Albert K. Liang

Performance of in-pixel circuits for photon counting arrays (PCAs) based on polycrystalline silicon TFTs

Photon counting arrays (PCAs), defined as pixelated imagers which measure the absorbed energy of x-ray photons individually and record this information digitally, are of increasing clinical interest. A number of PCA prototypes with a 1 mm pixel-to-pixel pitch have recently been fabricated with polycrystalline silicon (poly-Si)-a thin-film technology capable of creating monolithic imagers of a size commensurate with human anatomy. In this study, analog and digital simulation frameworks were developed to provide insight into the influence of individual poly-Si transistors on pixel circuit performance-information that is not readily available through empirical means. The simulation frameworks were used to characterize the circuit designs employed in the prototypes. The analog framework, which determines the noise produced by individual transistors, was used to estimate energy resolution, as well as to identify which transistors contribute the most noise. The digital framework, which analyzes how well circuits function in the presence of significant variations in transistor properties, was used to estimate how fast a circuit can produce an output (referred to as output count rate). In addition, an algorithm was developed and used to estimate the minimum pixel pitch that could be achieved for the pixel circuits of the current prototypes. The simulation frameworks predict that the analog component of the PCA prototypes could have energy resolution as low as 8.9% full width at half maximum (FWHM) at 70 keV; and the digital components should work well even in the presence of significant thin-film transistor (TFT) variations, with the fastest component having output count rates as high as 3 MHz. Finally, based on conceivable improvements in the underlying fabrication process, the algorithm predicts that the 1 mm pitch of the current PCA prototypes could be reduced significantly, potentially to between ~240 and 290 μm.

Performance analysis of several generations of flat-panel X-ray imagers based on polycrystalline silicon TFTs

Active matrix flat-panel imagers (AMFPIs) have become ubiquitous in medical imaging environments. AMFPIs are based on two-dimensional pixelated arrays coupled to various x-ray converter materials that provide either indirect or direct detection of the incident x-ray radiation. However, the capabilities of this technology are severely constrained by the underlying solid-state properties of the amorphous silicon semiconductor material employed in the thin-film transistors present in each array pixel. The considerably higher electron and hole mobilities of polycrystalline silicon, a semiconductor material that (like amorphous silicon) is well suited to fabrication of transistors for large area electronics, provide the potential to overcome these constraints by increasing the overall gain of the system relative to the electronic additive noise. To explore this potential, a series of prototype arrays based on increasingly complex pixel designs employing polycrystalline silicon transistors is under development by our collaboration. The designs include several generations of active pixel arrays that incorporate sophisticated pixel-level amplifier circuits with the goal of improving imaging performance. In this paper, an initial analysis of the noise and DQE performance of selected prototype pixel circuit designs will be presented. The results are based on a combination of Monte Carlo -based circuit simulations and cascaded systems analysis, supplemented with information obtained from measurements performed on poly-Si transistors. The paper concludes with a brief discussion of the potential for, and challenges associated with, the creation of single photon counting arrays based on poly-Si TFTs.

Noise performance limits of advanced x-ray imagers employing poly-Si-based active pixel architectures

A decade after the clinical introduction of active matrix, flat-panel imagers (AMFPIs), the performance of this technology continues to be limited by the relatively large additive electronic noise of these systems - resulting in significant loss of detective quantum efficiency (DQE) under conditions of low exposure or high spatial frequencies. An increasingly promising approach for overcoming such limitations involves the incorporation of in-pixel amplification circuits, referred to as active pixel architectures (AP) - based on low-temperature polycrystalline silicon (poly-Si) thin-film transistors (TFTs). In this study, a methodology for theoretically examining the limiting noise and DQE performance of circuits employing 1-stage in-pixel amplification is presented. This methodology involves sophisticated SPICE circuit simulations along with cascaded systems modeling. In these simulations, a device model based on the RPI poly-Si TFT model is used with additional controlled current sources corresponding to thermal and flicker (1/f) noise. From measurements of transfer and output characteristics (as well as current noise densities) performed upon individual, representative, poly-Si TFTs test devices, model parameters suitable for these simulations are extracted. The input stimuli and operating-point-dependent scaling of the current sources are derived from the measured current noise densities (for flicker noise), or from fundamental equations (for thermal noise). Noise parameters obtained from the simulations, along with other parametric information, is input to a cascaded systems model of an AP imager design to provide estimates of DQE performance. In this paper, this method of combining circuit simulations and cascaded systems analysis to predict the lower limits on additive noise (and upper limits on DQE) for large area AP imagers with signal levels representative of those generated at fluoroscopic exposures is described, and initial results are reported.

Theoretical investigation of the noise performance of active pixel imaging arrays based on polycrystalline silicon thin film transistors

Purpose Active matrix flat‐panel imagers, which typically incorporate a pixelated array with one a‐Si:H thin‐film transistor (TFT) per pixel, have become ubiquitous by virtue of many advantages, including large monolithic construction, radiation tolerance, and high DQE. However, at low exposures such as those encountered in fluoroscopy, digital breast tomosynthesis and breast computed tomography, DQE is degraded due to the modest average signal generated per interacting x‐ray relative to electronic additive noise levels of ˜1000 e, or greater. A promising strategy for overcoming this limitation is to introduce an amplifier into each pixel, referred to as the active pixel (AP) concept. Such circuits provide in‐pixel amplification prior to readout as well as facilitate correlated multiple sampling, enhancing signal‐to‐noise and restoring DQE at low exposures. In this study, a methodology for theoretically investigating the signal and noise performance of imaging array designs is introduced and applied to the case of AP circuits based on low‐temperature polycrystalline silicon (poly‐Si), a semiconductor suited to manufacture of large area, radiation tolerant arrays. Methods Computer simulations employing an analog circuit simulator and performed in the temporal domain were used to investigate signal characteristics and major sources of electronic additive noise for various pixel amplifier designs. The noise sources include photodiode shot noise and resistor thermal noise, as well as TFT thermal and flicker noise. TFT signal behavior and flicker noise were parameterized from fits to measurements performed on individual poly‐Si test TFTs. The performance of three single‐stage and three two‐stage pixel amplifier designs were investigated under conditions relevant to fluoroscopy. The study assumes a 20 × 20 cm2, 150 μm pitch array operated at 30 fps and coupled to a CsI:Tl x‐ray converter. Noise simulations were performed as a function of operating conditions, including sampling mode, of the designs. The total electronic additive noise included noise contributions from each circuit component. Results The total noise results were found to exhibit a strong dependence on circuit design and operating conditions, with TFT flicker noise generally found to be the dominant noise contributor. For the single‐stage designs, significantly increasing the size of the source‐follower TFT substantially reduced flicker noise – with the lowest total noise found to be ˜574 e [rms]. For the two‐stage designs, in addition to tuning TFT sizes and introducing a low‐pass filter, replacing a p‐type TFT with a resistor (under the assumption in the study that resistors make no flicker noise contribution) resulted in significant noise reduction – with the lowest total noise found to be ˜336 e [rms]. Conclusions A methodology based on circuit simulations which facilitates comprehensive explorations of signal and noise characteristics has been developed and applied to the case of poly‐Si AP arrays. The encouraging results suggest that the electronic additive noise of such devices can be substantially reduced through judicious circuit design, signal amplification, and multiple sampling. This methodology could be extended to explore the noise performance of arrays employing other pixel circuitry such as that for photon counting as well as other semiconductor materials such as a‐Si:H and a‐IGZO.

Exploration of maximum count rate capabilities for large-area photon counting arrays based on polycrystalline silicon thin-film transistors

Pixelated photon counting detectors with energy discrimination capabilities are of increasing clinical interest for x-ray imaging. Such detectors, presently in clinical use for mammography and under development for breast tomosynthesis and spectral CT, usually employ in-pixel circuits based on crystalline silicon – a semiconductor material that is generally not well-suited for economic manufacture of large-area devices. One interesting alternative semiconductor is polycrystalline silicon (poly-Si), a thin-film technology capable of creating very large-area, monolithic devices. Similar to crystalline silicon, poly-Si allows implementation of the type of fast, complex, in-pixel circuitry required for photon counting – operating at processing speeds that are not possible with amorphous silicon (the material currently used for large-area, active matrix, flat-panel imagers). The pixel circuits of two-dimensional photon counting arrays are generally comprised of four stages: amplifier, comparator, clock generator and counter. The analog front-end (in particular, the amplifier) strongly influences performance and is therefore of interest to study. In this paper, the relationship between incident and output count rate of the analog front-end is explored under diagnostic imaging conditions for a promising poly-Si based design. The input to the amplifier is modeled in the time domain assuming a realistic input x-ray spectrum. Simulations of circuits based on poly-Si thin-film transistors are used to determine the resulting output count rate as a function of input count rate, energy discrimination threshold and operating conditions.

Initial steps toward the realization of large area arrays of single photon counting pixels based on polycrystalline silicon TFTs

The thin-film semiconductor processing methods that enabled creation of inexpensive liquid crystal displays based on amorphous silicon transistors for cell phones and televisions, as well as desktop, laptop and mobile computers, also facilitated the development of devices that have become ubiquitous in medical x-ray imaging environments. These devices, called active matrix flat-panel imagers (AMFPIs), measure the integrated signal generated by incident X rays and offer detection areas as large as ~43×43 cm2. In recent years, there has been growing interest in medical x-ray imagers that record information from X ray photons on an individual basis. However, such photon counting devices have generally been based on crystalline silicon, a material not inherently suited to the cost-effective manufacture of monolithic devices of a size comparable to that of AMFPIs. Motivated by these considerations, we have developed an initial set of small area prototype arrays using thin-film processing methods and polycrystalline silicon transistors. These prototypes were developed in the spirit of exploring the possibility of creating large area arrays offering single photon counting capabilities and, to our knowledge, are the first photon counting arrays fabricated using thin film techniques. In this paper, the architecture of the prototype pixels is presented and considerations that influenced the design of the pixel circuits, including amplifier noise, TFT performance variations, and minimum feature size, are discussed.

Empirical and theoretical examination of the noise performance of a prototype polycrystalline silicon active pixel array

Active matrix flat-panel imagers (AMFPIs), which typically incorporate a single a-Si:H thin-film transistor (TFT) in each pixel, have become ubiquitous in diagnostic x-ray imaging by virtue of many advantages, including good radiation damage resistance and the economic availability of monolithic, large area backplanes. However, under conditions of low exposure per image frame, such as encountered in fluoroscopy, digital breast tomosynthesis and breast cone-beam CT, AMFPI performance degrades due to the effect of additive noise primarily originating from the acquisition electronics. To overcome this limitation, while retaining the advantages of AMFPIs, large area imagers can be fabricated using polycrystalline silicon (poly-Si) TFTs configured to form in-pixel amplifiers. Such active pixel (AP) circuits provide signal enhancement prior to readout, thereby largely overcoming the effect of additive noise, as well as facilitating correlated multiple sampling (CMS). In this paper, early results of an examination of the noise performance of a poly-Si AP prototype array are reported. The array consists of pixel circuit designs incorporating a single-stage amplifier with three TFTs and was operated at 25 fps using CMS techniques. Noise performance is compared to results obtained from sophisticated circuit simulations which account for TFT thermal and flicker noise. Noise is found to depend on many variables, including the size of the source-follower TFT, the reset voltage, the addressing time and the sampling technique – with noise levels from individual pixels as low as 715 e. The circuit simulations were found to reproduce the trends for noise as a function of the aforementioned variables.

TU-FG-209-03: Exploring the Maximum Count Rate Capabilities of Photon Counting Arrays Based On Polycrystalline Silicon

PURPOSE Photon counting arrays (PCAs) offer several advantages over conventional, fluence-integrating x-ray imagers, such as improved contrast by means of energy windowing. For that reason, we are exploring the feasibility and performance of PCA pixel circuitry based on polycrystalline silicon. This material, unlike the crystalline silicon commonly used in photon counting detectors, lends itself toward the economic manufacture of radiation tolerant, monolithic large area (e.g., ∼43×43 cm2) devices. In this presentation, exploration of maximum count rate, a critical performance parameter for such devices, is reported. METHODS Count rate performance for a variety of pixel circuit designs was explored through detailed circuit simulations over a wide range of parameters (including pixel pitch and operating conditions) with the additional goal of preserving good energy resolution. The count rate simulations assume input events corresponding to a 72 kVp x-ray spectrum with 20 mm Al filtration interacting with a CZT detector at various input flux rates. Output count rates are determined at various photon energy threshold levels, and the percentage of counts lost (e.g., due to deadtime or pile-up) is calculated from the ratio of output to input counts. The energy resolution simulations involve thermal and flicker noise originating from each circuit element in a design. RESULTS Circuit designs compatible with pixel pitches ranging from 250 to 1000 µm that allow count rates over a megacount per second per pixel appear feasible. Such rates are expected to be suitable for radiographic and fluoroscopic imaging. Results for the analog front-end circuitry of the pixels show that acceptable energy resolution can also be achieved. CONCLUSION PCAs created using polycrystalline silicon have the potential to offer monolithic large-area detectors with count rate performance comparable to those of crystalline silicon detectors. Further improvement through detailed circuit simulations and prototyping is expected. Partially supported by NIH grant R01-EB000558. This work was partially supported by NIH grant no. R01-EB000558.

Multi-energy imagers for a radiotherapy treatment environment

Over the last ~15 years, the central goal in external beam radiotherapy of maximizing dose to the tumor while minimizing dose to surrounding normal tissues has been greatly facilitated by the development and clinical implementation of many innovations. These include megavoltage active matrix flat-panel imagers (MV AMFPIs) designed to image the treatment beam, and separate kilovoltage (kV) AMFPIs and x-ray sources designed to provide high-contrast projection and cone-beam CT images in the treatment room. While these systems provide clinically valuable information, a variety of advantages would accrue through introduction of the capability to produce clinically useful, high quality imaging information at multiple energies (e.g., kV and MV) from a single detector along the treatment beam direction. One possible approach for achieving this goal involves substitution of the x-ray converters used in conventional MV AMFPIs with thick, segmented crystalline scintillators designed for dual-energy operation, coupled with the addition of x-ray imaging beams that contain a significant diagnostic component. A second approach involves introduction of a large area, monolithic array of photon counting pixels with multiple energy thresholds and event counters, which could provide multi-spectral views of the treatment beam with improved contrast. In this paper, the motivations behind, and the merits of each approach are described. In addition, prospects for such dual-energy imagers and photon counting array designs are discussed in the context of the radiotherapy environment.

TH-EF-BRB-03: Enhancement of Image Contrast Along the Beam’s-Eye-View (BEV) Direction Through Development of Imagers Based On Segmented Scintillators and Photon-Counting Arrays

Purpose: The contrast provided by conventional portal imagers is constrained by inefficient conversion of the incident radiation, and by the small diagnostic x-ray component of the treatment beam. Toward improved visualization of the tumor region prior to treatment, as well as improved tumor tracking during treatment, progress is reported on the design of a pair of novel detectors that would address these constraints. Methods: A dual-energy imager based on a segmented scintillator is being designed using a hybrid model to simulate radiation and optical transport. The simulations, which model operation with a megavoltage treatment beam and with a BEV imaging beam with a significant diagnostic component, employ parameters extracted from a previously developed prototype. Separately, the conceptual design of a second imager incorporating a 2D array of photon counting pixels is being explored through extrapolation of the design of the first prototype photon counting arrays fabricated using thin-film processing, a process scalable to very large area detectors. Results: The simulations identified a segmented scintillator imager design that, when operated with a BEV imaging beam, provides spatial resolution and contrast comparable to that of gantry-mounted kV imagers. When operated with a treatment beam, the design provides significantly improved DQE and contrast compared to conventional portal imagers. Extrapolation of initial photon counting array designs indicates that the circuitry necessary to provide energy-windowing of the diagnostic portion of a therapy beam can be achieved at the pixel pitch associated with current treatment room imagers. Conclusion: These early studies demonstrate potential for improved contrast in projection and volumetric images obtained with the megavoltage beam prior to treatment, while also providing good performance when used with BEV imaging beams. In addition, these studies also support the possibility of improved capability for tumor tracking with such dual-energy imagers as well as by photon counting array imagers. NIH grant R01-EB000558