Harold Rothfuss

Comparison of Fast Scintillators With TOF PET Potential

The renewed interest in time-of-flight (TOF) positron emission tomography (PET) has been accompanied by new research in the development of fast scintillators, mainly halides and/or lutetium-based compounds doped with Ce or Pr. In this work we measure some intrinsic properties of these materials, such as decay time and light output, which have a direct effect on time resolution, the key performance parameter for a TOF-grade detector. In particular, we report on measurements on LSO:Ce, LuAG:Pr, LuYAP:Ce, LaBr3:Ce and LaCl3:Ce. The scintillators are characterized in terms of absolute light yield, decay time, energy resolution, emission and excitation spectra, and time resolution. A new figure of merit to compare scintillators based on their performance in a TOF PET scanner is introduced. This figure of merit, the TOF effective sensitivity eta , includes both interaction probability and timing characteristics.

PET reconstruction with system matrix derived from point source measurements

The quality of images reconstructed by statistical iterative methods depends on an accurate model of the relationship between image space and projection space through the system matrix. A method of acquiring the system matrix on the CPS Innovations the HiRez scanner was developed. The system matrix was derived by positioning the point source in the scanner field of view and processing the response in projection space. Such responses include geometrical and detection physics components of the system matrix. The response is parameterized to correct point source location and to smooth projection noise. Special attention was paid to span concepts of HiRez scanner. The projection operator for iterative reconstruction was constructed, taking into account estimated response parameters. The computer generated and acquired data were used to compare reconstruction obtained by the HiRez standard software and produced by better modeling. Results showed that the better resolution and noise property can be achieved.

Effect of codoping on scintillation and optical properties of a Ce-doped Gd3Ga3Al2O12 scintillator

Single crystals of Gd3Ga3Al2O12 : Ce with different codopants were successfully grown using the Czochralski technique. Optical and scintillation properties of these codoped crystals were studied in detail including absorption, photoluminescence excitation and emission, decay time and thermoluminescence. This study revealed that while boron codoping improves the scintillation light output and energy resolution and decreases self-absorption in these crystals, calcium codoping affects their properties in the opposite manner. In addition to antisite defects, the effect of room temperature trap centres on the sensitivity of these crystals to light exposure is also reported for the first time. Light sensitivity was also found to be affected with the incorporation of codopants in the lattice. The effect of annealing in oxidizing and reducing atmospheres on the scintillation and optical properties of differently codoped crystals was also investigated in detail in order to better understand the defect structure of these crystals. All these measurements together are used to explain the effect of codoping on the crystal field and defect structure of these crystals.

Monte Carlo simulation study of LSO crystals

In order to study light transport in LSO crystals for PET, we simulated single crystals and block detectors using Detect2000. We devised basic experiments to evaluate fundamental physics parameters of the simulation and benchmarked the simulation with a set of experiments on single crystals. The block detectors that were studied were composed of Lutetium Oxyortho-Silicate crystals, a light guide and Photo Multiplier Tubes. The results of the simulation were compared to an experimental detector blocks position profile. By refining our simulation to model actual detectors, a tool was created that can be used to optimize the performance of detector blocks.

The Effect of

Low temperature (~35 K) measurements of the scintillation kinetics of Y2SiO5:Ce (YSO:Ce) have previously illustrated that shallow electron traps can play an important role in the scintillation mechanism. In addition, divalent calcium co-doping of isostructural Lu2SiO5:Ce (LSO:Ce) has been shown to eliminate shallow electron traps and decrease scintillation decay time while maintaining high light output. Here we investigate the effect of Ca2+ codoping on the trap populations and scintillation kinetics of YSO:Ce. Single crystals were grown with Ca2+ concentrations up to 0.5 at% relative to Y. Thermoluminescence measurements indicate a significant reduction in shallow traps, and a marked change in the scintillation kinetics can be seen in the scintillation time profiles as a result of Ca2+ codoping.

{\hbox {Ca}}^{2+}

LSO scintillators (Lu2Sio5:Ce) have a background radiation which originates from the isotope Lu-176 that is present in natural occurring lutetium. The decay that occurs in this isotope is a beta decay that is in coincidence with cascade gamma emissions with energies of 307, 202 and 88 keV. The coincidental nature of the beta decay with the gamma emissions allow for separation of the emission data originating from a positron annihilation event from transmission type data from the Lu-176 beta decay. By using the time of flight information, and information of the chord length between two LSO pixels in coincidence as a result of a beta emission and emitted gamma, a second time window can be set to observe transmission events simultaneously to emission events. Using the time when the PET scanner is not actively acquiring positron emission data, a continuous blank can be acquired and used to reconstruct a transmission image. With this blank and the measured transmission data, a transmission image can be reconstructed. This reconstructed transmission image can be used to perform emission data corrections such as attenuation correction and scatter corrections. It is observed that the flux of the background activity is high enough to create good transmission images with an acquisition time of 10 minutes.

Codoping on Shallow Traps in YSO:Ce Scintillators

A way to improve the spatial resolution in positron emission tomography (PET) is to determine the depth-of-interaction (DOI) in the detector. A way to achieve this is to use the phoswich approach, a detector with two or more layers of different scintillators. The layer identification is done by using differences in scintillation decay time and pulse shape discrimination techniques. The advantages of the concept have been demonstrated in the HRRT high resolution PET system using a LSO/LYSO combination giving a high spatial resolution uniformity of around 2.5 mm within a larger part of the imaged volume. A phoswich combination that lately has received attention is LuAP/LSO or LuYAP/LSO. The suggestions come from the crystal clear collaboration and there is a patent application for its use in PET. This particular combination of phoswich may, however, have a complication since both LuAP and LuYAP emit in the excitation band of LSO, thus making the functionality more complex. In the present paper we have looked into this and suggested different ways to overcome potential drawbacks.

LSO background radiation as a transmission source using time of flight.

The quest for the ideal inorganic scintillator is still active in PET. The aim of this investigation is to study the imaging performance of a 3D PET system as a function of the crystal when considering the HiRez scanner geometry and its Pico acquisition system. A realistic Monte-Carlo model of the scanner was developed using GATE. The model was tuned to measurements using the NEMA NU2-2001 protocol. Measured energy resolution, time window and energy window were inputs in the simulation, while measured front-end deadtime was introduced externally. Small discrepancies between experiment and simulation were described by relative efficiencies for singles and coincidences. The model was modified by replacing LSO by BGO, GSO, LaBr3 and CeBr3, for two different crystal lengths (20 and 30mm). Energy resolution, energy window, time coincidence window and deadtime parameters were adapted in each case. NEC curves were obtained by scaling the experimental HiRez sensitivity for singles and coincidences to the GATE results. The result shows that only the LSO 30 mm case provides superior NEC performance when the axial scanner length is fixed. Considering the HiRez geometry, LSO appears to yield the best imaging performance as measured by peak NEC, due to the combination of high speed and high stopping power.

Design Considerations of Phoswich Detectors for High Resolution Positron Emission Tomography

The main performance parameters in positron camera system design are sensitivity and spatial resolution. This paper concerns sensitivity, which is a function of the scintillation material, the solid angle subtended by the detector array, and the scintillator packing fraction. The solid angle can be increased by extending the axial extent of cylindrical detector systems. Most commercial positron camera systems are based on rings of detector blocks with lutetium oxyorthosilicate, LSO:Ce or LYSO:Ce, as the scintillator of choice. By adding more rings, the solid angle and thus the absolute sensitivity increases while the singles detection efficiency remains fairly constant assuming the same crystal thickness. It has been shown that Ca co-doping of LSO:Ce reduces the scintillation decay time to ~30 ns with a light output over 30000 ph/MeV. This improvement may give a time-of-flight (TOF) advantage with time resolution of 500 ps or less. If the count rate sensitivity of a large axial field-of-view (AFOV) system is combined with the TOF sensitivity increase, we have the means to create examination times in the sub-minute range with no compromise in image quality. In the present study we have compared the existing Siemens molecular CT (mCT) systems to future 6, 8, 12, 20 and higher block ring systems with and without TOF. The mCT 4 block ring system has been used as a reference. The time for acceptable image quality with this system is then extrapolated to other systems based on planar sensitivity. However, the planar sensitivity is related to the solid angle, and reaches saturation for large AFOVs. This implies that there is an upper count rate sensitivity limit. A 20 block ring system may cover a 70 cm examination range at a certain planar count rate and could provide acceptable quality images in approximately 10 seconds by combining the high planar sensitivity count rate provided by the multi-ring feature, the high stopping power of LSO and the TOF gain due to the improved timing resolution. The increased sensitivity may be used to reduce patient dose.

Influence of crystal material on the performance of the HiRez 3D PET scanner: A Monte-Carlo study

The temperature dependence of scintillation decay and rise time was measured for GGAG:Ce and Ca-codoped GGAG:Ce crystals in the temperature range of 10 K to 400 K. The decay and rise times were found to increase with decreasing temperature in the case of GGAG:Ce crystals, while Ca codoping modifies the kinetics such that decay and rise times are constant with the change in temperature. In order to obtain a better understanding of the role of trap centers in the temperature dependence of scintillation kinetics, spectrally resolved thermoluminescence (TL) properties were also studied for these crystals.