Figure 1. Dr. Little and a valve clinic patient discuss her anatomy with the help of a multi-material life-size 3D printed model of her heart.
As therapeutic cardiac interventions become increasingly more complex, the ability to create patient-specific physical models prior to intervention has become an area of intense interest. Models of normal anatomy, congenital abnormalities, acquired pathology, and even abnormal valve function have all recently been reported. Here we discuss how these models are created, discuss some of the strengths and limitations of each fabrication method, and provide examples of how these methods are being applied to teach, plan, and improve patient care. Three-dimensional (3D) printing, also called “additive manufacturing” or “rapid prototyping,” is a novel manufacturing technology that translates a digital 3D image data-set into a physical 3D object. 3D printing technology in the form of Stereolithography (STL) was introduced by Charles Hull in 1983. Stereolitography is based on the technique of exposing specific photopolymers to ultraviolet light (UV) to create rapid material hardening. By printing successive layers of these polymers with synchronized UV light exposure, a 3D model is constructed. Following this initial technique, many more 3D printing methods have been created and refined so that today, a 3D model can be created using a number of different (and competing) technologies including building successive layers of material by polymerization, by bonding agents, or by melting layers of material powder. An overview of these methods and applications are provided in Table 1.
Table 1. Classification and Characteristics of 3D Printing Technologies. Echo Magazine 2015.
A Brief History
In 1986, another 3D printing technology called Selective Laser Sintering was introduced by Carl Dechard and Joe Beaman from University of Texas. This method uses a laser for selective sintering of a wide range of material powders. In 1986, Charles Hull with 3D Systems introduced the first commercial printer along with standard STL file format that could translate a 3D digital model into 3D printable information. This was followed by development of Fused Deposition Modeling technology, invented by Scott Crump in 1988. This method relied on heating and extruding a thermoplastic filament to build up object layers. 3D printing technology has been increasingly commercialized with a new technique of agent binding introduced by MIT in 1993. Thermoplastic models and bio-printed scaffolds appeared in 1999, and the first research program on printing cells into the organs started in 2002. The first color high-definition 3D printer was introduced in 2005, and in 2007 PolyJet technology first offered the possibility of 3D printing using multiple different materials. However, many of these emerging technologies remained prohibitively expensive for widespread use. After 2010, these technologies became more cost effective, and there has since been an explosion in interest and applications. The first 3D printed car was made in 2010 and was soon followed by a bio-printed blood vessel (2010), chocolates and aircraft parts (2011), and the first patient-specific 3D printed lower jaw (2012). In the last few years, significant improvements in 3D printer resolution and the ability to print using a wide range of colors and materials, and material mixtures, have helped fuel the growing scientific interest in these new methods and their application in medicine. While initially utilized for commercial manufacturing and engineering applications, 3D printing soon found its way to the medical environment for the fabrication of dental implants, bone reconstruction, manufacturing specialized surgical instruments and customized prosthesis, and most recently for the printing of patient-specific complex anatomical structures. This technique is suitable for 3D printing the anatomically accurate, multi-material cardiac structures. Today, the choice of appropriate 3D printing technique depends on the specific features that the 3D printed object should contain. In Table 1, we linked each 3D printing technology with its main characteristics and applications.
Figure 2. 3D Patient-specific Model Reconstruction from CT Data. Construction process for a 3D patient-specific aortic valve. Echo Magazine 2015.
Clinical Images Exports
The process to create a 3D patientspecific anatomic model starts by acquiring volumetric (3D) images with the highest possible spatial resolution. Typically, magnetic resonance imaging (MRI) methods and computed tomography (CT) data sets have been employed for the reconstruction of anatomically accurate cardiovascular structures. More recently, 3D echocardiography has also been used as a source imaging method. The choice of imaging modality reflects the nature of the 3D model to be constructed but is often guided by the clinical indication for the imaging study. For example, MRI data for modeling congenital heart defects; CT data for modeling orthopedic replacements; and 3D echocardiographic data to model patient-specific mitral valve dysfunction. Using any of these clinical imaging modalities, the next step is to export a target image using the Digital Imaging and Communication in Medicine (DICOM) standard format into a specialized image processing software such as Mimics (Materialize, Belgium). Within this software environment, a skilled user performs anatomic identification and structural segmentation to ultimately create a digital model of the anatomy of interest. The complexity in creating these digital models can vary widely depending on their size and the nature of the target tissues for replication (e.g. bone only or a mixture of different “hard” and “soft” tissues). The final 3D digital model is transformed into the STL file format and fabricated using one of the several different 3D printing technologies discussed above. The development of these 3D printing methods is proceeding so rapidly that the state-of-the-art method is impossible to define. An outline of the design and fabrication process for a 3D printed patient-specific model is shown in Figure 1.
At our institution, we have partnered with a local company (3D Print Bureau of Texas) to fabricate patient-specific cardiac models used for pre-surgical planning to address congenital diseases (right ventricular outflow tract obstruction); to assess the size and attachment site of a right atrial malignancy; and to explore a method to replicate the severity of aortic valve stenosis by coupling a patientspecific 3D model of aortic stenosis to a functional flow loop and subjecting the modeled aortic valve to Doppler interrogation. We used clinically acquired CT data sets for reconstruction of the left ventricular outflow tract, aortic root, aortic valve leaflets, and ascending aorta (Figure 2). All our patient-specific models (Figures 2,5), were fabricated using a PolyJet based 3D printer (Figure 4). We have demonstrated that an anatomically accurate 3D model of a specific patient’s aortic pathology can be replicated using multiple materials to represent the calcified and non-calcified anatomic elements within the aortic valve and root complex. In addition, when matched to a clinically determined stroke volume, the 3D printed model replicated the degree of aortic stenosis recorded during a clinical Doppler echocardiogram. This was one of the first reports to show that a patient-specific 3D printed model could replicate both anatomy and function. It is important to note that aortic valve stenosis is a condition with relatively fixed valve motion, and the replication of anatomic function using 3D printing would be significantly more challenging if motion of the printed structures were required (e.g. mitral valve regurgitation). Nonetheless, we and other investigators are actively exploring the role of 3D printing for the replication of increasing complex functional challenges. Our group at the Houston Methodist DeBakey Heart and Vascular Center has begun to use 3D print methods to replicate functional, patientspecific models of the entire mitral valve apparatus (leaflets, annulus, and papillary muscle geometry) reconstructed from 3D transesophageal data. Our preliminary results have been encouraging and the developmental process of 3D patient-specific mitral valve models is outlined in Figure 3. Ultimately, we aim to employ these models to test current and emerging Doppler echocardiographic methods for the quantification of mitral valve regurgitation against in vitro flow reference standards.
Figure 3. Reconstruction of functional, multi-material model of the mitral valve with papillary muscles from 3D TEE images. Echo Magazine 2015.
Although introduced three decades ago, 3D printing technology has now become widely available and cost effective. The number of 3D printing applications has grown exponentially and is now defining its role in the medical sector. 3D printed patient-specific models of cardiac structures can be used as a novel 3D visualization tool to facilitate the planning of surgical procedures before entering the operation room. Having access to a fully functional valve model before performing an interventional procedure may prove to be remarkably beneficial to both surgeon and patient when the repair involves complex cardiac anatomy. In addition, the role of 3D printed models for the improvement of percutaneous procedural challenges continues to evolve. Without a doubt, 3D printed patientspecific models represent a powerful teaching tool for the education of residents and fellows, as well as a very intuitive tool to explain structural pathology or a planned procedure to a patient. (Figure 1) The integration of 3D modeling and in vitro studies represents an innovative path for medical device testing and optimization and may ultimately lead to the creation of customized prosthetics for the cardiac patient much like what is already offered to the orthopedic patient. We predict that for at least the next several years, 3D printing will remain at the crossroads of advanced cardiac imaging and structural interventions with a direct impact to improve clinical outcomes.