Why is Three-Dimensional Printing Superior to Viewing Three-Dimensional Images on a Two-Dimensional Flat Screen?

Although multiple different 3D imaging modalities are currently available, one must question the effectiveness of most of these techniques, which project a 3D structure on a two-dimensional (2D) screen. Three-dimensional echocardiography, for example, allows us to create a volumetric data set and then manipulate and crop it from different perspectives to focus on the area of interest. Can one truly consider such a structure to be effectively communicating to the observer in three dimensions? Previous research into the ability of “mental rotation,” or visualizing a 3D structure and then being able to imagine it from different perspectives, tells us that the answer is probably no. It has been demonstrated that examining a 3D structure in an actual 3D format, which can be held, is more useful in relaying anatomy than simply trying to understand the structure from 2D images. In addition, individuals vary in the strength of their visuospatial skills, so that 2D images presented to a group of colleagues may not be interpreted in the same way by all. The use of 3D printed cardiac models eliminates this possibility and leaves no aspect of the spatial relationships to the imagination, which can be invaluable in patients with complex anatomy.

Three-Dimensional Printing of Cardiac Models: Will it Be for Everyone?

The majority of work pertaining to creating cardiac models using 3D printing involves computed tomographic (CT) or magnetic resonance imaging (MRI) source data sets. The ability to create models from echocardiographic data brings forth a new level of accessibility. The steps in creating a 3D virtual model for the purpose of rapid prototyping start with the acquisition of the 3D imaging data set. Ideally, there is good blood pool to myocardial contrast. Olivieri et al . optimized their 3D echocardiographic images before processing, which is a critical step in both decreasing the time required to create a 3D virtual model and increasing its quality. Segmentation, the process by which we designate the range of gray values that highlights our area of interest, becomes much faster with such a set of images. The designation of the anatomy of interest to be printed is relatively simple and can be easily done either by pediatric cardiologists or sonographers. Although echocardiographic data do not allow us to visualize or print the whole heart in three dimensions, focusing on one part simplifies the process and makes it less time consuming and labor intensive. Once the area of interest is highlighted, the image-processing software creates a 3D structure that can be saved and then printed on a 3D printer to create a physical cardiac replica. Such a work flow brings us closer to the reality of a scenario in which we acquire images in a patient with newly diagnosed congenital heart disease on one day, with the physical replica in hand in time for discussion at a surgical conference the next day.

There are certainly challenges in making this technology accessible to all programs that would like to use it. The ideal scenario would involve predefined protocols for image acquisition, automated postprocessing, and subsequent creation of a 3D physical model on a cost- and time-efficient printer. Currently and unfortunately, there are no published recommendations on parameters that should be used during 3D image acquisition, for any modality. In addition, thin mobile structures, such as valves and the subvalvar apparatus, continue to be challenging to recreate. For patients with congenital heart disease, the automation of the segmentation process promises to be a challenge. With such a wide variation in cardiac anatomy for each lesion, it seems almost impossible for software to identify relevant structures for surgical planning, assign appropriate values for segmentation, and crop the virtual 3D model in a manner to best display the anatomy of interest. In their “Conclusions” section, Olivieri et al . refer to the technology used to create these models as “highly accessible.” It is difficult at this stage for this technology to be considered readily available. The cost of both the image postprocessing software and the 3D printer put the process even further from the reach of every program that may wish to use it. In addition, it remains unclear which patient populations will benefit most from these models. Although the report by Olivieri et al . includes mainly cases of patients with VSDs, it may be more intuitive to study patients with more complex intracardiac anatomy, in whom the models would add more spatial information. Nevertheless, the report by Olivieri et al . should be considered symbolic of the progress and burgeoning interest in creating 3D cardiac models from routine echocardiographic data sets.

Touching Three Dimensions to Explore Structural Heart Disease

The utility of this technology in multiple areas of medicine is increasingly being noted in the literature. The prospects for applications in transcatheter interventional procedures and presurgical planning for complex cardiac cases are particularly exciting ( Table 1 ). One of the earliest works, by Binder et al ., used a multiplane transesophageal echocardiographic probe to recreate 3D echocardiographic data sets at a time when the process was far from automated. Subsequent work by Gilon et al ., also using echocardiographic data sets and stereolithographic technology, demonstrated that the 3D aortic valve shape in aortic stenosis contributes to the lesions’ hemodynamic effects in addition to the limiting valve orifice.

In patients with complex congenital heart disease and complex intracardiac relationships ( Table 2 ), 3D printing removes the potential for misinterpretation that may occur between the presentation of images in two dimensions and the mental reconstruction that is undertaken to understand the relationships in three dimensions. For example, 3D printing has been used to demonstrate the location of major aortopulmonary collateral arteries in patients with tetralogy of Fallot and pulmonary atresia and to guide sternotomy in adults who have undergone coronary bypass surgery and subsequently required aortic valve replacement at a later date. These two scenarios, in addition to the patient cases presented by Olivieri et al ., demonstrate the use of 3D models for the purpose of directly visualizing anatomic relationships in a physical replica.

Table 2

Applications of 3D printing in congenital heart disease for preprocedural planning

Study	Purpose	Subjects	Principal findings
Ngan et al .	To assess utility and accuracy of solid anatomic models for surgical planning	Patients with pulmonary atresia, VSD, and MAPCAs	Three-dimensional printed anatomic models of the pulmonary vasculature from CTA data accurately represented MAPCAs and were found by the surgeons to be very useful in visualizing the vascular anatomy.
Noecker et al .	To demonstrate the utility of 3D cardiac modeling for medical device development	Pediatric patients with and without CHD	Three-dimensional models of pediatric cardiac and chest anatomy provided enhanced understanding and tactile representation of complex anatomy, which was found to be particularly useful during the development and placement of medical devices.
Sodian et al .	To explore the impact of using RP techniques for surgical planning and intraoperative orientation	Patients with complex congenital heart disease	Stereolithographic replicas are helpful for choosing treatment strategies, surgical planning of corrections, and intraoperative orientation, as demonstrated in a patient with aberrant retroesophageal left subclavian artery and right aortic arch and a patient with a VSD.
Schievano et al .	To determine if MRI data could be used to create rigid models which accurately represent the right ventricular outflow tract and pulmonary trunk for PPVI	Patients requiring PPVI	Using 3D RP models resulted in more accurate selection of patients for PPVI than did using magnetic resonance images.
Greil et al .	To demonstrate the creation of 3D physical models using high-resolution MDCT and MRI	Patients with congenital heart disease including tetralogy of Fallot, D-TGA, and DORV	High-resolution MDCT or MRI 3D data sets can be accurately reconstructed using laser sinter technique. Teaching, research, and preoperative planning may be facilitated in the future using this technique.
Jacobs et al .	To create anatomically correct RP models to facilitate planning and execution of the surgery	Patients with complex congenital heart disease and altered geometry of the atria and ventricles	The use of the 3D rapid prototyping model during resection of ventricular aneurysm and malignant cardiac tumors may facilitate the surgical procedure because of better planning and improved orientation.
Mottl-Link et al .	To improve spatial imagination for planning of surgical intervention	Patient with L-TGA, pulmonary atresia, atrial and VSDs, and dextrocardia	A 3D printed model in a patient with transposition of the great arteries, pulmonary atresia, atrial septal defect, and VSD were helpful in preoperative in surgical planning.
Vranicar et al .	To assess accuracy of 3D stereolithographic models created in patients with great vessels anomalies including coarctation of the aorta and vascular rings	Patients with anomalies of the great vessels	Three-dimensional printed models for patients with aortic coarctation and vascular rings were useful in planning transcatheter and surgical interventions.
Sodian et al .	To demonstrate the utility of 3D physical replicas in presurgical planning	Single-ventricle patients with failing ventricles requiring transplantation	Three-dimensional printed models were useful for perioperative planning and intraoperative visualization in patients with failing single ventricles undergoing cardiac transplantation.
Shiraishi et al .	To assess if a stereolithographic modelling technique can be applied for anatomical diagnosis and surgical simulation	Patients with complex congenital heart disease	Rubber-like urethane cardiac biomodels allowed surgeons to practice and actually suture and cut the flexible material.
Schmauss et al .	To present a single-center experience with a wide variety of cases using this technology in complex pediatric and adult cardiac cases	Patients with congenital heart disease and adults with structural heart disease	This group demonstrated its center’s experience and the feasibility of creating 3D models in a series of patients with congenital and structural heart defects in preparation for either surgical or transcatheter interventions.
Ryan et al .	To demonstrate a new approach to identify location of MAPCAs using RP that may preclude cardiac catheterization	Patient with tetralogy of Fallot, pulmonary atresia, and MAPCAs	Use of a 3D printed model allowed preoperative planning in a patient with tetralogy of Fallot, pulmonary atresia, and MAPCAs without cardiac catheterization.
Olivieri et al .	To demonstrate utility of a 3D printed model to assist in presurgical and interventional planning	Patient with D-TGA who had undergone a Mustard repair	The insight provided by 3D modeling may help increase procedural efficiency, decrease radiation exposure, and mitigate procedural complications.

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