Takaji Yamashita

A high resolution animal PET scanner using compact PS-PMT detectors

A new high resolution PET scanner dedicated to animal studies has been designed, built and tested. The system utilizes 240 block detectors, each of which consists of a new compact position-sensitive photomultiplier tube (PS-PMT) and an 8/spl times/4 BGO array. A total number of 7,680 crystals (480 per ring) are positioned to form a 508 mm diameter of 16 detector rings with 7.2 mm pitch and 114 mm axial field of view (FOV). The system is designed to perform activation studies using a monkey in a sitting position. The data can be acquired in either 2D or 3D mode, where the slice collimators are retracted in 3D mode. The transaxial resolution is 2.6 mm FWHM at the center of the FOV, and the average axial resolution on the axis of the ring is 3.3 mm FWHM in the direct slice and 3.2 mm FWHM in the cross slice. The scatter fraction, sensitivity and count rate performance were evaluated for a 10 cm diameter cylindrical phantom. The total system sensitivity is 2.3 kcps/kBq/ml in 2D mode and 22.8 kcps/kBq/ml in 3D mode. The noise equivalent count rate with 3D mode is equivalent to that with 2D mode at five times higher radioactivity level. The applicable imaging capabilities of the scanner was demonstrated by animal studies with a monkey.

Depth encoding multicrystal detectors for PET

A detector unit having a depth encoding scheme was designed and an experimental study has been performed to test the new idea for a positron camera detector. The detector unit consists of a multistage rectangular block of scintillation crystals optically coupled at the bottom face to four photomultiplier tubes (PMTs) in a 2/spl times/2 array or to a position sensitive photomultiplier tube. Each stage of the crystal block consists of four elements in a 2/spl times/2 array. Each scintillation event is mapped in a two dimensional (2D) distribution through the relative ratio of the output signals of the PMTs. The three dimensional location in the detector unit projects into the 2D map so that four top crystals correspond to four innermost regions in the map, and the four bottom crystals facing the PMTs correspond to four outermost regions. With appropriate light control in such a crystal block of Gd/sub 2/SiO/sub 5/:Ce coupled to four PMTs, good positioning performance to encode the depth-of-interaction was obtained for Cs-137 gamma-rays incidence.

High resolution block detectors for PET

New high-resolution blocks detectors in which BGO scintillators are coupled to a position-sensitive photomultiplier tube (PMT), were developed for positron emission tomography (PET) applications. The block detectors, which use two different BGO scintillator structures, were constructed and evaluated: one is a 1.7-mm-wide BGO array which provides a coincidence detector resolution of less than 2 mm FWHM (full width at half maximum) and the other is a BGO block having comb-shaped slits in it, which has a potential capability of improving the resolution uniformity of a PET system because of its ability to detect the depth-of-interaction of gamma -rays in the scintillator. The application of these detectors to a PET system for animal studies is discussed. >

A depth of interaction detector for PET with GSO crystals doped with different amounts of Ce

A new method for a four-stage depth of interaction (DOI) detector is proposed. The four-stage DOI detector is constructed with two kinds of Gd/sub 2/SiO/sub 5/:Ce (GSO) crystals doped with different amounts of Ce, 0.5 mol% and 1.5 mol%. The amount of Ce in GSO determines the scintillation decay time constant, and it is 60 ns for the 0.5 mol% GSO and 35 ns for 1.5 mol% GSO. This difference led to the introduction of pulse-shape discrimination, which would distinguish between two kinds of event data from respective GSOs and sort them into two groups. By independently applying Anger-type position arithmetic to the data of each group, two two-dimensional (2-D) histograms are obtained. The crystal of interaction can be identified on these histograms in which only 0.5 mol% or 1.5 mol% GSO crystal elements are expressed. To evaluate this method, we constructed the four-stage DOI detector by alternately stacking 1.5 mol% GSO crystal stages and 0.5 mol% stages. The result of a scanning measurement with a /sup 137/Cs gamma-ray beam proved that the DOI detector had enough accuracy in crystal identification.

Design of a depth of interaction detector with a PS-PMT for PET

A detector unit using a depth encoding scheme was designed and evaluated. The detector unit consists of four crystal blocks in a 2/spl times/2 array coupled to a position-sensitive photomultiplier tube (PS-PMT) having metal channel dynodes and 4/spl times/4 multianodes. Each crystal block consists of three stages of 2/spl times/2 GSO arrays of which element size is 3.8 mm/spl times/3.8 mm/spl times/10 mm. Each scintillation event is mapped on a two dimensional positioning histogram according to the relative ratio of the output signals of the PS-PMT. With appropriate light control in the crystal block, good positioning performance for Cs-137 photons was obtained.

Development of PET Detectors Using Monolithic Scintillation Crystals Processed With Sub-Surface Laser Engraving Technique

New monolithic scintillation detectors for PET have been developed, where the crystals are processed using internal focused laser processing technique, which is called subsurface laser engraving (SSLE) technique. When high intensity light pulses of short duration from a laser are focused 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 segmentation in the crystal can be formed without inter-pixel gaps. We have fabricated 2D segmented arrays engraved various patterns of micro-cracks inside monolithic LYSO crystal blocks by using a Nd:YAG laser. The processed crystal array segmented to 12 × 12 with 1.67 mm pitch have been evaluated by coupling to a position-sensitive photomultiplier tube (PS-PMT). The 2D position histograms were measured for uniform irradiation of gamma-rays and each crystal segment was clearly separated. The average energy resolution was 9.7%, similar to that of the conventional arrays, so that the laser processed LYSO crystals have kept their primary scintillation properties. We have also evaluated the laser processed crystals by using multi-pixel photon counters (MPPCs) to investigate the possibilities as a future PET detector. These results suggest that it is possible to fabricate high performance PET detectors using the SSLE technique.

Development of 3-D detector system for positron CT

A detector composed of a bundle of BGO (bismuth germanate) pillars and position-sensitive photomultiplier tubes is proposed. This detector has the ability to provide 3-D position information on gamma -ray absorption in the scintillator. It is expected that the use of this detector for positron-emission tomography (PET) will result in high spatial resolution and sensitivity. In particular, uniform and high spatial resolution over the entire field of view can be obtained because of the detectors ability to detect the depth of interaction of the gamma -rays in the scintillator. The fundamental characteristics were investigated by both experiment and simulation, and the results are discussed. >

Design of a Mosaic BGO Detector System for Positron CT

A new scintillation detector system has been designed for application in high spatial resolution positron CT (PCT) system. Many small BGO crystals are closely packed and coupled to a position-sensitive photomultiplier tube (PMT). The detector performance and feasibility for PCT are discussed based on both calculations and experiments. The coincidence response function of approximately 3.2mm FWHM and coincidence time resolution of less than 6ns FWHM were obtained. The results promise the possibility of a new high resolution PCT system.

Development of a high resolution PET

A high-resolution positron emission tomograph (PET) for brain studies has been developed. It consists of five detector rings (240 BGOs/ring). New multisegment photomultiplier tubes (PMTs) were adopted for the system with 5-mm-wide BGOs. The system is designed to examine a patient sitting or lying down on a chair/bed couch. The functions of PMT auto gain control and real-time image display are implemented in the system. The physical performance of the system was evaluated: the spatial resolution is 3.5 mm in the transaxial plane and 5.7 mm in the axial direction, and the total system sensitivity is 109 kcps/ mu Ci/ml for a uniform phantom 20 cm in diameter with a pulse height threshold of 350 keV. >

A new high resolution PET scanner dedicated to brain research

A high-resolution positron emission tomography (PET) scanner dedicated to brain studies has been developed and its physical performance was evaluated. The block detector consists of a new compact position-sensitive photomultiplier tube (PS-PMT, Hamamatsu R7600-C12) and an 8/spl times/4 bismuth germanate (BGO) array. The size of each crystal is 2.8 mm/spl times/6.55 mm/spl times/30 mm. The system has a total of 11 520 crystals arranged in 24 detector rings 508 mm in diameter (480 per ring). The field of view (FOV) is 330 mm in diameter/spl times/163 mm, which is sufficient to measure the entire human brain. The diameter of the scanners opening is equal to the transaxial FOV (330 mm). The system can be operated in three-dimensional (3-D) data acquisition mode, when the slice septa are retracted. The mechanical motions of the gantry and bed are specially designed to measure the patient in various postures; lying, sitting, and even standing postures. The spatial resolution of 2.9 mm in both the transaxial and axial directions is obtained at the center of the FOV. The total system sensitivity is 6.4 kc/s/kBq/ml in two-dimensional (2-D) mode, with a 20-cm-diameter cylindrical phantom. The imaging capabilities of the scanner were studied with the Hoffman brain phantom and with a normal volunteer.