Towards combined X-ray and optical mammography

• Main Author: Price, B. D.
• Published: University College London (University of London) 2011
• Subjects: 610
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.565177

Optical contrast, dependent upon haemodynamics and thus providing physiological information, is complementary to radiographic contrast. Combined x-ray and optical mammography screening could provide increased specificity over either system alone. Medical imaging equipment is routinely characterised and tested using tissue equivalent phantoms. A novel phantom material is presented: a solution of polyvinyl alcohol in ethanol and water freeze-thawed to produce a solid yet elastically compressible gel. The x-ray attenuation, mechanical and optical properties of these gels can be accurately adjusted over appropriate ranges so as to mimic cancerous or healthy breast tissues. Modulated imaging in both optical and x-ray acquisitions is also considered. An x-ray system capable of optimising dose distribution has previously been developed at UCL. Overall images are obtained by aligning multiple images from smaller sensors. The effects that this type of acquisition has on spatial resolution are discussed. Two considerations are made: (i) is there a minimum size sensor whose modulation transfer function (MTF) can accurately be determined? (ii) does the MTF of an overall image differ significantly from those of its constituent images? The smaller a sensor becomes, the harder it is to determine its MTF accurately, and the resolution of overall images is slightly poorer than those of individual sensor images. Nonetheless these effects are small and should not hinder the development of such systems. Whilst similar dose considerations do not apply to optical tomography, modulated imaging still presents potential benefits. A method of visualising intensity data in order to localise regions of heterogenous absorption is presented using both simulated and experimental data. Objective functions designed to quantify the visibility of these heterogeneities are proposed and it is shown that optimal distributions of source power, that maximise these, can be found. It is proposed that such techniques might allow optical acquisitions to be performed more rapidly.