Abstract
This thesis reports on improving the spectral performance of a MARS scanner to enhance soft tissue information within the human diagnostic energy range. The results presented in this thesis might lead to multiple clinical benefits such as tissue characterisation and monitoring the disease response to therapy non-invasively.
Clinical CT, equipped with scintillator detectors operating in energy-integrating mode, is unable to measure the spectral information of the transmitted x-ray photons. When using a polychromatic x-ray spectrum, CT data suffer from the beam hardening effects, which introduce inaccuracy in the measured x-ray attenuation values. In addition, different elements, such as calcium and iodine in an object, can have similar average x-ray attenuation values when using a polychromatic x-ray spectrum. Spectral information, acquired using a MARS scanner equipped with energy-resolving photon-counting detectors, has allowed myself and co-investigators to identify and measure the composition of tissues, and may allow demonstration of the effectiveness of the targeted drug against the diseases non-invasively.
This thesis reports on development of accurate and efficient techniques for calibrating the energy response of individual pixels of an energy-resolving detector using both x-ray fluorescence and g-ray from a radioisotope. Several high-Z semiconductor sensors bump bonded to either Medipix3.1 or Medipix3RX ASIC were evaluated in both Single Pixel Mode and Charge Summing Mode. This thesis also reports on development of a novel technique for calibrating the energy response of the detector using x-ray tube voltage. Similarly, the count rate capability of a CdTe-Medipix3RX using a polychromatic x-ray source was investigated.
This thesis also reports the evaluation of imaging performance of a MARS scanner. The potential of the MARS scanner in characterising the composition of ex-vivo human carotid atherosclerotic plaque was demonstrated by differentiating and visualising multiple intrinsic bio-markers including calcium, fat and water within firstly, pre-clinical small animal energy range (15 - 50 keV) and secondly, human diagnostic energy range (30 - 120 keV). Similarly, element-specific spectral x-ray imaging was performed to discriminate the K-edges of I, Gd and Au in a physical phantom simultaneously.
In conclusion, I have developed accurate and efficient techniques to characterise the energy response of detectors, and I have evaluated the imaging performance of the MARS scanner. The improvement of soft tissue information was demonstrated by characterising the composition of an ex-vivo human atherosclerotic plaque, and performing element-specific imaging of a multi-contrast phantom within the human diagnostic energy range. When translated to human imaging, this work could offer multiple clinical benefits such as in-vivo early detection of vulnerable plaque, and opens several possibilities of spectral molecular imaging.