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Biomedical subjects

H E Johns

Publications and source records attributed to H E Johns.

15 recordsLinked to original sources
PubMed1979

Variations in x-ray exposures to patients.

X-ray exposures to patients were measured in 30 rooms in Toronto hospitals. For a given procedure, the total exposure for a satisfactory examination differed from one room to another by as much as a factor of 30, skin exposures reaching 90 R and more for barium enemas and barium meals. The factors primarily responsible for these large differences in exposure were fluoroscopic exposure rate (0.65-12 R/min) and time 1.5-12 min), kVp and filtration (8:1), choice of screen-film combination (6:1) and attenuation of table tops and phototimers (4:1). The increase by a factor of 5 in exposure of pelvic organs, when using beams directed AP instead of PA, requires that remote-controlled GI machines be used cautiously, particularly on women of childbearing age. It is also important that these machines have tables with low attenuation. The use in some rooms of minimal fluoroscopic exposure rates and 70 mm fluorography at total exposures of less than 5 R shows that radiographic examinations can be made virtually "safe" with modern, properly adjusted equipment.

Barium Sulfate↗
PubMed1978

Edge contrast in ionography.

The effect of Mylar foil thickness on the appearance of ionography images has been investigated. Approximate expressions for the depletion width of strips and halfplanes of charge are given with supporting experimental measurements. In microdensitometer traces of charged strips and halfplanes the density is shown to be proportional to the vertical field profile of the charge pattern near the Mylar. This work suggests a simple means of altering edge contrast for ionography by using different thicknesses of Mylar. Ionography images of a hand phantom illustrate this effect.

Radiographic Image Enhancement↗
PubMed1976

New methods of imaging in diagnostic radiology Sylvanus Thompson Memorial Lecture.

A brief history of the recent developments in computer-assisted tomography (CAT) is presented. The development of whole-body scanners from the basic brain scanner, the trend towards higher-speed scanners using multiple detectors in a fan-type geometry, and some of the unsolved problems related to CAT scanners are discussed, as well as the potential use of whole-body scanning in radiotherapy treatment planning and in radiobiology. A review of recent developments in electrostatic imaging is presented and a new method of imaging called ionography is described in detail. It is shown how a single exposure can be used to produce a number of copies, each with a different amount of edge contrast, under the control of the radiologist. Further, it is shown how this can be done using a closed system so that the ionography chamber never needs to be opened. Some of the recent commercial developments in this field are described. It is suggested that in the future radiology departments may be replaced by departments of imaging, which will include all ways of obtaining diagnostic information and where such a department would be problem oriented rather than technique oriented. The need for basic scientific support by a medical physics and engineering group in such a department is emphasized.

Electrons↗
PubMed1975

New methods of imaging in diagnostic radiology.

The basic ideas of electrostatic imaging with special reference to ionography are reviewed. The concept of edge contrast is explained in terms of calculated powder particle trajectories and methods for controlling edge contrast are presented. Also, methods for reading a foil from outside the ionography chamber using the electric field extending through the foil are described. An example of an image taken with liquid in the chamber (liquid lonography) is presented, and the possible extension of liquid ionography to nuclear medicine is discussed.

Absorptiometry, Photon↗
PubMed1975

X-ray depth doses from linear accelerators in the energy range from 10 to 32 Mev.

The depth dose characteristics of the x-ray beam from a linear accelerator have been studied in the electron energy range from 10 to 32 MeV for various target and flattening filter combinations. At all energies the most penetrating x-ray beam is obtained with a low atomic number flattening filter. At energies below 15 MeV a high atomic number target should be used, while at energies above 15 MeV a low atomic number target should be employed to get the best depth dose distribution. The optimum target thickness is approximately equal to the mean range of electrons in the target material. The study of some physical parameters such as depth dose, average dose, and integral dose, suggests that the optimum energy for linear accelerators used in the x-ray mode is about 25 MeV.

Aluminum↗
PubMedDate not supplied

Electrostatic fields in ionography.

There is renewed interest in diagnostic radiology in electrostatic methods of imaging, such as xeroradiography and ionography. This is due to the fact that edge contrast can be achieved, aiding in the visualization of soft tissue tumors. In analyzing the image forming properties of these system, we chose to solve the electrostatic problems by the method of images. We present methods and recursion formulas for calculating electrostatic fields due to (a) any charge distribution on a slab of dielectric (the solution involves a single infinite series) or (b) the same as the above with the introduction of an additional ground plane parallel to the dielectric surface (the solution now involves double infinite series). Analysis of these fields suggests new methods of controlling edge contrast and development configurations where the field which penetrates through the foil is used to produce the final image rather than the field above the charged surface.

Electric Conductivity↗
PubMedDate not supplied

Closed-system ionography for diagnostic radiology.

Three methods are described whereby radiographic electrostatic images are transferred from inside of a liquid ionography chamber to the outside. One of these methods is implemented showing that multiple-charge images can be transferred from a single original without significant degradation of image quality and each copy may be developed with a different amount of edge contrast. This new method of imaging was applied to a radiograph of a test pattern and a hand phanton. An exposure of about 6 mR to the imaging chamber is needed to produce a useful image with a resolution of about 8 1p/mm.

Ions↗
PubMedDate not supplied

Spectroscopy of diagnostic x rays by a Compton-scatter method.

A method is described for measuring the energy spectra of diagnostic x rays using a high-resolution intrinsic germanium spectrometer. The method is applicable over the wide range of x-ray exposure rates normally used in radiography. Reduction of the high x-ray beam intensity to a level acceptable to the germanium spectrometer is achieved by measuring the spectrum of photons scattered through 90 degrees by a lucite disc. From this measured spectrum the primary spectrum is obtained by calculation. Some typical x-ray spectra are presented.

Radiography↗
PubMedDate not supplied

Use of electron filters to improve the buildup characteristics of large fields from cobalt-60 beams.

It has been shown that for Cobalt-60 units capable of producing large fields (up to 40 X 40 cm2)at 80 cm, the dose distribution in the buildup region is quite different from that observed with small fields. First, the maximum dose occurs at 1-2 mm instead of 5 mm; and second, the maximum does in some extreme cases is 15%-20% higher than the dose at 5 mm. Thus, the dose is decreased by more than 15%-20% by 3 mm of tissue, suggesting severe electron contamination. In this report, it is shown that this contamination can be best removed by a filter of thickness 0.4 g/cm2 of medium atomic number, placed some 25 cm from the source inside the collimator. For existing units, positioning the filter inside the collimator is not practical. Acceptable results, however, can be obtained by placing the filter just below the collimator.

Cobalt Radioisotopes↗