My Abstract List

NM2-7: Novel Front-End Pulse Processing Scheme for PET System Based on Pulse Width Modulation and Pulse Train Method

K. Shimazoe¹, H. Takahashi¹, B. Shi², T. Furumiya³, O. Jyunichi³, Y. Kumazawa³, H. Murayama⁴

¹Department of Bioengineering, Graduate School of Engineering,the University of Tokyo, Tokyo, Japan
²Department of Nuclear Engineering and Management, Graduate School of Engineering,the University of Tokyo, Tokyo, Japan
³Shimadzu Corporation, Kyoto, Japan
⁴National institute of radiological sciences, Chiba, Japan

Architecture of multi-channel Front-End system is very important for high-resolution PET system. We propose a novel Front-End pulse processing scheme with Pulse Width Modulation (PWM) for a PET system. Each channel of the proposed front-End consists of a preamplifier, a shaping amplifier, a comparator and digital circuit which generate a pulse train. Preamplifier/ Shaper/ Discriminator module generates a first trigger pulse using time over threshold (ToT) which contains energy information. Then the trigger pulse is processed through the digital circuit which adds the following pulses after the first trigger pulse to form the pulse train. These additional pulses encode the channel information, timing information etc. Output digital signals of each channel can be connected simply by wired-logic and the output is read out in one transmission line. This pulse processing scheme can easily realize multi-channel and low power consumption front-end which acquires pulse height (energy) and position information necessary for PET system and greatly decrease the number of output in ASIC. The pulse width encoding also simplifies the following digital processing parts. We have designed a new ASIC based on this concept and this chip will be used high resolution PET system which requires multi-channel ASIC.

M03-8: A DOI-Dependent Extended Energy Window Method to Control Balance of Scatter and True Events

E. Yoshida¹, K. Kitamura^2,1, K. Shibuya¹, F. Nishikido¹, T. Hasegawa³, T. Yamaya¹, H. Murayama¹

¹National institute of radiological sciences, Chiba, Japan
²Shimadzu Corporation, Kyoto, Japan
³Kitasato University, Kanagawa, Japan

In a conventional PET scanner, coincidence events are limited by an energy window for detection of photoelectric events. In contrast, Compton scatter events are occurred both in patient and crystal. Scatter within patient causes scatter coincidence, but scatter with crystal has useful information of activity distribution. The PET scanner with extended energy window has higher sensitivity but higher scatter fraction than the PET scanner with normal energy window. In this work, we develop scatter reduction method using depth-of-interaction (DOI) detector. DOI detector can make upper layer as absorber of patient scatter for lower layer. Therefore, crystal scatter with interacted lower layer have a potential to discriminate to patient scatter. This method has the different energy windows for each layer. The energy window of upper layer is limited the region of photoelectric events. The energy window of lower layer is extended the region of crystal scatter and photoelectric events. We applied the proposal technique for PET scanner using GATE simulation. From simulation result, PET scanner with DOI detector can be increased extra true events keeping low SF using this scatter reduction method.

N. Inadama¹, H. Murayama¹, T. Yamaya¹, F. Nishikido¹, K. Shibuya¹, E. Yoshida¹, C. F. Lam¹, K. Takahashi^2,1, A. Ohmura^3,1

¹Molecular Imaging Center, National Institute of Radiological Sciences, Chiba, Japan
²Graduate school of science and technology, Chiba University, CHIBA, Japan
³Science and Engineering, Waseda University, Tokyo, Japan

Previously, we showed depth of interaction (DOI) detection capability at the region between two distant position sensitive photomultiplier tubes (PS-PMTs) with a 2-layer DOI crystal array. Scintillation light path control in the crystal array was the key technique to identify the crystals in the region. In this study, we demonstrate a 2-layer DOI encoding on the detector composed of a 2 × 2 PMT array and scintillation crystals as another application of the technique.
A new way we propose to make distinction of crystal responses for crystal identification of the detector is insertion of reflector not only in between crystals but also between the crystal array and PMT, and the upper and lower layers of the crystal array. To make the distinction, scintillation light has to be spread and distributed among the four PMTs. It may be a standard way that controlling scintillation light path in the crystal array by appropriate reflector insertion in between crystals and the use of light guide; however, we found that the reflector insertion parallel to the PMT window influences the scintillation light distribution and accordingly crystal response distinction.

M13-221: Parallel Implementation of 3-D Dynamic RAMLA with Intra-node Image Update for the jPET-D4

C. F. Lam¹, T. Yamaya¹, T. Obi², E. Yoshida¹, N. Inadama¹, K. Shibuya¹, F. Nishikido¹, H. Murayama¹

¹Molecular Imaging Center, National Institute of Radiological Sciences, Japan, Chiba, Chiba, Japan
²Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Yokohama, Kanagawa, Japan

The use of small crystals in new PET scanners results in large number of line-of-response (LOR). Thus, fully 3D image reconstruction becomes time consuming especially for iterative algorithms. One of the methods to speed-up image reconstruction is by master-slave parallel computing. However, as number of computing nodes increases, the parallel efficiency decreases due to network transfer delay and other overheads. Network delay becomes significant particularly for high converging algorithm such as dynamic RAMLA, where image reconstruction is possible with large subset size. In this paper, we propose a way to reduce data transfer between nodes by allowing intra-node image update. In details, few intra-node updates for L-1 number of sub-iterations are performed in each slave thread before a global update is done at the master thread. We implemented a normal parallel version and the proposed intra-node update version of dynamic RAMLA for the jPET-D4 scanner by using MPICH2 library. We evaluated it with four servers, each contain two dual-core opteron 2.8GHz processors. Firstly, we compare the parallel efficiency (PE) gain by the proposed method with several L parameters: 1, 4, 8, and 12 for subset size of 192. PE gain is significant for L=4, after which no much gain is obtained as other factors such as load balancing become more prominent. Next, we accessed image quality of the proposed method with a hotspot phantom generated by GATE simulator, and a human brain experiment data. In both cases, no significant image artifact was found by using the proposed method. We showed that, by distributing the calculation load in a way where LORs in each slave thread covers the FOV substantially, it is possible to achieve almost the same image quality by allowing intra-node update. With proposed method, image reconstruction time is reduced by more than 22%.

M18-22: Preliminary Study of a DOI-PET Detector with Optical Imaging Capability

K. Takahashi^1,2, N. Inadama², H. Murayama², T. Yamaya², E. Yoshida², F. Nishikido², K. Shibuya², I. Oda³, H. Kawai⁴

¹Graduate school of science and technology, Chiba University, CHIBA, Japan
²National institute of radiological sciences, Chiba, Japan
³Shimadzu Corporation, Kyoto, Japan
⁴Faculty of Science, Chiba University, CHIBA, Japan

Abstract—Positron emission tomography (PET) and optical imaging are major techniques for molecular imaging. The PET scanner which can make an optical image simultaneously will be a powerful tool in the study field. We then propose a new detector that detects both signals: annihilation photons for PET and photons for optical imaging. The detector has four layer depth-of-interaction (DOI) encoding capability. Because the detector is required to be set close to an object for optical imaging, the DOI information is important to make uniform spatial resolution in PET imaging over the field of view. The PET/Optical imaging detector consists of a scintillation crystal block and a position sensitive photo multiplier tube (PS-PMT). The crystal block is covered with a reflector on the side faces and optically coupled to the PS-PMT through the bottom face. On the top face, a dichroic mirror is used. The dichroic mirror has transmission capability to fluorescence photons (longer wavelength than 600 nm) and reflectivity for scintillation photons for PET imaging (shorter than 600 nm). Owing to the dichroic mirror, both scintillation photons originated inside the crystal block and fluorescence photons coming from outside of the block can be detected by the same PS-PMT. We evaluated the prototype detector. The results show that the performance as a PET detector was improved by using the dichroic mirror.

M18-242: System Modeling of Small Bore DOI-PET Scanners for Fast and Accurate 3D Image Reconstruction

H. Takahashi¹, T. Yamaya², T. Kobayashi^1,2, K. Kitamura³, T. Hasegawa^4,2, H. Murayama², M. Suga¹

¹Graduate school of science and technology, Chiba University, CHIBA, Japan
²Department of Biophysics, Molecular Imaging Center, National Institute of Radiological Sciences, Chiba, Japan
³Shimadzu Corporation, Kyoto, Japan
⁴School of Allied Health Sciences, Kitasato University, Kanagawa, Japan

We are developing a small bore PET scanner with depth of interaction (DOI) detectors arranged in hexagonal and overlapped tetragonal shape for small animal imaging or mammography. DOI detector block consists of 4-layered array of 32x32 LSO crystals (1.4mm x 1.4 mm x 4.5mm) and a 256ch flat panel position sensitive photomultiplier tube. Small bore DOI-PET scanners have irregular detector sampling caused by inter-detector gaps. Therefore the accurate model of geometrical efficiency such as the geometrical crystal arrangement and the crystal penetration is of great importance. However, it is computational burden to calculate accurate system matrix for DOI-PET scanners which have huge number of Lines of Response (LORs). In this work, we propose a system model of small bore DOI-PET scanners for fast and accurate 3D image reconstruction. We investigate two items, system modeling to reduce the number of rays to trace and acceleration of ray tracing algorithms. For the first, We compared two imaging system models; (a)the parallel model where detector response functions (DRFs) are assumed to be uniform along LORs, (b)the divided model where each crystal is divided into a set of smaller volumes. These models were applied to the hexagonal scanner simulated by GATE. We showed that the quality of images reconstructed with these models were almost the same, however, computational time of the parallel model was a half of the divided model. For the second, we proposed accelerated ray-tracing method. Fully 3D images were reconstructed in 3 hours per OSEM iteration on a work station with 2 dual 3.0GHz Xeon CPUs. Using the DOI compression method to compress detected data into 1st-1st layer, calculation time per iteration will be reduced to11 minutes.

M19-7: Timing Resolution Improved by DOI Information in a Four-Layer LYSO PET Detector

K. Shibuya¹, F. Nishikido¹, N. Inadama¹, E. Yoshida¹, C. F. Lam¹, T. Tsuda², T. Yamaya¹, H. Murayama¹

¹Molecular Imaging Center, National Institute of Radiological Sciences, Anagawa 4-9-1, Inage, Chiba 263-8555, Japan
²Technology Research Labratory, Shimadzu Corporation, Hikaridai 3-9-4, Seika-cho, Kyoto 619-0237, Japan

Depth-of-interaction (DOI) PET detectors possess three-dimensional information on the interaction point of annihilation radiation and a scintillator crystal. The information has been used for improving PET spatial resolution especially at the edge of field-of-view. In this paper, DOI information is used for improving timing resolution of scintillation detectors. The annihilation radiation travels at the light speed, c, while scintillation photon travels at the speed of c/n in medium, where n is the refractive index. The difference in the propagation speed causes timing error which depends on the DOI. Given that the total length of the scintillator is 3.0 cm and the refractive index is 1.8, the error is estimated to be 80 ps or longer. We experimentally evaluated the timing error as the function of the DOI by using Pseudo-four-layer DOI detector and a fast digital oscilloscope. The detector was composed of an LYSO scintillator crystal and 255 dummy crystals made of silica. As the result, the error was estimated to be about 100 ps. Timing correction using DOI information would be an important technique when constructing high timing resolution PET detectors, especially ones for time-of-flight PET.

M19-11: Four-Layer DOI Detector with a Multi Pixel APD by Light Sharing Method for Small Animal PET

F. Nishikido¹, N. Inadama¹, K. Takahashi², K. Shibuya¹, E. Yoshida¹, T. Yamaya¹, C. F. Lam¹, I. Oda³, H. Murayama¹

¹National institute of radiological sciences, Chiba, Japan
²Shimadzu Corporation, Kyoto, Japan
³Chiba University, Chiba, Japan

Recently, avalanche photodiodes (APDs) are used as photo detector in positron emission tomography (PET) because APDs have many advantages over photomultipliers (PMTs) typically used in PET detector. We are also developing PET detector with a multi pixel APD for small animal PET scanner. In the previous study, we succeed in the identification of four-layer depth of interaction (DOI) with a position sensitive PMT coupled to the backside of a crystal array by only an optimized reflector arrangement. We adopt this method for the APD detector in replacement of the position sensitive PMT. While APDs have many advantages as photo detector, the performance of a detector with APDs is affected by electrical noise because of its low internal gain. Since scintillation lights are shared with many pixels by the presented method, the weaker signal in pixels far from interacting crystals is strongly affected by the noise. To evaluate the possibility of the four-layer DOI detector with APDs by light sharing method, we constructed a prototype DOI detector and tested its performance. We constructed the prototype detector consisting of four layers of a 6 × 6 array with Lu_2(1-x)Y_2x SiO₅ (LYSO) (Lu: 98 %, Y: 2 %) (Proteus Inc., U. S. A.) crystals, and a pixelized APD with a 4x8 pixels array (s8550, Hamamatsu Photonics K.K. ). The size of each crystal element is 1.46 mm × 1.46 mm × 4.5 mm and all surfaces of crystal elements were chemically etched. The experimental result indicates that the four-layer DOI detector with the multi pixel APD can be used as small animal PET detector. In addition, the result of the simulation for the APD detector was identical with experimental data.

T. Yamaya¹, T. Inaniwa², S. Minohara², E. Yoshida¹, N. Inadama¹, F. Nishikido¹, K. Shibuya¹, C. F. Lam¹, H. Murayama¹

¹Molecular Imaging Center, National Institute of Radiological Sciences, Chiba, Japan
²Research Center for Charged Particle Therapy, National Institute of Radiological Sciences, Chiba, Japan

A long patient port of PET scanner tends to put stress on patients, especially patients with claustrophobia. It also prevents doctors and technicians from taking care of patients during scanning. In this paper, we propose an "Open PET" geometry, which consists of two axially separated detector rings. Long and continuous field-of-view (FOV) including 360-degree opened gap space between two detector-rings can be visualized by 3D image reconstruction from all the possible lines-of-response (LORs). Based on the redundant characteristics of 3D PET, oblique LORs between two separated detector-rings compensate the missing LORs in the gap. In order to evaluate imaging performance of the open PET geometries, we simulated dual HR+ scanner (ring diameter=827mm, axial length=153mm x 2) separated by a variable gap. 3D OS-EM with geometrical system modeling was applied. The gap of 153mm was the maximum limit for the simulated scanner to have axially continuous FOV of 459mm though the maximum diameter of FOV at the central slice was limited to 414mm. The results show that high resolution images can be obtained with the gap up to 153mm, though missing LORs increase a little but almost invisible artifacts. We also tested open PET geometries using experimental data obtained by the jPET-D4, a prototype brain scanner. The jPET-D4 has 5 rings of 24 detector blocks. Therefore we simulated open PET data with a gap of 65mm by eliminating 1 block-ring from experimental FDG-PET data. Although some artifacts were seen at both ends of the gap, very similar images were obtained with and without the gap. The open PET geometry is expected to enable in-beam PET, which is a method for an in situ monitoring of charged particle therapy, by letting beams though the gap. The open PET geometry also enables a simultaneous PET/CT scanner to measure the same PET FOV as the CT FOV at the same time, in contrast to conventional PET/CT scanners where each FOVs are separated by several tens centimeters.