  Recently, 3D printing has been widely used to fabricate medical imaging
phantoms. So far, various rigid 3D printable materials have been investigated
for their radiological properties and efficiency in imaging phantom
fabrication. However, flexible, soft tissue materials are also needed for
imaging phantoms, which are used in various scenarios, such as anatomical
deformations to improve dynamic treatments and various needle-based surgeries
and training. Recently, various silicone additive manufacturing technologies
have been used to produce anatomical models based on extrusion techniques that
allow the fabrication of soft tissue materials. To date, there is no systematic
study in the literature investigating the radiological properties of silicone
rubber materials/fluids for imaging phantoms fabricated directly by extrusion
using 3D printing techniques. The aim of this study was to investigate the
radiological properties of 3D printed phantoms made of silicone in CT imaging.
The radiodensity as described as Hounsfield Units (HU) of several test phantoms
composed of three different silicone printing materials were evaluated by
changing the infill density to adjust their radiological properties. A
comparison of HU values with a Gammex Tissue Characterization Phantom was
performed. A scaled down anatomical model derived from an abdominal CT was also
fabricated and the resulting HU values were evaluated. For the three different
silicone materials, a spectrum ranging from -639 to +780 HU was obtained on CT
at a scan setting of 120 kVp. A good agreement was observed between the HU
target values in abdominal CT and the HU values of the 3D-printed anatomical
phantom in all tissues. Moreover, using different infill densities, the printed
materials were able to achieve a similar radiodensity range as obtained in
different tissue-equivalent inserts in the Gammex phantom (238 HU to -673 HU).
