3D printing customisable phantoms: Part 3

Matching bone, muscle and lung densities

Part 3/3 explores two 3D printing techniques utilised at the Peter MacCallum Cancer Centre, Physical Sciences Department. The techniques print object’s achievable Hounsfield Units (HU) that emulate lung and bone densities for Computed Tomography (CT) imaging studies.

1.0 Emulating lung tissue densities in CT

Standard 3D print techniques to emulate lung tissue densities in CT include the control of an infill percentage using line or grid structures. Here, we utilised a cellular structure called Gyroid as an alternative structure that emulates lung tissue density. In contrast to infill patterns using line and grids, Gyroid structures exhibit an isotropic structure which we found produces isotropic HU values at different scanning orientations, favourable for imaging studies [1].

We used a Gyroid structure in this work to mimic lung tissue (mean HU of -750 at 140 kVp CT energy) using a cell size and wall thickness of 6 and 0.6 mm, respectively (nTop 3.16.2, nTopology) (Figure 1.).

This nTop 3.16.2, nTopology Inc. tool is a subscription-based software package that permits valid students a free full-access license for a year. In recent times, open-source slicing software (i.e., IdeaMaker, Cura Slicer etc…) started implementing the use of Gyroids and other cellular structures as infill patterns (Figure 2.). However, they’re limited to using infill % only, which automatically determines cell size and wall thickness via their volume ratio.

2.0 Emulating bone densities in CT

Using the interlace deposition technique of Polylactic Acid (PLA) and Iron-reinforced PLA (Fe-PLA) filaments via dual-extrusion 3D printing, we can control the mean HU of a 3D printed volume by manipulating the layer thickness of the deposited PLA and Fe-PLA layers (see Figure 3. below) [2].

Using the interlace deposition technique, we can generate a bone model consisting of high- and low-density bone segments (see Figure 4.).

3.0 Customising your anthropomorphic phantoms for personalised treatment plans.

Using the MEX printing techniques outlined previously, we can develop  a relatively low cost customised anthropomorphic phantom which has similar CT attenuations as the commercial phantom slab and with patient-specific pathological features (Figure 5.) [3].

The soft tissue segment in Figure 5. was printed using PLA at 100% infill, lung tissue segments with ABS gyroid structures, lung lesion segments with ABS at 100% infill, and bone segments printed with the interlace deposition technique. By combining these techniques, we can achieve heterogeneous densities using material extrusion 3DP technology.

Figure 6. below shows a CT scan (transverse) of the customised 3D printed slab, containing two patient-specific lung lesion cases.

4.0 Concluding Remarks

3D printing provides a low-cost alternative to the fabrication of radiotherapy devices, enabling researchers the freedom to customise their devices. However, we should all keep in mind that 3DP technology, particularly for material extrusion 3D printing, comes with uncertainties in the form of geometrical inaccuracies and printing defects, to which they further vary depending on the printing parameters, as well as the brand and type of printer and materials used.

Quantifying and minimising these uncertainties will undoubtedly aid the development of proper 3D printing standards and regulations targeted for QA purposes and radiotherapy planning and treatment. 3DP technology for radiotherapy applications is still a rapidly evolving area of research. We should expect more innovations to come as research focuses on reducing:

• manufacturing costs;
• optimising processes for reproducibility and clinical functionality; and
• multi-centre prospective observational studies to quantify advantages 3D printed phantoms may have over commercial phantoms currently routinely used for a range of clinical radiotherapy.

It’s important to note that 3D printing technology does not have to fully replace the standard manufacturing process of radiotherapy devices – particularly phantoms. But there are situations where 3D printing can be usefully employed to aid existing anthropomorphic phantoms for imaging and dosimetry work.

6.0 References

[1]       Tino, R., et al., Gyroid structures for 3D-printed heterogeneous radiotherapy phantoms. Physics in Medicine & Biology, 2019.

[2]       Tino, R., et al., The interlace deposition method of bone equivalent material extrusion 3D printing for imaging in radiotherapy. Materials & Design, 2021. 199: p. 109439.

[3]       Tino, R.B., et al., A customizable anthropomorphic phantom for dosimetric verification of 3D-printed lung, tissue, and bone density materials. Medical Physics, 2022. 49(1): p. 52-69.

Rance Tino PhD, 4September 2022