Chris J. Vrettos

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.

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.