High-resolution pixel detectors for second generation digital mammography

Technological aspects of development of pixel and strip detectors based on CdTe and CdZnTe

A coincidence detection system between 350μm pitch pixelated Cadmium Zinc Telluride (CdZnTe) detector and a LSO-based Inveon PET detector was set up to investigate the performance of using highly pixelated CdZnTe detector as a PET insert for sub-500μm resolution PET imaging applications. 20×20×5mm^3 CdZnTe substrate was contacted with 350μm pitch pixels (250μm anode pixels with 100μm gap) and coplanar cathode. Based on the charge sharing measurement of the 350μm CdZnTe detector, we analyzed how charge sharing events from CdZnTe detector affect the PET image resolution. We recorded coincidence events between a central CdZnTe pixel and an Inveon LSO detector to image a NEMA NU4 Na-22 point source, which is 250μm in diameter and embedded in an acrylic cube. To interpolate the position of the charge sharing events between two neighbor pixels, we divided such events into five regions based on the ratio of signal amplitude of central pixel and neighbor pixels, and then reconstructed the PET image using events from different regions individually. Results show that the PET image resolution of coincidence events using the CdZnTe single-pixel photopeak events is 590μm FWHM, while the image resolution was decreased to 640μm FWHM if we included the charge sharing events. By interpolating the location of the charge sharing events, the reconstructed PET image resolution was improved from 640μm to 560μm. If we compensated the effective source dimension plus positron range, the image resolution was improved to 460μm FWHM. In a conclusion, the image resolution of an animal PET scanner can be improved to sub-500μm FWHM if we use 350μm pitch pixelated CdZnTe detector as a PET insert and integrate such device into the PET scanner.

We present a low-power, low-noise prototype pixel readout application specific integrated circuit (ASIC) for hyperspectral enegy-resolving X-ray imaging detectors. The ASIC provides 16-by-16 channels to read out positive and negative charges from 16-by-16 heagonal silicon or CZT detector arrays, at a pitch size of $250 \mu \mathrm{m}$, to achieve good spatial resolution and the ability to record the energy of a detected photon as well as its position. The readout is done by bumpbonding the anodes to the inputs of the ASIC. Each channel of the ASIC provides low-noise charge amplification, high-order shaping with baseline stabilization, discrimination, extraction of amplitude (with neighbour channels), multiplexing, and dissipates $\sim0.6$ mW. A smart readout of the triggered pixel and its adjacent six pixels in the hexagonal configuration allows reconstruction of events with charge sharing correction, and can be used to estimate the depth of the photon interaction and to suppress background events. The target equivalent noise charge (ENC) is $\sim10$ electrons for silicon detector pixel and $\sim15$ electrons for CZT detector pixel.

Cadmium zinc telluride (CdZnTe) detectors are known to suffer from polarization effects under high photon flux due to poor hole transport in the crystal material. This has led to the development of a high-flux capable CdZnTe material (HF-CdZnTe). Detectors with the HF-CdZnTe material have shown promising results at mitigating the onset of the polarization phenomenon, likely linked to improved crystal quality and hole carrier transport. Better hole transport will have an impact on charge collection, particularly in pixelated detector designs and thick sensors (>1 mm). In this paper, the presence of charge sharing and the magnitude of charge loss were calculated for a 2 mm thick pixelated HF-CdZnTe detector with 250 μm pixel pitch and 25 μm pixel gaps, bonded to the STFC HEXITEC ASIC. Results are compared with a CdTe detector as a reference point and supported with simulations from a Monte-Carlo detector model. Charge sharing events showed minimal charge loss in the HF-CdZnTe, resulting in a spectral resolution of 1.63 ± 0.08 keV Full Width at Half Maximum (FWHM) for bipixel charge sharing events at 59.5 keV. Depth of interaction effects were shown to influence charge loss in shared events. The performance is discussed in relation to the improved hole transport of HF-CdZnTe and comparison with simulated results provided evidence of a uniform electric field.

CdZnTe detector technology has been developed at NASA Goddard for imaging and spectroscopy applications in hard x-ray and gamma ray astronomy. A CdZnTe strip detector array with capabilities for arc second imaging and spectroscopy has been built as a prototype for a space flight gamma ray burst instrument. CdZnTe detectors also have applications for medical imaging, environmental protection, transportation safety, nuclear safeguards and safety, nuclear non-proliferation, and national security. This can be accomplished from space and also from portable detectors on earth. One of the great advantages of CdZnTe is that the detectors can be operated at room temperature which eliminates the need for cryogenic cooling. CdZnTe detectors have good energy resolution (3.6 keV at 60 keV) and excellent spatial resolution (<100 microns). NASA Goddard has developed the fabrication technology to make a variery of planar, strip, and pixel detectors and integrated these detectors to high density electronics. We have built a 2×2 and a large area (60 cm2, 36 detectors) 6×6 strip detector array. This paper will summarize the CdZnTe detector fabrication and packaging technology developed at Goddard.

PurposeThe implementation of photon‐counting detectors is widely expected to be the next breakthrough in X‐ray computed tomography (CT) instrumentation. A small number of prototype scanners equipped with direct‐conversion detectors based on room‐temperature semiconductors, such as CdTe and CdZnTe (CZT), are currently installed at medical centers. Here, we investigate the feasibility of using silicon photomultiplier (SiPM)‐based scintillation detectors in photon‐counting computed tomography (PCCT) scanners, as a potential alternative to CdTe and CZT detectors.MethodsWe introduce a model that allows us to compute the expected energy resolution as well as the expected pulse shape and associated rate capability of SiPM‐based PCCT detectors. The model takes into account SiPM saturation and optical crosstalk, because these phenomena may substantially affect the performance of SiPM‐based PCCT detectors with sub‐mm pixels. We present model validation experiments using a single‐pixel detector consisting of a 0.9 × 0.9 × 1.0 mm3 LuAP:Ce scintillation crystal coupled to a 1 × 1 mm2 SiPM. We subsequently use the validated model to compute the expected performance of the fast scintillators LYSO:Ce, LuAP:Ce, and LaBr3:Ce, coupled to currently available SiPMs, as well as to a more advanced SiPM prototype with improved dynamic range, for sub‐mm pixel sizes.ResultsThe model was found to be in good agreement with the validation experiments, both with respect to energy resolution and pulse shape. It shows how saturation progressively degrades the energy resolution of detectors equipped with currently available SiPMs as the pixel size decreases. Moreover, the expected pulse duration is relatively long (~200 ns) with these SiPMs. However, when LuAP:Ce and LaBr3:Ce are coupled to the more advanced SiPM prototype, the pulse duration improves to less than 60 ns, which is in the same order of magnitude as pulses from CdTe and CZT detectors. It follows that sufficient rate capability can be achieved with pixel sizes of 400 μm or smaller. Moreover, LaBr3:Ce detectors can provide an energy resolution of 11.5%‐13.5% at 60 keV, comparable to CdTe and CZT detectors.ConclusionsThis work provides first evidence that it may be feasible to develop SiPM‐based scintillation detectors for PCCT that can compete with CdTe and CZT detectors in terms of energy resolution and rate capability.

Interventional radiology has been beneficial for patients for over 30 years of age with the combination of diagnostic and therapeutic methods. The radiation affecting occupationally exposed workers should be evaluated by means of the energy spectra and flux of X-rays in the treatment room. The present study aims to obtain the energy spectra of interventional procedures and study the capability of some detectors to evaluate the dose in interventional procedures. These measurements were taken by silicon-drift, CdTe, and CdZnTe detectors. The energy spectra were corrected by the energy-response curve of each detector. The energy-response curves of silicon-drift and CdTe detectors provided by the manufacturers specification were used. The energy response of the CdZnTe detector was measured by $$^{133}\hbox {Ba}$$ and $$^{152}\hbox {Eu}$$ $$\gamma$$ sources. The experimental data were compared with the simulation results, and their perfect agreement provides a way to correct the energy or dose response, which can be used for the personal dosimeter developed by our group. Moreover, the measured energy spectra can be used in individual radiation protection. The present study shows that the CdZnTe detector is a good candidate detector in interventional procedures.

The Voxtel VX-798 is a prototype X-ray pixel array detector (PAD) featuring a silicon sensor photodiode array of 48 × 48 pixels, each 130 µm × 130 µm × 520 µm thick, coupled to a CMOS readout application specific integrated circuit (ASIC). The first synchrotron X-ray characterization of this detector is presented, and its ability to selectively count individual X-rays within two independent arrival time windows, a programmable energy range, and localized to a single pixel is demonstrated. During our first trial run at Argonne National Laboratory's Advance Photon Source, the detector achieved a 60 ns gating time and 700 eV full width at half-maximum energy resolution in agreement with design parameters. Each pixel of the PAD holds two independent digital counters, and the discriminator for X-ray energy features both an upper and lower threshold to window the energy of interest discarding unwanted background. This smart-pixel technology allows energy and time resolution to be set and optimized in software. It is found that the detector linearity follows an isolated dead-time model, implying that megahertz count rates should be possible in each pixel. Measurement of the line and point spread functions showed negligible spatial blurring. When combined with the timing structure of the synchrotron storage ring, it is demonstrated that the area detector can perform both picosecond time-resolved X-ray diffraction and fluorescence spectroscopy measurements.

The results of the application of optical methods are considered: spectral analysis of photoconductivity in the range 400–1800 nm and field measurements of stationar photoconductivity to study the bulk and surface characteristics of CdTe and CdZnTe gamma-ray detectors. The studies of the photoresponse of the samples were carried out in different irradiation modes: modulated at a frequency of 25 Hz and a constant light flux. The measurements were carried out at room temperature on domestic and foreign materials of detector quality. Using the spectral photoconductivity method, the parameters of deep levels, which affect the detector properties, have been estimated in the studied materials. Based on the calculations of the field dependences of the photoconductivity, the volume and surface parameters of the capture and recombination of non-equilibrium carriers in the investigated detectors were determined. The results of calculating the transport characteristics of charge transfer by electrons μτe in detectors are compared with similar parameters measured using the traditional method of irradiating the detector with alpha particles. To measure the band gap in CdTe and CdZnTe at room temperature the method was modified for calculating ΔEg. The possibility of monitoring the Zn content in CdZnTe detectors using the spectral photoconductivity method is shown.

We describe a stacked detector made of thin cadmium telluride (CdTe) diode detectors. By using a thin CdTe device, we can overcome the charge loss problem due to the small mobility and short lifetime of holes in CdTe or cadmium zinc telluride (CdZnTe) detectors. However, a CdTe detector with a thickness of more than 5 mm is needed for adequate detection efficiency for gamma-rays of several hundred keV. Good energy resolution and good peak detection efficiency are difficult to obtain using such a thick CdTe detector. The stacked detector enabled us to realize a detector with both high-energy resolution and good efficiencies for gamma rays up to several hundred keV. In order to verify this concept, we constructed a prototype made of ten layers of a 0.5-mm-thick CdTe diode detectors with a surface area of 21.5 mm/spl times/21.5 mm. With this, we have achieved 5.3-keV and 7.9-keV energy resolution [full width at half maximum (FWHM)] at 356 keV and 662 keV, respectively, at the temperature of -20/spl deg/C.

This report documents the results and findings of a one-year scoping study investigating multichannel readout application specific integrated circuits (ASICs) for interfacing to, and processing data from, silicon photomultiplier (SiPM) arrays. We document ASIC desired and required specifications for four applications supporting national security mission areas: neutron radiography, associated particle imaging, and two versions of kinematic neutron imaging cameras. While each application has a few unique requirements that stress capability, there is generally good agreement among most. Two recently developed ASIC devices were evaluated in a system-like configuration by interfacing these to scintillator crystals exposed to gamma and neutron sources. The 64-channel ORNL device delivered functional capability while meeting most mission requirements for neutron radiography. The Nalu Scientific device, a 32-channel full waveform digitizer, did not demonstrate reliable neutron / gamma separation but it is unclear if this was an ASIC issue or problems with test setup or firmware. A literature survey of other commercial and academic ASICs was undertaken to with the conclusion that existing devices do not meet all requirements.

Spectroscopy with pixelated CdZnTe gamma detectors – experiment versus theory

Over the past several years, digital mammography systems have been installed clinically across North America in small but growing numbers. A photostimulable phosphor-based full-field digital mammography image was evaluated in this investigation. Commonly known as computed radiography (CR), its use closely mimics the screen-film mammography paradigm. System performance using modulation transfer function (MTF) and detective quantum efficiency (DQE) metrics show MTF(2.5 mm(-1)) = 0.5, DQE(2.5 mm(-1)) = 0.3, and MTF(5.0 mm(-1)) = 0.2, DQE(5.0 mm(-1)) = 0.05, for a 26 kVp beam, 0.03 mm molybdenum tube filtration, 4.5 cm tissue attenuation, and 15 mR incident exposure to the detector. Slightly higher DQE values were measured at 32 kVp with 0.025 mm rhodium tube filtration. CR mammography advantages include the ability to use existing mammography machines, where multiple rooms can be converted to "digital" operation, which allows overall cost savings compared to integrated digital mammography systems. Chief disadvantages include the labor-intensive handling of the cassettes prior to and after the imaging exam, lack of a direct interface to the x-ray system for recording technique parameters, and relatively slow processing time. Clinical experience in an IRB-approved research trial has suggested that digital mammography with photostimulable storage phosphors and a dedicated CR reader is a viable alternative to conventional screen-film mammography.

Pixel radiation detectors for X-rays and gammarays are commonly built by hybridization of sensor and readout application-specific integrated circuit (ASIC) chips using flip-chip bonding. A typical example is pixelated high-Z semiconductor sensor material (e.g. CdTe) bonded to a matching pixel array on an ASIC to form a hybrid module. Conventionally, the I/O connections of the ASIC for such hybrids are wire bonded to a Printed Circuit Board (PCB) which subsequently is connected to a data acquisition system. The wire bond pads and PCB connections require a considerable amount of the module's footprint and usually protrude at one side of the detector module by approximately 1.5mm. The disadvantage of this practice is that those modules are only suitable for close proximity tiling modules along three sides of the detector (3-side buttable) to form larger sensor areas. In recent years Through-Silicon Via (TSV) technology was introduced to overcome this disadvantage which permits the fabrication of potentially 4-side buttable modules. Commonly a TSV-last process step in the ASIC production is integrated. However, ASICs with TSV are expensive and the TSV-last process step is prone to failure of the TSV metallization. The consequences of the latter are missing I/O connections. Alternatively, 4-side buttable hybrid detector modules can be fabricated using interposers as pitch convertors. Here the sensor has a larger pixel-pitch than the ASIC and therefore the sensor covers a larger area than the ASIC. A Re-Distribution-Layer (RDL) on the interposer facilitates connections to the I/O of the ASIC. Therefore, ASIC and all other wire bonded connections can be placed under the sensor with all four sides of the sensor being the boundary of the full-sized module. This was demonstrated in the past with a conversion from 250um-pitch on the ASIC to a 500um-pitch on the sensor using a layered PCB. In order to achieve detectors with smaller sensor pixel pitch, the layout of pitch re-routing on the interposer has to be considerably reduced in size. As a consequence, the fabrication of such interposers becomes more difficult. This paper studies desirable designs and interposer materials and compares these with the limitations of current fine-pitch interposer fabrication methods and design rules for a pitch convertor interposer. In addition, the paper will also present necessary changes such as indium deposition on individual sensor die instead of wafer-level fabrication that have to be taken into consideration for sensor/ASIC hybridization of fine-pitch radiation detectors.

The X-ray Astronomy and Observational Cosmology (XOC) group at Stanford University, in collaboration with the Massachusetts Institute of Technology (MIT) and MIT Lincoln Laboratory (MIT-LL), is developing next generation X-ray detector and readout technologies. Specifically, the XOC group is developing a fast, low noise readout application specific integrated circuit (ASIC) that is intended to be paired with X-ray charge-coupled device (CCD) detectors in development at MIT-LL. This readout ASIC is denoted as the MIT CCD Readout Chip or MCRC. The MCRC prototype is designed in a 350 nm technology node featuring 8 channels. Each channel is composed of two inputs. One is dedicated to read out the proven source follower-based (SF) CCD architecture with two selectable gain settings, an input referred noise of 1.63 e⁻_RMS, an input dynamic range of ±160 mV, channel to channel crosstalk less than -75 dBc, a power consumption of roughly 30 mW/channel, and a bandwidth of approximately 50 MHz, translating to an effective rise time of around 5 ns. Such a response can comfortably support readout speeds for large CCD pixel matrices in excess of 5 Mpixel/s/channel. The chip also features a second input which is an experimental drain-current readout (DR) topology to support Single electron Sensitive Read Out (SiSeRO) CCD stages currently in development at MIT-LL. The MCRC excels in speed while its input noise and bandwidth are on par with commercial discrete offerings, but with smaller power and real-estate footprints. Here we present the latest measurement results for this state-of-the-art prototype readout ASIC.