Amir H. Goldan

Amorphous selenium detector utilizing a Frisch grid for photon-counting imaging applications

Incomplete charge collection due to poor electron mobility in amorphous selenium (a-Se) results in depth-dependent signal variations. The slow signal rise-time for the portion of the induced charge due to electron-movement towards the anode and significant electron trapping cause ballistic deficit. In this paper, we investigate Frisch-grid detector design to reduce the depth dependent noise, increase the photon count-rate, and improve the spectral performance of positively biased amorphous selenium radiation detectors. In addition, we analyze the impact of using the Frisch grid detector design on x-ray sensitivity, detective quantum efficiency (DQE), modulation transfer function (MTF), and image lag of integrating-mode a-Se radiation detectors. Preliminary results based on theory are presented for emerging digital medical imaging modalities such as mammography tomosynthesis and fluoroscopy.

A counting and integrating pixel readout chip for amorphous selenium direct radiation detectors for medical imaging applications

A new pixel readout architecture is presented for amorphous selenium (a-Se) direct conversion radiation detectors. Photon-counting operation provides excellent sensitivity to low radiation doses but saturates the system at medium to high doses due to the poor charge transport properties of a-Se. Thus, we present an a-Se readout circuit design that is also capable of dynamically operating in integrating mode to enable the detection of high-dose radiation. This extends the resolvable dynamic range of the imaging system from very low gamma-ray count rates to very high flux x-ray radiations. The readout circuit is very promising for applications such as mammography tomosynthesis, and its benefits can also be extended to other radiation detectors. Finally, we present preliminary spectroscopy results with a-Se.

Selective photon counter for digital X-ray mammography tomosynthesis

Photon counting is an emerging detection technique that is promising for mammography tomosynthesis imagers. In photon counting systems, the value of each image pixel is equal to the number of photons that interact with the detector. In this research, we introduce the design and implementation of a low noise, novel selective photon counting pixel for digital mammography tomosynthesis in crystalline silicon CMOS (complementary metal oxide semiconductor) 0.18 micron technology. The design comprises of a low noise charge amplifier (CA), two low offset voltage comparators, a decision-making unit (DMU), a mode selector, and a pseudo-random counter. Theoretical calculations and simulation results of linearity, gain, and noise of the photon counting pixel are presented.

Single photon counter for digital x-ray mammography tomosynthesis

Photon counting is an emerging detection technique that is promising for mammography tomosynthesis imagers. In photon counting systems, the value of each image pixel is equal to the number of photons that interact with the detector. In this research, we introduce the design and implementation of a low noise, photon counting pixel for digital mammography tomosynthesis in 0.18μm crystalline silicon complementary metal-oxide semiconductor technology. The design comprises of a low noise, charge-integrating amplifier, a low offset voltage comparator, a decision-making unit, a mode selector, and a pseudorandom counter. Theoretical calculations and simulation results of linearity, gain, and noise of the photon counting pixel are presented.

Photon counting pixels in CMOS technology for medical X-ray imaging applications

Crystalline silicon (c-Si) technology is attractive for advanced large area imaging applications because of higher transistor mobility, smaller feature sizes and higher density of integration. These benefits of c-Si technology can help expedite the development of high performance circuitry required for better contrast, lower noise, and lower X-ray dose while providing small, high-resolution pixels. In this research, we examine the technology requirements of performing digital mammography tomosynthesis, an advanced diagnostic X-ray imaging modality, using c-Si semiconductor technology. We then present a novel photon counting pixel architecture in CMOS 0.18 mum c-Si technology to address the requirements posed by mammography tomosynthesis

Selenium coated CMOS passive pixel array for medical imaging

Digital imaging systems for medical applications use amorphous silicon thin-film transistor (TFT) technology due to its ability to be manufactured over large areas. However, TFT technology is far inferior to crystalline silicon CMOS technology in terms of the speed, stability, noise susceptibility, and feature size. This work investigates the feasibility of integrating an imaging array fabricated in CMOS technology with an a-Se detector. The design of a CMOS passive pixel sensor (PPS) array is presented, in addition to how an 8×8 PPS array is integrated with the 75 micron thick stabilized amorphous selenium detector. A non-linear increase in the dark current of 200 pA, 500 pA and 2 nA is observed with 0.27, 0.67 and 1.33 V/micron electric field respectively, which shows a successful integration of selenium layer with the CMOS array. Results also show that the integrated Selenium-CMOS PPS array has good responsivity to optical light and X-rays, leaving the door open for further research on implementing CMOS imaging architectures going forward. Demonstrating that the PPS chips using CMOS technology can use a-Se as a detector is thus the first step in a promising path of research, which should yield substantial and exciting results for the field. Though area may still prove challenging, larger CMOS wafers can be manufactured and tiled to allow for a large enough size for certain diagnostic imaging applications and potentially even large area applications like digital mammography.

Amorphous selenium lateral Frisch photodetector and photomultiplier for high performance medical x-ray and gamma-ray imaging applications

We propose a new indirect x-ray and gamma-ray detector which is comprised of a scintillating crystal coupled with an amorphous selenium (a-Se) metal-semiconductor-metal (MSM) photodetector. A lateral Frisch grid is embedded between the anode and the cathode to provide (1) unipolar charge sensing and (2) avalanche multiplication gain during hole transport inside the detection region. Unipolar charge sensing operation reduces the persistent photocurrent lag and increases the speed of the photodetector because most of the pixel charge is induced during carrier transport inside the detection region. Also, with proper biasing of the electrodes, we can create a high-field region between the lateral Frisch grid and the cathode for avalanche multiplication gain. Thus, we can convert the photodetector into a photomultiplier for higher signal-to-noise ratio and single photon-counting gamma-ray imaging. We present for the first time, a fabricated amorphous selenium lateral Frisch photodetector and present preliminary results of the measured photocurrents in response to a blue light emitting diode.

Amorphous silicon p-i-n photodetector with Frisch grid for high-speed medical imaging

In indirect digital x-ray detectors, photodetectors such as hydrogenated amorphous silicon (a-Si:H) p-i-n photodetectors are used to convert the optical photons generated by the scintillating material to collectible electron-hole pairs. A problem that arises during the collection of the charges is that the mobility and lifetime of both types of carriers (electrons and holes) differ. In a-Si:H, the mobility of holes is much lower than that of electrons which leads to depth-dependent signal variations and causes the charge collection time to be extensive. It has been shown that the use of a Frisch grid can reduce the effect of the slower carriers in direct x-ray detectors. The Frisch grid is essentially a conducting grid that shields carriers from the collecting electrode until they are in close proximity. When the pixel electrodes are properly biased, the grid prevents the slow moving carriers (traveling away from the collecting electrode) from being collected and puts more weight on the fast moving carriers, thus allowing the total charge to be collected in less time. In this paper we investigate the use of a Frisch grid in a-Si:H p-i-n photodetectors for indirect x-ray detectors. Through simulations and theoretical analysis we determine the grid line sizes and positioning that will be most effective for practical p-i-n photodetector designs. In addition we compare the results of photodetectors with and without the grid to characterize the improvement achievable.

Photon counting pixel architecture for x-ray and gamma-ray imaging applications

Photon counting is emerging as an alternative detection technique to conventional photon integration. In photon counting systems, the value of each image pixel is equal to the number of photons that are absorbed by the radiation detector. The proposed pixel architecture provides a method for energy windowing and serial readout for low-dose gamma-ray imaging. Each pixel is comprised of a radiation detector and integrated analog and digital circuitry. A prototype was developed on a printed circuit board (PCB) using discrete electronic components. In this research, we present the experimental results for the operation of the photon counting pixel with energy windowing and investigate the compromise between pixel noise level and photon count rate.

Characterization and comparison of lateral amorphous semiconductors with embedded Frisch grid detectors on 0.18μm CMOS processed substrate for medical imaging applications

An indirect digital x-ray detector is designed, fabricated, and tested. The detector integrates a high speed, low noise CMOS substrate with two types of amorphous semiconductors on the circuit surface. Using a laterally oriented layout a-Si:H or a-Se can be used to coat the CMOS circuit and provide high speed photoresponse to complement the high speed circuits possible on CMOS technology. The circuit also aims to reduce the effect of slow carriers by integrated a Frisch style grid on the photoconductive layer to screen for the slow carriers. Simulations show a uniform photoresponse for photons absorbed on the top layer and an enhanced response when using a Frisch grid. EQE and noise results are presented. Finally, possible applications and improvements to the area of indirect x-ray imaging that are capable of easily being implemented on the substrate are suggested.