Hamid Sabet

Fabricating high-resolution and high-sensitivity scintillator arrays using Laser Induced Optical Barriers

High-resolution positron emission tomography (PET) requires pixelated scintillators-however, if the pixelation process leaves inter-pixel gaps, the loss of material results in loss of sensitivity. PET sensitivity also requires scintillators such as LSO and L YSO to be thicker than 2 cm, due to the high penetrating power of 511 keV gamma rays. Fine pixelation of L YSO is difficult, since it is very hard material and is known to crack under thermal and mechanical stress. We have developed a method to introduce optical barriers within monolithic L YSO crystals to form pix elated arrays with small pixel size and large thickness. Arrays were fabricated using a high-frequency solid-state laser to form optical barriers (interpixel gaps), which can be as thin as 13 μm without affecting the transparency of the crystals. Our method yields near-perfect, extremely high aspect ratio pixels. By controlling parameters such as laser pulse repetition rate and energy density, LYSO crystals can be effectively pixelated with virtually no material loss. We have laser processed L YSO crystals ranging from 5 to 20 mm thick and 0.8×0.8 to 1.5×1.5 mm2 in pixel cross section. When a collimated beam (0.5 mm) of 70 kVp X-rays was incident on one pixel of a 10×10×20 mm3 scintillator array with 0.8×0.8 mm2 pixel size, an average optical crosstalk ratio was measured at 6.5:1, which shows excellent pixel separation. Our technique is ideal for fabricating scintillator arrays for clinical/pre-clinical PET and SPECT systems as well as photon counting CT detectors. Our technique is automated, and is cost-effective.

Novel laser-processed CsI:Tl detector for SPECT

PURPOSE The aim of this work is to demonstrate the feasibility of a novel technique for fabrication of high spatial resolution CsI:Tl scintillation detectors for single photon emission computed tomography systems. METHODS The scintillators are fabricated using laser-induced optical barriers technique to create optical microstructures (or optical barriers) inside the CsI:Tl crystal bulk. The laser-processed CsI:Tl crystals are 3, 5, and 10 mm in thickness. In this work, the authors focus on the simplest pattern of optical barriers in that the barriers are created in the crystal bulk to form pixel-like patterns resembling mechanically pixelated scintillators. The monolithic CsI:Tl scintillator samples are fabricated with optical barrier patterns with 1.0 × 1.0 mm(2) and 0.625 × 0.625 mm(2) pixels. Experiments were conducted to characterize the fabricated arrays in terms of pixel separation and energy resolution. A 4 × 4 array of multipixel photon counter was used to collect the scintillation light in all the experiments. RESULTS The process yield for fabricating the CsI:Tl arrays is 100% with processing time under 50 min. From the flood maps of the fabricated detectors exposed to 122 keV gammas, peak-to-valley (P/V) ratios of greater than 2.3 are calculated. The P/V values suggest that regardless of the crystal thickness, the pixels can be resolved. CONCLUSIONS The results suggest that optical barriers can be considered as a robust alternative to mechanically pixelated arrays and can provide high spatial resolution while maintaining the sensitivity in a high-throughput and cost-effective manner.

High-Performance and Cost-Effective Detector Using Microcolumnar CsI:Tl and SiPM

We are developing a technique to fabricate high spatial resolution and cost-effective photon counting detectors using silicon photomultipliers (SiPMs) and microcolumnar structured scintillator. Photon counting detectors using SiPMs are of much interest to the gamma- and X-ray detector community, but they have limitations at low energy due to their dark noise. In this paper, we report on vapor deposition of CsI:Tl directly onto a SiPM, a technique that improves optical coupling and allows for detection of low energy gamma- and X-rays. It simultaneously addresses related issues of light loss and light spread in the scintillator, thereby improving the performance of the detector. Devices made by this technique may be used for both photon counting and gamma- and X-ray imaging.

A Method for Fabricating High Spatial Resolution Scintillator Arrays

Scintillators like thallium doped cesium iodide (CsI:Tl) can be fabricated in microcolumnar form using physical vapor deposition (PVD). The microcolumns channel the scintillation light to the photodetector which results in an improved spatial resolution. This has lead to widespread use of microcolumnar CsI:Tl in digital X-ray radiography. We present here a PVD-based method to aggregate microcolumns into structures called macrocolumns to form scintillator arrays suitable for use in nuclear imaging. In this novel approach, patterned substrates with shallow grooves 20 μm wide, 50 μm deep, with pitch ranging 100 - 500 μm were fabricated and adopted. CsI:Tl scintillator was vapor deposited onto these substrates. The optimal deposition parameters resulted in microcolumnar CsI:Tl, which displayed a macrocolumnar structure dictated by the underlying pattern of the substrate. Scanning electron micrographs (SEM) show that the microcolumns within the macrocolumns are highly oriented and perpendicular to the surface of the substrates. Energy resolution approaching that of a single crystal CsI:Tl was achieved. Since the microcolumns are densely packed with minimal gap, they behave as a macrocolumn or a single pixel. Our technique for fabricating scintillator arrays is a cost-effective alternative to mechanical pixelation of scintillators. This technique results in a high fill factor scintillation detector with minimized inter-macrocolumn gap, and high-yield detector arrays without issues related to material loss in mechanical pixelation. Coupling these structured scintillators to silicon photomultipliers (SiPMs) and applying Anger logic, we resolved scintillator pixels that were almost 1/10th the size of the SiPM macro-pixels. Combining this structured CsI:Tl scintillator with SiPMs results in a compact detector that is ideal for X-ray, gamma-ray, and charged particle detection, such as beta and gamma imaging probes and hand held cameras.

A sub-mm spatial resolution LYSO:Ce detector for small animal PET

Current high-resolution scintillators are fabricated using mechanical pixelation technique. However the fabrication cost of finely pitched scintillator arrays together with their lack of flexibility to accommodate new depth of interaction designs has remained a significant issue with mechanical pixelation. Another pitfall of mechanically pixelated scintillators is their relatively large inter-pixel gaps that adversely affect their sensitivity to the incident gamma-ray. The main objective of our ongoing efforts is to fabricate high-spatial resolution and high sensitivity PET detectors with depth of interaction (DOI) capability and single-side readout in a cost-effective manner using laser-induced optical barriers (LIOB) technique. We have simulated the behavior of simple optical barriers in LYSO:Ce crystal using the DETECT simulation code. We have also created optical barriers with different size and barrier density in LYSO:Ce at various depths up to 20 mm to form pixel-like shapes similar to mechanically pixelated crystals. In order to process 20mm thick crystals we corrected for laser beam defocusing effect and its adverse effect on laser energy density which results in smaller barrier size and reflectivity. The fabrication time for 10×10×1 and 10×10×20 mm3 LYSO crystals (with 1mm pixels) was ~8 and 95 minutes respectively.

A Hand-Held, Intra-Operative Positron Imaging Probe for Surgical Applications

We have developed a prototype intra-operative β+ imaging probe to help tumor removal and malignant tissue resection. The probe can be used during surgery to provide clear delineation of malignant tissues. Our probe consists of a hybrid scintillator coupled to a silicon photomultiplier (SiPM) array with associated front-end electronics encapsulated in an ergonomic aluminum housing. Pulse shape discrimination electronics has been implemented and integrated into the downstream data acquisition system. The field of view of the probe is 10 ×10 mm2 realized by a 0.4 mm thick CsI:Tl scintillator coupled to a 1 mm thick LYSO. While CsI:Tl layer acts as β+ sensitive detector, LYSO detects gamma radiation where the gamma response can be subtracted from the total signal to improve SNR and contrast. The thickness of the LYSO scintillator is optimized such that it acts as light diffuser to spread the scintillation light generated in CsI:Tl over multiple SiPM pixels for accurate estimation of the β+ interaction location. The probe shows FWHM spatial resolution in the presence of large background radiation. The probe was used to study rabbits with tongue tumors. The experimental results show that the probe can successfully locate the tongue tumors in its active imaging area.

Laser pixelation of thick scintillators for medical imaging applications: x-ray studies

To achieve high spatial resolution required in nuclear imaging, scintillation light spread has to be controlled. This has been traditionally achieved by introducing structures in the bulk of scintillation materials; typically by mechanical pixelation of scintillators and fill the resultant inter-pixel gaps by reflecting materials. Mechanical pixelation however, is accompanied by various cost and complexity issues especially for hard, brittle and hygroscopic materials. For example LSO and LYSO, hard and brittle scintillators of interest to medical imaging community, are known to crack under thermal and mechanical stress; the material yield drops quickly with large arrays with high aspect ratio pixels and therefore the pixelation process cost increases. We are utilizing a novel technique named Laser Induced Optical Barriers (LIOB) for pixelation of scintillators that overcomes the issues associated with mechanical pixelation. In this technique, we can introduce optical barriers within the bulk of scintillator crystals to form pixelated arrays with small pixel size and large thickness. We applied LIOB to LYSO using a high-frequency solid-state laser. Arrays with different crystal thickness (5 to 20 mm thick), and pixel size (0.8×0.8 to 1.5×1.5 mm2) were fabricated and tested. The width of the optical barriers were controlled by fine-tuning key parameters such as lens focal spot size and laser energy density. Here we report on LIOB process, its optimization, and the optical crosstalk measurements using X-rays. There are many applications that can potentially benefit from LIOB including but not limited to clinical/pre-clinical PET and SPECT systems, and photon counting CT detectors.

Direct and indirect detectors for X-ray photon counting systems

Most currently available X-ray or gamma ray imaging detectors are based on energy integration over a certain period of time. We have been developing X-ray and gamma ray detectors based on the photon counting (with energy determination) concept using both direct and indirect radiation conversion, together with associated application-specific integrated circuits (ASICs). As an alternative to our ASIC design approach, we are also exploiting the potential of state-of-the-art silicon photomultipliers (SiPMs) and discrete electronics. In this paper we discuss the advantages and disadvantages of these two approaches and we report our latest results on our ASIC design efforts and our achievements on SiPM/CsI:Tl detector configurations. We will also discuss the potential uses and advantages that each offers to applications in medicine, imaging, homeland security and industry.

Multibeam healing for laser micromachining of scintillator arrays

Cost-effective, high-performance detectors are in high demand for use in positron emission tomography (PET) and other clinical and pre-clinical nuclear medicine imaging systems. Such detectors require high-resolution, high-sensitivity pixelated scintillator arrays. Scintillators with high stopping power, such as lutetium oxyorthosilicate (LSO) and lutetium-yttrium oxyorthosilicate (LYSO), are widely used in PET. However, the mechanical pixelation of such materials (especially to achieve <1×1 mm2 pixels), yield issues, and their assembly into arrays is expensive and results in small fill factors (large inter-pixel gaps) and, hence, lower detector sensitivity than desired. Laser pixelation of scintillator material can potentially increase scintillator fill factor, resulting in higher sensitivity and yield and lower costs. We have adopted a multibeam laser micromachining approach to pixelate LSO with enhanced yield and improved throughput. The results of the experiments and the finite-element modeling are discussed here.

Design and development of a dedicated portable gamma camera system for intra-operative imaging

PURPOSE We developed a high performance portable gamma camera platform dedicated to identification of sentinel lymph nodes (SLNs) and radio-guided surgery for cancer patients. In this work, we present the performance characteristics of SURGEOSIGHT-I, the first version of this platform that can intra-operatively provide high-resolution images of the surveyed areas. METHODS At the heart of this camera, there is a 43×43 array of pixelated sodium-activated cesium iodide (CsI(Na)) scintillation crystal with 1×1mm(2) pixel size and 5mm thickness coupled to a Hamamatsu H8500 flat-panel multi-anode (64 channels) photomultiplier tube. The probe is equipped with a hexagonal parallel-hole lead collimator with 1.2mm holes. The detector, collimator, and the associated front-end electronics are encapsulated in a common housing referred to as head. RESULTS Our results show a count rate of ∼41kcps for 20% count loss. The extrinsic energy resolution was measured as 20.6% at 140keV. The spatial resolution and the sensitivity of the system on the collimator surface was measured as 2.2mm and 142cps/MBq, respectively. In addition, the integral and differential uniformity, after uniformity correction, in useful field-of-view (UFOV) were measured 4.5% and 4.6%, respectively. CONCLUSIONS This system can be used for a number of clinical applications including SLN biopsy and radiopharmaceutical-guided surgery.