Positron Camera System Research Articles

Towards Sub-Minute PET Examination Times

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.

Numerical simulation design study of a positron camera for heavy-ion radiotherapy

The positron camera system has been designed to measure heavy-ion ranges in patients' bodies. The pencil-like beam of positron emitters, such as /sup 11/C, is used to check the range directly and precisely by detecting pairs of annihilation gamma-rays emitted from the end point of the beam trajectory. The positron camera consists of a pair of Anger-type scintillation cameras. The efficiency and the spatial resolution are modeled and simulated by a Monte Carlo method and a numerical calculation so that the positron camera has a high position accuracy for a small amount of irradiation dose. The simulation shows that the optimum effective diameter of the camera crystal is 500 mm and its thickness is 30 mm. The crystal diameter is concluded to be 600 mm by taking the outermost photomultiplier mounting into account. The simulation moreover, indicates that the range can be measured within an accuracy of 1 mm under the limitation that the irradiation dose has to be less than a few percent of the therapeutic one.

A simple data loss model for positron camera systems

A simple model to describe data losses in PET cameras is presented. The model is not intended to be used primarily for dead time corrections in existing scanners, even if this may be possible. Instead the model is intended to be used for data simulations in order to determine the figures of merits of future camera systems based on data handling state-of-art solutions. The model assumes the data loss to be factorized into two components, one describing the detector or block-detector performance and the other the remaining data handling such as coincidence determinations, data transfer and data storage. Two modern positron camera systems have been investigated in terms of this model. These are the new Siemens-CTI systems, Ecat Exact and Ecat Exact HR, both with an axial field-of-view (FOV) covering around 15 cm. They both have retractable septa and can acquire data from the whole volume within the FOV and can reconstruct volume image data. An example is given how to use the model for live time calculations in a futuristic large axial FOV cylindrical system. >

Evaluation of the Karolinska new positron camera system; the Scanditronix PC2048-15B

The PC2048-15B is the brain version of the new generation of Scanditronix positron camera systems, now being installed at the Karolinska Hospital in Stockholm, Sweden. The system has eight rings with 256 BGO crystals per ring. The dimensions of the crystals are 6*12*30 mm. Data corresponding to 15 image slices are simultaneously recorded. The detector rings are based on block-detector units, each with 16 scintillation crystals mounted on two dual photomultiplier tubes. The system is described, and test results including spatial resolution, sensitivity to true and random coincidences, scatter correction, count rate linearity, spatial independence, and reproducibility are presented. Clinical results are discussed, including rCBF studies and receptor studies, to cover the two most extreme situations met in brain PET (positron emission tomography) studies, i.e. high count rate and high resolution requirements.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Development studies for a high-rate positron camera based on a BaF 2/TMAE system

Clinical experience with our existing (Mark-II) positron camera has shown that the modest sensitivity and low maximum data rate of the device limit its clinical application. The latter problem is caused by the high singles rates encountered by the detectors and the sensitivity is limited by the conversion efficiency of the 511 keV gamma rays. We report the progress of our studies into a new positron camera system based on barium fluoride scintillators coupled to a low-pressure TMAE-filled MWPC which is read out by fast delay lines. According to our estimates this system will yield a sensitivity and a maximum data rate more than an order of magnitude higher than our existing system with comparable spatial resolution.

Nuclear medicine in the 1990s: a quantitative physiological approach.

This paper describes the potential advantages to medical diagnosis and treatment that might be obtained from the wider application of positron emission tomography as a clinical tool. Developments along the lines suggested here will require a radical change in thinking from both clinicians and the medically related scientific community in the UK and some enlightened and resourceful funding from a mixture of charitable, industrial and government sources. If these ideas are to be pursued successfully, then the work must start now on a much wider scale than is presently perceived in the UK, and close collaboration between physicists, engineers, chemists, biochemists, clinicians and industrialists is needed. Furthermore, it is imperative that the scientific developments now underway in silicon technology, parallel data processors, biochemical and pharmacological processes and even high-temperature superconductors be kept under close and constant review by those associated with the technological advancements of medicine, so that the value of such developments is rapidly transferred to applications to medicine. This must include closer relationships between academic medicine and science than is the general rule in the UK at present. In conclusion, the scenario presented here includes the installation of regional cyclotron facilities to provide a large number of institutions in the UK with positron-emitter labelled radiopharmaceuticals. Additionally, agents labelled with radionuclides from in-house generators and other already existing higher-energy cyclotrons will provide a versatile and valuable range of radiopharmaceuticals for the study of human disease. These developments must be supported by the manufacture of lower-cost positron camera systems, as suggested here, connected to high-data-rate parallel processors to provide images of body function and to determine the effects brought about by disease. These images may then be processed using algorithms based on kinetic models of the body systems to provide information about the basic biochemical and physiological processes of the body. Such a development could have a profound effect on our knowledge of human disease and on our ability to control and treat it successfully.

A preliminary evaluation of the Scanditronix PC2048-15B brain scanner.

The PC2048-15B is the brain version of the new generation of Scanditronix positron camera systems. It is based on a detection unit with 16 scintillating crystals mounted on 2 dual photomultiplier tubes. A system description is given and preliminary test results including spatial resolution, sensitivity to true and random coincidences, scatter correction, and count rate linearity are presented.

Image quantification with a large area multiwire proportional chamber positron camera (MUP-PET).

A large area multi wire proportional chamber positron camera system is under evaluation for clinical Nuclear Medicine investigation using isotopes both from the house generators (68Ga) and from remote cyclotrons (18F, 124I, 64Cu and 81Rb). Images are reconstructed using a fully three dimensional (3D) algorithm and exhibit equal resolution in all three orthogonal directions. The axial field of view of 15 cm is large enough to allow coverage of the whole brain. This paper discusses the performance of the MUP-PET system with particular emphasis on quantification and the development of an attenuation correction scheme for a large area detector system. The distribution of scattered and accidental events is investigated and observed to contribute a fairly uniform background to the image. Dead time correction factors are calculated from an analytical expression obtained by considering the various sources of dead time present in the system. Following correction for attenuation, accidentals, scattered events and dead time, reconstructed phantom images show a strong linear correlation (r = 0.998) between count density and regional isotope concentration. The extension of the methods to clinical studies is discussed.

A preliminary evaluation of a positron camera system using weighted decoding of individual crystals

The whole-body positron camera PC4096-15WB is based on a detection unit with 16 scintillating crystals mounted on two dual photomultiplier tubes. The design ideas are specified and a system description is given. Preliminary test results including spatial resolution, sensitivity to true and random coincidences, scatter correction, and count-rate linearity are presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A new image for positron emission tomography

A medical imaging technique for detecting tumours as small as 5 mm in diameter anywhere in the body looks likely to become more widely available with the development of the first low-cost positron camera system for clinical use. Researchers at the Institute of Cancer Research, the Royal Marsden Hospital in Surrey and the Rutherford Appleton Laboratory in Oxfordshire have developed a system that costs one tenth of those currently used for positron emission tomography (PET).

Design Studtes of Two Possible Detector Blocks for High Resolution Positron Emission Tomography of the Brain

Two possible detector designs for high resolution positron camera systems have been investigated. The goal is to achieve an instrument that can measure the whole brain with a spatial resolution of 5 mm FWHM in all directions. For both detectors BGO crystals are used, with the dimension 4.5*9.5*25 mm. One detector scheme utilizes the Anger principle for crystal identification with 16 crystals mounted on two dual PMT:s via a 3 mm light guide. The other detector scheme utilizes position sensitive PMT:s. The figures of merit for these two configurations are discussed in terms of high count rate capabilities and identification reliability.

Figures of Merit for Different Detector (Configurations Utilized in High Resolution Positron Cameras

A new positron camera system is currently being designed by our group. The goal is an instrument that can measure the whole brain with a spatial resolution of 5 mm FWHM in all directions. In addition to the high spatial resolution, the system must be able to handle count rates of 0.5 MHz or more in order to perform accurate fast dynamic function studies such as the determination of cerebral blood flow and cerebral oxygen consumption following a rapid bolus. An overall spatial resolution of 5 mm requires crystal dimensions of 6*6*L mm3, or less, L being the length of the crystal. Timing and energy requirements necessitate high performance photomultipliers. The identification of the small size scintillator crystals can currently only be handled in schemes based on the Anger technique, in the future possibly with photodiodes. In the present work different crystal identification schemes have been investigated. The investigations have involved efficiency measurements of different scintillators, line spread function studies and the evaluation of different coding schemes in order to identify small crystals.

Central Benzodiazepine Receptor Binding Studied with 11-C Labelled Ro 15-1788 and Positron Emission Tomography

The PC 384 positron camera system and tracer doses of 11-C labelled Ro 15-1788, a specific benzodiazepine (BZ) antagonist, were used to study BZ binding sites in the living brains of healthy control subjects and patients with chronic, severe anxiety disorder. The aim was to investigate the potential of the system for studies on the regional distribution and relative density of central BZ binding sites in the living human brain.

Performance study of the PC-384 positron camera system for emission tomography of the brain.

A positron camera system PC-384 has been designed and built for quantitative positron emission computed tomography of the brain. This head scanner consists of 384 bismuth germanate oxide scintillation detectors with a crystal packing fraction of 80%. The entire detector assembly rotates around a small circle (wobble) to achieve the necessary ray sampling. Seven simultaneous images are produced with four detector rings. Performances of the system are discussed and clarified based on experimental data.

A Four Ring Positron Camera System for Emission Tomography of the Brain

The exciting possibilities of regional, noninvasive and quantitative metabolic studies of the brain with positron emission tomography, have initiated the construction of a second generation positron camera system at the Department of Physics, University of Stockholm, in collaboration with the Departments of Neuroradiology, Clinical Neurophysiology and Radiation Physics at the Karolinska Hospital and Institute, and the Instrument AB Scanditronix. The camera is a four ring system with 96 bismuth germanate (BGO) detectors per ring. The spatial resolution is 7.7 mm FWHM and the sensitivity per ?Ci per ml for a 20 cm diameter cylinder phantom is 25000 c/s for direct slices and 35000 c/s for cross slices.

One Year Experience with a High Resolution Ring Detector Positron Camera System: Present Status and Future Plans

A ring detector positron camera system for brain research is described. The system utilizes 95 8 × 20 × 50 mm NaI (T1) detectors and is simultaneously recording coincidences from 1900 detection channels over a 30 cm diameter image field. To improve sampling two motions of the detector ring are utilized: a small rotation of half a crystal distance combined with a wobbling motion. The time window of the detection channels for accepting coincidences is 22 ns. Each detection channel corresponds to a unique address which labels any recorded event. Before being temporary stored in a buffer memory the event address passes through a PROM module which 1) removes events outside the specified image field and, 2) rearranges the event address to a projection address, i. e. a hard ware preprocessing of incoming data. The effective intrinsic resolution is 5.5 mm at the center of the image field and, together with a four point wobble, an overall resolution of 7 mm is obtained. The sensitivity is 5500 c/s for a 20 cm diameter cylinder containing 1 ?Ci/cc of a uniformly distributed positron activity in water. The system has been in clinical use since June 1978.

A Computer Assisted Ringdector Positron Camera System for Reconstruction Tomography of the Brain

A ring detector positron camera system for brain metabolism studies is being constructed at the Institute of Physics, University of Stockholm, in collaboration with the Departments of Neuroradiology and clinical Neurophysiology, Karolinska Sjukhuset, Stockholm, Sweden. The instrument utilizes 95 NaI(Tl) detectors and will simultaneously record coincidences from 1900 detector combinations. Each detector is coupled in coincidence with forty detectors on the opposite side of the ring. In the conventional stationary mode of operation the experimental system resolution is 10.5 mm. A new sampling technique has been developed to reduce the system resolution. Based on this technique, an experimental system resolution of 7 mm has been obtained.

A ring detector positron camera system: its merits: clinical experience.

A ring detector positron camera system for CT of the brain is described. The system uses 95 NaI (Tl) detectors, arranged with cylindrical geometry, for the simultaneous detection of coincidences from 1,900 detector combinations. With a new sampling technique an experimental system resolution of 7 mm FWHM has been obtained. The sensitivity was found to be 5,300 c/s with a 100-keV energy threshold and using a 19.2-cm-diameter cylindrical phantom with a homogeneous specific activity of 1 muCi/cm3. A clinical study of blood-brain barrier damage using 68Ga-EDTA is reported.

Performance and Evaluation of the Circular Ring Transverse Axial Positron Camera (CRTAPC)

Transverse axial positron annihilation coincidence detection tomography with a stationary circular ring posi tron camera system is described. A prototype constructed with 64 NaI(T1) crystal system has the capability of recording 736 - 1312 simultaneous coincidence pairs and is able to perform certain dynamic function studies.

Efficiency optimization analysis for dynamic function studies with 3-D transaxial positron cameras

Efficiency optimization problems for circular ring transverse axial positron camera systems (CRTAPC) have been investigated. The analyses include source activity, camera dimension, collimator and phantom sizes. The results indicate possibilities for fast dynamic function studies in the time region of a few seconds for reasonably small phantom sizes having a uniform activity distribution.