Tomoaki Tsuda

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.

Performance evaluation of a subset of a four-layer LSO detector for a small animal DOI PET scanner: jPET-RD

Previously, we proposed a new depth of interaction (DOI) encoding method and proved that it worked successfully with four-layered Gd/sub 2/SiO/sub 5/ crystals for a small animal positron emission tomography (PET) detector. We are now planning to develop a small animal PET scanner, jPET-RD (for rodents with DOI detectors), which has both high resolution and high sensitivity by the use of a DOI detector with a 32/spl times/32/spl times/4 crystal array. The scintillator for the detector will be Lu/sub 2(1-x)/Y/sub 2x/SiO/sub 5/ (LYSO). In this work, we evaluated performance of a DOI detector composed of four layers of a 12/spl times/12 LYSO (Lu: 98%, Y: 2%) crystal array by irradiating 511 keV gamma rays uniformly. The new encoding method was used for crystal identification. The size of each crystal was 1.46 mm/spl times/1.46 mm/spl times/4.5 mm. The crystal block was coupled to a 256-channel flat panel position sensitive photomultiplier tube, which has 16/spl times/16 multi anodes at intervals of 3.04 mm. As we expected, all crystals are expressed on a single two-dimensional position histogram without overlapping. Energy resolution of all events is 21.8% and time resolution of all events is 0.69 ns in FWHM. When layers are counted from the top, the energy resolutions of the first, second, third, and fourth layer events are 11.6%, 12.3%, 13.3%, and 19.1% and the time resolutions are 0.60ns, 0.59ns, 0.60ns, and 0.66ns, respectively.

Three-dimensional array of scintillation crystals with proper reflector arrangement for a depth of interaction detector

A new method to acquire four-layer depth of interaction (DOI) information is proposed for the next generation positron emission tomography scanner (jPET-D4) that realizes high resolution and high sensitivity. The detector module of the jPET-D4 is a 16/spl times/16/spl times/4 Gd/sub 2/SiO/sub 5/: Ce (GSO) multicrystal array coupled with a 256 ch flat panel position sensitive photomultiplier tube (256 ch FP-PMT) having large opening area. The first challenge to encode DOI information was carried out with 8/spl times/8 array of units consisted of 2/spl times/2/spl times/4 crystal elements. The unit is developed for four-layer DOI encoding in previous report. Its crystal identification performance is evaluated by uniform gamma ray irradiation. The measured scintillation events are mapped on a two-dimensional (2-D) position histogram according to the relative ratio of the multianode output of the FP-PMT. However, peaks corresponding to the crystal elements of one unit form a colony in the resultant 2-D position histogram and there is large space between adjacent colonies. In the new method, the reflector arrangement which makes proper light sharing in the multicrystal array decreases such wasted space. Consequently, peak-to-valley on the 2-D position histogram was improved to 3.3:1 from 1.8:1. We found energy performance was also enhanced by the new method.

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.

Preliminary resolution performance of the prototype system for a 4-Layer DOI-PET scanner: jPET-D4

We are developing a high-performance brain PET scanner, jPET-D4, which provides 4-layer depth-of-interaction (DOI) information. The scanner is designed to achieve not only high spatial resolution but also high scanner sensitivity with the DOI information obtained from multi-layered thin crystals. The scanner has 5 rings of 24 detector blocks each, and each block consists of 1024 GSO crystals of 2.9 mm/spl times/2.9 mm/spl times/7.5 mm, which are arranged in 4 layers of 16/spl times/16 arrays. At this stage, a pair of detector blocks and a coincidence circuit have been assembled into an experimental prototype gantry. In this paper, as a preliminary experiment, we investigated the performance of the jPET-D4s spatial resolution using the prototype system. First, spatial resolution was measured from a filtered backprojection reconstructed image. To avoid systematic error and reduce computational cost in image reconstruction, we applied the DOI compression (DOIC) method followed by maximum likelihood expectation maximization that we had previously proposed. Trade-off characteristics between background noise and resolution were investigated because improved spatial resolution is possible only when enhanced noise is avoided. Experimental results showed that the jPET-D4 achieves better than 3 mm spatial resolution over the field-of-view.

Annihilation photon acollinearity in PET: volunteer and phantom FDG studies

Annihilation photon acollinearity is a fundamental but little investigated problem in positron emission tomography (PET). In this paper, the cause of the angular deviation from 180.00 degrees is described as well as how to evaluate it under conditions of a spatially distributed radiation source and a limited acquisition time for the human body. A relationship between the shape of the photopeak spectrum and the angular distribution is formulated using conservation laws of momentum and energy over the pair annihilation. Then the formula is used to evaluate the acollinearity for a pool phantom and the human body with FDG injected. The angular distribution for the pool phantom agrees well with that for pure water which had been directly measured by Colombino et al in 1965 (Nuovo Cimento 38 707-23), and also with that for the human body determined in this study. Pure water can be considered as a good approximation of the human body regarding the angular deviation. The blurring coefficient to be multiplied by the ring diameter in calculations of the PET spatial resolution is experimentally determined for the first time as 0.00243 +/- 0.00014; this is 10% larger than the value widely used by investigators.

Preliminary evaluation of four-layer BGO DOI-detector for PET

We found that Bi/sub 4/Ge/sub 3/ O/sub 12/ (BGO) scintillator can be elements of a four-layer depth of interaction (DOI) detector and it was proved with a 12/spl times/12/spl times/4 array of BGO crystals in dimensions of 2.9 mm/spl times/2.9 mm/spl times/7.5 mm coupled to a 256-channel flat panel position sensitive photomultiplier tube. Appropriate reflector insertion in the array makes all crystal identification possible on one position histogram. Despite the large refractivity and small light output of BGO, the four-layer BGO detector showed no significant variation in the full energy peaks among all crystal elements. When no optical grease was used in the construction of the BGO DOI-block and irradiated with gamma-rays from /sup 137/Cs, a top layer crystal has 80% of light output relative to the bottom layer. The obtained two-dimensional position histogram by the irradiation was clear enough to allow identification of the crystals of interaction. Profiles of the histogram show peak-to-valley ratio of 1.9:1 for the top layer crystals and larger ratio for other layer crystals in the experiment.

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.

The jPET-D4: imaging performance of the 4-layer depth-of-interaction PET scanner

The jPET-D4 is a high-performance brain PET scanner which achieves not only high spatial resolution but also high scanner sensitivity by discriminating 4-layer depth-of-interaction (DOI) information. The scanner is designed to have 5 rings of 24 detector blocks each, and the detector block consists of 1,024 GSO crystals of 2.9 mm /spl times/ 2.9 mm /spl times/ 7.5 mm, which are arranged in 4 layers of 16 /spl times/ 16 arrays. In this paper, we investigated the imaging performance of the jPET-D4 prototype where one of the 5 block-detector rings was assembled. In order to reduce computational cost while retaining the advantage of DOI information in iterative image reconstruction, we have proposed the DOI compression (DOIC) method which reduces data dimensions with suppressing resolution loss. We have also proposed an approximated system model for the jPET-D4 which enables fast system matrix calculation while preserving image quality. The experimental results show that almost uniform spatial resolution of 2-3 mm is obtained over the field-of-view by using the 4-layer DOI information. A Huffman phantom image clearly shows the excellent imaging performance of the jPET-D4.

Preliminary design studies of a high sensitivity small animal DOI-PET scanner: jPET-RD

We present a preliminary study on the design of a high sensitivity small animal DOI-PET scanner: jPET-RD (for Rodents with DOI detectors), which is under development at the National Institute of Radiological Sciences in Japan. The scanner composed of two rings of six detector blocks arranged in a hexagonal shape with an inner radius of 88 mm. Each detector consists of a four-layer array of 32/spl times/32 scintillator crystals each measuring 1.44/spl times/1.44/spl times/4.5 mm/sup 3/ and a 256ch flat panel position sensitive photomultiplier tube (FP-PS-PMT). The FP-PS-PMT has a large effective area of 49/spl times/49 mm/sup 2/ and multi anodes of 16/spl times/16ch, which can be read out in parallel. We simulated the scanner for various parameters of the number of DOI channels and the length of crystal. Simulated data were reconstructed using the maximum likelihood expectation maximization with accurate system modeling. The trade-off results between background noise and spatial resolution shows that only shortening the length of crystal does not improve the trade-off at all, and that four-layer DOI information improves uniformity of spatial resolution in the whole FOV. Monte Carlo simulations were also performed using the EGS4 code to measure absolute sensitivity and count rate capability with a small cylindrical phantom. We also evaluated the impact of critical design parameters on the noise equivalent count rate (NECR) and LOR errors caused by inter-crystal Compton scatters. Simulation results show that this scanner can achieve high sensitivity and high NECR, which can be further improved by using an appropriate anode segmentation of the FP-PS-PMT.