Tomohide Omura

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.

A four-Layer depth of interaction detector block for small animal PET

We are now planning to develop a positron emission tomograph dedicated to small animals such as rats and mice which meets the demand for higher sensitivity. We propose a new depth of interaction (DOI) detector arrangement to obtain DOI information by using a four-layer detector with all the same crystal elements. In this DOI detector, we control the behavior of scintillation photons by inserting the reflectors between crystal elements so that the DOI information of four layers can be extracted from one two-dimensional (2D) position histogram made by Anger-type calculation. As a preliminary experiment, we measured crystal identification performance of the DOI detector which consists of four layers of a 16 /spl times/ 16 crystal array using Gd/sub 2/SiO/sub 5/ crystals with Ce concentration of 0.5 mol %. Each crystal is 1.42 mm /spl times/ 1.42 mm /spl times/ 4.5 mm. A crystal block is optically coupled to a 256-channel flat panel position sensitive photomultiplier tube whose opening area is 52.0 mm /spl times/ 52.0 mm. We obtained sufficient positioning performance for this four-layer DOI detector on the 2D position histogram. We concluded it would be a promising device to realize a small animal positron emission tomography scanner with high sensitivity and high resolution.

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.

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.

Performance of a PET detector with a 256ch flat panel PS-PMT

A 256ch flat panel position sensitive photomultiplier tube (FP-PMT) is a promising device for a PET detector because of its large opening area, 52mm /spl times/ 52 mm, and small dead space. The useful area of the FP-PMT is 89% to the opening area so that the FP-PMT affords optical coupling with a 16 /spl times/ 16 array or scintillation crystals having 3 mm /spl times/ 3 mm bottom area. Its 14.7 mm thickness will also ensure a compact volume and less weight for the PET apparatus. We evaluated performance by irradiating 511keV gamma ray onto GSO crystals coupled to a prototype of the FP-PMT. The resultant positioning image map assure its capability for crystal identification. In a series of measurements, we used a multilayer polymer mirrors for a reflector of the detector. It was cut or marked for folding in precise sizes using a CO/sub 2/ gas laser. Making folds on the reflector by a laser contributed to easier assembly of the detector composed of many small crystal elements and may potentially be utilized in various shaped detectors.

Development of a high-resolution four-layer DOI detector using MPPCs for brain PET

A new high-resolution four-layer DOl detector using MPPCs for brain PET scanner has been developed. The new depth of interaction (DOl) detector was designed to compose of four layers of detector units, which were lined up five axially. Each of the detector units consists of a LYSO scintillator array finely segmented of 1.2 mm and an 8 × 8 array of multi-pixel photon counters (MPPCs), which are one of the products of silicon photomultiplier family. The MPPC is so compact and insensitive to gamma-ray that the detector units can be piled up with a small gap between each scintillator array in the depth direction. In order to have the detector in every layer equally sensitive to gamma-ray, the scintillator thickness was designed at 3 mm, 4 mm, 5 mm and 8 mm toward the bottom respectively, and the total thickness was 20 mm. We adopted an internal focused laser processing technique to a monolithic LYSO scintillator and fabricated a 2D segmented array of 32 × 32 with 1.2 mm pitch in 38.4 mm square cross-section. Each detector layer has independently front end circuits including ASICs for MPPCs and signal processing circuits for crystal identification, energy and timing detection. Each data set of four layers are fed into data interface circuits placed behind detector layers and transferred to a data acquisition unit as formatted list-mode data. The performance of the four-layer DOl detector has been evaluated. The coincidence timing resolution of the detector, with a reference BaF2 detector, was obtained 850 ps FWHM. The average energy resolution value was 24.5% at 511 keV. The crystal separation with finely segmented LYSO scintillator was also good enough at each layer.

Development of a Small Animal PET Scanner Using DOI Detectors

A small animal PET scanner using 256-channel PS-PMTs has been designed, constructed, and evaluated. The scanner has twelve detector modules, each of which consists of a double-layer array of LYSO crystals and three PS-PMTs (Hama- matsu R8400-00-M256). In the LYSO crystal block, 32 times 53 crystal elements are optically coupled to 32 times 54 crystal elements with a shift of half the element pitch in the axial direction. The dimension of each crystal element is 1.275 times 2.675 mm2 in cross section and 7 mm in depth. The twelve detector modules are positioned on a 182 mm diameter ring to form 107 detector rings with 1.4 mm pitch. The transaxial FOV is 100 mm in diameter and the axial FOV is 151 mm, which is sufficient to cover the whole body of a mouse. In order to compensate for non-uniform outputs from the multi-anodes of the PS-PMTs, ASICs having 64-channel variable gain amplifiers and summing amplifiers are used in the front-end circuits. The preliminary experimental results are the transaxial resolution of 2.0 mm FWHM in the CFOV, and the axial resolution of 2.8 mm FWHM on the axis of the ring. The absolute coincidence sensitivity is 8.1% for a point source at the CFOV with setting an energy window of 350-750 keV and a timing window of 10 ns. The applicable imaging capability of the scanner was demonstrated by animal studies with a rat.

Development of a Multi-Pixel Photon Counter Module for Positron Emission Tomography

A new detector module for positron emission tomography using multi-pixel photon counters (MPPCs) was developed. The detection area is 40 mm × 40 mm using an 8 × 8 MPPC array. The active area of each MPPC is 3 mm × 3 mm. The detector module contains preamplifiers, digital-to-analog converters for setting the bias offset, a 2D resistor network for position encoding, a high-voltage module, a temperature sensor, and embedded microprocessor units. Since the MPPC gain depends on temperature, compensation was performed by varying the bias voltage. Two kinds of flood images of an 8 × 8 array (5-mm pitch) and a 32 × 32 (1.25-mm pitch) array were acquired and examined. The timing resolution and the effect of dark counts were evaluated. With a reference detector consisting of a BaF2 crystal coupled to a PMT, the timing resolution was 354 ps when a single 5 mm × 5 mm × 20 mm LYSO scintillator was coupled to one MPPC channel. However, the time resolution of the array deteriorated because of the increase in dark counts.

Development of a new brain PET scanner based on single event data acquisition

A new brain PET scanner based on single event data acquisition with on-the-fly coincidence detection has been developed. The new scanner was designed to form a detector ring of 430 mm in diameter with 32 detector modules. The single event data generated at each detector module were transferred to the data acquisition system through a fiber cable. The single event data from all detector modules were merged and processed to create coincidence event data in software with on-the-fly. The validation results indicated that the single count rate capability was expected over 220 Mcps. At experiments using 18F radioisotopes, the maximum single count rate was approximately 20 Mcps, and the maximum coincidence count rate with prompt was obtained 3.2 Mcps. The system timing resolution was 2.6 ns FWHM, and the optimum coincidence time window was 4.3 ns, obtained by adjusting the timing delay variations between each detector module.

Improvement of the depth of interaction detector for PET on full energy pulse height uniformity

As part of the next generation PET project, uniformity of full energy pulse height for all crystal elements was improved in the depth of interaction (DOI) detector constructed of three-dimensional crystal arrays. In our previous report, we found that the DOI detector constructed of four stages of a 2 /spl times/ 2 Gd/sub 2/SiO/sub 5/:Ce (GSO) crystal array provides good crystal identification performance but poor uniformity of the energy pulse height distribution. The upper stage crystal elements which stay further from the photocathode of a PMT have a tendency to show lower energy pulse height. For example, the ratio of the full energy peak of the top stage crystal to the bottom stage one was about 0.3. We designed a new DOI detector improved in the uniformity. By optimizing crystal surface finishes, reflector configurations, and optical coupling between crystal elements, we got comparable energy pulse height from the upper stage crystals to the bottom stage crystals. Despite this change of detector conditions, good separation between each area corresponding to crystal elements is maintained on two-dimensional histograms obtained by Anger-type position calculation. The uniform full energy pulse height of every stage crystal allows a narrower dynamic range of the electrical circuits, and may give a great advantage in getting an accurate scatter correction. It also improves energy and timing resolution.