8. A Depth Encoding Scintillation Detector Unit with a Position-sensitive Photomultiplier Tube
Hideo Murayama
Keywords: scintillation detector, positron emission tomography, nuclear medicine
A detector unit using a depth encoding scheme was designed. The unit consists of four Gd2 Si05:Ce (GSO) crystal blocks in a 2x2 array coupled to a position-sensitive photomultiplier tube (PS-PMT) having metal channel dynodes and 4x4 multi-anodes.
Figure 5 shows a schematic diagram of the detector unit. Element (i,j,k) represents the crystal segment which belongs to the i-th block and the j-th stage and is located on the k-th quadrant in a block. The four elements in the bottom stage in each block are optically coupled to the face plate of the PS-PMT, where each position of the sixteen bottom crystal elements in all the blocks corresponds to that of each anode segment. The position of a crystal element absorbing a gamma photon is detected by applying Anger-type position arithmetic to the output signals from PMT anodes. In each block, the crystal elements are coupled to each other with air gaps, various coupltng compounds, or reflectors, so that the light sharing among elements can be optimized to identify each crystal element clearly in the positioning logic.
The dimensions of GSO crystal elements used in the following experiments to evaluate our proposed detector unit were 3.8 mm x 3.8 mm x 10 mm. Each crystal element of the top stage was optically coupled to another with air gaps, while each crystal element of the middle and bottom stages was optically separated from other elements in the same stage with two layers of 0.1 mm thick PTFE (polytetrafluoroethylene, Teflon) tape. Each stage of the 2x2 array was wrapped with a 0.2 mm thick PTFE tape and stacked together to form a threestage crystal block with Silicone oil (refractive index = 1 .4) . Four crystal blocks were coupled to a multi-anode PS-PMT (Hamamatsu R5900-M16) The R5900-M16 had an outline cross section size of 26 x 26 mm2, with the photocathode sensitive area of 18 x 18 mm2. It had 16 discrete channels arranged in a 4x4 array with individual readout through the segmented anodes.
A 0.1 mCi Cs-137 (662 keV) point source was used for gamma ray irradiation. Four 2D positioning histograms were obtained with the anode signals in four individual quadrants of the PS-PMT. In each histogram, 12 peaks were clearly visualized corresponding to the individual crystal elements. The energy resolutions for the bottom, middle and top stages were 1 7, 2 1, and 21 % and the relative pulse height values for those stages were 1.0, 0.62 and 0.50, respectively. A fan beam of Cs-137 gamma rays was passed through a Pb slit collimator (slit width : 0.5 mm) and scanned along the side of the crystal blocks from the bottom stages to the top stages with 1.0 mm steps. The data were acquired in list mode and events were sorted using an island sorting method by which discrete islands were formed around each crystal element's position distribution in the positioning histograms. For each discrete island of the 2D positioning map, its energy threshold was set at 400 keV. The experimental resuits suggested that the light sharing method identi fied the depth of interaction should be a very reliable and sirnple solution suitable for volume PET devices since the proposed depth encodnng scheme us constructed with all the same crystal elements and does not need additional photo-detectors nor a combina tion of different types of scintillators.
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| Fig.5. Schematic drawing of the depth encoding detector unit with a PS-PMT. |
Publication:
Murayama, Fl., et al..: IEEE Trans. Nucl. Sci., 47 1045-1050 2000