Development of PET Detectors Using Monolithic Scintillation Crystals Procesd with Sub-Surface Lar Engraving Technique
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T. Moriya, K. Fukumitsu, T. Sakai, S. Ohsuka, T. Okamoto, H. Takahashi, M. Watanabe and T. Yamashita
Abstract– New monolithic scintillation detectors for PET have been developed, where the crystals are procesd using internal focud lar processing technique, which is called sub-surface lar engraving (SSLE) technique. When high intensity light puls of short duration from a lar are focud into a scintillation crystal, they induce multi-photon absorption at the focal point and result in refractive index changes or micro-cracks inside the crystal. By applying the SSLE technique to a monolithic scintillation block, fine gmentation in the crystal can be formed without inter-pixel gaps. We have fabricated 2D gmented arrays engraved various patterns of micro-cracks inside monolithic LYSO crystal blocks by using a Nd:YAG lar. The procesd crystal array gmented to 12 × 12 with 1.67 mm pitch have been evaluated by coupling to a position-nsitive photomultiplier tube (PS-PMT). The 2D position histograms were measured for uniform irradiation of gamma-rays and each crystal gment was clearly parated. The average energy resolution was 9.7%, similar to that of the conventional arrays, so that the lar procesd LYSO crystals have kept their primary scintillation pro
perties. We have also evaluated the lar procesd crystals by using multi-pixel photon counters (MPPCs) to investigate the possibilities as a future PET detector. The results suggest that it is possible to fabricate high performance PET detectors using the SSLE technique.
I.I NTRODUCTION
olecular and genomic imaging with positron emission tomography (PET) is one of the powerful techniques for drug discovery and elucidation of molecular functions. For adopting the applications, PET scanners dedicated to small laboratory animals, such as mice and rats, are required to have a high spatial resolution of less than 1 mm.
In order to achieve a high spatial resolution, PET detectors usually u gmented crystal blocks compod of small crystal elements [1]. However, there exists the loss of detection efficiency becau of inter-pixel gaps between each element in the scintillator array. Furthermore, it is difficult to accurately fabricate the crystal block with small crystal elements which are 1 mm2 or less in cross ction.
We propo to adopt an internal focud lar processing technique, which is called sub-surface lar engraving (SSLE) technique, to the fabrication of scintillator blocks. The technique could provid宠物保姆
e scintillation detectors having a high spatial resolution without complicated process. This
Manuscript received November 13, 2009.
T. Moriya (e-mail: t-moriya@jp), S. Ohsuka, H. Takahashi, M. Watanabe and T Yamashita are with the Central Rearch Laboratory, Hamamatsu Photonics K.K., Hamamatsu-City, 434-8601 Japan.
beloved宠儿K Fukumitsu, T Sakai and T Okamoto are with the Electron Tube Division, Hamamatsu Photonics K.K., Iwata-City, 438-0193 Japan. paper describes the SSLE technique and results of performance evaluation of the PET detectors using monolithic scintillation crystals procesd with the technique.
II.S UB-S URFACE L ASER E NGRAVING T ECHNIQUE
A. The Basic Principle of Lar Processing Techniques Lar processing techniques are widely ud in various fields becau of their fine accuracy, high speed and low running costs. There is a variety of lar processing techniques, for example lar ablation. The technique carves a groove on the surface by absorbing the lar beam at the surface of the material. Nagarkar et al. have made pixelated array by cutting grooves in the LSO scintillator using lar ablation technique [2].
On the other hand, the SSLE technique, which is an internally focud lar process, has been ud to create three-dimensional structures in a transparent material such as glass. The difference between the SSLE and lar ablation technique is the lar focal position. A lar beam is focud at an internal point in a material by an objective lens, where the material is normally transparent to its wavelength. When the lar beam is tightly focud in the bulk of a transparent material, the intensity at the focal region becomes high enough to induce nonlinear optical absorption such as multi-photon absorption. This phenomenon caus refractive index changes or micro-cracks inside the material locally. In particular, it is expected that multiple micro-cracks arranged at high density work efficiently as scattering walls for the visible photons. The SSLE technique could induce various three-dimensional patterns compod of optical scattering walls inside of transparent materials without surface damages.
B.Fabrication of Scintillator Blocks Using the SSLE Technique
A schematic drawing of the SSLE technique for processing a scintillation crystal is shown in Fig. 1. A monolithic scintillator sample is put on an XYZ stage, which is controlled by a personal computer The pul beam from Nd:YAG lar is focud through an objective lens in the interior of the sample. The lar emits high intensity puls with 10-20 ns in duration at a repetition rate of 1 kHz. F
igure
2 shows an optical microscope photograph of micro-cracks in
a LYSO crystal formed by an internally focud Nd:YAG lar. The size of most micro-cracks is tens of micrometers in diameter. The intervals of each are approximately 50 ȝm in
M
2009 IEEE Nuclear Science Symposium Conference Record M13-30
the horizontal direction, when the scan speed of the stage is 50 mm/s.
We have fabricated various patterns of 2D gmented arrays in monolithic LYSO scintillators using the SSLE technique and evaluated their characteristics by coupling to photo detectors.
Fig . 1. A schematic drawing of the SSLE technique for processing a scintillation crystal.
Fig. 2. Optical microscope photograph of micro-cracks in a LYSO crystal formed by internally focud Nd:YAG lar.
III. D ETECTOR C HARACTERIZATION
A. Evaluation by Coupling to a Position Sensitive Photomultiplier Tube (PS-PMT)
To evaluate the performances of the scintillators procesd by a Nd:YAG lar, a flat panel PS-PMT (Hamamatsu R8400-00-M256) was ud as a photo detector. The signals
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from 16 × 16 anodes of the PS-PMT are connected with a resistor chain network and outputted as four signals The signals were digitized with CAMAC analog-to-digital converters (ADCs), and then the data were transferred to a personal computer.
A 2D gmented scintillator array of 12 ×
12 with 1.67 mm pitch was fabricated in a 20 × 20 × 5 mm 3 monolithic LYSO crystal which is shown in Fig. 3. The boundaries of each gment are formed by den arrays of micro-cracks. The scintillator array was coupled to the PS-PMT and the 2D position histogram was measured with uniform irradiation of 662 keV gamma-rays, as shown in Fig. 4. The image profiles in the X and Y dir
ections are also shown in the figure. All of 144 gments are clearly parated even at the periphery FWHM values of the energy resolution for each gment are shown in Fig. 5. The results vary from 8.9% to 11.0%, and the average value is 9.7%.
In order to investigate the limitation of fine gmentation, ven types of gments were fabricated in a 20 × 20 × 5 mm 3 monolithic LYSO crystal, who pitch sizes are 0.63 mm, 0.83 mm, 1.00 mm, 1.25 mm, 1.67 mm, 2.00 mm and 2.50 mm, respectively The photograph of the crystal array is shown in Fig. 6. Figure 7 shows the 2D position histogram and the image profile of the region indicated in Fig. 7 (a) The figures show that the monolithic LYSO crystal gmented by boundaries compod of micro-cracks is optically parated even if the gment pitches are less than 1 mm. Figure 8 shows pul height distributions of area (c) and (d) indicated in Fig. 7. The characteristics of both areas between 0.63 and 2.50 mm gment pitch are similar with the peak channel, peak counts and the FWHM value. The results suggest that scintillation light is not absorbed at the boundaries and detection efficiency is almost unchanged when the micro-cracks are introduced.
Fig. 3. Photograph of a 2D gmented scintillator array of 12 × 12 with 1.67 mm pitch fabricated on a 20 × 20 × 5 mm 3 monolithic LYSO crystal.
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Fig. 4. (a) 2D position histogram of a gmented LYSO with 1.67 mm pitch for uniform irradiation of 662 keV gamma-rays. (b) Image profile in the venth row. (c) Image profile in the sixth column.
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Fig. 5. FWHM values of the energy resolution at 662 keV for each gment shown in Fig. 4.
Fig. 6. Photograph of a 2D gmented LYSO array, who pitch sizes are 0.63 mm, 0.83 mm, 1.00 mm, 1.25 mm, 1.67 mm, 2.00 mm and 2.50 mm, respectively
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Fig. 7. (a) 2D position histogram of the gmented LYSO array shown in Fig. 6 for uniform irradiation of 662 keV gamma-rays. (b) Image profile of the region indicated in Fig 7 (a).
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不假思索的读音Fig. 8. Pul height distributions of area (c) and (d) indicated in fig. 7. Peak
counts are sum of the counts in the channels within ± 15% of the peak
channel.
B.Evaluation by Coupling to a MPPC Array
The multi-pixel photon counter (MPPC) [3], which is one
of the products of Silicon Photomultiplier (SiPM) family, is a
promising photo detector for PET application. To investigate
the possibilities for a future PET detector, a procesd LYSO
scintillator was coupled to a 4 × 4 MPPC array and its
performance of gment identification was evaluated The
MPPC array is compod of surface mount type MPPCs
(Hamamatsu S10931-050P) as shown in Fig. 9. Each MPPC
has 3.0 mm × 3.0 mm active area and 3600 micro-pixels of
50 μm × 50 μm. The values of bias voltage supplied to each
element were tuned optimal, where the gains were
approximately 7.5 × 105. A 2D gmented 14 × 14 with 1.43
mm pitch scintillator array was fabricated on a 20 × 20 × 5
mm3 monolithic LYSO crystal.
Figure 10 shows the position histogram obtained by
uniform irradiation of 662 keV gamma-rays. The result
indicated that it is possible to identify each gment who
pitch size is less than 1.5 mm, when MPPC array is ud as
photo detector.
IV.D ISCUSSION AND C ONCLUSION
We have applied the SSLE technique to the processing for
scintillation crystals and 2D gmented LYSO scintillator
arrays have been fabricated by using Nd:YAG lar The
technique enables scintillation detectors to have a high spatial
resolution of less than 1 mm, without the deterioration of
primary scintillation properties and detection efficiency. In
addition, the production cost of scintillator arrays, especially
fine pitch arrays, will be reduced since the fabrication can be
performed automatically.
However, the SSLE technique still has some limitations,
when it is adopted for thicker monolithic scintillation crystals.
It is hard to focus the lar beam tightly at the point far from
the surface of materials, due to the large refractive index of
scintillation crystal. In order to overcome this problem, we
consider utilizing a spatial light modulator (SLM) device for
correctly focusing the lar beam inside the material [4]. The
(Hamamatsu, S10931-050P䋩
5.67 mm pitch
Nd:YAG
Fig. 9. Structures of a 2D gmented LYSO array and a 4 × 4 MPPC array.
Fig. 10. 2D position histogram obtained by coupling the gmented LYSO
array to the 4 × 4 MPPC array.
Peak to Valley Ratio
~ 11.4
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Fig. 11. Position histograms and image profiles of 3 × 3 with 3.0 mm pitch
LYSO gmented arrays of (a) 4mm and (b) 9mm thick.
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(b)
other problem is that the optical cross-talk between each gment increas with thicker scintillation crystals, becau the scattering wall made by the SSLE technique is still not enough compared to the conventional way, for example the u of reflection sheets. We have fabricated 3 × 3 with 3.0 m
m pitch LYSO gmented arrays of 4 mm and 9 mm thick and measured the position histograms for compared the cross-talk, shown in Fig. 11. The figures show that peak-to-valley ratio of the image profile of 9 mm thick LYSO was lower than that of 4 mm thick one. As one way to figure out this drawback, we could propo a multi-layer detector constructed with a pile of detectors which compod of thin scintillator arrays gmented by the SSLE technique and solid state photo detectors such as MPPCs.
The evaluation results have indicated that the SSLE technique could be adopted to fabricate high performance PET detectors.
R EFERENCES
[1]S. St. James et al., “Experimental characterization and system
simulations of depth of interaction PET detectors using 0.5 mm and 0.7
mm LSO arrays”, Phys. Med. Biol., vol. 54, pp. 4605-4619, 2009.
[2]V. V. Nagarkar et al., "A High Efficiency Pixelated Detector for Small
Animal PET", IEEE Trans. Nucl. Sci., vol. 51, pp. 801-804, 2004.
[3]K Yamamoto et al, “Development of Multi-Pixel Photon Counter
(MPPC)”, in Proc. IEEE Nuclear Science Symp., San Diego, CA, 2006, pp. 1094-1097
[4]H. Itoh et al., “Spherical aberration correction suitable for a wavefront
controller,” Optics Express, vol. 17, issue 16, pp. 14367-14373, 2009.
