Computer-Aided Design Principles for Anatomic Modeling

Introduction

The process of creating a physical three-dimensional (3D) printed model from medical imaging data is complicated and involves numerous steps. In order for a patient-specific anatomic model to be suitable for 3D printing, segmented anatomical regions of interest must be designed, prepped, and then converted into 3D file types that are recognized by vendor-specific 3D printing slicing software. Common, vendor neutral file formats for printing represent 3D geometry by a triangulated polygon mesh surface with node and vector data and include standard tessellation language or stereolithography (STL), alias wavefront object (OBJ), virtual reality modeling language (VRML), ZPR (a file format created by ZCorp), additive manufacturing (AMF), and 3D manufacturing format (3MF).

The STL file format, invented by the Albert Consulting Group for 3D Systems commercial printers in 1987, has been the predominant file format choice for years. There are two types of STL files: binary STL files that describe a single part and ASCII STL files that contain multiple independent parts. No color or texture is specified in the STL file format. A binary STL can be printed with a single material property, thus is ideal for printing a single component such as an organ, implant, or surgical guide. Multiple binary STL files can be combined or an ASCII STL file can be used to produce multicolored or multimaterial anatomical models such as a kidney and tumor (e.g., two different colors) or an aorta with calcification (e.g., a soft material for the aorta and hard material for the calcification). Since STL files do not encode color or material properties, those are selected using 3D printer-specific software.

In 2011, an initial effort was made to move away from STL files and the AMF was created. However, this file format was never fully adopted by the industry and 3D printer manufacturers. More recently, in 2015, an industrial effort led to the creation of the 3MF file format. This file format reduces the file size and enables the file to carry other data, such as units, color, lattices, and textures. Similar color and texture information is also incorporated in VRML files.

Regardless of the file format, in the preparation phase, minor changes may be necessary to make the model more suitable for printing or major modifications to the model might be needed to facilitate intervention planning. Additionally, model analysis, digital planning, and surgical simulations may be performed in computer-aided design (CAD) software and personalized surgical guides, templates, and molds may be designed. In this chapter, CAD principles and common tools/operations used for medical models will be discussed and clinical examples will be provided. Understanding these tools and methods is critical for any radiologist overseeing the creation of 3D printed models in a hospital setting and can help radiologists to work with surgeons to optimize treatment plans and execute surgeries.

CAD Principles

In medicine, CAD systems allow for the visualization and manipulation of 3D anatomical and associated structures as defined by geometrical parameters. There are many CAD programs where objects can be constructed and editing performed ( Table 4.1 ).

The majority of the CAD software uses solid modeling principles to create 3D representations of objects of interest. Apart from solid modeling, other techniques for the creation of objects include surface modeling (described below), parametric modeling (where a variety of parameters such as dimensions, features, and material properties are input to derive the required geometry), and 3D surface scanning.

Constructive solid geometry (CSG) and boundary representation are the two main methods used in solid modeling. CSG uses Boolean operations (merge, subtract, etc.) to create complex objects from instances of simpler forms such as cylinders, rods, and cubes. Two-dimensional (2D) representations may be swept along a plane or other trajectory to form more complex solid objects and then manipulated against other primitives.

Sweeping operations are also a feature of boundary representation modeling which connects faces, edges, and vertices with geometric surfaces, curves, and points. This technique offers a powerful method to create solid models of unusual shapes. Some well-known boundary representation systems are ROMULUS, Parasolid, and ACIS. The features of these systems form the basis, or are incorporated, into other CAD products.

Solid objects created using 3D modeling techniques can be combined with 3D models rendered from medical imaging, as well as STL files, to form composite images. In addition, 3D and four-dimensional (4D) data visualization, processing, and analysis software can produce additional objects, typically using surface mesh modeling where the surface of the object is represented by a simple surface mesh of vertices and edges. Image segmentation software, introduced in Chapter 3, along with associated modules can be used to create the integrated forms. This software can also post process medical image data using the following mesh modeling techniques. Once a segmented anatomical region of interest has been defined from volumetric medical images, there are two commonly used approaches to model parts by surface mesh modeling, tessellation and the marching cubes algorithm. The most widely used method is the marching cubes algorithm, where the extracted polygonal mesh of an isosurface from 3D voxels is divided into a discrete set of cubes from the input volume associated with that part. This was developed by William Lorensen and Harvey E. Cline as a result of their research at General Electric. The approach uses information from the original 3D imaging slices to derive interslice connectivity, surface locations, and surface gradient information with the resulting triangle model being displayed on conventional graphics display systems using standard rendering algorithms.

A second approach to CAD modeling is tessellation where the polygonal data are converted into a number of triangles proportional to the total surface area of a specific part in question. A higher number of triangles can more accurately represent the part, whereas a smaller number of triangles would decrease the surface detail, resulting in a less accurate representation of that part ( Fig. 4.1 ). Higher triangle counts need more computer processing power to render and are saved as larger file sizes due to the information stored from each triangle's vertices; therefore, a balance of surface quality and file size needs to be taken into account when selecting an appropriate number of triangles of a given part. Cut planes, to reduce the size of the model and surface connectivity, can also play a role in reducing the file size while still maintaining a higher level of surface quality. Decimation can be used, as well, to reduce the triangle count of a part. This allows users to select an amount of reduction of triangles in a specific part in order to reduce its size while still maintaining an appropriate number of triangles to represent the part.

Design Operations