Date Published: February 5, 2020
Publisher: Springer International Publishing
Author(s): Vahid Anwari, Ashley Lai, Ali Ursani, Karina Rego, Behruz Karasfi, Shailaja Sajja, Narinder Paul.
An anthropomorphic phantom is a radiologically accurate, tissue realistic model of the human body that can be used for research into innovative imaging and interventional techniques, education simulation and calibration of medical imaging equipment. Currently available CT phantoms are appropriate tools for calibration of medical imaging equipment but have major disadvantages for research and educational simulation. They are expensive, lacking the realistic appearance and characteristics of anatomical organs when visualized during X-ray based image scanning. In addition, CT phantoms are not modular hence users are not able to remove specific organs from inside the phantom for research or training purposes. 3D printing technology has evolved and can be used to print anatomically accurate abdominal organs for a modular anthropomorphic mannequin to address limitations of existing phantoms. In this study, CT images from a clinical patient were used to 3D print the following organ shells: liver, kidneys, spleen, and large and small intestines. In addition, fatty tissue was made using modelling beeswax and musculature was modeled using liquid urethane rubber to match the radiological density of real tissue in CT Hounsfield Units at 120kVp. Similarly, all 3D printed organ shells were filled with an agar-based solution to mimic the radiological density of real tissue in CT Hounsfield Units at 120kVp. The mannequin has scope for applications in various aspects of medical imaging and education, allowing us to address key areas of clinical importance without the need for scanning patients.
Since the discovery of x-rays in 1895, major advances have taken place in X-ray imaging including computed tomography (CT), dual energy (DE) imaging, cone beam CT (CBCT) and digital tomosynthesis (DT) [1–6]. Because these radiologic imaging technologies have been shown to expose the patient to harmful ionizing radiation, rigorous quality assurance (QA) testing is needed to minimize the radiation dose and maximize the diagnostic information from each scan . This process requires careful tailoring of the exposure parameters to the diagnostic task required and to the patient body habitus [8, 9]. An anthropomorphic X-ray phantom is a radiologically accurate and realistic model of the human body. Anthropomorphic phantoms have been used to provide realistic QA testing of medical imaging technologies and can be used to test new imaging protocols for radiation exposure, absorbed dose and effective dose . Anthropomorphic phantoms have also been used for education and training of imaging professionals in the operation of imaging equipment. However, current commercially available and research phantoms have significant limitations. Many phantoms are very expensive . Some anthropomorphic phantoms designed for X-ray or CT imaging and equipment calibration have provided a complex, detailed imaging target but remain fixed in their structure [12–14]. Other anthropomorphic phantoms have demonstrated greater scope for multi-modality imaging, but lack anatomical detail and radiological accuracy [10, 13]. There has been a particular lack of modular anthropomorphic abdominal phantoms that allow the user to remove and replace the organs to replicate different pathologies, and if required, to place foreign bodies such as dosimeters or surgical devices inside the abdominal cavity. Advances in 3D printing technology have increased the range of possibilities in the creation of innovative models for medical purposes. This includes the creation of realistic, anthropomorphic mannequins with various properties such as removable internal organs that are anatomically realistic compared to existing phantoms. The properties of such 3D-printed model(s) (3DPMs) depend on the desired medical application. In general, there are three main considerations for the selection of materials used in 3D printing of anatomical models.
Structural Properties: define the shape, size and anatomical detail.Mechanical Properties: define how the object responds to mechanical stress; these include strength, stiffness, and plasticity.Radiological Properties: define how the object interacts with X-rays; these include the material linear attenuation coefficient and the density measurement in Hounsfield Units.
Four different techniques were involved in creating CASMER: 1) tissue realistic 3D printing of abdominal organs, 2) material based molding of the pancreas, 3) beeswax sculpting of abdominal fat and 4) the use of off-the-shelf components for the bony skeleton and the outer shell. Almost all of the abdominal organs were 3D printed. The HU values of the abdominal organs were determined by placing several 10mm2 regions of interest in the abdominal viscera of 20 adults (10 males) with normal abdominal CT scans using an X-ray tube setting of 120kVp to determine mean (SD) HU values. The muscle and fat sections of the abdominal wall were sculpted from Clear Flex® urethane rubber (Smooth-ON, PA) and modeling beeswax respectively. We selected a variety of materials that had comparable atomic numbers to the principle attenuating tissue in the body organ of interest. All the materials underwent CT scanning using an X-ray tube setting of 120kVp. The materials that were selected closely mimicked the range of Hounsfield Unit (HU) values of the respective in vivo organs and tissues.
When the construction of the mannequin was complete, CT and X-ray scans were acquired to determine the radiological accuracy of the materials inside (Table 2). Figure 8a demonstrates the positioning of the mannequin for an anteroposterior (AP) radiographic view. The resulting radiographic image is shown in Fig. 8b. Figure 9 demonstrates a coronal view of the mannequin acquired with a CT scanner (Canon Medical Systems, Otawara, JP) using an abdominal clinical protocol at 120 kVP. Figure 10a, b demonstrates volume rendered images of the 3D printed organs (except the pancreas) using the Vitrea® software.
Table 2Measured Hounsfield Units of phantom components at 120kVpOrganHounsfield Unit (Mean, SD)Liver55.0 ± 18.0Liver Vasculature70.0 ± 15.0Kidneys40.0 ± 11.0 (inner), 55.0 ± 11 .0(outer)Spleen40.0 ± 15.0ColonToo variable to measurePancreas60.0 ± 14.0Intra-abdominal fat− 100 ± 15.0Muscle layers50 ± 10.0Fig. 8a: CASMER was positioned for an anteroposterior abdominal radiograph to determine radiological density. b: Anteroposterior X-ray of CASMER demonstrates the 3D printed organs and other structures as labelledFig. 9CT coronal view of CASMER demonstrates the positioned organs and surrounding intra-abdominal fat as labelledFig. 10a: Anteroposterior view of the volume rendered image of CASMER shows the labelled 3D printed organs with correct anatomical positioning. b: Posteroanterior view of the volume rendered image of CASMER shows the left and right kidneys
This manuscript outlined the specific steps involved in the manufacturing of a 3D printed, anthropomorphic, abdominal model using CT-based scans with radiologically accurate tissue characteristics. Table 3 lists the cost of materials, scanning and labour in the development of the model. Depending on the desired characteristics and the intended purpose of a model, certain steps in the 3D model’s preparation are more important than others. For example, educational models require structural accuracy. If the sole purpose of the 3D model is to educate patients about their disease, image post processing (i.e. segmentation) is the most important step to ensure that the anatomy of the model closely resembles the actual organ. Surgical models require accuracy in physical properties in addition to structural accuracy.
Table 3Manufacturing costsItemAmount (CAD)Raw materials: agarose gel, fiber, beeswax, synthetic bones, ABS plastic filament$2200Technologist time for segmentation, organ file processing, post print clinical use preparation$16003D printing of organs (liver, colon, kidneys, spleen)$900CT scanning time$3003D Printer (Rostock Max V2)$1400Total$6400
In this manuscript, the process of designing and validating a tissue realistic anthropomorphic abdominal mannequin was presented. There are several avenues for future uses of the model, some of which are mentioned below. CASMER will be available for training medical radiation technology (MRT) students in cross sectional anatomy of the abdomen and for radiation dosimetry calculations. We will also explore 3D printing of pathologies within organs to facilitate training in performing image guided procedures.