The PETRRA positron camera: design, characterization and results of a physical evaluation.

The PETRRA positron camera is a large-area (600 mm x 400 mm sensitive area) prototype system that has been developed through a collaboration between the Rutherford Appleton Laboratory and the Institute of Cancer Research/Royal Marsden Hospital. The camera uses novel technology involving the coupling of 10 mm thick barium fluoride scintillating crystals to multi-wire proportional chambers filled with a photosensitive gas. The performance of the camera is reported here and shows that the present system has a 3D spatial resolution of approximately 7.5 mm full-width-half-maximum (FWHM), a timing resolution of approximately 3.5 ns (FWHM), a total coincidence count-rate performance of at least 80-90 kcps and a randoms-corrected sensitivity of approximately 8-10 kcps kBq(-1) ml. For an average concentration of 3 kBq ml(-1) as expected in a patient it is shown that, for the present prototype, approximately 20% of the data would be true events. The count-rate performance is presently limited by the obsolete off-camera read-out electronics and computer system and the sensitivity by the use of thin (10 mm thick) crystals. The prototype camera has limited scatter rejection and no intrinsic shielding and is, therefore, susceptible to high levels of scatter and out-of-field activity when imaging patients. All these factors are being addressed to improve the performance of the camera. The large axial field-of-view of 400 mm makes the camera ideally suited to whole-body PET imaging. We present examples of preliminary clinical images taken with the prototype camera. Overall, the results show the potential for this alternative technology justifying further development.

A gas microstrip detector for XAS studies in the photon energy region 300-1500 eV.

The ability to perform X-ray absorption spectroscopy (XAS) in the 300-1500 eV energy range allows measurements to be made on transition metal compounds. This paper describes a detector and the technique used to perform fluorescent measurements on such materials. A variety of test sample results are shown to illustrate the low energy and energy-resolving capabilities of the detector (based on gas microstrip technology). Two possible applications are also demonstrated. The first shows how the detector can be used to gather X-ray absorption spectra for the L edges of transition metals and K edges of light elements (C, O and N). The other shows how the magnetic immunity of the detector can be exploited to study the magnetic properties of materials.

The effect of beam time structure on counting detectors in SRS experiments.

Counting detector systems are increasingly used in X-ray experiments because of their attractive properties as regards linearity, large dynamic range and simple noise properties. In synchrotron radiation source (SRS) applications of X-ray detectors, counting rates are generally high enough to require dead-time correction. The time structure of an SRS beam interacts with the dead-time characteristics of the detector in a way that the simple stochastic dead-time models cannot always handle. This report generates analytical and Monte Carlo mathematical models which describe the rate performance of any given detector system when used with the typical beam structures encountered in an SRS.

Measurement of small-angle photon scattering for some breast tissues and tissue substitute materials.

For photon energies encountered in diagnostic radiology the shape of the scattering distributions for low-atomic-number media exhibits peaks in intensity close to the forward direction that are not predicted by conventional theoretical models. The positions and shapes of the peaks depend upon the interatomic and intermolecular configurations of the scatterers. The phenomenon is of particular interest because of its relevance to the understanding and modelling of x-ray imaging processes and the possibility that the peaking may be characteristic of tissue type. In the present study, peaks in the forward scattering distributions have been demonstrated for 19 samples of breast tissue and three tissue substitute materials using a position-sensitive photon detector and a 60 kVp x-ray source. Prominent features were observed for all samples investigated. Large differences were found in the shapes of the distributions between adipose and fibroglandular tissues and only small differences were found between carcinomas and fibroglandular tissues.

Three-dimensional x-ray microtomography for medical and biological applications.

Three-dimensional (3D) x-ray microtomography is a technique for obtaining the 3D distribution of x-ray attenuation coefficients within small objects. To obtain microtomographic images apparatus has been developed which consists of a microfocal x-ray source, a computer-controlled stage for rotating the object, a 2D multi-wire gas proportional x-ray counter and a microcomputer to control image acquisition. Projection data were generated by rotating the object to discrete orientations around a single axis until of the order of 100 2D projection images of the object were collected. The projection images were transferred to a VAX 11/750 computer for subsequent 3D reconstruction using a convolution and back-projection algorithm in cone-beam geometry. The reconstructed data, comprising cubic voxels, may be displayed as sets of sequential transaxial, sagittal and coronal planes through the object. Alternatively, perspective displays of individual orthogonal sections may be formed with either intersecting planes or with these planes projected onto the surfaces of a box-like structure. The technique provides for the investigation of small-scale structures in biological specimens and we show some illustrative images of dead insects.

Quantitative autoradiographic imaging using gas-counter technology.

Gas counters based on the multiwire proportional counter (MWPC) and multistep avalanche (MSA) technologies can combine the functions of imaging and quantitation (or assay) of radioactively labelled electrophoretograms and blots. Submillimeter spatial resolutions combined with good quantitation have been achieved for the most common beta-emitting radio labels. The current state of development of this technology is reviewed and assessments of the performance of these devices relative to the more traditional systems (X-ray film and the scintillation counter) presented. Particular attention is paid to the potential of these devices to enhance the productivity of the scanning of blots and the performance of assays.

Long-term results of bipolar arthroplasty in osteoarthritis of the hip.

Bipolar arthroplasty of the hip was introduced by the first author in 1974. From that time until 1988, this type of arthroplasty has been carried out in 1213 hips at the authors' institution. Included is a group of 760 osteoarthritic hips on which follow-up studies have been done at six months, five years, ten years, and 15 years. Studies of the acetabulum show healthy bone preservation as long as 15 years after surgery. A process of floor reinforcement in certain states has also been identified. The relatively simple operative technique has resulted in few postoperative complications. Clinical results as long as 15 years postsurgery compare favorably with two-piece replacement techniques.

The performance of a multiwire proportional chamber positron camera for clinical use.

The Rutherford Appleton Laboratory clinical positron camera consists of two opposed multiwire proportional chambers (MWPCS) mounted on a rotating gantry capable of performing routine nuclear medicine studies. The system has operated since the end of 1986 with complete reliability. It has a sensitivity of 37 kcps MBq-1 cm3 per axial cm for a 20 cm diameter uniformly filled cylinder of activity. The best spatial resolution obtainable is about 6 mm, although in practice images are smoothed in order to reduce statistical noise with a corresponding decrease in resolution. Cross-plane rays are utilised during image reconstruction and the resulting three-dimensional images exhibit the same spatial resolution in three orthogonal directions over a large cylindrical field of view 15 cm high by 30 cm in diameter. The maximum data-taking rate is limited to 1.5 to 2 kcps at present due to deadtime in the read-out system. The performance of the system is described with particular reference to the problems of imaging with the large fractions of random and scattered events which are a consequence of using large-area detectors. Images of phantoms and patients are presented and proposed modifications to the camera are described.

The enhancement of radiographic images from a multiwire camera using a maximum entropy algorithm.

The multiwire camera (MWC) produces high speed, quantitative autoradiography of radiolabelled substances in two-dimensional systems. While greatly superior to film-based systems in respect of speed and quantitativity the MWC has significantly poorer spatial resolution (particularly for high energy beta-emitting radiolabels) and the performance is ultimately limited by the noise induced in the images by Poisson statistics and counter background. Processing the MWC images with a maximum entropy algorithm significantly improves the performance of the system in these respects. The algorithm has been tested using one-dimensional data taken from images of known tritium, 14C and 125I distributions. Processed images are visually more acceptable with improved quantitative accuracy and spatial resolution. Quantitative accuracy, calculated as the root mean square deviation between an image and the known sample activities, is 10-40% lower for processed images compared with original camera images. Spatial resolution, calculated from slopes in the images representing edges of activity in the sources, is improved by 20-40% for the processed images. The algorithm is used to improve a two-dimensional image from a biological study. The source distribution consisted of a set of circular dots of varying activity. The dots with lowest activity were barely discernible in the raw MWC image but are clearly resolved after processing. The algorithm used is simple and effective and executes acceptably quickly on a personal computer. It should prove useful in any context where the imaging performance of a system is limited by Poisson statistics.

Measurement of radiation dose to the thyroid using positron emission tomography.

Measurements of the functioning volume of thyroid tissue have been made in 22 patients undergoing radioiodine therapy for thyrotoxicosis, using a prototype multiwire proportional counter positron camera. Tomographic images were produced of the distribution of 124I in the thyroid. Functioning volumes were found to be in the range 21-79 cm3 with volume errors of the order of +/- 4% to +/- 14%. Radioiodine uptake varied from 28% to 98%. Using a value of 6 days for the effective half-life of radioiodine in hyperactive thyroids, radiation doses from a standard therapy administration of 75 MBq of 131I varied from 11 to 48 Gy (compared with a recommended 50-70 Gy). In five cases PET imaging showed a non-uniform distribution of radioiodine in thyroids thought to have uniform uptake from conventional pinhole scintigraphy.

The development of high-efficiency cathode converters for a multiwire proportional chamber positron camera.

A high-efficiency cathode converter for 511-keV photons has been developed for incorporation into a multiwire proportional chamber (MWPC) positron camera. The converter consists of a honeycomb pattern produced in a 1-mm-thick lead sheet to leave lead walls with a thickness of approximately 60 micron. The converter also serves as the cathode of an MWPC, the gap between the converter and the anode wire plane being 2.5 mm. This small gap results in a high secondary electron extraction efficiency without the need for additional drift voltages. Measurements of the efficiencies of a plane converter and of two types of structured converters in a single section MWPC are described and the efficiency is found to increase in proportion to the converter surface area. This result justifies the use of a simple theoretical model whereby an extrapolation to the efficiency of a detector consisting of a stack of 20 MWPC sections, each section having two converters, is made. The efficiency of this proposed system is calculated to be 17% for 511-keV photons.

Preliminary assessment of a multiwire camera for quantitative autoradiography of tritium-labelled substances.

We have assessed a multiwire camera for the high speed, quantitative autoradiography of tritium-labelled substances in two-dimensional systems. Exposure times for 3H were typically 1000 times less than film-based methods, with a detection efficiency of 19 +/- 3% for those beta particles predicted to escape into the sensitive region from uniformly 3H-labelled plastic segments of 30 micron thickness. The spatial resolving power was calculated to be 0.4 mm full width half maximum (FWHM), the background count rate was 1 CPM cm-2 and the lowest activity monitored for a biochemical study was 0.15 Bq cm-2. The camera is expected to have wide applications for detecting and quantifying biological substances in a wide range of separation media.

A clinical evaluation of a prototype positron camera for longitudinal emission tomography.

A multiwire proportional-chamber positron camera, developed at the Rutherford Laboratory, has been evaluated at The Royal Marsden Hospital. The prototype camera consists of two opposing 30 X 30 cm2 chambers. Longitudinal tomograms of a positron-emitting radioactive distribution placed between the detectors are obtained via back-projection and 2D-deconvolution. Due to the limited stereoscopic angle achieved with stationary detectors, only five planes parallel to the detector faces are reconstructed. A selection of images is presented of phantoms using 68Ga and of patients using 18F, 18F fluorodeoxyglucose and Na124I, to illustrate the tomographic performance of the positron camera. A comparison between back-projected and deconvoluted images shows that the 2D-deconvolution process, which includes filtration of image noise, successfully removes the background due to scattered photons. The spatial resolution achieved depends on the half-height frequency cut-off used in the filtering process, and this parameter was chosen according to the count density in the back-projected images. A qualitative visual comparison was made between the positron images and equivalent single photon studies on the same patient. The results justify further development of this new detector, especially for use with generator-produced positron-emitting radionuclides. Three-dimensional deconvolution resulted in improved tomographic performance for phantom data, but was less successful for patient data. These problems associated with limited-angle tomography will be overcome by multi-view acquisition. This study has shown that a relatively low-cost positron imaging system can be used for routine organ imaging. Further developments in hardware and software should yield images which are superior to those from single-photon planar or tomographic studies.

The influence of etiology on the results of coccygectomy.

The results of coccygectomy for coccygodynia seem not to have been correlated with etiologic factors and were investigated in 48 cases (10 men and 38 women) with an average follow-up period of seven years. The following four etiologic groups were designated: I, direct trauma; II, spontaneous in origin; III, coccygodynia associated with spinal surgery; and IV, postpartum. Groups I and IV had a 75% recovery rate from pain and Group II had a 58% recovery rate. No satisfactory results were obtained in Group III. The incidence of infection was 16.6%. Few satisfactory results were obtained in cases of deep infection. While this operation is effective in many cases, it is seldom successful for pain associated with lumbar disc disease requiring lumbar laminectomy and spine fusion. Prophylactic antibiotic therapy is clearly indicated.

Ollier's disease with multiple sarcomatous transformations.

A 53-year-old man had the monomelic form of Ollier's disease, which resulted in deformity of the left leg. The patient was otherwise well until pain and increasing size of the left thigh led him to seek treatment. Biopsy revealed chondrosarcomatous transformation in the distal left femur. Hip disarticulation was performed. This case is unusual in that multiple foci of chondrosarcomatous transformation at various stages of development were present throughout the left femur, tibia, and fibula.