Search

The response of a silicon photomultiplier (SiPM) to optical signals is inherently nonproportional due to saturation, afterpulsing, and crosstalk. Existing models of the SiPM response do not account for all of these effects, and therefore, these models are not sufficiently accurate for many applications. In this work, a comprehensive model of the SiPM response is developed that is generally applicable to exponentially decaying light pulses and that can be simplified in the case of very short (e.g., laser) light pulses. The model accounts for the total number and the temporal distribution of the incident photons as well as for the relevant SiPM parameters, viz. the recovery time, afterpulsing, crosstalk, and their cross correlations. The model is shown to correspond well with measurements on a SiPM-based scintillation detector. Furthermore, it is shown to be in agreement with several cases for which the SiPM response is known a priori. Having thus validated the model, its use is demonstrated by predicting the response of the Hamamatsu multipixel photon counter (MPPC) S10362-33-050C SiPM to several different scintillators., Radiation, Radionuclides and Reactors, Applied Sciences

The response of a silicon photomultiplier (SiPM) to optical signals is inherently nonproportional due to saturation, afterpulsing, and crosstalk. Existing models of the SiPM response do not account for all of these effects, and therefore, these models are not sufficiently accurate for many applications. In this work, a comprehensive model of the SiPM response is developed that is generally applicable to exponentially decaying light pulses and that can be simplified in the case of very short (e.g., laser) light pulses. The model accounts for the total number and the temporal distribution of the incident photons as well as for the relevant SiPM parameters, viz. the recovery time, afterpulsing, crosstalk, and their cross correlations. The model is shown to correspond well with measurements on a SiPM-based scintillation detector. Furthermore, it is shown to be in agreement with several cases for which the SiPM response is known a priori. Having thus validated the model, its use is demonstrated by predicting the response of the Hamamatsu multipixel photon counter (MPPC) S10362-33-050C SiPM to several different scintillators., Radiation, Radionuclides and Reactors, Applied Sciences

Charge Coupled Devices (CCDs) optically coupled to scintillation crystals can be used to construct high resolution gamma cameras. Previously, several groups have reported intrinsic detector spatial resolutions below 100 microns. When the CCD images can be read out fast enough, the energy and interaction position of individual gamma quanta can be estimated by a real-time image analysis of the scintillation light flashes. The Electron-Multiplying CCD (EMCCD) is well-suited for fast read out, since even at high frame rates it has extremely low readout noise. However, due to the often very low light levels present in scintillation gamma cameras, further reduction of noise is desirable. Here, the EMCCD is optically coupled to a 1000- µ-thick columnar CsI(Tl) crystal for Tc-99m and I-125 imaging.We investigate noise reduction through summing of signals in pixels before the gain register and readout circuit of the EMCCD (“pixel binning”). We compare the energy and spatial resolution, photopeak efficiency (PE) and signal-to-noise ratio (SNR) of an EMCCD-based gamma camera for the case of binning vs. non-binning. When pixels were read out simultaneously in groups of 4 the spatial resolution is slightly worse in the direction of binning, but the number of false-positive counts resulting from background noise for I-125 was reduced by 74% compared to the no binning case. We conclude that the use of charge binning when reading out EMCCDs may significantly improve the energy spectra and noise properties of CCD-based high-resolution gamma cameras., Radiation, Radionuclides and Reactors, Applied Sciences

With the steadily improving resolution of novel small-animal single photon emission computed tomography (SPECT) and positron emission tomography devices, highly detailed phantoms are required for testing and optimizing these systems. We present a three-dimensional (3-D) digital and physical phantom pair to represent, e.g., cerebral blood flow, glucose metabolism, or neuroreceptor binding in small regions of the rat brain. The anatomical structures are based on digital photographs of the uncut part of a rat brain cryosection block. The photographs have been segmented into ventricles and gray and white matter and have been stacked afterwards. In the resulting voxelized digital phantom, tracer concentration in gray and white matter can be scaled independently. This is of relevance since, e.g., cerebral blood flow or metabolism are much higher in gray than in white matter. The physical phantom is based on the digital phantom and has been manufactured out of hardened polymer using rapid prototyping, a process in which complicated 3-D objects can be built up layer by layer. X-ray computed tomography and high-resolution SPECT images of the physical phantom are compared with the digital phantom. The detailed physical phantom can be filled bubble-free. Excellent correspondence is shown between details in the digital and physical phantom. Therefore, this newly developed brain phantom will enable the optimization of high-resolution imaging for recovery of complex shaped molecular distributions., Radiation, Radionuclides and Reactors, Applied Sciences

Charge Coupled Devices (CCDs) optically coupled to scintillation crystals can be used to construct high resolution gamma cameras. Previously, several groups have reported intrinsic detector spatial resolutions below 100 microns. When the CCD images can be read out fast enough, the energy and interaction position of individual gamma quanta can be estimated by a real-time image analysis of the scintillation light flashes. The Electron-Multiplying CCD (EMCCD) is well-suited for fast read out, since even at high frame rates it has extremely low readout noise. However, due to the often very low light levels present in scintillation gamma cameras, further reduction of noise is desirable. Here, the EMCCD is optically coupled to a 1000- µ-thick columnar CsI(Tl) crystal for Tc-99m and I-125 imaging.We investigate noise reduction through summing of signals in pixels before the gain register and readout circuit of the EMCCD (“pixel binning”). We compare the energy and spatial resolution, photopeak efficiency (PE) and signal-to-noise ratio (SNR) of an EMCCD-based gamma camera for the case of binning vs. non-binning. When pixels were read out simultaneously in groups of 4 the spatial resolution is slightly worse in the direction of binning, but the number of false-positive counts resulting from background noise for I-125 was reduced by 74% compared to the no binning case. We conclude that the use of charge binning when reading out EMCCDs may significantly improve the energy spectra and noise properties of CCD-based high-resolution gamma cameras., Radiation, Radionuclides and Reactors, Applied Sciences

With the steadily improving resolution of novel small-animal single photon emission computed tomography (SPECT) and positron emission tomography devices, highly detailed phantoms are required for testing and optimizing these systems. We present a three-dimensional (3-D) digital and physical phantom pair to represent, e.g., cerebral blood flow, glucose metabolism, or neuroreceptor binding in small regions of the rat brain. The anatomical structures are based on digital photographs of the uncut part of a rat brain cryosection block. The photographs have been segmented into ventricles and gray and white matter and have been stacked afterwards. In the resulting voxelized digital phantom, tracer concentration in gray and white matter can be scaled independently. This is of relevance since, e.g., cerebral blood flow or metabolism are much higher in gray than in white matter. The physical phantom is based on the digital phantom and has been manufactured out of hardened polymer using rapid prototyping, a process in which complicated 3-D objects can be built up layer by layer. X-ray computed tomography and high-resolution SPECT images of the physical phantom are compared with the digital phantom. The detailed physical phantom can be filled bubble-free. Excellent correspondence is shown between details in the digital and physical phantom. Therefore, this newly developed brain phantom will enable the optimization of high-resolution imaging for recovery of complex shaped molecular distributions., Radiation, Radionuclides and Reactors, Applied Sciences

SPECT imaging with low dose or short acquisition time can lead to very noisy reconstructed images. With the USPECT-II system we study the effect of low-count scans, and 3D Gaussian blurring to suppress statistical errors. Three different groups of short-time cardiac rat scans were derived from a longtime scan. Images were reconstructed and then smoothed with 3D Gaussian filters with different kernel sizes. We used the standard deviations of voxel values in the reconstructions from each short-time group to indicate the quality of the blurred images. The results show that a good cardiac rat SPECT image can be obtained from a scan in the U-SPECT-II system within very short time (2 minutes) or with very low dose (0.30 mCi activity injected) if some additional blurring is accomplished to the reconstruction., Radiation, Radionuclides and Reactors, Applied Sciences

For multipinhole single-photon emission computed tomography (SPECT), iterative reconstruction algorithms are preferred over analytical methods, because of the often complex multipinhole geometries and the ability of iterative algorithms to compensate for effects like spatially variant sensitivity and resolution. Ideally, such compensation methods are based on accurate knowledge of the position-dependent point spread functions (PSFs) specifying the response of the detectors to a point source at every position in the instrument. This paper describes a method for model-based generation of complete PSF lookup tables from a limited number of point-source measurements for stationary SPECT systems and its application to a submillimeter resolution stationary small-animal SPECT system containing 75 pinholes (U-SPECT-I). The method is based on the generalization over the entire object to be reconstructed, of a small number of properties of point-source responses which are obtained at a limited number of measurement positions. The full shape of measured point-source responses can almost be preserved in newly created PSF tables. We show that these PSFs can be used to obtain high-resolution SPECT reconstructions: the reconstructed resolutions judged by rod visibility in a micro-Derenzo phantom are 0.45 mm with 0.6-mm pinholes and below 0.35 mm with 0.3-mm pinholes. In addition, we show that different approximations, such as truncating the PSF kernel, with significant reduction of reconstruction time, can still lead to acceptable reconstructions., Radiation, Radionuclides and Reactors, Applied Sciences

Radiation, Radionuclides and Reactors, Applied Sciences

For multipinhole single-photon emission computed tomography (SPECT), iterative reconstruction algorithms are preferred over analytical methods, because of the often complex multipinhole geometries and the ability of iterative algorithms to compensate for effects like spatially variant sensitivity and resolution. Ideally, such compensation methods are based on accurate knowledge of the position-dependent point spread functions (PSFs) specifying the response of the detectors to a point source at every position in the instrument. This paper describes a method for model-based generation of complete PSF lookup tables from a limited number of point-source measurements for stationary SPECT systems and its application to a submillimeter resolution stationary small-animal SPECT system containing 75 pinholes (U-SPECT-I). The method is based on the generalization over the entire object to be reconstructed, of a small number of properties of point-source responses which are obtained at a limited number of measurement positions. The full shape of measured point-source responses can almost be preserved in newly created PSF tables. We show that these PSFs can be used to obtain high-resolution SPECT reconstructions: the reconstructed resolutions judged by rod visibility in a micro-Derenzo phantom are 0.45 mm with 0.6-mm pinholes and below 0.35 mm with 0.3-mm pinholes. In addition, we show that different approximations, such as truncating the PSF kernel, with significant reduction of reconstruction time, can still lead to acceptable reconstructions., Radiation, Radionuclides and Reactors, Applied Sciences

SPECT imaging with low dose or short acquisition time can lead to very noisy reconstructed images. With the USPECT-II system we study the effect of low-count scans, and 3D Gaussian blurring to suppress statistical errors. Three different groups of short-time cardiac rat scans were derived from a longtime scan. Images were reconstructed and then smoothed with 3D Gaussian filters with different kernel sizes. We used the standard deviations of voxel values in the reconstructions from each short-time group to indicate the quality of the blurred images. The results show that a good cardiac rat SPECT image can be obtained from a scan in the U-SPECT-II system within very short time (2 minutes) or with very low dose (0.30 mCi activity injected) if some additional blurring is accomplished to the reconstruction., Radiation, Radionuclides and Reactors, Applied Sciences

Radiation, Radionuclides and Reactors, Applied Sciences

We demonstrate new technologies for SPECT imaging with unsurpassed resolution in mice and rats. Results of the imaging of living animals will be shown. In addition development of detectors for next generation systems with an even higher resolution will be shown., Radiation, Radionuclides and Reactors, Applied Sciences

We demonstrate new technologies for SPECT imaging with unsurpassed resolution in mice and rats. Results of the imaging of living animals will be shown. In addition development of detectors for next generation systems with an even higher resolution will be shown., Radiation, Radionuclides and Reactors, Applied Sciences