Single-side Readout Research Articles

Measurement of angular correlations of Compton-scattered gamma quanta from positron annihilation using GAGG:Ce scintillator matrices with single-side readout

We measured azimuthal correlations of the Compton-scattered gamma quanta from positron annihilation with two 8 × 8 matrices of Gadolinium Aluminum Gallium Garnet doped with Cerium. Each detector matrix contains 64 crystals of size 3 mm × 3 mm × 20 mm and it is read-out by a single Silicon Photo-multiplier (SiPM) array, with one SiPM matching one pixel. The studied single-side readout concept keeps the modules compact and cost-efficient on a large scale. Coincidence events were recorded using a Na-22 source placed between the modules. We clearly identify and reconstruct the Compton scattering events and we observe the excess of orthogonally scattered gammas over the ones with the parallel azimuthal angles. We present the measured azimuthal modulation factors for several kinematic selection criteria and different inter-pixel distances.

A layered single-side readout depth of interaction time-of-flight-PET detector

We are exploring a scintillator-based PET detector with potential of high sensitivity, depth of interaction (DOI) capability, and timing resolution, with single-side readout. Our design combines two previous concepts: (1) multiple scintillator arrays stacked with relative offset, yielding inherent DOI information, but good timing performance has not been demonstrated with conventional light sharing readout. (2) Single crystal array with one-to-one coupling to the photodetector, showing superior timing performance compared to its light sharing counterparts, but lacks DOI. The combination, where the first layer of a staggered design is coupled one-to-one to a photodetector array, may provide both DOI and timing resolution and this concept is here evaluated through light transport simulations. Results show that: (1) unpolished crystal pixels in the staggered configuration yield better performance across all metrics compared to polished pixels, regardless of readout scheme. (2) One-to-one readout of the first layer allows for accurate DOI extraction using a single threshold. The number of multi pixel photon counter (MPPC) pixels with signal amplitudes exceeding the threshold corresponds to the interaction layer. This approach was not possible with conventional light sharing readout. (3) With a threshold of 2 optical photons, the layered approach with one-to-one coupled first layer improves timing close to the MPPC compared to the conventional one-to-one coupling non-DOI detector, due to effectively reduced crystal thickness. Single detector timing resolution values of 91, 127, 151 and 164 ps were observed per layer in the 4-layer design, to be compared to 148 ps for the single array with one-to-one coupling. (4) For the layered design with light sharing readout, timing improves with increased MPPC pixel size due to higher signal per channel. In conclusion, the combination of straightforward DOI determination, good timing performance, and relatively simple design makes the proposed concept promising for DOI-Time-of-Flight PET detectors.

Improving depth-of-interaction resolution in pixellated PET detectors using neural networks

Parallax error is a common issue in high-resolution preclinical positron emission tomography (PET) scanners as well as in clinical scanners that have a long axial field of view (FOV), which increases estimation uncertainty of the annihilation position and therefore degrades the spatial resolution. A way to address this issue is depth-of-interaction (DOI) estimation. In this work we propose two machine learning-based algorithms, a dense and a convolutional neural network (NN), as well as a multiple linear regression (MLR)-based method to estimate DOI in depolished PET detector arrays with single-sided readout. The algorithms were tested on an 8× 8 array of 1.53× 1.53× 15 mm3 crystals and a 4× 4 array of 3.1× 3.1× 15 mm3 crystals, both made of Ce:LYSO scintillators and coupled to a 4× 4 array of 3× 3 mm3 silicon photomultipliers (SiPMs). Using the conventional linear DOI estimation method resulted in an average DOI resolution of 3.76 mm and 3.51 mm FWHM for the 8× 8 and the 4× 4 arrays, respectively. Application of MLR outperformed the conventional method with average DOI resolutions of 3.25 mm and 3.33 mm FWHM, respectively. Using the machine learning approaches further improved the DOI resolution, to an average DOI resolution of 2.99 mm and 3.14 mm FWHM, respectively, and additionally improved the uniformity of the DOI resolution in both arrays. Lastly, preliminary results obtained by using only a section of the crystal array for training showed that the NN-based methods could be used to reduce the number of calibration steps required for each detector array.

A new neutron detector based on ceramic THGEM and boron-coated meshes

Boron-coated micro-pattern gaseous detectors, such as gaseous electron multiplier (GEM), are widely studied as the neutron detectors. The existing solution to improve the neutron detection efficiency (NDE) is to coat the boron on the surfaces of the cascade GEM films. So the GEM films themselves will be affected by the complicated coating process and the boron layers, and the $$^{10}$$ B material will be wasted once the GEM films are damaged. Since the neutron conversion and the electron multiplication regions are not separated, the final gains of different layer ionized electrons are very difficult to control and keep consistent. A new neutron detector based on the ceramic thick GEM (THGEM) was proposed. The boron-coated cathode and multilayer meshes were used as the neutron conversion region and separated from the electron multiplication region consisting of only one THGEM film. The sawtooth surface treatment and the double-sided readout method were applied to improve neutron conversion efficiency (NCE). The neutron conversion and the electron collection efficiencies were simulated and studied based on the new structure. The prototype detector was test by using a $$^{252}$$ Cf neutron source. The simulation results show that the cascade boron-coated meshes have good neutron conversion and electron collection efficiencies, and the double-sided readout and sawtooth structure can greatly increase the NCE. The neutron test results show that the detection efficiency of the double-sided readout is 1.6 times higher than that of the common single-sided readout. These indicate that the new detector has high neutron conversion and detection efficiencies and is promising for practical applications.

Light Spread Manipulation in Scintillators Using Laser Induced Optical Barriers.

We are using the Laser Induced Optical Barriers (LIOB) technique to fabricate scintillator detectors with combined performance characteristics of the two standard detector types, mechanically pixelated arrays and monolithic crystals. This is done by incorporation of so-called optical barriers that have a refractive index lower than that of the crystal bulk. Such barriers can redirect the scintillation light and allow for control of the light spread in the detector. Previous work has shown that the LIOB technique has the potential to achieve detectors with high transversal and depth of interaction (DOI) resolution simultaneously in a single-side readout configuration, suitable for high resolution PET imaging. However, all designs studied thus far present edge effect issues similarly as in the standard detector categories. In this work we take advantage of the inherent flexibility of the LIOB technique and investigate alternative barrier patterns with the aim to address this problem. Light transport simulations of barrier patterns in LYSO:Ce, with deeper barrier walls moving towards the detector edge show great promise in reducing the edge effect, however there is a trade-off in terms of achievable DOI information. Furthermore, fabrication and characterization of a 20 mm thick LYSO:Ce detector with optical barriers forming a pattern of 1 × 1 × 20mm3 pixel like structures show that light channeling in laser-processed detectors in agreement with optical barriers with refractive index between 1.2 and 1.4 is achievable.

Simulation study of light transport in laser-processed LYSO:Ce detectors with single-side readout

A tightly focused pulsed laser beam can locally modify the crystal structure inside the bulk of a scintillator. The result is incorporation of so-called optical barriers with a refractive index different from that of the crystal bulk, that can be used to redirect the scintillation light and control the light spread in the detector. We here systematically study the scintillation light transport in detectors fabricated using the laser induced optical barrier technique, and objectively compare their potential performance characteristics with those of the two mainstream detector types: monolithic and mechanically pixelated arrays. Among countless optical barrier patterns, we explore barriers arranged in a pixel-like pattern extending all-the-way or half-way through a 20 mm thick LYSO:Ce crystal. We analyze the performance of the detectors coupled to MPPC arrays, in terms of light response functions, flood maps, line profiles, and light collection efficiency. Our results show that laser-processed detectors with both barrier patterns constitute a new detector category with a behavior between that of the two standard detector types. Results show that when the barrier-crystal interface is smooth, no DOI information can be obtained regardless of barrier refractive index (RI). However, with a rough barrier-crystal interface we can extract multiple levels of DOI. Lower barrier RI results in larger light confinement, leading to better transverse resolution. Furthermore we see that the laser-processed crystals have the potential to increase the light collection efficiency, which could lead to improved energy resolution and potentially better timing resolution due to higher signals. For a laser-processed detector with smooth barrier-crystal interfaces the light collection efficiency is simulated to >42%, and for rough interfaces >73%. The corresponding numbers for a monolithic crystal is 39% with polished surfaces, and 71% with rough surfaces, and for a mechanically pixelated array 35% with polished pixel surfaces and 59% with rough surfaces.

Development of a high resolution module for PET scanners

Positron Emission Tomography (PET) scanners require high performances in term of spatial resolution and sensitivity to allow early detection of cancer masses. In small animal and organ dedicated PET scanners the Depth of Interaction (DOI) information has to be obtained to avoid parallax errors and to reconstruct high resolution images. In the whole body PET, the DOI information can be useful to correct for the time jitter of the optical photons along the main axis of the scintillator, improving the time performances. In this work we present the development of PET module designed to reach high performance as compared to the current scanners while keeping the complexity of the system reasonably low. The module presented is based on a 64 LYSO (Lutetium-yttrium oxyorthosilicate) crystals matrix and on a 4×4 MPPC (Multi Pixels Photon Counter) array as detector in a 4 to 1 coupling between the crystals and the detector and a single side readout. The lateral surfaces of the crystals are optically treated to be unpolished. The DOI and the energy resolution of the PET module are presented and a fast method to obtain the DOI calibration is discussed.

Development and evaluation of a practical method to measure the Depth of Interaction function for a single side readout PET detector

In small animal and organ dedicated PET scanners, the knowledge of depth of interaction (DOI) of the gamma ray along the main axis of the scintillator is a fundamental information in order to avoid parallax error and to achieve high performances in terms of spatial resolution. Recently we developed a new method to obtain the DOI function for a single side readout PET module, recirculating the scintillation light in the matrix by means of a mirror placed on top of the module. In a complete PET scanner, periodical DOI calibrations have to be performed to prevent time dependent miscalibrations and performance degradations. The current DOI calibration relies on a coincidence system between the module and an external scintillator to provide a priori the DOI information and it is clearly not feasible in a real system without unpractical disassemblies of the scanner. In this paper we develop instead a fast and precise calibration method based on uniform irradiation of the scintillators. Three irradiation modalities are presented, in particular one where the source is placed on top of the module, one with the source placed on one side of the module and one that exploits the internal radioactivity of the scintillator. The three different procedures are evaluated and the calibration method is validated by comparing the information provided by the coincidence setup.

A new method for depth of interaction determination in PET detectors

A new method for obtaining depth of interaction (DOI) information in PET detectors is presented in this study, based on sharing and redirection of scintillation light among multiple detectors, together with attenuation of light over the length of the crystals. The aim is to obtain continuous DOI encoding with single side readout, and at the same time without the need for one-to-one coupling between scintillators and detectors, allowing the development of a PET scanner with good spatial, energy and timing resolutions while keeping the complexity of the system low. A prototype module has been produced and characterized to test the proposed method, coupling a LYSO scintillator matrix to a commercial SiPMs array. Excellent crystal separation is obtained for all the scintillators in the array, light loss due to depolishing is found to be negligible, energy resolution is shown to be on average 12.7% FWHM. The mean DOI resolution achieved is 4.1 mm FWHM on a 15 mm long crystal and preliminary coincidence time resolution was estimated in 353 ps FWHM.

Development of high-resolution detector module with depth of interaction identification for positron emission tomography

We have developed a Time-of-flight high resolution and commercially viable detector module for the application in small PET scanners. A new approach to depth of interaction (DOI) encoding with low complexity for a pixelated crystal array using a single side readout and 4-to-1 coupling between scintillators and photodetectors was investigated. In this method the DOI information is estimated using the light sharing technique. The detector module is a 1.53×1.53×15mm3 matrix of 8×8 LYSO scintillator with lateral surfaces optically depolished separated by reflective foils. The crystal array is optically coupled to 4×4 silicon photomultipliers (SiPM) array and readout by a high performance front-end ASIC with TDC capability (50ps time binning). The results show an excellent crystal identification for all the scintillators in the matrix, a timing resolution of 530ps, an average DOI resolution of 5.17mm FWHM and an average energy resolution of 18.29% FWHM.

Single Side Readout Depth of Interaction Method With Wavelength Discrimination

Depth of interaction (DOI) information is important in the development of high-resolution detectors as it helps to reduce parallax error in positron emission tomography (PET) systems. Further, the determination of depth in the crystal is important for making scintillation crystal-based detectors in many other applications, such as the Compton imager and multi-radiation imagers. In this paper, a novel DOI method based on discrimination of the wavelength of the scintillation light emitted from a single side readout system is proposed. The single side readout facilitates flexibility in the final system in a geometrical configuration. The proposed method can be applied to stacked detectors with emissions of various wavelengths in crystals (e.g., LYSO and GAGG) and long monolithic detectors utilizing the wavelength dependence of internal light absorption. Simulations performed of the proposed discrimination principle via Monte Carlo simulation based on SLITRANI and experiments conducted using two silicon photomultipliers (SiPMs) with wavelength filters verify the feasibility of the proposed method.

Improving the Time Resolution of TOF-PET Detectors by Double-Sided Readout

State-of-the-art scintillation detectors for time-of-flight positron emission tomography (TOF-PET) typically employ scintillation crystals with a high aspect ratio (e.g. 4 ×4 ×22 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ) read out on one of the small crystal surfaces. This single-sided readout (SSR) geometry may be unfavorable in terms of the coincidence resolving time (CRT) that can be achieved because of its effect on the light collection efficiency; the spread in the scintillation photon propagation times; and depth-of-interaction (DOI) related, variable detection delays. In this work it is investigated to which extent these effects can be mitigated by applying a fast photosensor on each of the small crystal surfaces. Such double-sided readout (DSR) has been introduced previously to counter the issue of DOI-related parallax errors. For the present purpose, we used Hamamatsu MPPC-S10362-33-050 C silicon photomultipliers (SiPMs) optically coupled to LSO:Ce,Ca scintillators. For polished 3 ×3 ×20 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> crystals with SSR, CRT values of 184 ±6 ps FWHM and 215 ±6 ps FWHM were determined for irradiation head-on and from the side, respectively. In this case, DSR improved the CRT measured under side irradiation to 174 ±6 ps FWHM. A much more substantial improvement was observed for equally sized crystals having etched side surfaces. For these crystals the CRT changed from 358 ±5 ps FWHM (head-on irradiation) and 343 ±6 FWHM (side irradiation) with SSR to 180 ±5 ps FWHM with DSR (side irradiation). These values are compared with the time resolution of detectors employing LSO:Ce,Ca crystals with a size of 3 ×3 ×5 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ( CRT = 121 ±2 ps FWHM) and 3 ×3 ×10 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ( CRT = 162 ±9 ps FWHM and CRT = 183 ±3 ps for polished and etched crystals, respectively).

Clinical evaluation of a dual-side readout technique computed radiography system in chest radiography of premature neonates

Recently, the dual-side readout technique has been introduced in computed radiography, leading to an increase in detective quantum efficiency (DQE) compared with the single-side readout technique. To evaluate if the increase in DQE with the dual-side readout technique results in a higher clinical image quality in chest radiography of premature neonates at no increase in radiation dose. Twenty-four chest radiographs of premature neonates were collected from both a single-side readout technique system and a double-side readout technique system. The images were processed in the same image-processing station in order for the comparison to be only dependent on the difference in readout technique. Five radiologists rated the fulfillment of four image quality criteria, which were based on important anatomical landmarks. The given ratings were analyzed using visual grading characteristics (VGC) analysis. The VGC analysis showed that the reproduction of the carina with the main bronchi and the thoracic vertebrae behind the heart was better with the dual-side readout technique, whereas no significant difference for the reproduction of the central vessels or the peripheral vessels could be observed. The results indicate that the higher DQE of the dual-side readout technique leads to higher clinical image quality in chest radiography of premature neonates at no increase in radiation dose.

A 7 k-pixel×3 line color linear sensor with single-side readout method

A 7 k-pixel/spl times/3 line color linear sensor has been developed for color image scanners. The sensor pitch, 8 /spl mu/m, is the smallest size among devices with a single-sided readout method. A wide blooming margin and dynamic range is achieved. A gain-select function and a shutter function is adopted. This sensor reads A4-size documents at a density of 800 DPI.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Capacitively coupled Si strip detectors on a 100 mm wafer

Silicon strip detectors with single-sided readout were designed and processed on 100 mm silicon wafers. Detectors with integrated coupling capacitors and polysilicon bias resistors were tested by electrical measurements. A 200 nm gate oxide thickness was chosen to provide a coupling capacitance of 9.75 pF/cm. A full depletion voltage of 80 V and leakage currents of 0.29, 0.64, 0.97, and 1.01 nA/strip for strip lengths of 2, 5, 7, and 8 cm, respectively were measured.