A. Jeavons

Increased sensitivity and field of view for a rotating positron camera

Discusses two approaches towards improving the sensitivity and enlarging the useful field of view for a rotating positron camera by removing the constraint on phi . Weighted backprojection appears superior in that the camera response is more uniform and the images are less noisy than for the post-reconstruction scaling method. Fourier deconvolution reconstruction may be retained since the response function of the overall system is still shift invariant. In a typical imaging situation, the increase in sensitivity may be as much as a factor of 2.5, which represents a significant improvement when considering the poor statistics of nuclear medicine images. As a consequence of decoupling the sensitivity from the diameter of the field of view, transverse sections through internal organs with dimensions approaching those of the detectors may therefore be accurately imaged with good sensitivity. The weighted backprojection method is now applied in routine clinical imaging (Frey et al., 1984).

The High-Density Avalanche Chamber for Positron Emission Tomography

The development of wire chambers for positron emission tomography continues at CERN. In its present form, the basic detector is called a HIgh Density Avalanche Chamber (HIDAC). Owing to the Penning effect, electron avalanche multiplication is obtained in the gamma-ray converter: coincidence time resolution is reduced to 20 ns and stable chamber operation is achieved with a safe, non-polymerizing gas mixture. Spatial resolution remains at 2-3 mm FWHM. A rotating camera consisting of two 20 × 20 cm chambers has now been under evaluation at Geneva Hospital for one year. Multilayer printed-circuit techniques are now used to construct chambers with multiple converters, thus raising detection efficiency from 7.5 to 20%. Read-out electronics and back-projecting memory are being developed to handle the high data rate from these chambers. A four-chamber positron camera designed to achieve 100,000 coincidences s-1 is nearing completion.

The high-density multiwire drift chamber

Abstract A multiwire proportional chamber, with a high-density drift space attached, has been developed as a position-sensitive detector for non-ionizing radiation. A chamber with a 5 g/cm 3 drift space, comprising a lead-bismuth matrix of 1 mm square holes on a 1.5 mm pitch has been investigated. For 0.66 MeV protons a detection efficiency of 5%, combined with a spatial resolution of 1.3 mm fwhm, is obtained. Much higher efficiencies are possible. A theoretical analysis of the detection efficiency as a function of photon energy is presented. It agrees well with experimental results. The chamber offers new possibilities for photon and neutron detection, and as a shower detector for high-energy particles.

The spherical drift chamber for x-ray imaging applications

Properties of proportional chambers with spherical drift spaces have been investigated. An experiment of X-ray diffraction in crystals shows that an accuracy of 0.5 mm, at 20° inclination with respect to the axis of the chamber, can be obtained with 4 cm of drift length. Such chambers have many applications: X-ray diffraction patterns, pin-hole imaging, angular distributions of cascades of X-rays in nuclear physics, etc.

A High-Resolution Proportional Chamber Positron Camera and Its Applications

A positron camera consisting of two high density proportional chambers is described. It provides a spatial resolution of 2.4 mm FWHM, a maximum data rate of 3000 c.p.s. and a sensitivity of 25 c.p.s. per ¿Ci. Results of its application to angular correlation studies of condensed matter and to phantom and in vivo medical imaging are presented. A Fourier deconvolution technique is described for obtaining three-dimensional medical images. It uses a generalized matrix inversion by singular value decomposition to modify the low frequency Fourier components.

A proportional chamber positron camera for medical imaging

Abstract A proportional chamber positron camera based on the principle of the high-density drift space has been constructed for medical imaging. Each of the two chambers has a 1 cm thick converter, 20 × 20 cm 2 in size, with a photon detection efficiency of 8.5% at 0.5 MeV and a time resolution of 100 ns. With a new mode of operation, using neon gas, less than 20 ns has now been achieved. Spatial resolution for a 1 mm line source in plastic is 2 mm fwhmfor 22 Na and 3.5 mm fwhm for 68 Ga. A source of 300 μCi activity, surrounded by a 20 cm diameter plastic scattering bolus, provides a count rate of 4000 coincidences per second, of which 50% are random. This will improve to 20% with the 20 ns time gate. A three-dimensional image comprising 16 slices may be obtained in 20 min. Bone and organ imaging is presented.

Image Reconstruction for a Rotating Positron Tomograph

A high-resolution positron tomograph consisting of two high-density avalanche chambers mounted on a rotating gantry has been installed in the Nuclear Medicine Department of the Cantonal Hospital in Geneva. Positron annihilation data are collected from six detector positions and back-projected in real time to form a single three-dimensional image. Up to 16 transverse or longitudinal sections may be imaged simultaneously. The derivation of the appropriate deconvolution filter is described in this paper, and the first images taken in the transverse mode are presented. It is shown that quantitative information can be obtained from such images.

A new position-sensitive detector for thermal and epithermal neutrons

Abstract A new two-dimensional position-sensitive neutron detector is described. It is based on (n, γ) neutron resonance capture in a foil with subsequent detection of internal conversion electrons with a high-density proportional chamber. Large-area detectors with a 1 mm spatial resolution are feasible. A detection efficiency of 50% is possible for thermal neutrons using gadolinium-157 foil and for epithermal neutrons using hafnium-177.

Object reconstruction from focused positron tomograms

A major problem with the reconstruction of three-dimensional object distributions from focused tomographic images using Fourier transforms is the amplification of statistical noise in certain frequency components. The use of a generalised matrix inversion technique to limit noise amplification to a level related to the spatial resolution of the imaging system is described. The reconstruction method is applied to a simulated positron camera, and results are presented on the imaging of an extended, three-dimensional object distribution. A significant improvement in the elimination of the background is achieved.

The high-density proportional chamber and its applications

Abstract The recent development of the high-density proportional chamber - a proportional chamber with solid converters - has provided important, new imaging possibilities for low-energy neutral particles. The principal device realized to date is a high spatial resolution (2 mm fwhm) positron camera. This has been used for angular correlation studies of positron annihilation radiation for condensed matter physics research, and for three-dimensional medical imaging of positron radioisotope distributions. For these applications, a new converter, made of bismuth-doped araldite, is described, which combines good detection efficiency (10%) and coincidence time resolution (100 ns) when operated with carbon-dioxide gas. The addition of a gadolinium foil to a high-density proportional chamber affords an efficient detector for thermal neutrons that maintains the good spatial resolution at large angles of neutron incidence. With a hafnium foil, the same proporties are possible for epithermal neutrons. Solar neutrino detection, electrophoresis, and radiochromatography are other fields of possible application.