Nuclear Instruments and Methods in Physics Research A 621 (2010) 590 594 Contents lists available at ScienceDirect Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima Preliminary experimental results of a quasi-monolithic detector with DOI capability for a small animal PET Yong Hyun Chung a,b,n, Cheol-Ha Baek a,b, Seung-Jae Lee a,b, Key Jo Hong c, Ji Hoon Kang d, Yong Choi c a Department of Radiological Science, College of Health Science, Yonsei University, 234 Meaji, Heungup, Wonju, Kangwon-Do 220-710, Republic of Korea b Institute of Health Science, Yonsei University, Wonju 220-710, Republic of Korea c Department of Electric Engineering, Sogang University, Seoul 121-742, Republic of Korea d Department of Nuclear Medicine, Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul 135-710, Republic of Korea article info Article history: Received 16 January 2010 Received in revised form 5 April 2010 Accepted 12 April 2010 Available online 20 April 2010 Keywords: PET Maximum likelihood position estimation Monolithic crystal Depth of interaction abstract In our previous work, the new detector module with depth of interaction (DOI) based on a quasimonolithic crystal array and maximum-likelihood position-estimation (MLPE) algorithm was designed and the methodology for 3D event positioning in the detector was established by using Monte Carlo simulation. The aim of this study was to evaluate the performance of our detector module experimentally. The detector module consisted of a 1D array of eight LYSO crystals of 20.0(X) 2.0(Y) 10.0(Z) mm 3, optically coupled to a Hamamatsu H7546B position sensitive photomultiplier tube (PSPMT). The PSPMT was read out using a resistive charge divider, which multiplexes 64 anodes into 8(X)+8(Y) channels. Gaussian-based MLPE methods have been implemented using experimentally measured detector response functions (DRFs). The results demonstrated that the detector module could identify the position of the gamma ray interaction inside the crystal in all three directions. The new DOI detector for a small animal PET was developed and verified experimentally with a view towards achieving high sensitivity as well as high and uniform radial resolution. & 2010 Elsevier B.V. All rights reserved. 1. Introduction The demand for a high performance positron emission tomography (PET) is motivated by the need to perform molecular imaging of small animal models. However, thick scintillation crystals for high sensitivity in PET scanners cause radial elongation or parallax errors. To correct these problems and to improve the resolution uniformity across the field of view (FOV), there has been a growing interest in the design of small animal PET with the depth-of-interaction (DOI) determination capability. Most of developed DOI detector modules used matrices of small scintillating crystals, but the detection efficiency in such designs is reduced by the dead space between crystal elements [1 9]. As an alternative, monolithic crystal detectors with statistical positioning algorithms have been proposed. These detectors use several cubic centimeters of scintillating material coupled on one or more sides to photo sensor. These techniques require look-up tables (LUTs) containing information on the light distribution of scintillation events in the crystal as a function of source position, which are generated through computer simulation or machine training corresponding to a specific angle of incidence. Our group has introduced a new DOI detector concept based on an array of quasi-monolithic crystals and multi-channel photosensors used with a maximum-likelihood position-estimation (MLPE) algorithm [10 12]. The main advantage of the quasimonolithic detector is that the dependence of the light response on an event location can be measured experimentally, because the crystal elements are separated from each other in the axial direction and are monolithic in the trans-axial direction. In our previous work, we evaluated the performances through DETECT2000 simulation and the results showed that the detector could determine the position of the gamma events inside the crystal with a resolution of 2.0 mm in all three dimensions. The purpose of this study is to develop the proposed detector module and to evaluate the performances of it experimentally. 2. Materials and Methods 2.1. Quasi-monolithic detector n Corresponding author at: Department of Radiological Science, College of Health Science, Yonsei University, 234 Meaji, Heungup, Wonju, Kangwon-Do 220-710, Republic of Korea. Tel.: +82 33 760 2477; fax: +82 33 760 2815. E-mail address: ychung@yonsei.ac.kr (Y. Hyun Chung). Quasi-monolithic detector module employs a volumetric slab configuration, rather than a matrix of pixelated crystal elements. The module consists of a 1D array of eight LYSO crystals with 0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.04.039
Y. Hyun Chung et al. / Nuclear Instruments and Methods in Physics Research A 621 (2010) 590 594 591 20.0(X) 2.0(Y) 10.0(Z) mm 3 size. The crystal elements are discrete in the Y direction and are monolithic in the X direction, as shown in Fig. 1. The 2.0 16.0 mm 2 bottom surface of the crystal is polished and directly coupled to a position-sensitive PMT (PSPMT) or an array of silicon multipliers (SiPMs). The large sides are wrapped with Teflon reflector so that the crystals are separated from each other in the Y direction. To reduce the degradation of the resolution near the edge of the crystal, the short sides are painted with absorbent material [10]. All 64 channels of the PMT are acquired for each event. To compute the event positions in the Y direction, eight outputs on the same row are summed, and the eight summed signals along the Y direction are compared to each other. After determining the Y position of the event, the MLPE algorithm is employed to extract the X and Z positions using eight output signals in the selected monolithic crystal. 2.2. MLPE algorithm The MLPE method uses the likelihood between the detector response function (DRF) of each PMT channel and the event position. Because of the stochastic nature of signal formation in a Fig. 1. (a) Schematic diagram of a quasi-monolithic crystal module and (b) the simulated apparatus for training. detector, the probability of measuring a given set of PMT output signals M¼M 1, M 2, y, M 8 for a given event position x in the crystal is described by the probability distribution as Pr(M9x). Assuming that the noise process in the detector response follows a Gaussian distribution, the likelihood is given by Y N Pr M9x ¼! 1 pffiffiffiffiffiffi exp ðm i m i ðxþþ 2 s i ðxþ 2p 2s 2 i ðxþ where m i (x) is the mean and s i (x) is the standard deviation of the ith output of PMT as a function of event position. Performing the maximization with respect to log likelihood, the resulting approximation is X n lnpr M i 9x ¼ ðm i m i ðxþþ 2 2s 2 i ðxþ þ Xn lns i ðxþ The MLPE solution is achieved by minimizing the value between parenthesis in Eq. (2). 2.3. Experimental setup for training DRFs of each crystal element were determined by experimental training. The single LYSO crystal of size 20.0(X) 2.0(Y) 10.0(Z) mm 3 was optically coupled to a Hamamatsu H7546B PSPMT. The PSPMT was read out using a resistive charge divider, which multiplexes 64 anodes into 8(X)+8(Y) channels. Fig. 2 is a diagram of the experimental apparatus employed. During the experimental training, only one crystal element was coupled to PSPMT to generate gamma events at any location. Fig. 3 shows the DRF training system consisting of a collimated source, motorized x z stage, LYSO crystal, PSPMT, signal processing electronics and data acquisition system. F-18 source was housed in a 5 cm thick lead block and collimated into a pencil beam 1 mm in diameter. To characterize DRFs, data were acquired on 4 5 evenly spaced points with 2.0 mm spacing in the X and Z directions using a computer controlled stepper motor stage and collimated F-18 source. Then DRFs were generated using Gaussian fitting and bicubic spline interpolation.! ð1þ ð2þ Fig. 2. Block diagram of the circuitry of a detector module. Fig. 3. Detector and training system consist of collimated source, motorized x z stage, LYSO crystal, PSPMT, signal processing electronics and data acquisition board.
592 Y. Hyun Chung et al. / Nuclear Instruments and Methods in Physics Research A 621 (2010) 590 594 Fig. 4. The point source image and global energy spectrum for one training position. Fig. 5. Energy spectra of eight X signals were plotted and the mean and standard deviation were extracted to generate LUT by Gaussian fitting. 2.4. Performance measurements After generating DRFs for a crystal element, the point source imaging was performed. To measure the intrinsic spatial resolution of the detector in the X direction, the image of the collimated source was obtained and the full-width at half-maximum of the profile was calculated. The measured DOI (Z-axis) positions were compared to the true positions of the collimated source. The misclassification rates, defined as the number of mis-assigned events in Z direction divided by the total number of events, were calculated for each source position. 3. Results Fig. 4 illustrates the point source image and global energy spectrum for one training position. Energy spectra of eight X signals were plotted and the mean and standard deviation were extracted to generate LUT by Gaussian fitting as shown in Fig. 5.
Y. Hyun Chung et al. / Nuclear Instruments and Methods in Physics Research A 621 (2010) 590 594 593 Fig.6. The measured images of point sources located at nine different positions with 2.0 mm spacing. The numbers represent the positioning accuracies. Fig.6 represents the images of true source positions and the acquired images by MLPE algorithm. The positioning accuracies in the X and Z directions were calculated. The positioning accuracies for each source position were 91.7%, 97.8%, 99.8%, 95.5%, 98.6%, 99.7%, 96.8%, 91.3% and 99.9%, respectively, and the average is 96.8% in the X direction. The average misclassification rates in the DOI direction was 45% in the depth direction. 4. Discussion and conclusions A new DOI detector based on quasi-monolithic LYSO crystal and an MLPE algorithm was developed and tested. The source positions were accurately identified with 2.0 mm resolution in the X and Y directions, but the positioning accuracy in the depth direction was about 55%. Since the quasi-monolithic crystal elements were continuous in the X direction and separated in the Y direction, the spatial resolution in the Y direction was geometrically limited to 2.0 mm in this detector module. However, the spatial resolution can be improved by increasing the number of LUT sampling points and by reducing the physical crystal size in Y direction. The main advantage of the proposed detector module is that the LUTs needed for position estimates can be generated experimentally. For a monolithic crystal, generating a gamma event at a specific position is impossible, and for a pixelated crystal, the light distributions from events at different depths are indistinguishable. Also, the correction method for the nonuniform signals from PSPMT is not needed in this design because MLPE involves the use of LUTs to map from measured PSPMT signals to position estimates. Other advantages of this design include simple fabrication, higher packing fraction and high sensitivity. The positioning accuracy is affected by the light distribution in the crystal, which is largely varied by the surface conditions, especially for the DOI direction. In our previous simulation [10], the top surface of crystal was designed as diffuse reflecting material with a reflection coefficient (RC) of 0.5 and the position misclassification rate was calculated as 8% for the DOI direction. However, in the experiment, it is very hard to accurately wrap the crystal with specific RC value. Also, the wrapping material such as Teflon tape may introduce irregular dead spaces between crystals. Therefore, the optimization of the surface treatment of quasimonolithic crystal, a reflector material and an accurate surface treatment technique are required for further study to improve depth resolution. In this paper, PSPMT was considered as a photo-sensor in PET detector, but it can be replaced by an array of avalanche photodiodes (APDs) and SiPMs [8 9] in the future development due to their high quantum efficiency and positioning accuracy. In conclusion, using the quasi-monolithic detector, a small animal PET system with high sensitivity and uniform radial resolution might be realized. Acknowledgments This work was supported by a Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea Government Ministry of Education, Science and Technology (MEST) (2009-0083262).
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