Materials Development for Additive Manufacture of Magnetic Resonance Imaging (MRI) Phantoms

Magnetic resonance imaging (MRI) is an important non-invasive probe used extensively for medical diagnostics. Phantoms are manufactured devices of controlled specific composition, dimensions and shape that are used for: instrument calibration, to provide common reference samples to allow the calibration of multisite research collaborations, they can act as replacements for human volunteers or live animal models during operator training or technique development. Additive manufacture is an attractive method for the production of phantoms with better physiological shape reproduction, if appropriate printable materials can be developed that provide the appropriate imaging contrast to mimic healthy and diseased tissue.<br/>Phantoms are more difficult to develop for MRI applications than other medical imaging techniques, such as X-Ray or ultrasonic imaging, because the imaging contrast is provided by the relaxation times, T1 and T2) of the nuclear magnetic resonance excitation of protons in biological systems. These relaxation times (T1 and T2) in biological tissue are much longer than in conventional polymeric materials because of the presence of water and the soft matter gel nature of biological tissue. Hence, conventional MRI phantom materials are either aqueous solutions or weak gels of biologically extracted hydrogels such as agarose gels, carageens and alginates, possibly doped with paramagnetic ions to further tune the relaxation times. These phantoms have a very limited shelf-life and are normally formulated prior to use and disposed of afterwards. They are not particularly stable and the relaxation may be affected by the local environment, e.g. humidity, ambient temperature or oxygen pressure (altitude of the site).<br/>Here we demonstrate a family of printable materials that have been developed using formulations of silicone resins that can access the T1 and T2 parameter space that encompasses most medically important organs and tissues. This tunability can be achieved using conventional silicone formulations and commercially available resins usi.ng simple variation of polymer and oligomer molecular weight and cross-linking density. A second strategy is also presented that uses a multiphase or composite approach with blends of immiscible materials. These formulations are shown to have the rheological properties appropriate for extrusion additive manufacturing methods. Simple demonstrator structures have been produced that show good imaging quality and stability.