Count Rate Losses Research Articles

Design and Construction of a PET-Compatible Double-Tuned 1H/31P MR Head Coil.

Simultaneous MR-PET is an increasingly popular multimodal imaging technique that is able to combine metabolic information obtained from PET with anatomical/functional information from MRI. One of the key technological challenges of the technique is the integration of a PET-transparent MR coil system, a solution to which is demonstrated here for a double-tuned 1H/31P head coil at 3 T. Two single-resonant birdcage coils tuned to the 1H and 31P resonances were arranged in an interleaved fashion and electrically decoupled with the use of trap circuits. All high 511 keV quanta absorbing components were arranged outside the PET field-of-view in order to minimize count rate reduction. The materials inside the PET field-of-view were carefully evaluated and chosen for minimum impact on the PET image quality. As far as possible, the coil case was geometrically optimized to avoid sharp transitions in attenuation, which may potentially result in streaking artefacts during PET image reconstruction. The coil caused a count rate loss of just above 5% when inserted into the PET detector ring. Except for the anterior region, which was designed to maintain free openings for increased patient comfort, an almost uniform distribution of 511 keV attenuation was maintained around the circumference of the coil. MR-related performance for both nuclei was similar or slightly better than that of a commercial double-tuned coil, despite the MR-PET coil having a close-fitting RF screen to shield the PET and MR electronics from possible electromagnetic interferences.

Performance characteristic evaluation of a bismuth germanate-based high-sensitivity 5-ring discovery image quality positron emission tomography/computed tomography system as per National Electrical Manufacturers Association NU 2-2012.

National Electrical Manufacturers Association (NEMA) provides guidelines to assess the performance of Positron Emission Tomography (PET). A PET/CT scanner, Discovery IQ, GE Medical systems, Milwaukee, USA was installed in our department which has high a sensitivity PET component. We have performed the NEMA NU-2 2012 quality control tests to evaluate this system on site before clinical use. Performance measurements of the PET scanner were made using the NEMA NU2-2012 procedures for spatial resolution, scatter fraction, sensitivity, count rate loss and random coincidence estimation, Noise Equivalent Count Rate (NECR) and image quality. As per NU2 2012, spatial resolution was measured at 1 cm, 10 cm and 20 cm vertically from the centre and at each of these points resolution was measured at tangential, radial and axial directions. Sensitivity was measured at centre and 10 cm off center vertically from the center. The system sensitivity is reported as an average of the two measured values. Scatter fraction and NECR measurements, Image quality test was also performed. The tangential, radial and axial FWHM were 4.99 mm, 4.20 mm and 4.79 mm at 1 cm off centre, 5.49 mm, 4.69 mm and 4.81 mm at 10 cm off centre and 7.99 mm, 5.07 mm and 4.95 mm at 20 cm off centre respectively. The absolute sensitivity of this scanner was found to be 20.1 cps/kBq. The scatter fraction calculated from the decay method was 37.94% and NECR was 125 kcps. The peak NECR was achieved at activity concentration of 8.7 KBq/ml and the count loss below the peak NECR was found to be 0.68%. Image quality test for, contrast recovery, background variability and lung error residual mean met all specifications. Overall PET performance of Discovery IQ whole-body scanner was satisfactory and the scanner met all the performance specifications required by NEMA 2012.

Performance evaluation of quantitative SPECT/CT using NEMA NU 2 PET methodology

Although PET is routinely evaluated using NEMA NU2 as standard in the clinic, standard methodology for evaluating the performance of quantitative SPECT systems has not been established. In this study, the quantitative performance of the Symbia Intevo SPECT/CT was evaluated for two common isotopes (99mTc, 177Lu) and benchmarked against the performance of a PET/CT. A further aim was to demonstrate the utility of adapting NEMA NU2 PET measurements to SPECT. In addition, dead-time and resolution recovery were evaluated to provide more complete system evaluations. Spatial resolution of the SPECT system at 1 cm from the center in the transverse direction was 13.1 mm and 22.4 mm for 99mTc and 177Lu respectively, compared with 4.3 mm (18F) and 5.8 mm (68Ga) for PET. Sensitivity at the center of the FoV was 119 cps MBq−1 and 48 cps MBq−1 (99mTc, 177Lu) for SPECT and 9632 cps MBq−1 and 8216 cps MBq−1 (18F, 68Ga) for PET. Scatter fraction was 0.25 and 0.36 (99mTc, 77Lu) for SPECT and 0.32 and 0.29 (18F, 68Ga) for PET. Contrast recovery coefficient in the largest spheres was 0.79 and 0.65 (99mTc, 177Lu) for SPECT, 1.00 and 0.97 (18F, 68Ga) for PET and the background variability was 2.7%, 6.5% (99mTc, 177Lu), 1.5% and 1.6% (18F, 68Ga), respectively. Partial volume effect was evaluated using the NEMA IQ phantom with six sphere inserts (diameter: 37 mm, 28 mm, 22 mm, 17 mm, 13 mm and 10 mm). Full contrast recovery was reached with the 17 mm for 18F, while SPECT did not reach full recovery for any sphere. Count rate losses were 2% for 99mTc at 1 GBq and 11% for 177Lu at 8.5 GBq which are well below the typical activities for clinical applications. We concluded NEMA NU2 methodology can be easily adapted to SPECT/CT as a routine quality assurance procedure in the clinic.

314. A multi-centre study to evaluate physical acceptance reference values, tolerances and action levels for a new PET/CT scanner

Purpose A multi-centre study to evaluate performances of PET/CT scanners and to draft a unique QC protocol for five departments was performed. The variability in quantitation of PET acceptance tests was investigated to define acceptance limits of these data establishing reference values and new tolerances to compare with the technical specifications provided by the manufacturer. Methods Spatial resolution, scatter fraction, sensitivity, count rate loss and random coincidence estimation, Noise Equivalent Count Rate (NECR) and Image quality parameters were calculated following NEMA NU2-2012 [1] standard on the new PET/CT scanners Biograph mCT Siemens installed in five different nuclear medicine departments. For each parameter, we assessed the consistency of the related data. Results The acceptance test results demonstrated comparable output for all parameters investigated. A summarize schematic results were proposed and their relative variations and tolerances were discussed. As expected, the spatial resolution showed the least variability amongst all features related to its operator-independent preparation and strictly depending on equipment construction. Conversely, the sensitivity was the most variable parameter correlated to very low activity concentration required by NEMA protocol. We reported, for one scanner, an unexpected value in the count rate accuracy test (relative mean error @ peak NEC), despite the consistency of the values of peak NEC rate obtained. Conclusions Based on this investigation, new tolerances of performance measurements were calculated and suggested employing a common platform for quality assurance procedures to minimize variability of physical reference values. The results showed the need for a multi-centre inter-comparison especially in the tests without the tolerances of technical specifications suggested by the manufacturer.

XPS spectrometer transmission function optimization by the differential evolution algorithm

An XPS containing XSAM 800 setup, produced by Kratos, was upgraded. This mainly included the reconstruction of the acquisition and control systems. The detection system non-linearity, and the transmission function of the upgraded system were measured. The dead time of the detection system was determined using two independent approaches, which provided practically the same result. The knowledge of the detector dead time allows us to anticipate the count rate loss. The transmission function was measured using the well-established first principles method. Since this approach did not provide satisfactory results for all pass energies, a novel procedure, here denoted as sample-biasing method, was proposed and successfully applied. The novel approach is much faster, and significantly reduces the measurement error as compared to the first principles method. The transmission function was then optimized by applying a differential evolution algorithm, which provided its relative increase in the range from 10%–110% depending on the fixed analyzer transmission mode and the electron kinetic energy. The optimization process is fully automatized, and can be readily applied for other similar problems such as tuning of charged particle beams.

On the stability of 3H and 63Ni Ultima Gold liquid scintillation sources

The stability of 3H and 63Ni samples, made with Ultima Gold and Ultima Gold AB using eight aqueous fractions ranging between 0.1% and 10%, was monitored over a period of 32 weeks. Counting rate losses were measured in all cases, but were found to be particularly severe for samples with low aqueous fractions. Hydrodynamic diameter measurements of the reverse micelles of 3H and 63Ni in Ultima Gold and Ultima Gold AB samples were performed with dynamic light scattering. For Ultima Gold, the micelle diameters were measured to be about 2nm and slightly dependent of the aqueous fraction, while for Ultima Gold AB they were found to vary strongly with the aqueous fraction and range between 1 and 4nm. The hypothesis that the counting rate instabilities derive from changes in the sizes of micelles was tested by measuring them four times over the monitoring period. The measurement results do not support this claim.

Performance studies of boron lined proportional counters for reactor applications

Boron lined proportional counters of standard cylindrical geometry were developed and characterized for reactor applications. It is observed that the life of boron lined proportional counters is significantly lower if irradiated in high neutron flux keeping the HV bias “ON” compared to the irradiation with HV bias “OFF”. When operated in gamma radiation, the boron lined proportional counters suffer count rate loss and underestimate the neutron flux. The loss in the count rate in gamma radiation is attributed to the space charge effect. The space charge effect is theoretically calculated for a known gamma radiation. The estimated values compared well with the experimentally observed values. Neutron sensitivity is estimated by modeling the boron lined proportional counters in FLUKA Monte-Carlo simulation code. The estimated neutron sensitivity values are comparable to the experimental and theoretical values.

A Depth-Encoding PET Detector Inserting Glass Plate Between Crystal Layers

This study introduces a depth-encoding PET detector inserting a glass plate between the pixilated scintillation crystal layers. The principle of the proposed design was that the relative amount of light received by each photosensor will be altered by using the glass plate. This change in the light distribution can generate a pattern diagram of the 2D flood histogram that identifies depth position as well as X-Y position of γ-ray interaction. A Monte Carlo simulation was conducted for the assessment of the DOI-PET detector of 4 ×4 array photosensor coupled with 2-layers of LSO arrays which consist of 4 ×4 arrays of 3 ×3 ×10 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> discrete crystals. The traced light distribution for each event was converted by the modified resistive charge division networks into the 2D flood histogram. Optical glass plates with 11 different thicknesses that range from 0 to 10 mm with a 1 mm step were modeled. This was done, to estimate the thickness which allows the extraction of the depth information from the 2D flood histogram. An experimental study was performed to acquire the flood histograms of the DOI-PET detectors with 3 and 5 mm thick glass plate. The effect of glass plate on light loss and count rate loss were assessed for two detector configurations with and without glass plate. The simulation results showed that the flood histogram without overlapping of each crystal position can be generated for the detectors by inserting the glass plates with thickness of 3 ~ 10 mm. They were also demonstrated in the acquired representative flood histograms which were obtained by the experimental study. The light and count rate losses measured from DOI-PET detector with 3 mm thick glass plate was ~ 5% and ~ 2%, respectively. This study demonstrated that the proposed DOI-PET detector can extract the 3D γ-ray interaction position without considerable performance degradations of PET detector from the 2D flood histogram.

The piecewise-linear dynamic attenuator reduces the impact of count rate loss with photon-counting detectors

Photon counting x-ray detectors (PCXDs) offer several advantages compared to standard energy-integrating x-ray detectors, but also face significant challenges. One key challenge is the high count rates required in CT. At high count rates, PCXDs exhibit count rate loss and show reduced detective quantum efficiency in signal-rich (or high flux) measurements. In order to reduce count rate requirements, a dynamic beam-shaping filter can be used to redistribute flux incident on the patient. We study the piecewise-linear attenuator in conjunction with PCXDs without energy discrimination capabilities. We examined three detector models: the classic nonparalyzable and paralyzable detector models, and a ‘hybrid’ detector model which is a weighted average of the two which approximates an existing, real detector (Taguchi et al 2011 Med. Phys. 38 1089–102 ). We derive analytic expressions for the variance of the CT measurements for these detectors. These expressions are used with raw data estimated from DICOM image files of an abdomen and a thorax to estimate variance in reconstructed images for both the dynamic attenuator and a static beam-shaping (‘bowtie’) filter. By redistributing flux, the dynamic attenuator reduces dose by 40% without increasing peak variance for the ideal detector. For non-ideal PCXDs, the impact of count rate loss is also reduced. The nonparalyzable detector shows little impact from count rate loss, but with the paralyzable model, count rate loss leads to noise streaks that can be controlled with the dynamic attenuator. With the hybrid model, the characteristic count rates required before noise streaks dominate the reconstruction are reduced by a factor of 2 to 3. We conclude that the piecewise-linear attenuator can reduce the count rate requirements of the PCXD in addition to improving dose efficiency. The magnitude of this reduction depends on the detector, with paralyzable detectors showing much greater benefit than nonparalyzable detectors.

Decaying source model: Alternative approach to determination of true counting rates at X and gamma ray counting systems

The counting systems consisting of electronic devices are used for detection of radiation due to X or gamma rays. The dead time of the counting system is based on time limitations of these electronic devices. The dead time causes counting losses. Determination of counting rate losses in quantitative and qualitative analysis become a vital step for correction of analysis. Therefore, compensating for counting rate losses is of great importance. These counting rate losses are due to piled up reject time, paralyzable or non-paralyzable system dead time or a combination of these mechanisms. Paralyzable and non-paralyzable models are well-known and frequently used for correction of counting rate losses dependent on the system dead time. However, these two models do not provide enough correction at medium and high counting rates. Therefore, the new models for corrections of counting rate losses are needed. For this reason, both an alternative approach is proposed and a simulation program is coded for counting rate losses in this study. A good agreement is obtained between theoretical model and simulation program.

Pulse pileup statistics for energy discriminating photon counting x‐ray detectors

Energy discriminating photon counting x-ray detectors can be subject to a wide range of flux rates if applied in clinical settings. Even when the incident rate is a small fraction of the detector's maximum periodic rate No, pulse pileup leads to count rate losses and spectral distortion. Although the deterministic effects can be corrected, the detrimental effect of pileup on image noise is not well understood and may limit the performance of photon counting systems. Therefore, the authors devise a method to determine the detector count statistics and imaging performance. The detector count statistics are derived analytically for an idealized pileup model with delta pulses of a nonparalyzable detector. These statistics are then used to compute the performance (e.g., contrast-to-noise ratio) for both single material and material decomposition contrast detection tasks via the Cramdr-Rao lower bound (CRLB) as a function of the detector input count rate. With more realistic unipolar and bipolar pulse pileup models of a nonparalyzable detector, the imaging task performance is determined by Monte Carlo simulations and also approximated by a multinomial method based solely on the mean detected output spectrum. Photon counting performance at different count rates is compared with ideal energy integration, which is unaffected by count rate. The authors found that an ideal photon counting detector with perfect energy resolution outperforms energy integration for our contrast detection tasks, but when the input count rate exceeds 20% N0, many of these benefits disappear. The benefit with iodine contrast falls rapidly with increased count rate while water contrast is not as sensitive to count rates. The performance with a delta pulse model is overoptimistic when compared to the more realistic bipolar pulse model. The multinomial approximation predicts imaging performance very close to the prediction from Monte Carlo simulations. The monoenergetic image with maximum contrast-to-noise ratio from dual energy imaging with ideal photon counting is only slightly better than with dual kVp energy integration, and with a bipolar pulse model, energy integration outperforms photon counting for this particular metric because of the count rate losses. However, the material resolving capability of photon counting can be superior to energy integration with dual kVp even in the presence of pileup because of the energy information available to photon counting. A computationally efficient multinomial approximation of the count statistics that is based on the mean output spectrum can accurately predict imaging performance. This enables photon counting system designers to directly relate the effect of pileup to its impact on imaging statistics and how to best take advantage of the benefits of energy discriminating photon counting detectors, such as material separation with spectral imaging.

Effects of energy threshold and dead time on Compton camera performance

We study the effects of energy threshold and dead time on the sensitivity and image resolution of the Compton camera. The simulation model includes the decay times, detection time jitters, energy threshold and detector dead time as well as basic detector parameters such as Doppler broadening, energy resolution and finite detector resolution. The GEANT4 toolkit was used to model the camera geometry and performance for two common nuclear medicine energies that correspond to 99mTc (140.5 keV) and 18F (511 keV) radiotracers. Results without the energy threshold and time effects show good agreement with previous studies. For 140.5 keV, the inclusion of energy threshold improved image resolution from 10.7 to 9.5 mm with a source-to-detector distance of 5 cm, while the inclusion of time effects made no further difference on resolution. The energy threshold reduced the sensitivity by 48%, and subsequent inclusion of time effects further reduced the sensitivity by 17%. At 511 keV, the application of energy threshold reduced the sensitivity by 6%, while the time effects dominated count rate losses with further reduction of 13%. However, the inclusion of the two effects had negligible impact on the resolution.

Random coincidence summing of gamma rays and dead time count loss corrections in HPGe-detector based gamma ray spectrometry

Random coincidence summing of gamma rays and dead time count losses are a potential source of errors in gamma ray spectrometry. In this work, the magnitudes of count rate losses due to the random coincidence summing and dead time effect, alongwith corresponding corrections, have been studied. Experiments were performed in two independent gamma ray spectroscopic laboratories (referred as LAB-A & LAB-B), utilizing low activity reference nuclides 65 Zn and 60 Co. The low activity reference radionuclide 65 Zn was produced by using 10 MW nuclear reactor (PARR-1) at PINSTECH for use in one Lab. The gamma ray spectra were acquired using HPGe-detector based gamma ray spectrometers at fixed shaping times of the amplifier and positions of the reference nuclides. The MCA dead times of the spectrometers were varied to incur the count rate losses to the peaks of interest. Three procedures were adopted to determine the dead time and random summing correction factors. The corrections to count rate data measured at LAB-A, which represented a maximum of 46% losses, were obtained within standard deviation figures of 2.9%, 1.3% and 0.4% and in LAB-B case the losses of 41%, were corrected to within 1.8%, 1.1% and 0.6%, respectively with the described three procedures. The best-achieved precision (difference between the measured count rate with negligible dead time and the average of corrected count rates) in the case of LAB-A and LAB-B were within 1 x 10 ―3 and 2 x 10 ―2 %. The new procedure on the basis of fitting function is also evolved as a result of this study, which has produced the highly consistent results. The experimental data from the literature are also incorporated in this study for comparison purposes.

Improvements to the Leake neutron detector II: Extension to 10 GeV energy

We report on an improved design of neutron ambient dose equivalent (DE) detector, SNS-LINUS, consisting of a spherical polyethylene detector with internal shields of boron and lead based on the LINUS model thus extending the response to the GeV region. The computed Monte Carlo neutron ambient DE energy response is within a factor ±2.4 from 0.1 keV to 10 GeV with a detector weight of 7.8 kg that is significantly lower than other LINUS designs. The ambient DE response in the 50 keV–10 MeV range is within a factor ±1.36 (i.e. ±30%). The main applications are likely to be around particle accelerators, in aircraft or in spacecraft. The improved response in the 20 MeV energy region should make it attractive for use with fission sources. The computed responses of the SNS NGREM and LINUS detectors to pulsed neutrons and limits for their prudent use in portable monitors in such fields are given. The effect of large gamma bursts on the measurement of pulsed neutrons is discussed with recommendations made to enable operation up to 10 μGy of gamma radiation per burst. We propose a method of extending the dynamic range for pulsed neutron detection in which the neutron decay within the moderator is used to make measurements at a time at which the count rate losses are acceptable.

Quantification of radiotracer uptake with a dedicated breast PET imaging system

Tomographic breast imaging techniques can be used to quantify radiotracer uptake in breast and tumor tissue. However, physical processes common to PET imaging can confound accurate quantification. In this investigation, we assessed the effects of these phenomena and tested correction schemes for our new positron emission mammography-tomography system (PEM-PET). The PEM-PET scanner utilizes two sets of rotating planar detector heads. Each unit consists of a 4 x 3 array of Hamamatsu H8500 flat panel position sensitive photomultipliers coupled to a 96 x 72 array of 2 x 2 x 15 mm3 LYSO detector elements (pitch = 2.1 mm). Image reconstruction is performed with a 3D-OSEM algorithm parallelized to run on a multiprocessor computer system. The reconstructed field-of-view is 15 x 15 x 15 cm3. Much of the testing procedures were based on NEMA-NU2/2001 protocols. Count rate losses due to pulse pile-up, image contamination due to acceptance of random coincidences and Compton scatter, and image artifacts produced by photon attenuation were measured. It was found that the system was susceptible to count rate losses when moderate levels of radiation were present in the scanner due to the current design of the event trigger electronics. Application of corrections for Compton scattering, photon attenuation and dead time resulted in improved estimations of 18F concentration in simplified phantom studies. Results from these preliminary studies indicate that the PEM-PET scanner will be useful for the quantification of radiotracer uptake in breast tumors, possibly facilitating early assessment of cancer treatments.

Count Rate Performance of the MicroPET Focus 220 Animal Scanner in Singles Transmission Scanning Mode

Count rate loss in singles transmission scans due to dead time can decrease the accuracy of attenuation correction. In addition, for very low transmission source activities measured attenuation may be affected by noise and background radiation emitted by radioactive lutetium-176 in the scintillator material. The aims of this study were to investigate count rate performance in singles transmission scanning for the microPET Focus 220 animal scanner, to examine the effect of energy window width on noise, scatter and dead time, and to determine the optimal range of activity for accurate attenuation correction. Transmission measurements were performed with a decaying <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">99m</sup> Tc point source and using wide (21%) and narrow (4%) energy windows. We observed count rate losses at the detector level and in the front end electronics during energy discrimination. Further losses emerged due to multiplexing at the detector head interface for activities higher than 150 MBq for the wide energy window. Substantial count rate losses during transfer of list mode data from the microPET to the PC were also observed for this condition. Lutetium-176 background caused a significant deviation of measured attenuation for activities lower than 10 MBq for both energy windows. No significant bias due to scatter was introduced when using the wide energy window. We conclude that accurate attenuation data can be obtained with single photon transmission source activity in the range 10 to 150 MBq without requiring deadtime correction.

Measurement of cosmic ray muon flux in the Belgrade ground level and underground laboratories

The cosmic-ray muon count rate and energy loss spectra in plastic scintillator detectors are recorded in the Institute of Physics, Belgrade. Measurements are performed at two different levels: at the surface level (78 m a.s.l.) and in the Belgrade underground laboratory at the depth of 25 m.w.e. From experimental data and with the use of GEANT4 computer simulation the flux and vertical intensities have been determined. At ground level the muon flux is determined to be J 1 G = ( 1.37 ± 0.06 ) × 10 - 2 s - 1 cm - 2 and vertical intensity I vG = ( 8.4 ± 0.4 ) × 10 - 3 s - 1 cm - 2 sr - 1 while the same quantities underground are J 1 U = ( 4.5 ± 0.2 ) × 10 - 3 s - 1 cm - 2 and I vU = ( 2.5 ± 0.2 ) × 10 - 3 s - 1 cm - 2 sr - 1 . Their relation to the results of similar measurements worldwide is discussed.

Validation of a Monte Carlo simulation of the Philips Allegro/GEMINI PET systems using GATE

A newly developed simulation toolkit, GATE (Geant4 Application for Tomographic Emission), was used to develop a Monte Carlo simulation of a fully three-dimensional (3D) clinical PET scanner. The Philips Allegro/GEMINI PET systems were simulated in order to (a) allow a detailed study of the parameters affecting the system's performance under various imaging conditions, (b) study the optimization and quantitative accuracy of emission acquisition protocols for dynamic and static imaging, and (c) further validate the potential of GATE for the simulation of clinical PET systems. A model of the detection system and its geometry was developed. The accuracy of the developed detection model was tested through the comparison of simulated and measured results obtained with the Allegro/GEMINI systems for a number of NEMA NU2-2001 performance protocols including spatial resolution, sensitivity and scatter fraction. In addition, an approximate model of the system's dead time at the level of detected single events and coincidences was developed in an attempt to simulate the count rate related performance characteristics of the scanner. The developed dead-time model was assessed under different imaging conditions using the count rate loss and noise equivalent count rates performance protocols of standard and modified NEMA NU2-2001 (whole body imaging conditions) and NEMA NU2-1994 (brain imaging conditions) comparing simulated with experimental measurements obtained with the Allegro/GEMINI PET systems. Finally, a reconstructed image quality protocol was used to assess the overall performance of the developed model. An agreement of <3% was obtained in scatter fraction, with a difference between 4% and 10% in the true and random coincidence count rates respectively, throughout a range of activity concentrations and under various imaging conditions, resulting in <8% differences between simulated and measured noise equivalent count rates performance. Finally, the image quality validation study revealed a good agreement in signal-to-noise ratio and contrast recovery coefficients for a number of different volume spheres and two different (clinical level based) tumour-to-background ratios. In conclusion, these results support the accurate modelling of the Philips Allegro/GEMINI PET systems using GATE in combination with a dead-time model for the signal flow description, which leads to an agreement of <10% in coincidence count rates under different imaging conditions and clinically relevant activity concentration levels.

A β-spectrometer for searching effects of finite neutrino masses

A detailed description of a Tret’yakov-type magnetic β-spectrometer with electrostatic acceleration is presented. The instrument is being used for high precision measurements of the β-spectra of various isotopes and to search for neutrino mixing and heavy neutrinos with masses up to 100 keV/ c 2. The relative momentum resolution, obtained with an extended source ( 3×2 cm 2) and a large silicon strip detector, is δ p/ p=6.9×10 −4 (FWHM). The width of the energy band, which can simultaneously be measured with this resolution, is typically 50 times larger than the corresponding width of the energy resolution. The detector delivers pulse-height and one-dimensional position information. The read-out electronics provides a separate channel for each detector strip, allowing measurements to be made with high count rate and small dead-time losses. The detection of scattered electrons was minimized by the arrangement of baffles and by the use of materials with small atomic number. Experimentally, scattering in the spectrometer was investigated by accelerating photoelectrons up to 60 keV energy. A system for measuring the corresponding high voltage is described. Extensive Monte Carlo simulations of scattering in the spectrometer have been performed. Good agreement with the experimental data was obtained and there was no need to adjust any scattering cross section to the particular experimental situation. We conclude that calculated scattering probabilities can reliably be used in the analysis of experimental β-spectra, which is important as otherwise scattering would constitute a major uncertainty.

Performance stability of SHR-2000 high resolution PET for animal research.

The performance of a high resolution positron emission tomography (PET) system SHR-2000 for animal studies was re-evaluated six years after its installation. The system employs a detector array consisting of BGO crystals that are 1.7 mm (transaxially) by 10 mm (axially) by 30 mm (deep). A block detector, which is a position-sensitive photomultiplier tube (PMT) coupled to 4 arrays of BGO crystals has been adopted to the system. There are 15 block detectors positioned to form a 35 cm diameter ring with a field of view (FOV) of 17 cm by 4.6 cm axially, giving the system a 7 slice imaging capability. For six year workload in spatial resolution (FWHM), there were approximately a 2.6% increase at tangential FOV and a 7.5% increase at radial FOV. In axial resolution (FWHM) there was almost no change. The count rate loss for the true count rate increased 1.3% at 200 kBq/ ml. The average slice sensitivity showed a decrease of approximately 4.1%, and in scatters it showed an increase of approximately 1.4%. In animal experiments, the bones of guinea pigs were clearly identified with 18F fluoride ion. These experiments show that after a six year workload, the system also maintains good performance and has good stability.