Needs of improved medical diagnostics, specially for early and reliable breast cancer detection, lead us to consider developments in scintillation crystals and position sensitive photomultiplier tubes (PSPMT) in order to develop a high resolution medium field g-ray imaging device. However, gamma rays detector need to find a compromise between many conflicting requirements. In order to optimize different parameters involved in the detection process, we have developed a Monte Carlo simulation software. Its aims were to optimize a gamma ray imaging system based on pixellated scintillation crystal coupled to a PSPMT array. Several crystal properties were taken into account as well as the measured intrinsic response of PSPMTs. Images obtained by simulations are compared with experimental results. Agreement between simulation and experimental results validate our simulation model.
Although gamma cameras have emerged in the sixties, their spatial resolution is still not sufficient to detect small tracer concentration abnormalities. Examinations like scintimammography requires high spatial resolution and then the possibility to position the detector as close to the explored organ as possible . The emergence of the new position sensitive photomultipliers tubes(PSPMT), from HAMAMATSU, permitted us to develop a compact gamma ray imaging probe which fulfils these requirements. The major interest of the new R8520-00-C12 PSPMT generation is their very low height (27mm) which allows to build a very compact and relatively light gamma ray detector. Their square shape (25.7x25.7mm2) and their very thin dead edges (1.85mm) authorize their juxtaposition in order to obtain a large detection area. In this study we investigate the characteristics of a prototype using a square 2x2 array of HAMAMATSU R8520 position sensitive photomultiplier tubes coupled to a pixelated NaI(Tl) crystal array containing 24x24 pixels each made of 2 x 2 x 5 mm3 crystals with 2.2 mm centre to centre spacing. We present the first results regarding intrinsic spatial resolution, energy resolution and homogeneity . Illuminating the detector, without scintillating crystal, with a light source simulating a scintillation at 140kev, we obtain an intrinsic spatial resolution better than 1mm on the whole field of view also including dead areas between PSPMTs. By coupling this detector to the crystal scintillator previously described, an energy resolution better than 10% FWHM at 140kev is obtained in PSPMT centers. These performances and the inherent scalability of detectors built using arrays of square tubes, make it an attractive choice for use in dedicated nuclear medicine instruments, including small animal imaging.
Due to its functional capabilities, gamma imaging is an interesting tool for medical diagnosis. Recent developments lead to improved intrinsic resolution. However this gain is impaired by the poor activity detected and the Poissonian feature of gamma ray emission. High resolution gamma images are grainy. This is a real nuisance for detecting cold nodules in an emitting organ. A specific translation wavelet filter which takes into account the Poissonian feature of noise, has been developed in order to improve the diagnostic capabilities of radioisotopic high resolution images. Monte Carlo simulations of a hot thyroid phantom in which cold spheres, 3-7 mm in diameter, could be included were performed. The loss of activity induced by cold nodules was determined on filtered images by using catchment basins determination. On the original images, only 5-7 mm cold spheres were clearly visible. On filtered images, 3 and 4 mm spheres were put in prominent. The limit of the developed filter is approximately the detection of 3 mm spherical cold nodule in acquisition and activity conditions which mimic a thyroid examination. Furthermore, no disturbing artifacts are generated. It is therefore a powerful tool for detecting small cold nodules in a gamma emitting medium.
Scintimammography is a promising technique for breast cancer detection. Scintimammography uses radiotracer containing 99mTc that emits 140 keV gamma photons. We developed a small field of view gamma ray imaging probe called IRIS. A possible application of this probe is scintimammography. IRIS is composed by a single NaI(Tl) scintillator coupled to a 5 inch round PSPMT. In order to optimize compromise between resolution and detection efficiency, we developed a Monte Carlo code modeling light transport in NaI crystals. The thickness of the scintillator (4 mm) was optimized for 99mTc imaging. We also designed a high-resolution collimator with a 35 mm thickness and 1.7 mm hole diameter. Detection efficiency of the crystal is 65% at 140 keV. IRIS shows a 2.5 mm global spatial resolution in contact. Energy and spatial corrections allow a +/- 5% uniformity and an energy resolution better than 10% at 140 keV. IRIS has a 10 cm field of view and a 13 cm external diameter at the entrance face. The small size of the detector head allows placing the detector close to the breast, improving global spatial resolution. The high-resolution gamma ray imaging probe IRIS shows physical characteristics well suited for 99mTc breast imaging.
The new high performance radioisotopic imager IRIS we recently developed has an intrinsic spatial resolution of about 1.7 mm. To obtain a good sensitivity, it is necessary to use a collimator with holes greater than this spatial resolution. However, this induces artifacts in images. In this paper, we describe a new concept: a moving collimator. Deterministic and Monte Carlo simulations demonstrate that it is possible to obtain simultaneously global high spatial resolution and good sensitivity in radioisotopic imaging.
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