Stereolithography (medicine)

Stereolithographic model of a skull, using an infrared system

Stereolithographic models have been used in medicine since the 1990s,^[1] for creating 3D corporeal models of various anatomical regions of a patient, based on datasets from CT-scans.

Usage

• Stereolithography is a Rapid prototyping process that creates solid physical models directly from computer data. In industry this data comes from 3D computer-aided design (CAD) data. The process can also be used to build highly accurate replicas of human (or animal) anatomy by using computer images from medical scanners.^[2] Typically Computed tomography (CT) is used but Magnetic Resonance Imaging (MRI) can also be used. Models have also been made from Ultra Sound and more recently from lower cost Cone Beam CT scanners. Medical models can also be made using a range of other Rapid Prototyping processes although stereolithography remains popular.

• Medical models are used in medicine and surgery to provide surgeons with a better appreciation of the anatomical situation of a patient, before surgery. Although the advent of improved 3D computer reconstruction and virtual surgical planning means that in some cases models are not needed they remain popular for complex surgeries particularly in cranial surgery, maxillofacial surgery, oral surgery and neurosurgery.

• Stereolithographic models are used as an aid to diagnosis, preoperative planning and implant design and manufacture. This might involve for example planning and rehearsing osteotomies. Surgeons use models to help plan surgeries but prosthetists and technologists also use models as an aid to the design and manufacture of custom-fitting implants. Medical models are frequently used to help in the construction of Cranioplasty plates for example.

Medical Modelling Process

The process of medical modelling involves several stages including image acquisition, image segmentation, data translation, model building and post-processing.^[3] Medical modelling involves first acquiring a 3D CT scan (or other form of scan data). The CT data should be in a suitable format and acquired using suitable parameters to obtain a high quality model.^[4] This data consists of a series of cross sectional images of the human anatomy. In these images different tissues show up as different levels of grey. Selecting a range of grey values enables specific tissues to be isolated. A region of interest is then selected and all the pixels connected to the target point within that grey value range are selected. This enables a specific organ to be selected. Most frequently this will be bone but it could be any tissue that can be identified in the scan image. This process is referred to as segmentation. The segmented data may then be interpolated and have other processes performed on it to translate it into a format suitable for the stereolithography process.

Whilst the stereolithography process is inherently accurate the accuracy of a medical model depends on many factors, especially the operator performing the segmentation correctly. There are potential errors possible when making medical models using stereolithography but these are easy to avoid with practice and well trained operators.^[5]

Commercial Services

There are several specialist companies that provide medical modelling services such as for example PDR in the United Kingdom, Medical Modeling Inc. in the USA and Materialise in Belgium. As Rapid Prototyping machines become more affordable many hospitals are investing in their own medical modelling facilities.

References