3D Printing: Which Materials Resemble Tissue on CT?

A 3D-printed part that looks uniform to the eye may appear differently on a computed tomography image than a designer expects. That matters when printed materials are used for medical models, devices, or phantoms that support imaging and radiation-dose research. A 2022 study in Radiography measured the CT numbers of 15 commercially available additive manufacturing materials at several CT tube voltages.

Key findings

• The study tested 15 materials: 7 made using FDM and 8 using SLA.
• FDM materials ranged from −188.0 to 189.1 HU; SLA materials ranged from −15.8 to 167.3 HU.
• The specimens were 20 × 20 × 20 mm blocks printed at 100% infill density.
• CT scans were acquired at 80, 100, 120, and 135 kV; CT numbers increased with tube voltage for most materials.
• Higher measured material density was associated with a higher CT number.
• Some measurements overlapped with ranges reported for human tissues, but the values depended on the material and scan voltage.

Why CT numbers matter for 3D-printed materials

A CT number, expressed in Hounsfield units (HU), describes how a material attenuates X-rays relative to water. Different materials can therefore produce different values and contrast on a CT image. Understanding this behaviour is useful when designing 3D-printed parts that will be imaged or placed near anatomical structures.

One important application is phantom production. A phantom is a physical model used to test medical imaging equipment, image quality, or dosimetry. Knowing a material’s CT number can help in choosing a material for a particular part of a model and estimating how it may appear relative to surrounding structures. The paper also identifies potential uses in anatomical and therapeutic phantoms, as well as immobilization devices that can be distinguished on CT images.

Human tissues have different CT number ranges. The paper lists water at −4 to 4 HU, fat at −100 to −80 HU, and liver at 50 to 70 HU. Spongious bone is listed at 50 to 300 HU. These ranges help explain why material selection affects image contrast, but they do not mean that a 3D-printed material is a universal substitute for tissue under every imaging condition.

What the study tested

The study covered two additive manufacturing technologies. Fused deposition modelling (FDM) builds a part by depositing material layer by layer. Stereolithography (SLA) uses liquid resin that is cured during fabrication. Infill density describes how much of a part’s interior is filled. The study used 100% infill and avoided an internal infill pattern that could introduce air spaces and change the material density.

Each test object was a 20 × 20 × 20 mm block. The FDM materials were Novamid 1030-CF10, polypropylene, DISCO PLA, PLA, TOUGH PLA, ABS-X, and PLACTIVE AN1, also identified as C-PLA in the results. The SLA materials were Flexible 50A, Durable, Tough 1500, Elastic 50A, Tough 2000, High Temp, Clear, and Grey resin.

The study used Form 3B, Ultimaker S3, and Ultimaker S5 printers. The specimens were scanned on an Aquilion Prime CT scanner, using a 3 mm slice thickness and tube voltages of 80, 100, 120, and 135 kV. CT numbers were measured in a circular region of interest at the centre of each block. Density was estimated from measurements of mass and volume. The paper reports a 0.15 mm layer height for the printed materials.

Results: density, voltage, and CT number

For FDM, measurements ranged from −188.0 HU for polypropylene at 80 kV to 189.1 HU for C-PLA at 120 kV. The SLA range was −15.8 HU for Elastic 50A at 80 kV to 167.3 HU for Tough 2000 at 120 kV. These are measurements for the tested specimens and conditions, not guaranteed values for every product sold under the same material name.

For most specimens, the CT number rose as tube voltage increased. PLA, for example, increased from 132.4 HU at 80 kV to 151.6 HU at 135 kV. Flexible 50A rose from 43.2 HU to 80.5 HU over the same voltage range. The response was not the same for every material. C-PLA measured 185.5 HU at 80 kV and 188.4 HU at 135 kV, while Tough 2000 measured 165.1 HU at 80 kV and 165.8 HU at 135 kV. The paper describes the voltage-related changes for these two materials as negligible.

The statistical analysis found an association between higher density and higher CT number for both FDM and SLA materials. Density is not the only influence, however. Chemical composition, X-ray beam energy, and specimen thickness can also affect the measurement. The paper specifically notes that C-PLA contains copper, a high-atomic-number element that can affect X-ray absorption.

Several materials produced values within ranges similar to those reported for particular tissues. FDM materials DISCO PLA, PLA, TOUGH PLA, and PLACTIVE AN1 overlapped with the range listed for spongious bone. ABS-X had values similar to the range listed for the pancreas. Depending on the voltage, some SLA materials had values close to those listed for the kidney, pancreas, or liver. These are comparisons of CT numbers, not evidence of biological or complete radiological equivalence.

What the findings mean in practice

For engineers and designers, the measurements can inform material selection when a printed part needs to be visible or distinguishable on a CT image. In an anatomical phantom, for example, different materials can help create different levels of image contrast. For devices or supports that are imaged with a patient, knowing the CT number can help predict how a material will appear and whether it may be differentiated from its surroundings.

For students, the study shows why the material name or printing technology alone is not enough to predict CT appearance. Infill density, material composition, and scan settings also matter. It also connects 3D printing with medical imaging: a designed part is not only a shape, but also a material that affects the resulting image.

Researchers can use the measured values as starting information when selecting materials for phantoms and test objects. The paper points to potential applications in medical-modality testing and research into doses in diagnostic radiology and radiotherapy. For a specific application, however, the material and manufacturing and scanning conditions should be checked directly rather than relying only on measurements from this study.

Limitations and next steps

The study included 15 materials, but only tested blocks printed at 100% infill. The study did not compare different infill patterns, infill densities, or layer heights, because that would require producing many combinations. The results therefore cannot automatically be applied to parts with a different internal structure or different printing settings.

CT number also depends on tube voltage: measurements were taken at four settings between 80 and 135 kV. The paper notes that the composition of some resins is not fully known and that the methods used to measure specimen density and dimensions have limited precision. The paper also discusses the potential effects of specimen thickness and high-atomic-number elements on CT values.

The paper reports plans to extend the work to additional additive manufacturing materials and biomaterials and to develop a larger database of CT numbers. The paper also identifies mixed, graded, and multipart materials as areas for further study. Such data could support the development of phantoms with a wider range of contrasts.

Conclusion

The study shows that commercially available FDM and SLA materials can have a wide range of CT numbers, and that values often change with scan voltage. For the tested specimens, the ranges were −188.0 to 189.1 HU for FDM and −15.8 to 167.3 HU for SLA. These measurements provide a useful starting point for choosing materials for 3D-printed medical models, devices, and phantoms, but they should be interpreted in the context of the specific material, printing process, and CT protocol. The paper is attributed to Adi Pandžić, PhD.

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Author: Adi Pandžić, PhD in Mechanical Engineering

This article summarises a published research paper; please refer to and cite the original paper. About the author

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