A high spatial resolution and cost-effective amorphous selenium multi-layer detector architecture was previously proposed for high-energy applications including single-photon emission computed tomography (SPECT). To overcome the complexity of reading out signal from the multi-layer detector, we propose a resistive microstrip detector readout using charge division. The charge induced on the readout electrode undergoes charge division along the electrode and is proportional to the physical location of absorbed photons, enabling position sensitivity. A 1 mm collimated blue LED is employed to illuminate the electrode, which has a resistance of 242 kΩ. To improve the linearity of the position sensing, the microstrips were deposited underneath a-Se in order to move bonding pads far away from the active area. A low deposition temperature evaporated Cs-doped and As-stabilized a-Se layer is used as a hole blocking layer to ensure that the resistivity of microstrips is not affected and to prevent crystallization of the temperature sensitive a-Se layer. The detector structure was fabricated on a flexible substrate that can potentially be incorporated, in the future, into a curved detector structure for large area SPECT systems. The results presented demonstrate that the detected position is linearly proportional to the location of absorbed photons with a spatial resolution of 1 mm.
KEYWORDS: Sensors, Modulation transfer functions, X-rays, X-ray detectors, Prototyping, Signal to noise ratio, X-ray imaging, Signal detection, Photons, Selenium
An x-ray detector’s ability to produce high signal-to-noise ratio (SNR) images for a given exposure is described by the detective quantum efficiency (DQE) as a function of spatial frequency. Current mammography and radiography detectors have poor DQE performance at high frequencies due to noise aliasing when using a high- resolution converter layer. The Apodized-Aperture Pixel (AAP) design is novel detector design that increases high-frequency DQE by removing noise aliasing using smaller sensor elements (eg. 5 - 50 μm) than image pixel size (eg. 50 - 200 μm). The purpose of this work is to implement the AAP design on a selenium (Se) CMOS micro-sensor prototype with 7.8 × 7.8 μm size elements. Conventional (binned) and AAP images with 47 μm pixel size were synthesized and used to measure the modulation transfer function (MTF), normalized Wiener noise power spectrum (NNPS) and DQE. A micro-focus x-ray source (with a tungsten target) and a 60kV beam filtered with 2mm of aluminum was used to measure performance with DQEPro (DQEInstruments Inc., London, Canada) in a dynamic image acquisition mode at a high exposure level (9.7mR). The AAP design has 1.5x greater MTF near the image cut-off frequency (uc = 10.6 cyc/mm) than conventional design. DQE near ucwas 2.5x greater with the AAP design than conventional, and specimen imaging of a kidney stone shows greater SNR of fine-detail in the AAP image.
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