High-Flux Multi-Spectral X-Ray Imaging with Energy-Sensitive CZT Detectors

Conventional medical x-ray imaging systems are equipped with: i) a powerful x-ray tube, mounted on a fast rotating gantry, which generates polychromatic radiation characterised by a broad spectrum of energies, and ii) an x-ray detector, which records the total energy of all the x-rays that transmitted through the body of the patient. The attenuation of the x-ray beam, as it passes through the patient's body, depends on the photon energy and this energy dependence is different for different materials, tissues and elements. Therefore, the energy of each detected photon contains additional valuable information about the elemental composition of the scanned object. The current x-ray detectors are mostly insensitive to this spectral information, because their signal output is proportional to the total energy deposited within the active area of the detector, while a detector with energy-discrimination capabilities can provide the solution for enhanced exploitation of this additional spectral information by recording all these different energy photons and arranging them into respective spectral bins. Direct conversion CdZnTe (CZT) semiconductor detectors with high sensitivity, high stopping power, high spatial resolution and excellent energy resolution have emerged as the dominant solid-state room temperature detectors in a wide range of spectroscopic and imaging applications. Most recently, there has been a growing interest in using the CZT detectors for the next generation of high-flux multi-energy x-ray imaging systems, with a particular emphasis on Computed Tomography (CT) and 3D x-ray breast imaging. Both these imaging modalities have common goals, the ability to make quantitative measurements, and therefore, the enhancement of diagnostic capability at low patient doses. However, these applications require very fast data acquisition, and hence, there is a need for detectors that can efficiently operate at a high photon flux, almost 100 million photons per second per square millimetre. The design and fabrication of energy-sensitive CZT detector arrays for high-flux photon-counting multi-spectral x-ray imaging pose significant technological challenges and issues, which are the focus of the investigations of this proposal. The main objectives of this project are: 1. the innovative design specifications of a photon-counting CZT detector for high-flux multi-energy x-ray imaging; and 2. the optimum architecture of a proof-of-concept photon-counting spectral x-ray imaging system based on optimised CZT detectors for multi-energy x-ray spectral CT and 3D x-ray breast imaging. The main motivation of this work is to investigate to which extent photon-counting, energy discriminating CZT detectors are capable of overcoming fundamental performance limits and carrying out quantitative imaging revealing the additional spectral information, and therefore, improving conventional x-ray medical imaging. If energy information is recorded alongside the intensity, x-ray medical imaging modalities could increase diagnostic accuracy through soft-tissue differentiation, material decomposition, tumour characterisation, target quantification and development of disease-specific targeted contrast agents and drugs. The latter could improve low-contrast resolution and overall image quality at significantly reduced radiation doses and lead to superior diagnostic performance with lower cost. Spectral x-ray imaging can become an important imaging technique providing material-specific quantitative information in combination with high spatial resolution imaging, and therefore, leading to a paradigm shift in x-ray medical diagnostics.