were asked if that had added to their understanding of the valve regarding the following categories: (1) anatomy in general, (2) area and prolapse of the lea ets, and (3) shape of the annulus.
Finally, the corresponding 3D printed MV models were handed over to the participants for tangible assessment. The same questionnaire was administered once again following their assessment of the printed model.

Statistical analysis:
Data were entered and analyzed using the Statistical Package for Social Sciences program (SPSS Inc, Chicago, IL). For each case, the number and percentage of fellows giving the correct answer and whether it improved their understanding of MV anatomy and pathology was calculated and presented as a mean and standard deviation. Each fellow was asked to assign a response to each of the four questions describing how a certain image modality (3D model or 3D printed respectively) improved their understanding of MV anatomy and pathology. The available answers were: 'extremely', 'very', 'moderately', 'slightly', and 'not at all'. For the presentation of the perception data, we have created three groups of responses. A negative perception as indicated by "not at all" and "slightly" answers, a neutral perception as indicated by "moderately" answer, and a positive perception as indicated by "very" and "extremely"", Percentages of negative, neutral and positive responses were calculated for each category in all 6 cases.
In order to assess the overall responses and compare outcomes between adult and pediatric fellows on one hand, and between junior ( rst-year trainees) and senior fellows (second and third year trainees) on the other; number and percentages of positive answers were compared and the difference in mean scores was calculated using the Mann-Whitney test, where p-value < 0.05 was used to indicate statistical signi cance.

1-Video Clips:
Based on the video clips of the valves (Table 1), fourteen participants (87.5%) identi ed the normal case correctly. Twelve (75%) and seven (44%) of them named the correct prolapsing segment in the two models with one pathological segment. However, six (37.5%), four (25%) and only one (6.3%) participant identi ed correctly, all the pathological segments in the last three complex cases (with more than one prolapsing segment present).     (Table 3)

Discussion
The complexity of the mitral valve has been challenging to image ever since the initial stages of echocardiography. Advancement in 2DE during the last three decades; however, has revolutionized the cardiologists' understanding of the mitral valve over the initial "B" and "M" echocardiographic imaging modes. The introduction of the mono-and biplane electronic phased-array probes in 1982, on the other hand establishes a new a landmark in the clinical use of Trans Esophageal Echocardiograms (TEE) [6]. Two dimensional TEE and its application on the mitral valve imaging [7,8] has expanded the scope of mitral valve repair surgery [9]. Before the introduction of 3D TEE, the combination of perioperative TEE data with the results of direct intraoperative inspection of the MV in an arrested heart was the standard practice surgeons used in making decision regarding mitral valve repair. Despite all advancement, the understanding of the MV pathology requires the assimilation of two-dimensional images with direct intraoperative valve ndings. Ahmed et al reported that this approach could be challenging for less experienced echocardiographers; especially in cases of complex MV prolapse involving multiple MV segments [10].
Training a cardiologist on echocardiography is a tedious and lengthy process. The biggest challenge for a trainee is to mentally reconstruct a 3D image from multiple slices of 2D planes of a given structure. To achieve such purpose, the advantage of 3DE over 2D TEE is obvious, especially for a complex structure like the MV. Furthermore, La Canna et al. eluded to an additional diagnostic value of 3D TEE, in that it provides more accurate mapping of MV prolapse and further details of MV anatomy [11].
Currently, three-dimensional images rendered from 3DE are projected on 2D monitors. The mere fact of projecting a three-dimensional structure on a at screen is a signi cant shortcoming, despite the use of different hues and shadows to improve depth perception.
Recently, 3D printing has gained momentum in the medical eld. Anatomical structures from different systems of the body are printed in a very high degree of accuracy [4,5]. Moreover, the price of printing has decreased signi cantly, and most centers nowadays can print their 3D images.
While printing cardiac structures from CT and MRI scans are done routinely nowadays; printing from echocardiographic data has not been perfected yet. This is mainly due to the relatively low special resolution of 3DE images. Mahmoud et al [11] and others have printed 3D models of the mitral valve generated from echocardiographic data. The resulting models closely mimicked the original echocardiographic pictures. This imaging modality however still requires some re nement to better describe lea et coaptation, chordae, papillary muscles and left ventricular geometry [13].
In the eld of surgery, the improvement in MV surgical outcomes coinciding with the use of 3D based images was highlighted by Beraud and his group [14]. Hien et al. had shown in a multicenter study that the use of 3DE of the Mitral valve improves the understanding of prolapse segments [15]. The fact that this was true for beginners and experienced echocardiographers alike was of interest. Taking it one step further, Hadeed and his group in a brief communication, proposed the use of printed 3D models of complex congenital lesions as a roadmap for surgical repair [16]. Recently, Premyodhin et al. published their experience in the use of a specially constructed MV molds, that simulated natural tissue in the preoperative simulation of robotic mitral valve repair [17].
From the educational point of view; Biglino et al, in a recent publication showed the bene t of using 3D models of the heart as an educational tool for the training of both adult and pediatric cardiac nurses [18]. Costello et al. found that the even pediatric residents' understanding of lesions like ventricular septal defect could be further enhanced by the printed model as compared to educational seminars in anatomy and echocardiography. They proposed that 3D printed heart models can be effectively incorporated into a simulation-based congenital heart disease and critical care training curriculum for pediatric resident physicians [19].
One of the advantages of the physical inspection of the 3D model over a 2D projection of its image could be explained by the loss of the depth perception that is intrinsic to projecting a 3D structure onto a 2D screen.
In this study, we sought to prove that a physical model of the mitral valve, printed based on MV echocardiographic data, can enhance the understanding of the normal and abnormal anatomy of mitral valves even at a high level of training. Our data clearly shows that the MV model, when physically examined by the cardiology fellows, did signi cantly improve their understanding even after seeing a 3D image of the same model. This observation was not only valid for the prolapsing segments but also for the area and shape of the valve, and the complex anatomy of the annulus. This was the case irrespective of the level of training of the cardiology fellow.
In the future, further advancement in the spatial and temporal resolution of echo generated 3D images along with the advancement in 3D printing, will permit the printing of actual structures of normal as well as congenitally deformed hearts from echocardiographic data in high delity and accuracy. Eventually, combining valves and subvalvular apparatus printed from echocardiographic data with heart models printed from CT or MRI scans will further improve our understanding of congenital heart disease as well as surgical planning.

Study Limitations:
Although the number of study participants was small, the results were coherent for the superiority of the 3D printed model. This study could thus serve as a starting point for larger studies to assess the effectiveness of 3 D models integration in medical education.

Conclusion
Three-dimensional models of MV manufactured based on echocardiographic data and printed by means of 3D technology have a positive impact on the understanding of adult and pediatric cardiology trainees, of the MV anatomy and pathology. It might further enhance the diagnostic power of 3DE especially in complex cases of MV disease.

Consent for Publication
Not applicable.

Availability of data and material
The datasets generated and/or analyzed during the current study are available from the corresponding author on reasonable request

Con icts of interest/ Competing Interests
The authors declare they have no competing interests The two lea ets are divided into a total of six scallops: A1, A2, A3 (anterior) and P1, P2, P3 (posterior)  Questionnaire designed to elicit whether the 3D printed model improved participant perception and knowledge about the mitral valve