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M O Leach

Publications and source records attributed to M O Leach.

At least 163 records · Page 9Linked to original sources
PubMed1985

An x-ray detector system and modified simulator providing CT images for radiotherapy dosimetry planning.

A high count rate position-sensitive detector was developed in order to obtain CT information using a modified radiotherapy simulator. The detector is attached to one arm of the simulator gantry and acquires projection data from a fan beam of X-rays derived from the simulator X-ray tube. The scanner is designed principally for planning the dosimetry of breast cancer radiotherapy. During the collection of transmission projection data, suitable for the digital reconstruction of patient cross sections, the simulator gantry rotates continuously through about 360 degrees. A film cassette holder can be positioned above the detector to provide conventional simulator check films. Cross sections through patients receiving post-operative radiotherapy for breast cancer were reconstructed from projections acquired with the patient lying in the treatment position. These scans are an average over the breathing cycle. The outlines of internal anatomy and the patient external contour were used to produce treatment plans which include tissue inhomogeneity corrections. The data produced by this wide aperture CT scanner are suitable for planning those conditions in which radiotherapy is delivered to an organ such as the breast or the whole body, rather than to a localised lesion.

Breast Neoplasms↗
PubMed1984

The release rate of 37Ar from human subjects following intravenous injection.

Two measurements of the exhalation rate of 37Ar following an intravenous bolus injection have been made on a volunteer, and compared with the exhalation rate of 133Xe for the same subject. The results are also compared with a previously reported pilot 37Ar injection study on a second volunteer. During the first 100 min following injection, the 37Ar washout rate was shown to resemble closely the 133Xe washout rate. This result shows that for short time periods after injection, the behaviour of 133Xe may be used to predict 37Ar behaviour.

Argon↗
PubMed1984

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.

Bone Marrow↗
PubMed1983

The retention and release of 37Ar from samples of human bone examined in vitro and a review of the implications for argon transfer from bone in vivo.

The release rates and long term retention of 37Ar produced by the reaction 40Ca(n, alpha) 37Ar, following fast neutron irradiation, have been measured in vitro, in three samples of dried human femur. For each sample, the release rates were fitted to three exponential curves, of half-times approximately 2, 20 and 300 min. Some 60-80% of the 37Ar released was contained in the first component. The release rates for samples of trabecular and cortical bone were qualitatively similar, but after some 20 h the retained fraction of 37Ar was 28 +/- 0.6% in a sample of predominantly trabecular bone, and 33 +/- 0.7% in samples of predominantly cortical bone. The processes affecting the trapping and release of 37Ar from bone crystals are discussed. It is suggested that 37Ar escapes from bone mineral by back-diffusion along recoil damage tracks. The factors affecting the release of 37Ar from bone into the venous blood supply are discussed in the context of in vivo measurements of body calcium by the 37Ar method. The physiological processes and compartments likely to give rise to the long 37Ar exhalation half-times following irradiation in vivo: transfer from the mineralised bone matrix to blood capillaries, and uptake and washout from bone marrow, are considered. Uptake in bone marrow may be responsible for both of the observed half-times.

Argon↗
PubMed1983

A comparison of attenuation correction methods for quantitative single photon emission computed tomography.

Transverse section tomograms of experimental phantoms and patients have been obtained using a GE 400T camera and a filtered back-projection reconstruction technique. These tomograms have been compared with the corresponding sections reconstructed from the same tomographic projection data, but using iterative algorithms with correction for photon attenuation. The comparison assesses the importance of including a correction for attenuation as well as demonstrating how closely a simple geometric attenuation correction, applied to the filtered back-projection reconstruction method, approximates to a more accurate correction incorporated in the computation of line integrals during iterative reconstruction. A comparison is also made between the behaviour of reconstruction algorithms with simulated projection data and real data in terms of convergence properties, and some shortcomings arising from simulation are noted.

Humans↗
PubMed1983

A comparison between 180 degrees and 360 degrees data reconstruction in single photon emission computed tomography of the liver and spleen.

Single photon emission computed tomography (SPECT) of the liver and spleen has been studied using a GE400T/STAR rotating gamma camera/computer system. We have compared section image contrast, in both phantom and patient studies, produced by 180 degrees and 360 degrees reconstructions of data from a full 360 degrees data acquisition. There is little difference in overall image quality between the two reconstruction modalities. Quantitatively, however, appropriate 180 degrees reconstructions significantly improve lesion contrast in both organs by as much as a factor of two. In all cases studied, the values of image contrast achieved when the 180 degrees arc of reconstruction was centred about the region of interest were higher than the values obtained from the 360 degrees reconstructions. These results are shown to be independent of the corrections made for photon attenuation. Clinically this technique may prove useful in clarifying the presence or absence of disease, particularly in equivocal cases where lesions have poor contrast or are small in size. An extended clinical trial of the technique is now in progress.

Adult↗
PubMed1983

The spatial resolution of a rotating gamma camera tomographic facility.

An important feature determining the spatial resolution in transverse sections reconstructed by convolution and back-projection is the frequency filter corresponding to the convolution kernel. Equations have been derived giving the theoretical spatial resolution, for a perfect detector and noise-free data, using four filter functions. Experiments have shown that physical constraints will always limit the resolution that can be achieved with a given system. The experiments indicate that the region of the frequency spectrum between KN/2 and KN where KN is the Nyquist frequency does not contribute significantly to resolution. In order to investigate the physical effect of these filter functions, the spatial resolution of reconstructed images obtained with a GE 400T rotating gamma camera has been measured. The results obtained serve as an aid to choosing appropriate reconstruction filters for use with a rotating gamma camera system.

Air↗
PubMed1983

Pharmacokinetics of therapeutic doses of isotopically labeled platinum antitumor agents in the mouse and rat.

The pharmacokinetics of the antitumor agents ethylenediamine platinum dichloride and cisplatin have been investigated in mice bearing the ADJ/PC6 plasma cell tumor after therapeutic doses of these agents, labeled either with 191Pt or in the ligand with 14C. The distribution of the radioactive labels has been estimated in tumor, kidney, liver, and intestine in treated animals from 1 hour to 15 days after administration. The two labels, on ligand and on Pt, were equivalent only in some cases with respect to the total radioactivity within a tissue. Patterns of isotope distribution varied with time and tissue and suggested that the metal-ligand bond was dissociated as early as 1 hour after administration.

Animals↗
PubMed1978

In vivo measurement of calcium by the 37Ar method: a study of the effect of recirculating breath collection systems on the exhalation rate.

A study has been made of the effect that different methods of breath collection have on the measurement of calcium in vivo by the 40Ca(n,alpha) 37Ar method. The effect of open and rebreathing breath collection methods has been examined by using intravenous injections of 133Xe. This study has indicated that comparison of calcium measurements between individuals and between centres is likely to be accurate only if breath is collected on 'open circuit'.

Argon↗
PubMed1978

Problems in the interpretation of the in vivo measurement of calcium by the argon-37 method: an investigation of inert-gas elimination in humans.

An investigation has been made of some physiological problems associated with the interpretation of in-vivo measurements of calcium by the argon-37 method. Inert-gas elimination in humans over a period of several days was studied using i.v. injections of Xe-133. The results imply that the exhalation rate of A-37 formed in bone will be affected by the individual's composition, in particular body fat. Comparison of calcium measurements between individuals and between laboratories is meaningful only if corrections are made for differing individual composition.

Argon↗
PubMedDate not supplied

Probing tumor microvascularity by measurement, analysis and display of contrast agent uptake kinetics.

This paper describes a measurement protocol for acquiring quantitative dynamic MRI data and novel analysis and display software (Magnetic Resonance Imaging Workbench (MRIW)). Proton density-weighted and T1-weighted two-dimensional gradient echo images are used to quantify tissue contrast agent concentration. The dynamic studies last approximately 7 minutes, with 10-second temporal resolution. Analyses of signal and concentration changes with time are performed, allowing capillary permeability-surface area product, tissue leakage space, enhancement onset time, mean enhancement gradient and maximum enhancement level to be mapped as false-color parametric overlays registered with anatomic images. Quantification of permeability and leakage space provides a method for comparing physiology in patients between visits or for intersite comparisons.

Astrocytoma↗