Cases were selected for 3D modeling in at least one of three ways: 1) Request of the primary cardiologist or surgeon 2) request of the cross-sectional imaging team and/or 3) meeting procedural criteria as part of a standardized 3D modeling pathway. (Figure 1) Through the standardized 3D modeling pathway, patients undergoing one of a specific set of procedures AND had high-resolution cross-sectional imaging, would routinely have 3D modeling performed. The procedure-based criteria for this pathway were developed organically over a 12-month period (in 2018), as a result of trial-and-error, tracking requests, modeling a wide variety of anatomies and iterative conversations with the cardiothoracic surgeons as to the relative added value of the models. Ultimately, the following procedures were integrated into the modeling pathway in January of 2019: 1) Multiple muscular ventricular septal defects (VSD) 2) Determination of feasibility of biventricular repair 3) Tetralogy of Fallot / Pulmonary Atresia with Major Aortopulmonary Collaterals (TOF PA MAPCA) 4) Transcatheter Innominate Vein Turndown.
The institutional review board at the Children’s Hospital of Philadelphia approved this study.
CMR studies were all performed on a 1.5 Tesla scanner (Magnetom Avento, Siemens Healthcare) with either gadobutrol (Gadavist, Bayer HealthCare, New Jersey) of ferumoxytol (Feraheme, AMAG Pharmaceuticals, Massachusetts) as the contrast agent. Retrospective imaging studies were deemed to be adequate for 3D modeling purposes if there was a cardiac gated magnetic resonance angiography (MRA) sequence performed with a maximum voxel size of 1.2 mm3. Computed tomography (CT) studies were performed on a SOMATOM Definition Flash (Siemens Healthcare) with an iodinated contrast agent (Iohexol, Omnipaque, GE Healthcare Inc.). Similarly, studies were deemed adequate if CT angiography was acquired with a voxel size ≤ 1mm3. In addition, regardless of modality, there could not be any imaging artifact in the anatomic region of interest.
Prospective image acquisition for the purpose of 3D modeling entailed a ferumoxytol-enhanced CMR with a navigated cardiac inversion recovery FLASH sequence in both systole and diastole. The maximum interpolated voxel size for patients less than 10kg was 0.9mm3 and 1.2mm3 for patients ≥ 10kg.
Selected digital imaging and communications in medicine (DICOM) files were imported into FDA approved software (Mimics Innovation Suite, Materialise, Belgium). Semi-automatic segmentation techniques were used to create masks and subsequent 3D volumes of the blood pool and if indicated, the myocardium, semilunar valve annuli and atrioventricular (AV) valve annuli. (Supplemental Data: Figure 1) The 3D volumes were post processed to create a hollow blood pool with uniform thickness ≤ 1.0mm, that was then merged with the myocardial volume to create a whole heart model. The post-processed 3D volumes were used to create stereolithography (STL) files. When clinically indicated, both systolic and diastolic models were created for a single case.
All segmentation and post-processing were performed by an advanced imaging cardiology fellow (RMG) or clinical engineer (ES). Prior to clinical use, digital models were verified for anatomic accuracy within the region of interest, by the advanced imaging fellow and/or cardiac MRI attending.
3D Visualization Tools for Procedural Planning
1) 3D printing
3D printing was performed by the Children’s Hospital Additive Manufacturing for Pediatrics (CHAMP) Lab, a central 3D printing lab administered by the Department of Radiology at CHOP. A material jetting printer, (Objet500 Connex 3 (prior to July 2019) and J750, Stratasys, Eden Prairie, MN), was used to create physical models at 100% scale. Prior to 2020, models were created using rigid photopolymers with a minimum thickness of 1.0mm. After January 2020, the option to print models with a minimum thickness of 0.75mm and using flexible rubber photopolymers was available. (Figure 2a, Supplemental Figure 2)
2) 3D PDF / flat screen visualization
The generated STL files were visualized on a flat (2D) screen (Figure 2b) using one of two methods. 1) The files were converted to a 3D portable document file (PDF) format and viewed using a free Adobe Acrobat Reader (Adobe Systems, San Jose CA). 2) Starting in October 2020, the ability to view STL files using a recently developed Dynamic Modeling Module15 in 3D Slicer (www.slicer.org) was available.
3) CAD-based modeling
Standard CAD operations were employed using 3Matic software (Materialise, Belgium) to digitally design surgical repair options when clinically indicated. Sweep-loft functions were used to design Fontan and right ventricle to PA conduits, as well as simulate interposition grafts. Simple VSD patches were created according to the Radiologic Society of North America 2015 primer. 16 Intracardiac baffles were designed using a combination of spline creation, sweep-loft and hollow functions.
With the release of the Baffle Tool in the SlicerHeart extension for 3D Slicer (www.slicer.org), all intracardiac baffle design was performed using this software due to its simplicity and superior functionality. Description of this modelling tool has been published previously. 15 (Figure 2d)
4) Virtual Reality
The STL files were visualized in virtual reality using the SlicerVR module in 3D Slicer (www.slicer.org) running on a standard PC with a RTX 2080TI graphics processor and a Valve Index Headset (Valve Corporation Bellevue, WA). 17 (Figure 2c)
5) 3D Modality Utilization
Utilization of the above-mentioned four 3D modalities was based on both availability of the technique and proceduralist request. From program inception through July 2019 the only available visualization modalities were 3D printing and digital 3D PDF. In July of 2019, CAD-based modeling of surgical repairs as well as the ability to view STLs in virtual reality became available.
An 8 ft. x 15 ft. room was designated by the cardiac center for image review. (Figure 3) A computer with two monitors was used for interactive work with a remote wall mounted HD TV for group review. Two SteamVR 2.0 Base Stations (Valve Corporation Bellevue, WA) were mounted on opposite sides of the room. The majority of the room’s footprint was left open to allow ease of movement when utilizing the VR system. The room was situated between the interventional cardiology and cardiothoracic surgery office suites, allowing for proximity to the proceduralists.
Images and models were reviewed in multiple forums and formats. Physical and/or on-screen models were presented at the weekly cardiothoracic surgical conference. The 3D modelling was reviewed in further detail with a small group of case-specific multidisciplinary team members, using all available 3D visualization formats. At these meetings, all surgical approach options were discussed and visualized using the models. Afterwards, 3D PDF or screen recording of model manipulation was sent to all team members for reference.