A. Pergola

Multi-PSPMT scintillating camera

Gamma ray imaging is usually accomplished by the use of a relatively large scintillating crystal coupled to either a number of photomultipliers (PMTs) (Anger Camera) or to a single large Position Sensitive PMT (PSPMT). Recently the development of new diagnostic techniques, such as scintimammography and radio-guided surgery, have highlighted a number of significant limitations of the Anger camera in such imaging procedures. In this paper a dedicated gamma camera is proposed for clinical applications with the aim of improving image quality by utilizing detectors with an appropriate size and shape for the part of the body under examination. This novel scintillation camera is based upon an array of PSPMTs (Hamamatsu R5900-C8). The basic concept of this camera is identical to the Anger Camera with the exception of the substitution of PSPMTs for the PMTs. In this configuration it is possible to use the high resolution of the PSPMTs and still correctly position events lying between PSPMTs. In this work the test configuration is a 2 by 2 array of PSPMTs. Some advantages of this camera are: spatial resolution less than 2 mm FWHM, good linearity, thickness less than 3 cm, light weight, lower cost than equivalent area PSPMT, large detection area when coupled to scintillating arrays, small dead boundary zone (<3 mm) and flexibility in the shape of the camera.

SCINTILLATING ARRAY GAMMA CAMERA FOR CLINICAL USE

Abstract Dedicated gamma cameras for specific clinical application are representing a new trend in Nuclear Medicine. They are based on Position Sensitive Photo Multiplier Tubes (PSPMT). The main intrinsic limitation of large area PSPMT (5″ diameter) is the photocathode glass window. Coupling to a planar scintillation crystal strongly affects the useful active area and the intrinsic spatial resolution. To overcome this limitation at University of Rome “La Sapienza” was developed the first 5″ diameter gamma camera consisting of a Hamamatsu R3292 PSPMT coupled to 50 × 50 YAP: Ce scintillating array. The array pixel size is 2 × 2 mm2 and the overall dimension of multi-crystal is 10 × 10 × 1 cm3. Resistive chains were used to calculate the centroid. The scintillating array produces a focused light spot minimising the spread introduced by PSPMT glass window. The intrinsic spatial resolution varied between 2 and 2.7 mm. The position linearity and useful active area resulted in good agreement with intrinsic one obtained by light spot irradiation. The real limitation was the poor energy resolution of an individual crystal (40%) and the poor uniformity response of PSPMT (within ±15%). A correction matrix was then carried out by which a 57% of total energy resolution was obtained for the whole matrix. The camera is currently operating as Single Photon Emission Mammography (SPEM) and it is producing breast functional images for malignant tumour detection using the same geometry as standard X-ray mammography.

Multi-crystal YAP: Ce detector system for position sensitive measurements

Abstract Yttrium aluminum perovskite (YAP:Ce) scintillation crystal has a light efficiency of about 40% relative to NaI. Because of the yttrium atomic number ( Z = 39) and the relatively high density (5.37 g/cm 3 ) it has a good gamma-ray absorption. Furthermore it is not hygroscopic and is inert. Its peculiarity consists of material processing that provides us with crystal pillars down to 0.3 × 0.3 mm 2 aperture size and up to some centimeters in length. An array consisting of 11 × 22 YAP:Ce elements was made where each crystal has an aperture of 0.6 × 0.6 mm 2 and a length of 7 mm. Each scintillation crystal is optically separated by a reflective material resulting in a separation layer between elements of about 5 μm. The multicrystal detector was optically coupled to a Hamamatsu Position Sensitive Photomultiplier Tube (R2486). The intrinsic spatial resolution of the PSPMT is better than 0.3 mm but it is strongly dependent on the Point Spread Function (PSF) generated on the photocathode. The multicrystal detector very well matched the PSPMT characteristics resulting in a spatial resolution of about 0.7 mm at 140 keV ( 99m Tc) gamma irradiation.

First Results from a YAP:Ce Gamma Camera for Small Animal Studies

The YAP (yttrium aluminum perovskite) camera is a novel gamma camera with intrinsic submillimeter spatial resolution and detection efficiency comparable to a standard Anger camera. At the first stage, it is a miniature gamma camera with a field of view of 4/spl times/4 cm/sup 2/ and is currently utilized for radio tracer studies on small animals. The YAP camera consists of a multicrystal array coupled to a position sensitive photomultiplier tube (Hamamatsu R2486) with a parallel hole collimator. The preliminary results are presented and discussed, in particular the intrinsic characteristics of the scintillating array, which are measured, as well as the position linearity and the spatial resolution, with a parallel collimator. Images were obtained from /sup 99m/Tc line source and nude Balb C mice, which have been injected with /sup 99m/Tc MDP, a bone-seeking agent. The observed images show the importance of the small-field YAP camera in radiopharmaceutical research.

Toward a nuclear medicine with sub-millimiter spatial resolution

Abstract The HIRESPET Collaboration is developing a new concept of a gamma camera with sub-millimiter spatial resolution. The first prototype consists of a small field size gamma camera based on a Position Sensitive Photo-Multiplier Tube (PSPMT) coupled to a novel scintillation crystal. The intrinsic spatial resolution of the PSPMT is better than 0.3 mm. The scintillation crystal consists of yttrium aluminium perovskit (YAP:Ce). It has a light efficiency of about 40% relative to NaI, a good gamma radiation absorption ( Z = 39) and a high density (5.37 g/cm 3 ). It is inert and not hygroscopic. To match the PSPMT characteristics, a special crystal assembly has been made consisting of a bundle of YAP pillars, where a single crystal has the transversal dimension of 0.6 × 0.6 mm 2 and a thickness ranging between 1 mm and 28 mm. Each scintillation pillar is optically separated from the other by a reflective layer of 5 μm thick. The preliminary results obtained from the gamma camera prototype (YAP camera) show spatial resolution values ranging between 0.6 mm and 1 mm and an intrinsic detection efficiency comparable with a standard Anger camera.

YAP multi-crystal gamma camera prototype

The Anger camera principle has shown a practical limit of a few millimeters spatial resolution. To overcome this limit, a new gamma camera prototype has been developed, based on a position-sensitive photomultiplier tube (PSPMT) coupled with a new scintillation crystal. The Hamamatsu R2486 PSPMT is a 76-mm diameter photomultiplier tube in which the electrons produced in the conventional bi-alkali photocathode are multiplied by proximity mesh dynodes and form a charge cloud around the original coordinates of the light photon striking the photocathode. A crossed wire anode array collects the charge and detects the original position. The intrinsic spatial resolution of PSPMT is better than 0.3 mm. The scintillation crystal consists of yttrium aluminum perovskit (YAP:Ce or YAlO/sub 3/:Ce). This crystal has a light efficiency of about 38% relative to NaI, no hygroscopicity and a good gamma radiation absorption. To match the characteristics of the PSPMT, a special crystal assembly was produced by the Preciosa Company, consisting of a bundle of YAP:Ce pillars where single crystals have 0.6/spl times/0.6 mm/sup 2/ cross section and 3 mm to 18 mm length. Preliminary results from such gamma camera prototypes show spatial resolution values ranging between 0.7 mm and 1 mm with an intrinsic detection efficiency of 37/spl divide/65% for 140 keV gamma energy. >

NEW GENERATION POSITION-SENSITIVE PMT FOR NUCLEAR MEDICINE IMAGING

Recently, Hamamatsu has developed a new dynode structure in a position-sensitive photomultiplier tube (PSPMT) based on metal channel technology, producing a very focused charge distribution. Hamamatsu R5900-C8 is the first metal channel dynode PSPMT with a crossed wire anode. The outline dimension is 28 × 28 × 20 mm3 with an active area of 21 × 22 mm2 and four wire anodes (4X + 4Y). A weighted summing device was realised to calculate the charge distribution centroid where each anode was directly connected to a preamplifier and amplifier. Tests of position linearity, energy response and spatial resolution were carried out coupling the PSPMT to an NaI(Tl) planar crystal and to a YAP:Ce scintillation array. To test the influence of charge spread on position linearity, the photocathode was irradiated by a light spot with 1 mm of aperture size. Position non-linearity resulted when a scintillating array with 1 × 1 mm2 pixel size was coupled to PSPMT. On the contrary, increasing the spread of charge distribution by a planar scintillation crystal, a good linearity response was obtained. Impressive spatial resolution values were obtained from the YAP:Ce scintillating array. They ranged between 0.5 and 0.2 mm irradiating the crystals by 57Co and 137Cs source, respectively.

A detector for submillimeter gamma cameras

Anger cameras (SPECT etc.) presently used in nuclear medicine employ as active detector NaI crystals, obtaining intrinsic spatial resolutions ≥ 3mm. Arrays made of optically isolated single crystal elements of YAP:Ce, having sub-millimeter aperture size, read out by position sensitive photomultipliers, allow to build active detectors to employ in SPECT systems, with intrinsic spatial resolution below the millimeter, and with time resolution of the order of tens of nanoseconds. In this paper preliminary results of measurements carried on different kinds of YAP:Ce arrays are reported. The measurements have been performed aiming to optimize the geometrical and physical parameters of the crystals in order to accomplish a SPEM (Single Photon Emission Mammography) camera detector.

8 inch diameter PSPMT for gamma ray imaging

This work presents preliminary measurements taken with the first prototype Hamamatsu (R6970) 8-inch Position Sensitive Photomultiplier Tube (PSPMT). These measurements are compared with similar ones obtained using a 5 inch PSPMT (Hamamatsu R3292). The new 8 inch PSPMT has 12 dynode stages, 11 with a proximity mesh structure whilst the last one is a back reflector. The entrance window is 7.5 mm thick and has an active area of 180 mm diameter. The crossed-wire anode of the 8 inch tube consists of 36/spl times/36 wires on a 4 mm pitch. These are paired together to give an 18/spl times/18 wire outputs. Both PSPMTs were coupled to a 110 mm diameter, 3 mm thick CsI(Tl) scintillating array in which each pixel has dimensions of 2/spl times/2 mm/sup 2/. Two read-out methods are compared in this paper. The first being the conventional resistive-divider technique. The second method uses a new multi-wire readout technique in which, the charge on each anode wire is individually read out and digitized. Measurements of the spatial resolution, position linearity, energy resolution and intrinsic charge distribution were carried out for both tubes using both read-out systems. Spatial resolution values of approximately 2 mm FWHM were obtained using the 8-inch PSPMT and the multiwire read-out technique. The other measured characteristics were similar to those obtained using the 5-inch PSPMT. These results obtained using the prototype 8-inch PSPMT underline the potential of this detector in the field of imaging in Nuclear Medicine.

The role of Compton scattering in scintimammography

Functional breast imaging using /sup 99m/Tc MIBI is showing that this technique is able to detect cancer with more than 90% specificity. Using a dedicated gamma camera, with the breast under compression, i.e. in similar conditions of mammography, the detection of sub-centimeter cancers can be improved. A number of factors affects the detection of small cancers as: thickness of the breast, distance between tumor and collimator, cardiac activity. Radioactivity emitted from the body can obscure the breast activity mainly due to the large difference on radioactivity concentration. In this work we analyze the intensity and the energy distribution of Compton scattering coming from the breast by a Germanium detector and by a dedicated imager with a small FOV placed in a geometrical condition similar to mammography. In vivo measurements were performed in patients with a breast cancer ranging between 8 mm and 15 mm. Intensity of Compton scattering from 4 to 10 times greater than full energy peak events resulted. Measurements were compared with ones obtained by a phantom simulating the breast without chest activity demonstrating how large is the Compton contribution from the chest.