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J Cormack

Publications and source records attributed to J Cormack.

8 recordsLinked to original sources
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↗
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↗