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Dual-modality imaging of a compressible breast phantom with realistic optical and x-ray properties

Medical imaging equipment is routinely characterised and tested using tissue equivalent phantoms. Combined x-ray and optical mammography could provide increased screening specificity over either system alone. The ongoing evaluation of this approach depends upon the development of phantoms with simultaneously breast tissue equivalent optical and x-ray properties. Furthermore deformation models used in the registration of optical and x-ray images, which are acquired at differing levels of breast compression, require validation through phantoms which are also mechanically tissue equivalent. As well as static imaging, dynamic optical imaging of blood flow whilst breast compression is applied has been proposed as a method of enhancing screening specificity. The effect of changes in blood flow and volume on optical tomography still need to be established. A novel phantom material created by freezing and thawing a solution of polyvinyl alcohol (PVAL) in ethanol to create a solid yet elastically compressible gel is described. These gels have x-ray attenuation coefficients equivalent to those of breast tissues whilst their optical and mechanical properties are readily modified. Titanium dioxide is added to the optically non-scattering and colourless gels to obtain the transport scattering coefficient required. Cancerous tissues are often many times stiffer than healthy. Similar differences in stiffness are achieved between gels by varying PVAL concentration. The first x-ray and optical images of an anthropomorphically shaped breast phantom made from this gel are presented. This contains a lesion filled with blood equivalent dye whose volume changes upon compression of the phantom.