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G Muehllehner

Publications and source records attributed to G Muehllehner.

At least 19 recordsLinked to original sources
PubMed1991

Factors affecting accuracy and precision in PET volume imaging.

Volume imaging positron emission tomographic (PET) scanners with no septa and a large axial acceptance angle offer several advantages over multiring PET scanners. A volume imaging scanner combines high sensitivity with fine axial sampling and spatial resolution. The fine axial sampling minimizes the partial volume effect, which affects the measured concentration of an object. Even if the size of an object is large compared to the slice spacing in a multiring scanner, significant variation in the concentration is measured as a function of the axial position of the object. With a volume imaging scanner, it is necessary to use a three-dimensional reconstruction algorithm in order to avoid variations in the axial resolution as a function of the distance from the center of the scanner. In addition, good energy resolution is needed in order to use a high energy threshold to reduce the coincident scattered radiation.

Algorithms↗
PubMed1991

Standards for performance measurements of PET scanners: evaluation with the UGM PENN-PET 240H scanner.

A standard set of performance measurements is proposed for use with positron emission tomographs. This set of measurements has been developed by the Computer and Instrumentation Council of the Society of Nuclear Medicine and the National Electrical Manufacturers Associations. These measurements are discussed and compared to the set of standard measurements being proposed by the Instrumentation Task Group of the European Economic Community Concerted Action of Cellular Regeneration and Degeneration. The performance of the PENN-PET 240H scanner from UGM Medical Systems is tested with this set of measurements. The PENN-PET scanner consists of six continuous position-sensitive NaI(T1) detectors, which results in a 50 cm transverse field-of-view and a 12.8 cm axial field-of-view. The fine spatial sampling in the axial direction allows the data to be sorted into as many as 64 transverse planes, each 2 mm thick. A large axial acceptance angle, without inter-plane septa, results in a high sensitivity, with a low scatter and randoms fraction, due to the use of a narrow photopeak energy window. This paper emphasizes those performance measurements which illustrate the special characteristics of a volume imaging scanner, compared to a more traditional multi-ring scanner.

Equipment Design↗
PubMed1990

Continuous-slice PENN-PET: a positron tomograph with volume imaging capability.

The PENN-PET scanner consists of six hexagonally arranged position-sensitive Nal(TI) detectors. This design offers high spatial resolution in all three dimensions, high sampling density along all three axes without scanner motion, a large axial acceptance angle, good energy resolution, and good timing resolution. This results in three-dimensional imaging capability with high sensitivity and low scatter and random backgrounds. The spatial resolution is 5.5 mm (FWHM) in all directions near the center. The true sensitivity, for a brain-sized object, is a maximum of 85 kcps/microCi/ml and the scatter fraction is a minimum of 10%, both depending on the lower level energy threshold. The scanner can handle up to 5 mCi in the field of view, at which point the randoms equal the true coincidences and the detectors reach their count rate limit. We have so far acquired [18F]FDG brain studies and cardiac studies, which show the applicability of our scanner for both brain and whole-body imaging. With the results to date, we feel that this design results in a simple yet high performance scanner which is applicable to many types of static and dynamic clinical studies.

Equipment Design↗
PubMed1989

The high count rate performance of a two-dimensionally position-sensitive detector for positron emission tomography.

In positron tomographs using a small number of position-sensitive detectors, each detector must operate at high singles event rates, especially during dynamic studies. To enable the PENN-PET tomography to perform studies involving high data rates, the high count rate behaviour of the position-sensitive scintillation detector used in the tomograph was investigated at singles rates in excess of 2 million counts per second (MCPS). Detector dead-time, minimised through the use of pulse clipping (clipping time, 120 ns), is a maximum of 20% at the highest data rates. At 2 MCPS and 240 ns pulse integration time, the full width at half maximum of the point spread function (PSF) worsens by approximately 20% over its low count rate value of 5.2 mm. Furthermore, at high count rates, pulse pile-up produces long tails in the PSF along the detector's long axis. These tails were reduced or eliminated through the use of a shortened pulse integration time (160 ns instead of 240 ns), an upper level energy discriminator and a local centroid event positioning algorithm. Detector performance was characterised for different combinations of these event processing techniques, and the mechanisms by which pulse pile-up distorts the high count rate PSF were investigated using computer simulations. With the incorporation of the high count rate event processing techniques, the detector's count rate capability enables the PENN-PET tomograph to handle most current imaging protocols.

Scintillation Counting↗
PubMed1987

Treatment of axial data in three-dimensional PET.

Improved axial spatial resolution in positron emission tomography (PET) scanners will lead to reduced sensitivity unless the axial acceptance angle for the coincidences is kept constant. A large acceptance angle, however, violates assumptions made in most reconstruction algorithms, which reconstruct parallel independent slices, rather than a three-dimensional volume. Two methods of treating the axial information from a volume PET scanner are presented. Qualitative and quantitative errors introduced by the approximations are examined for simulated objects with sharp boundaries and for a more anatomically realistic distribution with smooth activity gradients.

Algorithms↗
PubMed1986

Positron emission tomography imaging--technical considerations.

Positron imaging instrumentation has improved rapidly in the last few years. Scanners currently under development are beginning to approach fundamental limits set by positron range and noncolinearity effects. This report reviews the latest developments in positron emission tomography (PET) instrumentation, emphasizing the development of coding schemes that reduce the complexity and cost of high-resolution scanners. The relative benefits of using time-of-flight (TOF) information is discussed as well.

Biophysical Phenomena↗
PubMed1986

A positron camera using position-sensitive detectors: PENN-PET.

A single-slice positron camera has been developed with good spatial resolution and high count rate capability. The camera uses a hexagonal arrangement of six position-sensitive NaI(Tl) detectors. The count rate capability of NaI(Tl) was extended to 800k cps through the use of pulse shortening. In order to keep the detectors stationary, an iterative reconstruction algorithm was modified which ignores the missing data in the gaps between the six detectors and gives artifact-free images. The spatial resolution, as determined from the image of point sources in air, is 6.5 mm full width at half maximum. We have also imaged a brain phantom and dog hearts.

Animals↗
PubMed1985

Effect of resolution improvement on required count density in ECT imaging: a computer simulation.

The effects of changes in spatial resolution and total number of counts on image quality were investigated for positron and single photon emission computed tomography (ECT) systems. A variety of high contrast phantoms were generated in a computer simulation and count density and spatial resolution were varied independently over a wide range. As system spatial resolution is improved, significantly fewer counts are needed to give images of comparable visual quality. Using 100% object contrast, it was found that the number of counts could be reduced by a factor of four for a 2 mm improvement in spatial resolution over a wide range of parameters. This is due to the fact that image contrast increases rapidly with spatial resolution improvements in high contrast objects such as those used in this simulation and typically encountered in brain and cardiac ECT studies.

Computers↗
PubMed1985

Performance of a position-sensitive scintillation detector.

The spatial resolution of a NaI(T1), 25 mm thick bar detector designed for use in positron emission tomography has been studied. The position along the 500 mm long detector is determined from the centroid of the light distribution in the crystal as measured by a linear array of photomultiplier tubes. A Monte Carlo computer simulation was performed to investigate the factors limiting the spatial resolution. The program allowed us to study the effect of various phototube configurations and crystal surfaces. Since the resolution is affected by the width of the light distribution, we studied the effect of sharpening the distribution by modifying the front crystal surface with grooves cut perpendicular to the long axis of the crystal and by using non-linear preamplifiers. The simulation predicts a spatial resolution (FWHM) of 3 mm with this crystal. Experimental measurements of spatial resolution were performed concurrently with the simulations. In particular, a modified grooved crystal was measured to have 4.0 mm spatial resolution, an improvement over the original crystal without grooves. With delay line pulse shortening, which increases the count rate capability of the detector, the grooved crystal was measured to have 5.5 mm spatial resolution.

Animals↗
PubMed1976

Influence of hole shape on collimator performance.

The imaging properties of triangular and hexagonal hole shapes were compared for low energy scintillation camera collimators. The response of both hole shapes was calculated using a ray-tracing computer program, corresponding collimators were constructed and their performance was evaluated by measuring line spread functions and imaging bar patterns. The triangular hole shape was found to give better results above a spatial frequency of 1-5 cycles cm-1, below that frequency the hexagonal hole shape is superior. The differences, however, are small and the superior performance of the hexagonal hole collimator at clinically significant spatial frequencies is barely visible in bar pattern images.

Computers↗
PubMed1975

Positron camera with extended counting rate capability.

Positron emitters may be imaged using two opposing scintillation cameras without collimators. The counting rate limitation of this approach can be largely largely overcome by using graded absorbers to reduce scattered radiation from the patient and using not only photopeak events but Compton events in the scintillator as well. This increases the useful counting rate by more than a factor of 5. By combing this technique with the use of fast electronics, useful images have been obtained in the presence of scattering material at counting rates above 7,500 cps.

Elementary Particles↗