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Biomedical subjects

A C Ralston

Publications and source records attributed to A C Ralston.

8 recordsLinked to original sources
PubMed1994

Monte Carlo modelling of neutron depth dose distributions in optimizing prompt gamma in vivo neutron activation analysis systems.

Optimization of prompt gamma in vivo neutron activation analysis systems is best achieved using Monte Carlo simulation. In this study the modelling of the dimensions and materials for source holders and collimators is described and compared with experimentally derived results where feasible. Results show that valid depth doses are obtained by modelling only the central part of an IVNAA system and that the use of borated paraffin as a reflector provides acceptable thermal fluence and depth dose.

Americium↗
PubMed1992

Film dosimetry for the junction region of four compensated photon beams.

A set of four 4 MV photon beam compensators were produced, one for each field of a Hodgkins Disease treatment plan. The resultant dose profiles at various depths were measured by an ion chamber in water and by Kodak X-Omat V film in a Solid Water phantom, and compared to the doses calculated by a GE RT/Plan treatment planning computer. After normalisation and correction for the film's non-linear dose response, the film and ion chamber results compared well with each other. They both showed cold spots of 80% in the junction region of the four fields which were not shown on the computed isodose plan. Film dosimetry is faster than ion chamber dosimetry and is shown to be accurate enough to use for measuring the dose uniformity of compensated beams.

Film Dosimetry↗
PubMed1991

A comparison of the performance of 20 pulse oximeters under conditions of poor perfusion.

The performance of 20 pulse oximeters with finger probes was evaluated by comparison of their readings with directly measured arterial blood oxygen saturations. The samples were taken from patients who had undergone cardiac surgery under hypothermic cardiopulmonary bypass and had poor peripheral perfusion. The mean difference (bias, accuracy), standard deviation (precision) and drop-out rate for each pulse oximeter was determined. An overall ranking of performance of each pulse oximeter was calculated using five criteria (accuracy, precision, number of readings within 3% of standard, percentage of readings given within 3% of standard, expected overread limit in 95% of cases). Two pulse oximeters achieved a combination of accuracy and precision such that 95% of measurements would be expected to be within 4% of the co-oximeter value; these two also had the lowest drop-out rate.

Carboxyhemoglobin↗
PubMed1991

Potential errors in pulse oximetry. I. Pulse oximeter evaluation.

There is no absolute reference for oxygen saturation, although multiwavelength in vitro oximeters are accepted as the 'gold standard'. Regardless of whether fractional or functional saturation is used by manufacturers to calibrate their oximeters, evaluation against fractional saturation is recommended since this is the clinically relevant variable. The use of standard notation and comparisons based on bias and precision is recommended. The accuracy of pulse oximetry is intrinsically limited by the use of only two wavelengths, and is dependent on the initial calibration population. The empirical algorithms used to convert the signal to its 'readout value' and the quality control of hardware may both be important sources of variability between oximeters. Change in blood temperature may introduce errors in pulse oximeter and in vitro oximeter saturation readings, but these will be clinically insignificant. Changes in blood pH should not decrease pulse oximetry accuracy.

Humans↗
PubMed1991

Potential errors in pulse oximetry. II. Effects of changes in saturation and signal quality.

The published studies of pulse oximeter performance under conditions of normal, high and low saturation, exercise, poor signal quality and cardiac arrhythmia are reviewed. Most pulse oximeters have an absolute mean error of less than 2% at normal saturation and perfusion; two-thirds have a standard deviation (SD) of less than 2%, and the remainder an SD of less than 3%. Some pulse oximeters tend to read 100% with fractional saturations of 97-98%. Pulse oximeters may be suitable hyperoxic alarms for neonates if the alarm limit chosen is directly validated for each device. Pulse oximeters are poorly calibrated at low saturations and are generally less accurate and less precise than at normal saturations; nearly 30% of 244 values reviewed were in error by more than 5% at saturations of less than 80%. Ear, nose and forehead probes respond more rapidly to rapid desaturation than finger probes, but are generally less accurate and less precise. Ear oximetry may be inaccurate during exercise. Low signal quality can result in failure to present a saturation reading, but data given with low signal quality warning messages are generally no less accurate than those without. Cardiac arrhythmias do not decrease accuracy of pulse oximeters so long as saturation readings are steady.

Animals↗
PubMed1991

Pulse oximeter probes. A comparison between finger, nose, ear and forehead probes under conditions of poor perfusion.

The performances of 10 pulse oximeters using finger probes were compared with the same pulse oximeters using alternative probes (eight finger probes, two nose probes and a forehead probe) in poorly perfused patients. All readings were then compared with directly measured arterial blood oxygen saturations. The mean difference (bias, 'accuracy'), standard deviation (precision) and 'drop out' rate for each pulse oximeter combination was determined. An overall ranking of performance of each pulse oximeter was calculated using five criteria (accuracy, precision, number of readings within 3% of standard, percentage of readings given within 3% of standard, expected overread limit in 95% of cases). Nose and forehead probes performed poorly. Some ear probes performed well compared to some finger probes, but the overall performance of probes in other sites compared to finger probes was worse, (p = 0.05). Two of eight ear probes and no nose or forehead probes would be expected to be within 4% of the reference value in 95% of readings. The use of finger probes rather than probes in other sites is recommended in the patient with poor peripheral perfusion.

Adult↗
PubMed1991

Potential errors in pulse oximetry. III: Effects of interferences, dyes, dyshaemoglobins and other pigments.

Electrosurgery, patient motion and some types of lighting can cause errors in saturation readout; it is recommended that probes should be shielded from ambient lighting. Intravenous dyes can introduce gross but transient errors, which may also be present in in vitro measurements. Carboxyhaemoglobin causes overestimation of fractional saturation by an amount less than, but possibly close to, the percent of carboxyhaemoglobin present. Methaemoglobin causes the pulse oximeter readout to tend towards 85%. Fetal haemoglobin and bilirubin introduce no significant error, although they may interfere with in vitro measurements. Skin pigmentation can result in a slight decrease in accuracy. Nail polish may cause up to 6% underestimation of saturation; it is recommended that probes should be mounted sideways on fingers with nail polish or long nails. Adhesive tape or a vinyl glove across the probe has no demonstrable effect on accuracy. A blood sample should be analysed by a multiwavelength in vitro oximeter when an erroneous pulse oximeter reading is suspected, although errors may be introduced in the in vitro reading by fetal haemoglobin, bilirubin and intravenous dyes.

Coloring Agents↗