In Computed Tomography applications a major opportunity has been identified in the exploitation of the spectral information inherently available due to the polychromatic emission of the X-ray tube. Current CT technology based on indirect-conversion and integrating-mode detection can be used to some extent to distinguish the two predominant physical causes of energy-dependent attenuation (photo-electric effect and Compton effect) by using dual-energy techniques, e.g. kVp switching, dual-source or detector stacking. Further improvements can be achieved by transitioning to direct-conversion technologies and counting-mode detection, which inherently exhibits a better signal-to-noise ratio. Further including energy discrimination, enables new applications, which are not feasible with dual-energy techniques, e.g. the possibility to discriminate K-edge features (contrast agents, e.g. Gadolinium) from the other contributions to the x-ray attenuation of a human body. The capability of providing energy-resolved information with more than two different measurements is referred to as Spectral CT. To study the feasibility of Spectral CT, an energy-resolving proprietary photon counting ASIC (ChromAIX) has been designed to provide high count-rate capabilities while offering energy discrimination. The ChromAIX ASIC consists of an arrangement of 4 by 16 pixels with an isotropic pitch of 300 μm. Each pixel contains a number of independent energy discriminators with their corresponding 12-bit counters with continuous read-out capability. Observed Poissonian count-rates exceeding 10 Mcps (corresponding to approximately 27 Mcps incident mean Poisson rate) have been experimentally validated through electrical characterization. The measured noise of 2.6 mVRMS (4 keV FWHM) adheres to specifications. The ChromAIX ASIC has been specifically designed to support direct-converting materials CdZnTe and CdTe.

Hybrid CdZnTe and CdTe pixel detector arrays that convert X-rays directly into charge signals are under development at Nova for applications to digital mammography. CdZnTe and CdTe have superior X-ray conversion efficiency compared to either emulsion based film, phosphor-based detectors or other low-Z, solid-state detectors such as silicon. In this paper, latest results from thin (0.15 to 0.2 mm) CdZnTe and CdTe detectors will be presented in terms of MTF, DQE and phantom images. Single crystal CdZnTe detectors yield better results in DQE as well as phantom images, compared to the poly-crystal CdZnTe detectors. This is due to the non-uniformities in the poly-crystal that degrade the charge transport properties. Because of the charge-coupling limitation of the readout ASIC that was originally designed for Si detectors, the detector is biased to collect holes from the front side. This charge collection mode limits the CdZnTe detector performance. Their DQE measurements yield 25% and 65% for the poly-crystal and single-crystal CdZnTe detectors respectively. Poly-crystal CdTe test detectors were also hybridized to the same type charge readout chip. Since CdTe has much longer hole-propagation lengths compared to CdZnTe, it shows better performance in the hole-collecting mode. However, severe polarization effect degrades performance of the present device. Excellent images were also obtained from the CdTe detectors. Future work to redesign the readout ASIC and thus improve the detector performance is discussed. Applications to industrial imaging such as Non-Destructive Evaluation (NDE) and Non-Destructive Inspection (NDI) are also discussed.

A linear CdZnTe pad detector array with approximately 1 mm(2) pad area has been developed. The detector has a wide energy range from about 20 to 200 keV. To read out these detector arrays, a fast, low-noise monolithic mixed signal ASIC chip has been developed. A prototype x-ray imaging system consisting of the CdZnTe detector array and the monolithic ASIC chip has been fabricated and tested. In this system, the detectors are abutted against each other to form an approximately 1 m long linear array. The system has been used to take preliminary scanned images of complex objects at various energies. New results from this system will be presented.

Hybrid pixel detector arrays that convert X-rays directly into charge signals are under development at NOVA for application to digital mammography. This technology also has wide application possibilities in other fields of radiology and in industrial imaging for applications in nondestructive evaluation and inspection. These detectors have potentially superior properties compared to either emulsion based film, which has nonlinear response to X-rays, or phosphor-based detectors in which there is an intermediate step of X-ray to light photon conversion. Potential advantages of direct conversion detectors are high quantum efficiencies (QE) of 98% or higher (for 0.3 mm thick CdZnTe detector with 20 keV X-rays), improved contrast, high sensitivity and low intrinsic noise. These factors are expected to contribute to high detective quantum efficiency (DQE). The prototype hybrid pixel detector developed has 50/spl times/50 microns pixel size, and is designed to have linear response to X-rays, and can support a dynamic range of 14 bits. Modulation Transfer Function (MTF) is measured on a l-mm silicon detector system where 10% or better modulations are obtained at 10 lp/mm spatial frequency. Preliminary DQE measurements of the same system yields a value of 55% at zero spatial frequency. Here, the authors report data of their first full size prototype readout ASIC chips hybridized with both silicon and CdZnTe detector arrays and present initial MTF and DQE measurement results as well as some test images.

Hybrid pixel detector arrays that convert X-rays directly into charge signals are under development at NOVA for application to digital mammography. This technology also has wide application possibilities in other fields of radiology and also in industrial imaging for applications in nondestructive evaluation and inspection. These detectors have potentially superior properties compared to either emulsion based film, which has non-linear response to X-rays, or phosphor-based detectors in which there is an intermediate step of X-ray to light photons and light photons to electron-hole pairs conversion. Potential advantages of direct conversion detectors are high quantum efficiencies (QE) of 98% or higher (for 0.3 mm thick CdZnTe detector with 20 keV X-rays), improved contrast, high sensitivity and low intrinsic noise. These factors are expected to contribute to high detective quantum efficiency (DQE). The prototype hybrid pixel detector developed has 50×50 microns pixel size, is designed to have linear response to X-rays, and can support a dynamic range of 14 bits. The detector arrays are lightweight and can be packaged compactly, facilitating their incorporation into a digital mammography system or into other clinical radiology and industrial imaging applications where high sensitivity, excellent contrast, high resolution, and high dynamic range are required. Here, the authors report on tests of the first full size prototype readout ASIC chips hybridized with either silicon or CdZnTe detector arrays and present initial results and images