Pixel detector readout architectures

Hybrid pixel detectors are gaining importance within the area of radiation imaging applications. In such devices, a pixel detector material is bonded to readout electronic circuits. It is possible to use both integrating and photon counting methods to read out the information from the detector. However, only the photon counting technique will enable energy discrimination.

Readout circuits for photon counting image sensors consist of a complex mixture of analog and digital behavior. State of the art photon counting pixel detector systems such as the Medipix 2 have previously been presented. In this system, the analog channel consists of a pre-amplifier, a pulse-shaping amplifier and comparators in addition to pulse pileup rejecters and base-line restorers. The digital part consists of pulse discrimination logic, counters and associated readout logic. Whereas the integrating version contains a small number of transistors, photon counting pixels contain complex circuitry. This means that the pixel design is driven mainly by area, power consumption and mixed mode design constraints. The area constraint is crucial because of the pixel size and hence the spatial resolution is a fundamental imaging property.

One method of enabling color imaging, that is, resolving multiple energy ranges, is to use the same methods as those used in image sensors for light. This is a solution with low circuit complexity since it discriminates only one energy range in each pixel. The color for each pixel is then extrapolated from a predetermined pattern during post processing. Another approach is to have multiple energy channels in all pixels. This will give full resolution but it also involves high circuit complexity. In this paper we present an architecture that utilizes the best of both these techniques. The proposed architecture uses full spatial resolution for the intensity and it uses sub-sampling for resolving the energy levels. The results from standard image benchmarks show that the pixel architecture produces images with almost the same quality as for those from a full sampling architecture and with almost the same circuit complexity as is necessary for a single energy range architecture.

Energy levels in a 3-level imaging system.