Abstract
New tissue substitute materials known as Cross-linked Hydrophilic Copolymers have been investigated in order to determine their suitability as phantoms for mammography applications. The main requirement of a radiation phantom is that the radiation interactions within the phantom should match those interactions that would take place in the simulated body organ or tissue. The potential of cross-linked hydrophilic copolymers, as tissue substitute materials, in simulating the photon attenuation and scattering responses of human soft tissues has been evaluated. The cross-linked hydrophilic copolymers used in this study, designated as ED IS and ED4C, were based upon a combination of polyvinylpyrrolidone: methyl methacrylate and are able to take up a controllable amount of water without being dissolved in it. The hydrophilic materials in dry and hydrated states have been irradiated in order to estimate their mass and linear photon attenuation coefficients. The materials were hydrated in double distilled water and various solutions of sodium hydroxide (NaOH), hydrochloride (HCl) and copper sulphate (CUSO4) as well as plasma tissue substitute material known as HaemaccelRTM (Polygeline). The effect of the pH value of the absorbent on the linear expansion of the hydrophilic materials was investigated. Moreover, the attenuation properties of the hydrophilic materials were verified at a range of hydration levels where double distilled water was used as an absorbent. The XCOM computer code was used to calculate the mass photon attenuation coefficients of the hydrophilic materials and the results were compared to the measured values. Furthermore, the electron densities of the cross-linked hydrophilic copolymers were determined. Normal and pathological breast tissue samples (adipose, glandular and tumour) excised from nine postmenopausal breast cancer patients undergoing mastectomies were collected from the Histopathology Department at the Royal Surrey County Hospital as fresh and blood free samples. The linear photon attenuation coefficients and the electron densities of the breast tissue samples were independently measured and compared to both the hydrophilic materials and the published data for breast tissues. The breast tissue samples were examined after freeze drying and the results were corrected for the water reduction by the use of the Mixture Rule. The water content of each tissue type was determined as the weight lost during the freeze during process. The technique of Photon Transmission Tomography (PTT) was used to measure the linear attenuation coefficients of the materials and the tissue samples whereas their electron densities were determined by the employment of the technique of Compton Scattering Densitometry (CSD). For both the PTT and CSD techniques, an encapsulated americium-241 radiation source of photons of energy 59.50 keV and a high purity germanium (HPGe) detector were used. Chemical substances of known mass and linear photon attenuation coefficients and electron densities were employed in order to investigate the performance of the PTT and CSD systems.