Roger Steadman

Experimental feasibility of multi-energy photon-counting K-edge imaging in pre-clinical computed tomography

Theoretical considerations predicted the feasibility of K-edge x-ray computed tomography (CT) imaging using energy discriminating detectors with more than two energy bins. This technique enables material-specific imaging in CT, which in combination with high-Z element based contrast agents, opens up possibilities for new medical applications. In this paper, we present a CT system with energy detection capabilities, which was used to demonstrate the feasibility of quantitative K-edge CT imaging experimentally. A phantom was imaged containing PMMA, calcium-hydroxyapatite, water and two contrast agents based on iodine and gadolinium, respectively. Separate images of the attenuation by photoelectric absorption and Compton scattering were reconstructed from energy-resolved projection data using maximum-likelihood basis-component decomposition. The data analysis further enabled the display of images of the individual contrast agents and their concentrations, separated from the anatomical background. Measured concentrations of iodine and gadolinium were in good agreement with the actual concentrations. Prior to the tomographic measurements, the detector response functions for monochromatic illumination using synchrotron radiation were determined in the energy range 25 keV-60 keV. These data were used to calibrate the detector and derive a phenomenological model for the detector response and the energy bin sensitivities.

Status of Direct Conversion Detectors for Medical Imaging With X-Rays

Imaging detectors for medical X-ray and computed tomography (CT) applications have undergone many improvements and technology changes over time. But most (dynamic) detectors sold in this field still rely on indirect conversion, using scintillators and photodiodes to convert the X-ray quanta ultimately into electrical signals. Direct conversion detectors promise very high spatial resolution and high signal-to-noise ratios. Some direct conversion materials may allow for counting or even energy resolving detection of the X-ray quanta. Based on this, for example spectrally resolving CT systems are becoming an interesting option for the next decade. This contribution highlights the requirements of advanced medical X-ray and CT imaging and reviews examples of status and progress in the field. The emphasis is on the direct conversion sensors for pixelated detectors, but considerations on read-out concepts and on associated challenges such as interconnects will also be presented. Finally, the most burning issues, such as count rate limitations and polarization effects, will be discussed from an application point of view.

A CMOS photodiode array with in-pixel data acquisition system for computed tomography

A CMOS photodetector array with in-pixel electronics has been developed for computed tomography (CT) applications. Current CT detectors are based on two discrete components: a photodiode array and a data acquisition system (amplifier). Both elements have to fulfill a series of severe requirements. CT scanners are moving toward larger detectors and higher speed, and yet lowering costs and improving performance. This contribution relates to the integration of both elements into a standard CMOS process to fulfill future CT scanner specifications and for a cost-effective solution. A series of limitations have been overcome to integrate both the photodiode and a charge-sensing amplifier at pixel level. In order to balance the limited responsivity of standard CMOS photodiodes, a new low-capacitance device has been devised so that low-noise design is possible while providing enough gain in the amplifier. Since a good geometric detective quantum efficiency is desired (>60%), the available area for electronics is very limited. In order to achieve a necessary dynamic range of 17 bits in such reduced area, a single-stage amplifier with automatic gain-switching has been devised. Consisting of a 10/spl times/20 pixel array, in-pixel electronics has been designed to achieve a quantum limited system under CT operating conditions.

ChromAIX: a high-rate energy-resolving photon-counting ASIC for spectal computed tomography

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.

Towards direct conversion detectors for medical imaging with X-rays

Imaging detectors for medical X-ray and Computed Tomography (CT) applications have undergone many improvements and technology changes over time. But most (dynamic) detectors sold in this field still rely on indirect conversion, using scintillators and photodiodes to convert the X-ray quanta ultimately into electrical signals. Direct conversion detectors promise very high spatial resolution and high signal-to-noise ratios. Some direct conversion materials may allow for counting or even energy resolving detection of the X-ray quanta. Based on this, for example spectrally resolving CT systems are becoming an interesting option for the next decade. This contribution highlights the requirements of advanced medical X-ray and CT imaging and shows examples of status and progress in the field. The emphasis is on the direct conversion sensors for pixelated detectors, but considerations on read-out concepts and on associated challenges such as interconnects will also be presented. Finally, the most burning issues, such as count rate limitations and polarization effects, will be discussed from an application point of view.

Performance of prototype modules of a novel multislice CT detector based on CMOS photosensors

A novel CT detector based on CMOS photodiodes has been developed. A detector module comprises two identical photosensor arrays mounted to a ceramic substrate. Each sensor has a matrix of 20 by 10 pixels. Pixels are 1 mm (channel direction) x 1.8 mm (slice) large and consist of a photodiode, charge integration unit and a sample and hold stage. An automated switching between a low and a high sensitivity mode allows for a dynamic range of 17 bits. The integrated signals are read out, transferred to a printed circuit board (at a rate of 2463 Hz per pixel) and here converted into a digital data stream. The structured cadmium tungstate scintillator features lead stripes between pixels to reduce x-ray crosstalk and to shield the underlying in-pixel electronics. During assembling care was taken to ensure that the lead stripes of the scintillator entirely cover the pixel electronics underneath. Several prototype modules have been assembled and their performance concerning linearity, noise, crosstalk, and temperature dependence has been evaluated.

Testing an Energy-Dispersive Counting-Mode Detector With Hard X-Rays From a Synchrotron Source

A counting-mode line detector has been evaluated at a synchrotron radiation source in order to assess its performance for imaging applications. The x-ray detector is based on 3 mm thick CdZnTe arrays with 1 mm pixel pitch and multi-threshold counting electronics. Data readout has been performed in threshold scan mode to provide maximum energy dispersion. The acquired energy spectra are interpreted regarding the characteristics of a periodic source and pulse pile-up in the detector. In particular, energy response and rate behavior of single-photon and double-photon events are investigated. Count rate dynamics are studied up to incident rates of . A further subject of the investigation is the variance of the observed counting rate. Linearity in the energy domain is tested between 40 keV and 140 keV.

Low noise, large area CMOS x-ray image sensor for C.T. application

In this paper, we describe a novel CMOS X-ray active pixel sensor for indirect C.T. X-ray detection. Considerable noise reduction has been achieved by lowering the detector junction capacitance. For this purpose a new low capacitance, low dark current dot type photodiode based on minority diffusion has been developed. The dynamic range is expanded to 17 bit by the use of individual in-pixel automatic gain control. A photon noise limited detector exhibiting a 20 /spl times/ 10 pixel array with a frame rate of 3000 frames/sec has been realized in a 1.2 /spl mu/m CMOS process.

ChromAIX: Fast energy resolved photon-counting readout electronics for future human Computed Tomography

ChromAIX is an ASIC for fast single photoncounting designed to study the feasibility of energy-resolved photon-counting human Spectral CT. It allows for reading out 4 × 16 pixels of a direct converting sensor made of Cd[Zn]Te with a pixel pitch of about 300μm. It supports K-edge imaging [3]. After thorough electrical testing reported in [6] characterization under g- and X-rays was done giving promising results as to the behavior of the ASIC, and revealing the known limitations of direct conversion material (flux limitation, polarization behavior). Measurements with g- and X-rays are compared with simulation results, and good correspondence is obtained.

An In-Pixel Current-Mode Amplifier for Computed Tomography

A high-dynamic-range current-mode detector for a computed-tomography application is shown. A regulated current-mirror structure that provides a 17-bit dynamic range and a noise floor below 3 pArms has been implemented at pixel level. Nonlinearity is kept below 2%, and the signal bandwidth is higher than 10 kHz. A test structure with a 4 times 4 pixel array is presented in this paper. Both the photodiode and the current-mode amplifier have been integrated into the same CMOS standard process