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Biomedical subjects

J Cormack

Publications and source records attributed to J Cormack.

14 recordsLinked to original sources
PubMed1994

Gene marker loss induced by the transposable element, En, in maize.

The En/Spm transposable element system in maize includes the functional element, En/Spm and the receptor element I/dSpm. An En receptor has been found that shows En-induced breakage. This En-responsive receptor (designated I836518) is located on the short arm of chromosome 9, proximal to Wx. In the presence of En, markers distal to the receptor show a loss of gene expression. Kernels heterozygous for aleurone and endosperm marker genes have a variegated appearance. The hypothesis is advanced that this variegation represents a physical loss of the chromosome segments carrying the genes distal to the receptor position. It is the first case of an En-controlled breakage event.

Chromosome Mapping↗
PubMed1991

Computer generation of random deviates.

The need for random deviates arises in many scientific applications, such as the simulation of physical processes, numerical evaluation of complex mathematical formulae and the modeling of decision processes. In medical physics, Monte Carlo simulations have been used in radiology, radiation therapy and nuclear medicine. Specific instances include the modelling of x-ray scattering processes and the addition of random noise to images or curves in order to assess the effects of various processing procedures. Reliable sources of random deviates with statistical properties indistinguishable from true random deviates are a fundamental necessity for such tasks. This paper provides a review of computer algorithms which can be used to generate uniform random deviates and other distributions of interest to medical physicists, along with a few caveats relating to various problems and pitfalls which can occur. Source code listings for the generators discussed (in FORTRAN, Turbo-PASCAL and Data General ASSEMBLER) are available on request from the authors.

Algorithms↗
PubMed1990

Factors affecting the precision of bone mineral measurements. Part 1: Review of experimentally derived results obtained from single photon absorptiometry.

An evaluation of the factors which influence the precision of bone mineral measurements using single photon absorptiometry (SPA) is presented. This incorporates several techniques which have been developed over the past few years to improve the reliability of such measurements. As such, the figures obtained should provide an objective and up-to-date basis for critical comparison with other modalities such as quantitative computerized tomography (QCT), dual photon absorptiometry (DPA) using Gd-153, and quantitative digital radiography (QDR). Under optimum conditions, a precision of 0.3% was achievable for in vitro phantom measurements. Under realistic working conditions, however, a precision of 1.0% for phantoms and 1.4% for clinical studies was found to be more typical for bone mineral content (BMC) measurements. Derived parameters such as bone mineral density (BMD) were generally less reliable, with a precision of 4.7%. It is unlikely that these values can be improved upon substantially with current SPA technology.

Absorptiometry, Photon↗
PubMed1990

Factors affecting the precision of bone mineral measurements. Part 2: Some statistical notes relating to photon counting statistics and rates of bone loss.

This paper discusses some statistical aspects of absorptiometric bone mineral measurements. In particular, the contribution of photon counting statistics to overall precision is estimated, and methods available for carrying out statistical comparisons of bone loss and determining their precision are reviewed. The use of replicate measurements as a means of improving measurement precision is also discussed.

Absorptiometry, Photon↗
PubMed1983

Geometric determination of left ventricular volume from gated blood-pool studies using a slant-hole collimator.

A geometric method of measuring absolute left ventricular volumes from gated blood-pool studies obtained in a single-plane modified left anterior oblique view was evaluated prospectively in 30 patients who also underwent single-plane contrast ventriculography. The gated studies used a 30 degrees straight-bore slant-hole collimator with the holes slanted caudally. Left ventricular end-diastolic volume was calculated using the area-length method, with semiautomatic definition of the left ventricular region of interest and the maximum length of the left ventricle. Ejection fraction was determined from the left ventricular time/activity curve. The left ventricular end-systolic volume was derived using the end-diastolic volume and ejection fraction. Correlation coefficients between the two methods were 0.93 for end-diastolic volume, 0.95 for end-systolic volume, and 0.91 for ejection fraction. This method provided accurate and highly reproducible measurements of actual left ventricular volumes and was easily applicable in routine clinical studies.

Adult↗
PubMed1980

Minimisation of data transfer losses in the display of digitised scintigraphic images.

The transfer of scintigraphic data to any type of display, and ultimately to the eye, is a data compression procedure which invariably leads to information loss. Unless this information loss can be minimised, valuable image data may well be discarded. Attempts to achieve data loss minimisation have led to procedures such as the use of statistically equal levels for display, and the more recent histogram modification techniques such as equalisation and hyperbolisation. The method discussed here uses information theory to obtain the mean uncertainty (or entropy) per pixel in the intensity of the displayed image. Statistical noise and the characteristics of the image are taken into account. Calculation of the best choice of grey or colour levels for transfer loss minimisation is then possible: that is, the best conditions for data transfer to the display can be set up. A computer algorithm which carries out this function has been written. Several different types of simulated phantom, and clinical images, have been investigated. Improved perceptibility of many of these images has been obtained, correlating with reduction in mean pixel uncertainty. Although the technique still requires some refinement, it appears that optimisation of display characteristics for any transmitted image is potentially feasible.

Computers↗
PubMed1980

Cardiac tamponade: accurate diagnosis by radionuclide angiography.

This study was performed to define the signs of cardiac tamponade on radionuclide angiography. The first pass studies were performed on a large field gamma camera, using a bolus of technetium pertechnetate. Three hundred and seventy-two studies were analysed. Cardiac tamponade requiring surgical intervention was present at the time in 28 studies. The diagnostic signs of cardiac tamponade on radionuclide angiography were: (1) Right ventricular compression--sensitivity 82%, specificity 97%, predicitive value of a positive result 72%, predictive value of a negative result 99%, and (2) Right atrial compression--sensitivity 21%, specificity 99%, predictive value of a positive result 75%, predictive value of a negative result 94%. Cardiac tamponade can be accurately diagnosed on radionuclide angiography and thus indicate those pericardial effusions which require urgent drainage.

Cardiac Tamponade↗
PubMed1977

Experimental fundoplication: comparison of results of different techniques.

After the creation of a hypotensive lower esophageal sphincter (LES) by circular myectomy, several techniques of fundoplication were employed to assess the effect of variations in the degree and length of fundoesophageal encirclement on LES function. In series A, 360 degrees wraps were constructed around varying lengths of esophagus: group I, 1 cm; group II, 2 cm; and group III, 3 cm. In series B, 2 cm long fundoplications of varying circumference were performed: group I, 90 degrees; group II, 180 degrees; and group III, 360 degrees. LES evaluation included measurement of LES amplitude and length, the adaptive response of the LES to increased intragastric pressure, pH reflux testing, and LES response to parenterally administered pentagastrin. In series A, 2 and 3 cm wraps restored resting LES pressures to normal (18.8 +/- 0.5 and 20.7 +/- 0.3 cm H2O) in contrast to the 1 cm wrap (12.9 +/- 0.8 cm H2O), and both provided more effective protection against reflux when compared to the shorter variation (pH, 6.2 +/- 0.1 and 6.6 +/- 0.1 versus 4.2 +/- 0.1). In series B, the 360 degrees wrap proved to be more effective than partially encircling techniques. Fundoplication optimally restores normal LES function when it encircles the esophagus completely over a length equivalent to the normal high-pressure zone (1.8 +/- 0.1 cm in the cat).

Animals↗