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Biomedical subjects

D R Shearer

Publications and source records attributed to D R Shearer.

At least 19 recordsLinked to original sources
PubMed2000

Dose rate limitations of integrating survey meters for diagnostic x-ray surveys.

Medical and health physicists are often called upon to measure scatter and evaluate shielding for diagnostic x-ray installations. As exposure times are typically too short to allow rate meters to be successfully used, integrating survey meters are often employed. Pressurized survey meters are particularly valuable because of their increased sensitivity. In practice, some integrating survey meters may give erroneous results under typical measurement conditions.

Radiation Dosage↗
PubMed1995

Effects of long-term rigid contact lens wear on the endothelium of corneal transplants for keratoconus 10 years after penetrating keratoplasty.

Contact lenses do not appear to cause changes in endothelial cell density in normal individuals. In eyes with abnormal corneal endothelium, however, the effects of contact lens wear may be more severe. We studied eyes with abnormally enlarged endothelial cells resulting from penetrating keratoplasty (PK). We identified 34 patients with phakic corneal transplants for keratoconus that had the following characteristics: first graft, no rejection episodes, no reoperations, and follow-up examinations for at least 5 postoperative years. Fifteen of these patients had been fit with daily wear rigid contact lenses (12 rigid gas permeable, 3 polymethylmethacrylate) postoperatively. Contact lens fitting occurred anywhere from 3 months to 2 years post-PK; 19 control subjects had not been fit with contact lenses. Twenty-one of the patients (nine contact lens wearers and 12 control subjects) were examined 10 years after keratoplasty. The contact lens group developed polymegethism, but there was no significant difference from the controls in endothelial cell density at 5 or 10 years post-PK. The cell density at 10 years was 824 +/- 156 cells/mm2 (mean +/- SD) in the contact lens group and 833 +/- 167 cells/mm2 in the control group (P = 0.90). The endothelial cell loss 10 years after keratoplasty was 73% for both groups. Thus, we found no effect on endothelial cell density from at least 7 years of daily rigid contact lens wear in eyes with corneal transplants for keratoconus.

Adult↗
PubMed1994

Human corneal endothelial tolerance to glycerol, dimethylsulfoxide, 1,2-propanediol, and 2,3-butanediol.

We exposed human corneas to various concentrations of four cryoprotectants by one of two methods: a gradual increase to the final concentration (ramp method) and a series of steps to the final concentration (step method). Endothelial damage was manifest as a decrease in the number of endothelial cells per unit area. The highest concentrations that did not cause a loss of endothelial cells by the ramp and step methods, respectively, were 4.3 and 2.0 M glycerol, 2.0 and 4.3 M dimethylsulfoxide, 2.0 and 3.0 M 1,2-propanediol, and 2.0 and 2.5 M 2,3-butanediol. The ramp method achieved higher final concentrations with the more slowly permeating glycerol, but required low toxicity. The step method achieved higher final concentrations with the more toxic cryoprotectants by limiting the exposure time, but required more rapid permeation. None of the four cryoprotectants was tolerated at concentrations sufficient for vitrification at practical cooling and warming rates.

Adolescent↗
PubMed1994

Anomalous primary radiation from the Leksell Gamma Knife Unit.

The Leksell Gamma Knife Unit, Type U, utilizes 201 separate 60Co sources intersecting at a common focus for radiosurgical treatment of the brain. It has been generally understood that all primary unattenuated radiation beams are confined within the protective housing during all phases of Gamma Knife operation. We have found that when the unit's shielding door is lowered for treatment, twelve primary beams exit the unit's door opening. Furthermore, in the original Gamma Knife unit design, two of the twelve primary beams failed to be attenuated by either the unit base or the unit wings. This has implications for the design of structural shielding and the protection of personnel during emergency procedures. Engineering modifications to eliminate the problem are discussed.

Cobalt Radioisotopes↗
PubMed1992

Radioactive contamination in hospitals from nuclear medicine patients.

Comparatively little research has been published on radioactive contamination from nuclear medicine patients using hospital toilet facilities. The present study was undertaken to obtain an estimate of the amount of restroom contamination found in a typical 700-bed community hospital as a result of nuclear medicine procedures. Wipe tests were performed on designated areas for two 1-wk periods approximately 6 mo apart. For a typical workload of approximately 25-30 procedures per day, it was found that radioactive contamination ranged from approximately 10(3) Bq 100 cm-2 99mTc for the nuclear medicine restrooms to approximately 10(4) Bq 100 cm-2 for men's toilet facilities. The major component of the activity was found in the vicinity of the toilet. Although this contamination is minimal, it is recommended that nuclear medicine patients be directed to designated restroom facilities while awaiting examinations.

Female↗
PubMed1991

A simple method of conjunctival dissection for Gunderson flaps.

In this study we describe a method for simplifying the often frustrating and time-consuming task of dissecting conjunctiva from underlying Tenon's capsule during Gunderson flap procedures. Using gentian violet marks as a guide, a needle and two pairs of scissors are used in series to separate superior bulbar conjunctiva from the superficial layers of Tenon's capsule along a series of semimeridians. The techniques provides the surgeon with important landmarks, reduces the risk of inadvertent buttonholing, and minimizes conjunctival retraction caused by uneven dissection of Tenon's capsule.

Conjunctiva↗
PubMed1989

Fetal dose estimates for CT pelvimetry.

Fetal and maternal dose estimates for computed tomographic pelvimetry have been obtained from phantom measurements. Use of routine abdomen imaging techniques may result in localized fetal doses in excess of 13 mGy (1.3 rad). With the use of a low-exposure (40-mAs) technique, it is possible to obtain images of acceptable quality for the necessary measurements. The resulting dose to the fetus is approximately 2.3 mGy (0.23 rad).

Female↗
PubMed1988

Radiation dose from radiopharmaceuticals contaminated with molybdenum-99.

Sixteen patients undergoing routine nuclear imaging procedures were injected with 99mTc-labeled radiopharmaceuticals containing 99Mo which exceeded the recommended limit of 1 microCi of 99mMo per mCi of 99mTc. The kinetics of the resulting 99Mo distribution in 14 of these patients were studied over a period of several weeks. The mean biologic half-life [T 1/2b] ranged from about 19.3 days to 11.2 days depending on the model used. Similarly, the mean radiation dose to the liver ranged from approximately 0.02 rad/microCi of 99Mo to 0.05 microCi of 99Mo.

Humans↗
PubMed1986

Lag in radiographic imaging systems: simple methods for evaluation.

Lag is an important measure of performance in radiographic imaging systems that are used for dynamic studies. This paper describes some simple, noninvasive techniques that can be used to evaluate lag under clinical conditions. The image receptor is exposed through a copper plate perforated with regularly spaced holes or slits and rotating at a known angular velocity. The resulting image can be evaluated visually to determine the point at which separate holes appear to blur. Alternatively, a light-sensitive diode connected to an oscilloscope can be used to determine the decay rate of the signal from an individual slit. A correlation between these two techniques has been obtained that enables lag to be estimated from a visual assessment of the image of the rotating plate.

Angiography↗
PubMed1981

A comparative physical evaluation of four X-ray films.

In this study, four general purpose radiographic films (Agfa Gevaert Curix RP-1, duPont Cronex 4, Fuji RX, and Kodak XRP-1) were compared using three independent techniques. By examining the characteristic curves for the four films, film speed and contrast were compared over the diagnostically useful density range. These curves were generated using three methods: (1) irradiation of a standard film cassette lined with high-speed screens, covered by a twelve-step aluminum wedge; (2) direct exposure of film strips to an electro-luminescent sensitometer; and (3) direct irradiation of a standard film cassette lined with high-speed screens. The latter technique provided quantitative values for film speed and relative contrast. All three techniques provided virtually properly identical results and indicate that under properly controlled conditions simplified methods of film testing can give results equivalent to those obtained by more sophisticated techniques.

Evaluation Studies as Topic↗