LSO Scintillator Research Articles

The SiPM — A new Photon Detector for PET

We have studied a novel semiconductor photon detector (silicon photomultiplier) as a readout for LSO scintillator crystals used in PET detectors. Silicon photomultipliers (SiPMs) of a ( 1 × 1 ) mm 2 area were coupled to LSO crystals of ( 2 × 2 × 10 ) mm 3 . Two elements were exposed to a 22Na positron source emitting two 511 keV gamma quanta. Coincidence studies yielded a FWHM of the photopeak spectrum around 35% and a time resolution of 3 ns. Extrapolating from the large area mismatch in the readout area allows us to conclude that the novel SiPM is a very promising element for PET applications.

Preliminary study for the development of a tweezers-type coincidence detector for tumor detection

We have conducted a preliminary study for development of a tweezer-type coincidence detector for tumor detection in procedures such as FDG-guided surgery. The detector consists of a pair of LSO scintillators, optical fibers, a pair of photo-multiplier tubes (PMTs), and a coincidence circuit. Because the LSO scintillators are located on the tips of tweezers, a target organ such as a lymph node or the colon can be positioned between them. The size of a single LSO was 3.7 mm×3.7 mm×10 mm, and the scintillation photons are transferred to the PMTs via 2-mm-diameter, 1-m long optical fibers. The results show that the light loss due to the fiber was significant but there was sufficient light to observe the photo-peak of the 511-keV gamma photons. Sensitivity response function perpendicular to the detector has a full-width at half-maximum (FWHM) of 2.5 mm, while that parallel to the detector has a FWHM of 5.5 mm. Background counts due to the natural radioisotope in 176Lu can be observed when the distance between these two scintillators is small. Results also show that the absolute sensitivity was 0.057% at the center of the detector when the two LSOs were 10 mm apart and that the optical fiber was insensitive to bending up to a radius of 10 cm. From these results, we conclude that the proposed tweezers-type coincidence detector could be some interest for tumor detection using FDG, such as that in radio-guided surgery.

CT acquisition using PET detectors and electronics

The emergence of positron emission tomography/computerized tomography (PET/CT) multimodality imaging has provided the ability to sequentially obtain anatomic and functional information using adjacent PET and CT scanners without having to move the patient from the bed. To avoid the need for successive PET and CT scans, we have investigated the possibility of acquiring both the anatomic and functional images using the same detection system, based on PET detectors and electronics operated in photon-counting mode. The detector consisted of a high-luminosity LSO scintillator individually coupled to an avalanche photodiode (APD) to enable low-energy X-ray detection at a high-count rate. A simulator was set up to collect tomographic data using a monochromatic 60 keV source (/sup 241/Am) to irradiate a phantom made of tissue-equivalent materials. The observed spatial resolution with this nonoptimized setup was better than 2 mm, demonstrating the capability to provide fairly accurate anatomical localization in CT counting mode. The three main constituents of biological tissues (bones, water, and air) could be clearly identified in the images with a dose significantly lower than with conventional CT operated in current mode. These preliminary results demonstrate the feasibility of dual-modality PET/CT imaging based on PET detectors and electronics, and suggest that substantial dose reduction would be possible by acquiring the CT image in photon-counting mode.

The ECAT HRRT: an example of NEMA scatter estimation issues for LSO-based PET systems

The ECAT high-resolution research tomograph (HRRT) is a three-dimensional (3-D)-only dedicated brain positron emission tomograph with LSO and GSO scintillators. In this paper, the system has been looked at as an example of issues that need to be addressed when evaluating LSO-based system following the recent NEMA NU 2-2001 protocols. The LSO scintillators contain the isotope /sup 176/Lu which is radioactive and creates a small amount of single counts and, depending on the low level discriminator threshold for validated singles, also true coincidence events from a cascade of gamma rays following the beta decay of /sup 176/Lu into /sup 176/Hf. The presence of intrinsic random and true coincidence events has an effect on the scatter fraction determination if using the guidelines according to NU 2-2001. We show here that these guidelines have to be changed in order to obtain accurate determination of the scatter fraction. In this paper, we have determined the scatter fraction for different low level discriminator settings. Since the scatter fraction determinations also comprise full count rate studies for the NEMA phantom, we have in addition extracted the sensitivity information for singles, true + scatter, and for the NEC data.

Radiation sensitivity of GSO and LSO scintillation detectors

Radiation resistance of 4×4×30 mm 3 GSO and LSO imaging scintillation detectors has been studied for low-energy gamma-ray doses of 10 4 Gy (10 6 rad) and 10 5 Gy (10 7 rad). Radiation hardness was determined by the measurement of optical transmission through GSO and LSO scintillation crystals before and after irradiations with 60Co gamma-rays. The results have been analysed in terms of the radiation-induced absorption coefficients and compared with radiation sensitivity measurements of small BGO scintillation crystals. The recovery time of irradiated small GSO and LSO crystals has also been determined.

Performance measurements of a depth-encoding PET detector module based on position-sensitive avalanche photodiode read-out

We are developing a high-resolution, high-efficiency positron emission tomography (PET) detector module with depth of interaction (DOI) capability based on a lutetium oxyorthosilicate (LSO) scintillator array coupled at both ends to position-sensitive avalanche photodiodes (PSAPDs). In this paper we present the DOI resolution, energy resolution and timing resolution results for complete detector modules. The detector module consists of a 7 × 7 matrix of LSO scintillator crystals (1 × 1 × 20 mm3 in dimension) coupled to 8 × 8 mm2 PSAPDs at both ends. Flood histograms were acquired and used to generate crystal look-up tables. The DOI resolution was measured for individual crystals within the array by using the ratio of the signal amplitudes from the two PSAPDs on an event-by-event basis. A measure of the total scintillation light produced was obtained by summing the signal amplitudes from the two PSAPDs. This summed signal was used to measure the energy resolution. The DOI resolution was measured to be 3–4 mm FWHM irrespective of the position of the crystal within the array, or the interaction location along the length of the crystal. The total light signal and energy resolution was almost independent of the depth of interaction. The measured energy resolution averaged 14% FWHM. The coincidence timing resolution measured using a pair of identical detector modules was 4.5 ns FWHM. These results are consistent with the design goals and the performance required of a compact, high-resolution and high-efficiency PET detector module for small animal and breast imaging applications.

Lutetium oxyorthosilicate block detector readout by avalanche photodiode arrays for high resolution animal PET

Avalanche photodiodes (APDs) have proven to be useful as light detectors for high resolution positron emission tomography (PET). Their compactness makes these devices excellent candidates for replacing bulky photomultiplier tubes (PMTs) in PET systems where space limitations are an issue. The readout of densely packed, 10 × 10 lutetium oxyorthosilicate (LSO) block detectors (crystal size 2.0 × 2.0 × 12 mm3) with custom-built monolithic 3 × 3 APD arrays was investigated. The APDs had a 5 × 5 mm2 active surface and were arranged on a 6.25 mm pitch. The dead space on the edges of the array was 1.25 mm. The APDs were operated at a bias voltage of approximately 380 V for a gain of 100 and a dark current of 10 nA per APD. The standard deviation in gain between the APDs in the array ranged from 1.8 to 6.5% as the gain was varied from 50 to 108. A fast, low-noise, multi-channel charge sensitive preamplifier application-specific integrated circuit (ASIC) was developed for the APD readout. The amplifier had a rise time of 8 ns, a noise floor of 515 e− rms and a 9 e− pF−1 noise slope. An acquired flood image showed that all 100 crystals from the block detector could be resolved. Timing measurements with single-channel LSO-APD detectors, as well as with the array, against a plastic scintillator and PMT assembly showed a time resolution of 1.2 ns and 2.5 ns, respectively. The energy resolution measured with a single 4.0 × 4.0 × 10 mm3 LSO crystal, wrapped in four-layer polytetrafluoroethylene (PTFE) tape and coupled with optical grease on a single APD of the array, yielded 15% (full width at half maximum, FWHM) at 511 keV. Stability tests over 9 months of operation showed that the APD arrays do not degrade appreciably. These results demonstrate the ability to decode densely packed LSO scintillation blocks with compact APD arrays. The good timing and energy resolution makes these detectors suitable for high resolution PET.

A small prototype of LSO-SDD anger camera

In this paper, we present a small prototype of Anger camera based on a monolithic array of silicon drift detectors (SDDs) coupled to a single LSO scintillator. The SDD photodetector consists of an array of 19 hexagonal units with on-chip JFET and has about 1 cm/sup 2/ of total sensitive area. The SDD array was already successfully tested with a CsI(Tl) crystal. However, the long scintillation decay time of CsI(Tl) requires the use of a long shaping time to reduce the effect of the ballistic deficit. This requirement does not allow to fully exploit the low electronics noise of the SDD, which, on the contrary, reaches its minimum noise at short shaping times. Moreover, the use of a faster scintillator would improve the overall counting rate capability of the Anger Camera. On the other side, the LSO scintillator is characterized by a lower scintillation conversion efficiency with respect to CsI(Tl) and by a natural background. In this work the performances which could be expected from a LSO-SDD Anger Camera are first evaluated. Then, the results achieved in the experimental characterization of a prototype of this detector are presented. A position resolution better than 1 mm FWHM using a /sup 57/Co source (122 keV) has been measured.

The LSO/APD array as a possible detector for in-beam PET in hadron therapy

We have studied the performance of finger-like LSO:Ce (LSO) crystals coupled one by one to pixels of avalanche photodiode detector (APD) arrays during their operation in coincidence at /sup 12/C ion beams of parameters being typical for tumor irradiations. In a first step of these experiments the parameters of the detectors and the signal processing setup have been characterized off-beam, i.e., by means of /spl beta//sup +/ radioactive sources (/sup 22/Na, /sup 68/Ge). Afterwards, the apparatus was installed at the medical beam line of the heavy ion synchrotron (SIS) of the Gesellschaft fu/spl uml/r Schwerionenforschung (GSI) at Darmstadt, Germany. Here the /spl beta//sup +/ activity produced by nuclear fragmentation reactions of /sup 12/C 200.3 AMeV heavy ion beams with polymerized methyl methacrylate phantoms were measured. Furthermore, a /sup 68/Ge source was included into the in-beam experiment, in order to check the stability of the setup and to compare energy and time resolution before, during, and after the phantom irradiations. Additionally, it could be demonstrated by means of high resolution /spl gamma/-ray spectroscopy that LSO is not activated by the light projectile fragments escaping the patients downbeam during therapeutic irradiations The experimental results indicate that LSO scintillator is a suitable material for in-beam positron emission tomography (PET) and, furthermore, the LSO/APD array is a feasible detector concept for in-beam monitoring of the dose application by means of PET.

Properties of LYSO and recent LSO scintillators for phoswich PET detectors

The luminescence and nuclear spectroscopic properties of the new cerium-doped rare-earth scintillator lutetium-yttrium oxyorthosilicate (Lu/sub 0.6/Y/sub 1.4/Si/sub 0.5/:Ce, LYSO) were investigated and compared to those of both recent and older LSO crystals. UV-excited luminescent spectra outline important similarities between LYSO and LSO scintillators. The two distinct Ce1 and Ce2 luminescence mechanisms previously identified in LSO are also present in LYSO scintillators. The energy and timing resolutions were measured using avalanche photodiode (APD) and photomultiplier tube (PMT) readouts. The dependence of energy resolution on gamma-ray energy was also assessed to unveil the crystal intrinsic resolution parameters. In spite of significant progress in light output and luminescence properties, the energy resolution of these scintillators appears to still suffer from an excess variance in the number of scintillation photons. Pulse-shape discrimination between LYSO and LSO scintillators has been successfully achieved in phoswich assemblies, confirming LYSO, with a sufficient amount of yttrium to modify the decay time, to be a potential candidate for depth-of-interaction determination in multicrystal PET detectors.

Performance characteristics of a new generation of processing circuits for PET applications

An electronics architecture for a PET camera is presented which utilizes a newly developed ASIC, CFD, and TDC to fully utilize the timing advantages of an LSO scintillation crystal. Measured intrinsic timing resolution for a LSO/PMT detector against plastic/PMT detector is 900 ps FWHM using the newly developed ASIC electronics. The energy and pixel location are derived using continuous sampling ADCs with FPGA digital processing algorithms. Sampled PMT signals are rescaled using the CFD trigger to recalibrate the energy and position since the ADCs are asynchronous with the events. The overall linearity of the energy channels is 1.8% for a 6:1 input dynamic range. The electronics are designed to be compatible with large and small area LSO or BGO detectors used in PET imaging.

Measurement of the depth of interaction of an LSO scintillator using a planar process APD

The authors have evaluated the performance of an avalanche photodiodes (APD)/lutetium oxyorthosilicate (LSO) module for use in positron emission tomography systems. They have used a recently developed APD detector in combination with a 3 /spl times/ 3 /spl times/ 30 mm/sup 3/ LSO scintillator to measure the depth of interaction of 511 keV photons from a /sup 22/Na source. The detectors were built using standard planar technology for silicon devices. Photodiodes with 3 mm diameter active area have been produced by deep boron diffusion, followed by shallow boron and phosphor diffusion. Because the structure has not been optimized yet, the authors only obtained a gain of 6 at 1300 V. A simple noise analysis of the detector characteristics indicate that they are still capable of measuring 511 keV photons with sufficient energy resolution for depth of interaction position measurement from a long LSO scintillator.

Pulse shape discrimination of LSO and LuYAP scintillators for depth of interaction detection in PET

A feasible way to gain the depth of interaction information in a positron emission tomography scanner is the use of phoswich detectors. In general, the layer of interaction is identified from the pulse shape of the corresponding scintillator material. In this work, pulses from LSO and LuYAP crystals were investigated in order to find a practical method of distinguishing. It turned out that such a pulse processing could be kept simple because of an additional slow component in the light decay of the LuYAP pulse. At the same time, the short decay time guarantees that the major amount of the light output is still collected within a short pulse recording time.

Hamamatsu S8550 APD arrays for high-resolution scintillator matrices readout

The performance of Hamamatsu S8550 avalanche photodiode (APD) arrays for scintillator matrices readout has been evaluated. The S8550 device is a monolithic 8×4 pixels structure with an active area of 2.56mm2 for each pixel. The device allows stable operation at gains up to 74, with a detection efficiency of about 60% for photons of 420nm wavelength. It is characterized by a low noise equal to 27 electrons equivalent noise charge at room temperature. The energy resolution of 14.6%, for the 511keV peak from a 22Na source has been recorded with a 2×2×10mm3 LSO crystal coupled to one pixel. The number of electron–hole pairs produced by the 511keV photopeak absorbed in LSO is equal to 4830±240e–h/MeV. Coupling LSO scintillator crystals to individual pixels of the APD array a coincidence timing resolution of 3.0±0.2ns FWHM has been measured for the 511keV γ-rays from a 22Na source. Finally, we compared the characteristics and readout performance of the Hamamatsu array with the results measured earlier under the same conditions for the quadrant large area avalanche photodiodes of Advanced Photonics Inc.

The ECAT HRRT: NEMA NEC evaluation of the HRRT system, the new high-resolution research tomograph

The ECAT HRRT (high-resolution research tomograph) is a three-dimensional(3-D)-only dedicated brain positron emission tomograph (PET) with LSO and GSO scintillators. The system is based on eight panels of detectors. The HRRT's imaging performance has previously been tested with phantoms and FDG scans performed in animal and human brains that showed significantly improved spatial resolution, below 2.5 mm for animal studies and below 3 mm for brain studies. The NEC count rate performance has been evaluated based on the NU 2-2001 protocol. The results show a peak NEC approximately 30% higher than the performance of the ECAT HR+, this in spite of a more shallow detector for the HRRT, only 15 mm relative to the 30 mm for the HR+. However, peak NEC as derived from the 70-cm line source phantom is not optimized for the performance of dedicated brain scanner. NEC data derived with a 20-cm diameter, 20-cm long phantoms show a peak NEC more than 60% higher than for the HR+. The reasons for the high performance are due to several factors, the large axial coverage, the short timing window of 6 ns, and the low detector dead time, all factors that imply the use of fast LSO and GSO scintillators.

Evaluation of LAAPD arrays for high-resolution scintillator matrices readout

In this paper, we evaluate the performance of recently released avalanche photodiode arrays from Advanced Photonix, Inc., for scintillator matrices readout. The large-area avalanche photodiode (LAAPD) quadrant device is a monolithic 2 /spl times/ 2 pixel structure with an active area of 5.3 mm/sup 2/ per pixel. The device allows stable operation with high gains up to 200 and detection efficiency of 73/spl sim//spl plusmn/ 10% for 420-nm photons. It is furthermore characterized by a pixel linear fill factor of 90% and low noise equal to 16 electrons equivalent noise charge measured at room temperature. We have measured pixel-to-pixel gain nonuniformity smaller than 1.5% and interpixel crosstalk of 1.7% in pulse-mode operation with a device gain of 50. An energy resolution of 12.3 /spl plusmn/ 0.5% was achieved for the 511-keV photopeak from a /sup 22/Na source placed on top of a 2 /spl times/ 2 /spl times/ 10 mm/sup 3/ LSO crystal coupled to one pixel of a quadrant LAAPD. An array of LSO crystals was later coupled to the device allowing individual crystal identification. A coincidence timing resolution of 1.9 /spl plusmn/ 0.1 ns full-width at half-maximum has been obtained for quadrant pixel with LSO scintillator for the 511-keV peak from a /sup 22/Na source. Finally, we compared the characteristics and readout performance of the LAAPD array with published results of some commercially available avalanche photodiode arrays.

Optimization of fiber-optic readout of LSO scintillation crystals with acid etching

Optimizing the collection of scintillation light is essential for good positron emission tomography scanner performance, and even more so when the crystals are read out through optical fibers. Acid etching has been proposed as a cost-effective alternative to mechanical polishing, but there are discrepancies between the results published so far. The aim of this paper is to gain a better understanding of acid etching and its effects on the light yield, and to assess its application to fiber-optic-based configurations. We have examined the surface state of LSO crystals etched for various times and measured their light output in different configurations, with both direct and fiber-optic readout. Our results indicate that crystal etching is a complex process, where different crystal faces may be etched in different ways. Acid etching always resulted in an improved light yield and energy resolution of the crystals, and was as efficient as, or in many cases superior to, mechanical polishing. While this improvement was somewhat limited for the configurations with direct readout of the crystals we tested, it was much larger for the fiber-optic-based configuration.

Comparative study of avalanche photodiodes with different structures in scintillation detection

The performance of beveled-edge large-area avalanche photodiodes (LAAPDs) produced by Advanced Photonix, Inc. (API), Hamamatsu SPL 2560 APDs, and APDs from the Institute of Electron Technology (ITE) was studied in scintillation detection using CsI(Tl), BGO, LSO, and YAP scintillators. Measurements covered DC gain characteristics, relative response to X-rays and light, and energy resolution for 5.9-keV X-rays from a /sup 55/Fe source. We also determined the electron-hole (e-h) pair numbers for the studied scintillators and the detector energy resolution for 662-keV /spl gamma/-rays from a /sup 137/Cs source. The highest numbers of e-h pairs were measured with the LAAPD equaling 33 800 e-h/MeV for CsI(Tl) and 10 200 e-h/MeV for the YAP crystal. In the case of the Hamamatsu APD, the respective numbers were 30 900 e-h/MeV and 4700 e-h/MeV. Finally, the APD from ITE measured 16 500 e-h/MeV with CsI(Tl). The best energy resolution of 4.9% for 662 keV was observed with CsI(Tl) coupled to the LAAPD. The second best resolution was with the Hamamatsu APD showing a result of 5.8%.

Spectral characterization of a blue-enhanced silicon photodetector

In this paper, we report on spectral response data and gamma ray spectroscopy measurements using two newly developed silicon photodetectors that are designed to have enhanced sensitivity in the blue spectral region. The enhanced sensitivity is a result of our newly developed ion implantation profile used to create the active area of the photodetector. The quantum efficiency of the new photodetectors (without any optimized antireflective coating) has been measured to be /spl sim/40% at a wavelength of 420 nm. Gamma ray spectroscopy experiments have been performed using a thallium doped cesium iodide, [CsI(Tl)], and a cerium doped lutetium oxy-orthosilicate, (LSO) crystal excited by a /sup 137/Cs or /sup 22/Na source and read out by the new photodetectors. We have measured an energy resolution of 7.7% and 22.7% full-width at half-maximum (FWHM) for the 662-keV gamma rays from a /sup 137/Cs for the CsI(Tl) and LSO scintillator crystal respectively. We intend to use the photodetectors, in the form of a detector array optically coupled to CsI(Tl) or LSO, in the development of a new scintillator detector module for use in positron emission tomography (PET).

Pulse recording by free-running sampling

Pulses from a position-sensitive photomultiplier (PS-PMT) are recorded by free-running ADCs at a sampling rate of 40 MHz. A four-channel acquisition board has been developed which is equipped with four 12-bit ADCs connected to one field programmable gate array (FPGA). The FPGA manages data acquisition and the transfer to the host computer. It can also work as a digital trigger, so a separate hardware trigger can be omitted. The method of free-running sampling provides a maximum of information, besides the pulse charge and amplitude also pulse shape and starting time are contained in the sampled data. This information is crucial for many tasks such as distinguishing between different scintillator materials, determination of radiation type, pile-up recovery, coincidence detection or time-of-flight applications. The absence of an analog integrator allows very high count rates to be dealt with. Since this method is to be employed in positron emission tomography (PET), the position of an event is also important. The simultaneous readout of four channels allows localization by means of center-of-gravity weighting. First results from a test setup with LSO scintillators coupled to the PS-PMT are presented here.