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Biomedical subjects

J A Rawlinson

Publications and source records attributed to J A Rawlinson.

At least 19 recordsLinked to original sources
PubMed1999

Detective quantum efficiency of an amorphous selenium detector to megavoltage radiation.

The spatial frequency dependent detective quantum efficiency (DQE(f)) of a high-resolution selenium-based imaging system has been measured at megavoltage energies. These results have been compared with theoretical calculations. The imaging system was a video tube with a 5 microm amorphous selenium (a-Se) target which was irradiated by 1.25 MeV gamma-rays. The modulation transfer function (MTF) decreased rapidly with spatial frequency (determined by spread of electrons in the build-up material) while the noise power spectrum was constant as a function of spatial frequency. The DQE obtained from these MTF and noise power measurements was compared with a Monte Carlo model of the pulse height spectrum of the detector. The DQE(0) model accounted for the interaction of x rays with the detector as well as the energy-dependent gain (charge generated/energy deposition). Good agreement between the calculated and measured DQE(0) was found. The model was also used to estimate the DQE(f) of a metal plate + a-Se detector which was compared with a metal plate + phosphor system of the same mass thickness. The DQE(f) s of both detectors are very similar, indicating that the choice of which detector is better will be based upon criteria other than DQE(f), such as read-out approach, ease of manufacture or sensitivity.

Biophysical Phenomena↗
PubMed1998

Sensitivity of amorphous selenium to x rays from 40 kVp to 18 MV: measurements and implications for portal imaging.

Recently, the clinical application of electronic portal imaging devices has enabled more frequent verification of patient setup for radiation treatment. However, the image quality has sometimes proven to be inadequate, motivating the investigation of alternative sensors with better image quality. Amorphous selenium (a-Se) is potentially one such sensor since the electrostatic image formation process has high resolution. To fully evaluate the potential of a-Se for portal imaging, it is necessary to investigate all the imaging properties at high x-ray energies. Here, measurements of the sensitivity of a-Se to incident x-ray spectra ranging in energy from 40 kVp to 18 MV and for a-Se thicknesses ranging from approximately 10 to 300 microns under full buildup conditions are described. When x rays or energetic electrons deposit energy in a photoconductor with an applied electric field, F, electrons and holes are released. The x-ray conversion sensitivity may be defined as 1/W +/-, where W +/- is the energy required to release an electron-hole pair. Consistent with the results of previous investigators, W +/- is found to vary approximately with F-2/3. Unexpectedly, over the energy range of 40 kVp to 18 MV, W +/- was found to decrease by a factor of nearly 3. These dependencies are compared to the predictions of two competing charge recombination models, geminate and columnar. The results are explained by a microdosimetric model in which the sensitivity at megavoltage energies is governed by geminate recombination, but at lower energies, both mechanisms are involved. Thus, the sensitivity of a-Se to x rays spanning the diagnostic and radiotherapy range has been measured and the physical basis for this behavior established.

Dose-Response Relationship, Radiation↗
PubMed1996

Measurement of quantum noise in fluoroscopic systems for portal imaging.

In fluoroscopic portal imaging systems, a metal plate is bonded to a phosphor screen and together these act as the primary x-ray sensor. The light from the screen is collected and imaged by a lens on the target of a video camera. The demagnification (M) between the large area of the phosphor being imaged and the small active area of the video camera results in poor optical coupling between the screen and the video camera. Consequently x-ray quantum noise is small compared to other noise sources. By reducing the demagnification, the light from the screen is collected more efficiently, so we were able to increase the x-ray quantum noise relative to other noise sources and thus unambiguously identify it. The noise power spectrum was measured as a function of M to determine the relationship between the x-ray quantum noise. shot noise, and amplifier noise. It was found by extrapolation to clinical demagnifications that the amplifier noise dominates x-ray quantum noise, at all spatial frequencies, but the shot noise was less than the x-ray quantum noise at low spatial frequencies. For low spatial frequencies, this implies that a secondary quantum sink can be avoided. If amplifier noise could be sufficiently reduced, x-ray quantum limited images could be obtained in clinical systems at low spatial frequencies.

Equipment Design↗
PubMed1990

A digital fluoroscopic imaging device for radiotherapy localization.

We have been developing a digital fluoroscopic imaging system to replace the portal films that are currently used to verify patient positioning during radiotherapy treatments. Our system has a number of modifications compared to previously reported devices. The detector, which consists of a copper plate with Gd2O2S:Tb phosphor bonded directly to the copper, has been designed to maximize light output from the phosphor by increasing the phosphor thickness. The operation of the T.V. camera has been modified so that the light signal is accumulated on the target of the T.V. camera for periods of 0.2-2.0 seconds. Accumulation of the light increases the video signal relative to the fixed noise current generated by the camera, and thus minimizes the camera noise. The resulting image quality is comparable to film, so the imaging system represents a promising alternative to film as a method of verifying patient positioning in radiotherapy.

Fluoroscopy↗
PubMed1984

Partial bolussing to improve the depth doses in the surface region of low energy electron beams.

In many low energy electron beams the surface dose is considerably less than the maximum dose, making them unsatisfactory for clinical application. A method is described for producing better surface dose uniformity in such beams. The method makes use of bolus applied to the patient for a fraction of each daily electron treatment. The technique is shown to be simple and practical. This approach is compared to more conventional techniques of using bolus in electron beams.

Electrons↗
PubMed1981

Ontario accelerator dose intercomparison study.

An ionometric dose intercomparison has been carried out on eight accelertors, with maximum photon energies from 6 to 32 meV, at five radiotherapy centres in Ontario. The ratio of the dose based on the clinically-employed rad/monitor unit to the dose measured by the committee representative had a mean value of 0.994, with a range of 7.3% and a coefficient of variation of 2.5%. The ratio of the dose measured by the institution's physicist to the dose measured by the committee representative had a mean value of 1.000 with a range of 6.9% and a coefficient of variation of 2.3%. Eight recommendations regarding dose calibration procedures are presented.

Humans↗
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

Theoretical and experimental investigation of dose enhancement due to charge storage in electron-irradiated phantoms.

Recent measurements have shown that significant errors in radiation dosimetry can arise by the use of insulating plastic phantoms which have been exposed to electron beams. The effect has been attributed to the generation of large electric fields in the phantom by charge storage causing alteration of electron trajectories and an increase in the measured dose. In this report, we examine this hypothesis theoretically by calculating the change in response to radiation of an ion chamber in a cylindrical cavity in an electron-irradiated polymethylmethacrylate phantom. The electric field distribution is determined using a model which allows for charge leakage by radiation-induced conductivity, and the dose in the cavity is determined by a Monte Carlo simulation using the EGS (electron gamma shower) code modified to account for electron trajectories in the electric field. The theoretical results are shown to agree well with new and previously published experimental dose enhancement data. The agreement is taken as confirmation of the reported explanation of the effect. The use of conducting phantoms in radiation dosimetry is advocated.

Electric Conductivity↗