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Monolithic scintillator detectors, consisting of several [cm.sup.3] of scintillating material coupled to one or more Hamamatsu S8550 avalanche photodiode (APD) arrays, are proposed as detectors for high resolution positron emission tomography (PET). In this work, the factors contributing to the variance on the signals are investigated, and their effects on the energy, time and spatial resolutions are analyzed. Good agreement was found between a model of the energy resolution and experiments with a 20 x 10 x 10 [mm.sup.3] LYSO:Ce crystal coupled to a single channel large-area AID (LAAPD). With the same crystal coupled to an APD array, differences between model and experiment were observed at high APD gain. The measured energy resolution of ~11% FWHM was dominated by scintillation photon statistics, with less important roles for the APD excess noise factor and electronic noise. On the other hand, electronic noise was an important factor both for the time and the spatial resolutions. The time resolution was found to depend strongly on the APD bias voltage, and was best at the highest bias. A time resolution of 1.6 ns full width at haft maximum (FWHM) was measured against a [BaF.sub.2]-PMT detector. The best spatial resolution measured was 1.64 mm FWHM, without correction for the ~0.9 mm FWHM measurement beam. It is estimated that an intrinsic spatial resolution of 1.26 mm FWHM can be achieved at the center of the detector with an infinitely narrow test beam. Index Terms--Avalanche photodiode (APD), monolithic scintillator detector, positron emission tomography (PET), signal to noise ratio (SNR).

Positron emission tomography detectors based on monolithic scintillation crystals exhibit good spatial and energy resolution, intrinsically provide depth-of-interaction information, have high γ -photon capture efficiency, and may reduce the manufacturing costs compared to pixelated crystal arrays. Here, we present the characterization of a detector consisting of a 18.0 mm×16.2 mm×10.0 mm monolithic LaBr3:5%Ce scintillator directly coupled to a 4×4 array of silicon photomultipliers. An energy resolution of 6.4% full-width-at-half-maximum (FWHM) was obtained. The point-spreadfunction (PSF) was determined for different regions of the detector. The fullwidth-at-half-maximum (FWHM) of the PSF was measured to be <1.5 mm at the center of the detector and <1.7 mm averaged over the entire crystal. Both values are not corrected for the ˜0.6 mm FWHM test beam diameter. Furthermore, the influence of edge effects was investigated. We found that near the edges of the detector the spatial resolution degrades to 2.2 mm (FWHM), and a bias in the position estimates, up to 1.5 mm, was observed. Moreover, the coincidence resolving time for two identical detectors in coincidence was measured to be as small as ˜198 ps FWHM. [ABSTRACT FROM AUTHOR]

Despite the large interest in the application of LaBr3:Ce^3+, little is known yet about its optical properties, as measurements are hampered by the hygroscopicity of LaBr3:Ce^3+ and because it is not trivial to produce crystals with optically polished surfaces. Here, the absorption and scattering lengths as well as the refractive index of LaBr3:5\%Ce^3+ are determined experimentally for the first time. The refractive index is found to vary from 2.25 to 2.40 in the Ce^3+ emission wavelength region depending on crystal orientation. Furthermore, a model of the Ce^3+ absorption and emission probability as a function of wavelength, Ce^3+ concentration, and scintillation photon traveling distance is developed. This model is used in combination with the measured absorption and scattering lengths to obtain the intrinsic emission spectrum of LaBr3:Ce^3+ from measured emission spectra. Additionally, the model is used to illustrate the importance of the investigated crystal properties for scintillation detector design. It is demonstrated that for crystals with dimensions in the order of a few centimeters, the fraction of scintillation photons undergoing scattering and/or absorption before reaching the photosensor can be several tens of percents depending on the Ce^3+ concentration. Finally, it is shown that self-absorption and re-emission of the scintillation photons can have a non-negligible effect on the timing resolution of LaBr3:Ce^3+ scintillator detectors. [ABSTRACT FROM PUBLISHER]

Monolithic scintillator detectors have been shown to provide good performance and to have various practical advantages for use in PET systems. Excellent results for the gamma photon interaction position determination in these detectors have been obtained by means of the k-nearest neighbor (k-NN) method. However, the practical use of monolithic scintillator detectors and the k-NN method is hampered by the extensive calibration measurements and the long computation times. Therefore, several modified k-NN methods are investigated that facilitate as well as accelerate the calibration procedure, make the estimation algorithm more efficient, and reduce the number of reference events needed to obtain a given lateral (x,y)-resolution. These improved methods utilize the information contained in the calibration data more effectively. The alternative approaches were tested on a dataset measured with a SiPM-array-based monolithic LYSO detector. It appears that, depending on the number of reference events, \sim 10\% to \sim 25\% better spatial resolution can be obtained compared to the standard approach. Moreover, the methods amongst these that are equivalent to calibrating with a line source may allow for much faster and easier collection of the reference data. Finally, some of the improved methods yield essentially the same spatial resolution as the standard method using \sim200 times less reference data, greatly reducing the time needed for both calibration and interaction position computation. Thus, using the improvements proposed in this work, the high spatial resolution obtainable with the k-NN method may come within practical reach and, furthermore, the calibration may no longer be a limiting factor for the application of monolithic scintillator detectors in PET scanners. [ABSTRACT FROM AUTHOR]