Phantom Development

Solid phantoms

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We have built a wide range of solid phantoms based on an epoxy resin. These can be cast into a wide range of shapes and sizes from cylinders to realistic body shapes (Figure 1) [1, 2].

These phantoms have been widely used for studies to test instrumentation advances and to predict the image quality we might be able to expect prior to clinical imaging of the head and breast.

Liquid phantoms

For imaging the difference between two states, it is important that the coupling between the optical fibres and the object being imaged does not change. For experiments such as these, we often use hollow solid phantoms, filled with either liquid resin or a mixture intralipid with water. Intralipid is an inttravenous nutrient which contains fat suspended in water. When diluted appropriately, its optical scattering properties are close to tissue-equivalent and optical absorption can be modified by the addition of dye. Liquid resin (the same epoxy resin as described above, but without hardener) can be used as this gives constant refractive index between the solid resin outer case of the phantom and the liquid centre, but it is more difficult to work with.

Gel phantoms

For some applications, particularly simulating optical mammography where the breast need to be gently compressed, we have developed a gel phantom which has tissue-equivalent optical, x-ray and elastic properties [3]. The recipe is based on polyvinyl alcohol (PVA) slime, a viscoelastic fluid which readily deforms under moderate pressure. Scattering particles and absorbing compounds can be added to provide a uniform material with stable and reproducible optical properties. A linear relationship between the concentration of scattering particles (either titanium dioxide or microspheres) and the transport scatter coefficient is demonstrated. Phantoms of an arbitrary size and shape may be produced by containing the slime within a thin latex shell, and a stability over a period of at least 3 months has been established.

Multimodality phantoms

Multimodality phantoms can be made by adding a contrast agent which is optically clear but provides contrast for the complementary modality. For example, we have used copper sulphate to provide MR contrast and various additives for x-ray contrast.

References

  1. Firbank, M, and Delpy, DT (1993): A design for a stable and reproducible phantom for use in near-infrared imaging and spectroscopy. Physics in Medicine and Biology 38, 847-853. Download PDF file.
  2. Firbank, M, Oda, M, and Delpy, DT (1995): An improved design for a stable and reproducible phantom material for use in near-infrared spectroscopy and imaging. Physics in Medicine and Biology 40, 955-961. Download PDF file.
  3. Hebden J C, B D Price, A P Gibson, G Royle (2006). "A soft deformable tissue-equivalent phantom for diffuse optical tomography" Phys. Med. Biol. 51 p5581-5590. Full paper