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The design of detectors for radio-imaging of small animals is challenging because of the high spatial resolution required, possibly coupled with high efficiency to allow dynamic studies. Spatial resolution and sensitivity are difficult to attain at the same time with single photon imaging techniques because collimators define and limit performance. In this paper we first describe a simple desktop gamma imager equipped with a pinhole collimator and based on a pixellated NaI(TI) scintillator array coupled to a Hamamatsu R2486 PSPMT. The limits of such a system as well as the way to overcome them in future systems is shown next. Better light sampling at the anode level would allow better pixel identification for a higher number of pixels, which is one of the parameters defining image quality and improving spatial resolution. The performance of such a design is compared with other designs using other PSPMT types with different light sampling schemes at the anode level. Finally, we show how the substitution of the pinhole collimator with a coded aperture collimator can result in a substantial improvement in system sensitivity while maintaining very good spatial resolution, possibly at a sub-millimeter level. Calculations and simulations of a particular solution show that sensitivity can improve by a factor of nearly 30. Index Terms--Nuclear imaging, scintillation detectors, sensitivity, spatial resolution.

Background: Sphincter of Oddi dysfunction is diagnosed at manometry and, after cholecystectomy, non-invasively at quantitative choledochoscintigraphy. Patients may benefit from endoscopic sphincterotomy. Aims: The aim of this study was to [...]

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 (

Prone scintimammography with Tc-99m SestaMIBI has been proposed as a method of detecting breast cancers in women with dense breasts. Clinical reports have shown that this technique lacks sensitivity for cancer smaller than 1 cm. To overcome some limitations offered by Anger camera on breast imaging was realized a new scintigraphic detector dedicated to SPEM (Single Photon Emission Mammography). This novel small FOV device, shows high spatial resolution performances (< 2 mm), but poorer energy resolution values, than Anger camera (17% vs. 10%, respectively). In this paper we examine the spatial distribution of Compton-to-photopeak ratio in planar scintimammographic images in order to elucidate the causes of lack of scintimammography in visualizing deep or posteriorly located cancers introduced by the scatter from the patient's body. Phantom and clinical studies with several instruments including a general purpose gamma camera, a dedicated camera and a non imaging Germanium detector were performed. The results obtained suggest that scatter from extramammary sources plays an important role in breast imaging with radiotracers. By using gentle breast compression in cranio-caudal view the energy resolution attained with the dedicated camera is adequate to differentiate between photopeak and Compton counts; the Compton-to photopeak mean ratio resulted 4.5 and 6.9 for mild portion and posterior breast respectively.

For scintimammography (SMM) to have wide clinical use, physicians must be able to rapidly direct biopsy of small cancers. We therefore undertook a theoretical and experimental study of lesion visibility vs. acquisition time and tumor size. Theoretical considerations as to acquisition time and contrast suggested that compression would significantly improve lesion visibility for small cancers. These predictions were confirmed with phantom studies. Patient studies showed that high confidence images of small primary breast tumors can be produced with the addition of breast compression. We conclude that breast compression would improve SMM detection of small primary breast cancers.

The YAP (Yttrium Aluminum Peroskite) 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 x 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 photo multiplier tube (Hamamatsu R2486) with a parallel hole collimator. The preliminary results are presented and discussed, in particular the intrinsic characteristics of scintillating array, which are measured, as well as the position linearity and the spatial resolution with a parallel collimator. Images were obtained from 99mTc line source and nude Balb C mice, which have been injected with 99mTc MDP, a bone-seeking agent. The observed images show the importance of the small-field YAP camera in radiopharmaceutical research.

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 YAl[O.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 x 0.6 m[m.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 / 65% for 140 keV gamma energy.