Biomedical subjects

D A Jaffray

Publications and source records attributed to D A Jaffray.

42 records · Page 3Linked to original sources

PubMed1995

Extrafocal radiation: a unified approach to the prediction of beam penumbra and output factors for megavoltage x-ray beams.

An extrafocal source model has been developed to explain the dependence of head scatter and beam penumbra on field size. In this model, the x-ray source of a medical linear accelerator is described by two components: a small but intense focal component; and a broadly distributed extrafocal component of low intensity. The extrafocal component is so large that it can be "eclipsed" by the field-defining collimators. Extrafocal radiation was found to account for 12% of the energy fluence on the central axis of the 6 MV x-ray beam from a Varian Clinac 2100 c accelerator. Head scatter factors were calculated "in-air" for symmetric, asymmetric, and half-blocked fields. Calculations agreed with measured values to better than 0.5%, on average. However, head scatter factors for asymmetric fields were underestimated by 1.2% when one of the field dimensions was reduced to 4 cm (the minimum jaw setting that was tested). The extrafocal source model was combined with a convolution/superposition dose calculation algorithm to calculate dose-per-monitor-unit calibration (output) factors and beam dose profiles in water. These dose calculations predict the degradation of the field edge as a function of field size, and calculate output factors to within 0.5%, on average. In the most extreme case of a 4 cm field width, output factors were underestimated by 2%. Dose profiles are predicted without the aid of an empirical fit to measured beam penumbra data. The extrafocal source model will be particularly useful for fields defined by independent jaw and multileaf collimation systems.

Air↗

PubMed1994

Optimal radiographic magnification for portal imaging.

Two approaches to estimate the optimal radiographic magnification for a TV camera-based portal imaging system and portal films have been used. The first approach optimizes signal transfer while the second optimizes signal-to-noise ratio (SNR) transfer. In order to perform these optimization calculations, the physical characteristics of the imaging system (modulation transfer function and noise power spectrum) as well as the sizes of the radiation sources of our medical linear accelerators have been measured. Using these data, the optimal magnification considering signal transfer alone (M signal) has been calculated to range between 2.0 and 2.3 for the TV camera-based imaging system and is about 1.0 for portal films. Conversely, the optimal magnification considering SNR transfer (MSNR) has been calculated to range between 1.5 and 1.7 for the TV camera-based imaging system and is about 1.0 for portal films. The results suggest that most portal imaging systems are operated close to their optimal radiographic magnification.

Humans↗

PubMed1994

X-ray scatter in megavoltage transmission radiography: physical characteristics and influence on image quality.

The physical characteristics of x rays scattered by the patient and reaching the imaging detector, as well as their effect on verification (portal) image quality, were investigated for megavoltage (0.1-20 MeV) x-ray beams. Monte Carlo calculations and experimental measurements were used to characterize how the scatter and primary fluences at the detector plane were influenced by scattering geometry and the energy spectrum of the incident beam. The calculated scatter fluences were differentiated according to photon energy and scattering process. Scatter fractions were measured on a medical linear accelerator (Clinac 2100c, 6 MV) for a typical imaging geometry using an ionization chamber and a silicon diode. After correction for the energy dependence of the chamber and diode, the scatter fractions generated by the Monte Carlo simulations were found to be in excellent agreement with the measured results. In order to estimate the effect of scatter on image quality, the scatter and primary signals (i.e., energy deposited) produced in five different types of portal imaging detectors (lead plate/film, storage phosphor alone, lead plate/storage phosphor, compton recoil-electron detector, and a copper plate/Gd2O2S phosphor) were calculated. The results show that, for a specified geometry, the scatter fraction can vary by an order of magnitude, depending on the sensitivity of the imaging detector to low-energy (< 1 MeV) scattered radiation. For a common portal imaging detector (copper plate/Gd2O2S phosphor), the scattered radiation (i) reduced contrast by much as 50% for a fixed display-contrast system, and (ii) decreased the differential-signal-to-noise ratio (DSNR) by 10%-20% for a quantum-noise-limited portal imaging system. For currently available TV-camera-based portal imaging systems, which have variable display contrast, the reduction in DSNR depends on the light collection efficiency and the noise characteristics of the TV camera. Overall, these results show that scattered radiation can reduce contrast significantly in portal films while deteriorating image quality only moderately in on-line systems.

Biophysical Phenomena↗

PubMed1992

Daily monitoring and correction of radiation field placement using a video-based portal imaging system: a pilot study.

We have developed a video-based portal imaging system for radiotherapy localization. The system can acquire high quality portal images automatically using short (1-3 monitor unit) irradiations and immediately display the images. The major advantage of the imaging system is that it can be used routinely to check and correct patient positioning before much of the daily irradiation has been delivered. The portal imaging system has been used in a pilot study to monitor five patients during each of their daily treatments. The study has shown that: (i) image quality is sufficiently high to detect discrepancies in field placement from that prescribed on the simulator film; (ii) discrepancies in field placement occur frequently; and, (iii) routine correction of patient and block positioning can reduce the size of these discrepancies. This is the first time that field placement in radiation therapy has been checked and corrected routinely, before the treatment irradiation. However, limitations in the size of the field of view and in the methods of extracting and presenting the geometric information to the users limits the clinical utility of the imaging system. Solutions to these limitations are currently under development.

Computer Terminals↗

PubMedDate not supplied

Activity distribution of a cobalt-60 teletherapy source.

In the course of quantifying the effect of radiation source size on the spatial resolution of portal images, a concentric ring structure in the activity distribution of a Cobalt-60 teletherapy source has been observed. The activity distribution was measured using a strip integral technique and confirmed independently by a contact radiograph of an identical but inactive source replica. These two techniques suggested that this concentric ring structure is due to the packing configuration of the small 60Co pellets that constitute the source. The source modulation transfer function (MTF) showed that this ring structure has a negligible influence on the spatial resolution of therapy images when compared to the effect of the large size of the 60Co source.

Cobalt Radioisotopes↗

PubMedDate not supplied

X-ray sources of medical linear accelerators: focal and extra-focal radiation.

A computerized tomography (CT) reconstruction technique has been used to make quantitative measurements of the size and shape of the focal spot in medical linear accelerators. Using this technique, we have measured the focal spots in a total of nine accelerators, including (i) two Varian Clinac 2100c's, (ii) two Atomic Energy of Canada Ltd. (AECL) Therac-25's, (iii) two AECL Therac 6's, (iv) a Siemens KD-2, (v) a Varian Clinac 600c (4 MV), and (vi) an AECL Therac-20. Some of these focal spots were monitored for changes over a 2-yr period. It has been found that (i) the size and shape of the source spot varies greatly between accelerators of different design ranging from 0.5 to 3.4 mm in full width at half maximum (FWHM); and (ii) for accelerators of the same design, the focal spots are very similar. In addition to the measurements of the focal spot, a new technique for measuring the magnitude and distribution of extra-focal radiation originating from the linear accelerator head (flattening filter, primary collimator) has also been developed. The extra-focal radiation produced by a Varian Clinac 2100c accelerator was measured using this technique and it was found that the extra-focal radiation accounts for as much as 8% of the total photon fluence reaching the isocenter. The majority (75%) of this extra-focal radiation originates from within a circle 6 cm in diameter at the target plane. The source MTFs for each of the measured focal spots have been calculated in order to assess their influence on the spatial resolution of verification images. The limiting spatial resolution (i.e., 10% modulation) for all the source MTFs is 1.8 mm-1 or greater when used for transmission radiography at a magnification of 1.2. The extra-focal radiation, which produces a low-frequency drop in the source MTFs of up to 8%, changes with field size. As a result, the source MTFs of linear accelerators depend not only on the design of individual accelerators and image magnification, but also on the field size used when forming an image.

Biophysical Phenomena↗