Brian E. Harwood

A new 2D-tiled detector for multislice CT

The tremendous increase in speed with which the body can now be scanned using multislice CT has improved the diagnostic ability of the modality, especially in time critical applications involving contrast injection. Advances in photodiode and front-end electronics technology now allow a CT detector module to be made that can be tiled in two dimensions. An array of such modules can be used to easily make a CT scanner with hundreds of slices with the promise of scanning whole organs with a single revolution and further improving diagnostic ability. Recently, a back-illuminated photodiode for CT has been developed which has its electrical connections on the underside. With all four sides of the silicon chip free, the photodiodes can be tiled in two dimensions. In addition, improvements in front-end electronics now allow the A/D converters for all photodiode elements to be placed completely behind the photodiode. A prototype detector module has been constructed and tested. Measurements of DQE, MTF, dynamic range and temporal response are presented showing that the module has the same high performance as detectors found in current diagnostic CT scanners. A dynamic range of 250,000:1 at a frame rate of 10,000 fps has been achieved. Alternatively a dynamic range of 1,000,000:1 can be achieved at 2,500 fps. This new compact 2D tiled detector with digital data output can be used as a basic building block for future multislice detection systems enabling larger coverage and the promise of improved diagnostic ability.

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.

Two-dimensional anti-scatter grids for computed tomography detectors

The use of two-dimensional, focused, anti-scatter-grids (ASGs) in computed tomography is one essential solution to reduce the scatter radiation for large area detectors. A detailed analysis of the requirements and related image quality aspects lead to the specification of the two-dimensional focused geometry of the X-ray absorbing grids. Scatter simulations indicated trade-off conditions and provided estimations for the expected scatter reduction performance. Different production technologies for focused two-dimensional structures have been evaluated. The presented technology of Tomo Lithographic Molding (TomoTM) shows good fulfilment of the specifications. TomoTM is a synthesis of lithographic micromachining, precision stack lamination, molding, and casting processes with application-specific material systems. Geometry, material properties, and scatter performance have been investigated. Different analysis methods will be presented and results of the investigations demonstrate the performance capability of this two-dimensional grid technology. Material composition of the tungsten-polymer composite, homogeneity of wall thickness, and precision of the focusing have the biggest influence on the X-ray behavior. Dynamic forces on the anti-scatter-grid during CT operations should not lead to dynamic shadowing or intensity modulation on the active pixel area. Simulations of the wall deformation have been done to estimate the maximum position deviation. Prototype two-dimensional ASGs have been characterized and show promising results.

Design and performance of a 32-slice CT detector system using back-illuminated photodiodes

A complete 32 slice CT detector system has been constructed which uses back illuminated photodiodes (BIPs). Individual detector modules in the system incorporate the BIPs along with highly integrated A/D conversion electronics on the same substrate. A symmetrical mechanical structure allows the system to be compact and lightweight for use at high rotational speeds. The unique design also has the advantage of having no internal cables. The current BIP exhibits a higher level of crosstalk between photosensitive elements when compared to a conventional photodiode. Differences in the crosstalk level at detector module boundaries can cause artifacts unless the crosstalk can either be reduced or a software correction made. In order to show that the BIP is a viable technology for use in multislice CT, a performance evaluation of the complete BIP system along with its associated mechanical, electrical and software components is required. The 32 slice detector system has been mounted to a rotating CT scanner for image performance evaluations. Measurements of low contrast sensitivity, MTF, limiting resolution and other parameters have been done. A crosstalk correction algorithm has also been developed and evaluated under different conditions. Low contrast sensitivity, MTF and limiting resolution of the system match those of a current conventional CT scanner of similar geometry. The crosstalk correction effectively eliminates artifacts caused by non-uniform crosstalk at module boundaries. MTF and noise properties before and after crosstalk correction match theoretical values.