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P Munro

Publications and source records attributed to P Munro.

At least 37 records · Page 2Linked to original sources
PubMed1995

Portal Imaging Technology: Past, Present, and Future.

Many different electronic portal imaging devices (EPIDs) have been developed to improve geometric accuracy in radiation therapy. This article describes the two types of EPIDs that have become available commercially-the television camera-based EPID and the matrix ion chamber EPID-as well as describing the amorphous silicon array, a device that may become available in the future for portal imaging. In addition, the various image registration techniques that identify geometric errors from the portal images are described. These include interactive techniques, landmark-based techniques, contrast-based techniques and hybrid techniques. Although great improvements in portal imaging technology have been made, more development needs to be directed towards making portal imaging convenient and reliable. Image quality must be improved further, to improve the robustness of image registration techniques and more thought must be given to integrating and automating the various steps in the image registration process. Otherwise, too much time will have to be devoted to these tasks. Finally, and most importantly, users will have to decide what is the best way of using EPIDs clinically. Much development is required before the full potential of this exciting technology can be realized.

Journal Article↗
PubMed1995

Tests for evaluating laser film digitizers.

A number of tests to evaluate the image transfer characteristics of laser film digitizers have been developed and these tests have been used to evaluate the performance of a Konica KFDR-S laser film digitizer. These tests were designed to be simple in nature and to use materials readily available in clinical departments. The tests examined (i) the geometric accuracy of the laser film digitizer; (ii) the linearity of the digitizer's characteristic curve; (iii) the temporal/spatial response of the digitizer to abrupt changes in optical density; and (iv) the noise added by the digitizer to the digitized images. The tests were easy to use and could be performed in a short period of time. Results of the tests for the Konica laser film digitizer revealed several problems including minor geometric inaccuracies, structural noise added by a shading correction circuit in the digitizer, and spread of the laser beam before reaching the film plane, possibly caused by the f-theta lens of the digitizer. The light spread made the optical density of small, high density structures measured by the digitizer dependent upon the optical density of the surrounding regions. Therefore, under some circumstances, film digitization with the Konica laser film digitizer could lead to erroneous results. In addition, our measurements suggest that these problems are not unique to the Konica digitizer. Thus, the testing of laser film digitizers is recommended, especially if the digitizer is to be used for quantitative measurements.

Humans↗
PubMed1995

Monte Carlo studies of x-ray energy absorption and quantum noise in megavoltage transmission radiography.

The subject contrast of bony anatomy in megavoltage medical radiographs is very low, making detection of bony landmarks difficult if additional noise sources are introduced into the images. One source of noise, which is inherent to the x-ray detection process, is x-ray energy absorption noise. X-ray energy absorption noise results from variations in the amount of energy deposited in the imaging detector per interacting x ray. These variations increase the noise content of the image. In this study, EGS4 Monte Carlo simulations of x-ray interactions in metal plate phosphor screen detectors have been performed to determine the distribution of energy absorption events within the phosphor screen. From these "absorbed energy distributions (AEDs)", the x-ray energy absorption noise and the quantum absorption efficiency of the detector are determined. These calculations are performed for a range of detector thicknesses (0.1-4 mm) and x-ray energies (0.1-10 MeV). A number of conclusions can be drawn from these investigations. (i) The x-ray absorption noise reduces the detective quantum efficiency (DQE) of metal plate/phosphor screen detectors by as much as 50% at energies used in megavoltage imaging (1-10 MeV). (ii) It is important to include secondary particle (electron) transport in estimating the quantum absorption efficiency of these detectors. For instance, the quantum efficiency of a typical portal detector is approximately 2%, even though 4%-5% of the incident photons are attenuated. (iii) The metal "conversion" plate commonly used in megavoltage imaging enhances the DQE of the phosphor screen by increasing the quantum absorption efficiency and reducing the magnitude of the x-ray absorption noise.

Biometry↗
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

Extraocular muscle problems in thyroid eye disease.

In this paper methods of visualisation of the extraocular muscle changes in thyroid eye disease are discussed. The histopathology of extraocular muscle biopsies has been studied by both light and electron microscopy to show the type of cellular infiltration and the amorphous material in the extracellular matrix. A series of questions to which answers have not yet been found concerning thyroid eye disease are posed which may help to direct new research projects. Finally, in the last part of the paper, the surgical results in a series of 41 patients having ocular muscle surgery for diplopia and/or compensatory head postures due to thyroid eye disease are described. The conclusions drawn from these results are that one should maintain the patient euthyroid, establish by orthoptic measurements that the ocular movements have been stable for at least 6 months, treat by recessing tight muscles using adjustable sutures, and aim to undercorrect the vertical deviation at the time of adjustment.

Diplopia↗
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↗
PubMed1994

A semiautomatic method for registration of portal images.

A method to register pairs of portal images quickly and accurately has been developed. The approach uses a cross-correlation operator to find the optimal match between corresponding anatomic regions that have been selected by a user on pairs of portal images. The cross-correlation operator determines the translation that best registers each pair of anatomic features independently, and then the images are translated, rotated, and scaled so that the least squares difference between the coordinates of all of the paired regions is minimized. Tests using simulated images have shown that the accuracy of the algorithm is dependent on (i) the size and shape of the structures within the paired regions; (ii) the subject contrast of the anatomic features being matched; (iii) the rotational difference between the anatomic features; and, (iv) the noise in the image. Tests using contrast-detail phantom images have shown that the semiautomated registration algorithm is only slightly less accurate than human observers, but is considerably faster. The semiautomated algorithm shows great promise as a method of quickly and accurately estimating discrepancies in patient positioning when used in conjunction with an on-line portal imaging system.

Algorithms↗
PubMed1993

Dynamic gender-related differences in dichotic listening performance.

This experiment investigated dynamic gender-related differences in perceptual asymmetry (PA) in a dichotic task. Twenty right-handed males and 20 right-handed females performed a 200 trial directed attention dichotic listening task using consonant-vowel-consonant nonsense syllables. Men showed a greater right-ear advantage (REA) at the beginning of the test than did women. Over the course of the test the REA in men declined significantly (P < 0.001) whilst in females the REA showed a significant increase (P < 0.001). Thus by the end of the test the REA was greater in females than males. However, the reduced REA in males was entirely due to improved left-ear (LE) performance and the increased REA in females was due to improved right-ear (RE) performance. The change in PA in women is consistent with a dynamic improvement in left hemisphere (LH) syllable processing and in men a dynamic improvement in the transmission of information through the right hemisphere (RH) to the language processing areas in the LH.

Adult↗
PubMed1993

Detection of pneumothorax by accident and emergency officers and radiologists on single chest films.

To assess whether an accurate diagnosis of pneumothorax can be made on a single chest film, 233 pairs of inspiratory (I) and expiratory (E) chest films taken in an accident and emergency (A&E) department for suspected pneumothorax were reviewed by two A&E officers and three radiologists. The films were assessed for the presence of pneumothorax by viewing the I film in isolation and, after an interval, by viewing the paired I and E films together. Fifty-four of the patients had a pneumothorax. The five observers missed 23 pneumothoraces (8.5% of total) on the I film alone which were correctly diagnosed on the paired I and E films, the three radiologists missed 10/162 pneumothoraces on the I film alone which were correctly identified on the I and E films (6%) and the two A&E officers 13/108 (12.5%). The use of a single inspiratory chest film for suspected pneumothorax could result in pneumothoraces being missed, particularly by less experienced observers and therefore we believe that paired I and E films should continue to be used routinely for suspected pneumothorax.

Adolescent↗
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↗
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↗
PubMedDate not supplied

The development of improved ultrasound heaters suitable for superficial tissue heating.

The acoustical fields generated by several ultrasound heaters have been measured to determine their suitability for heating superficial lesions in cancer patients. Three different acoustical sensors were used in the study, and one sensor, consisting of a thermocouple embedded in a sound-absorbing plastic, proved to be the most practical device for routine use. The method employed to mount the piezoelectric crystal in the heater was found to have a profound effect on the acoustical field generated by the heater. Clamping the crystal close to its edge resulted in a field with a prominent acoustical maxima on the crystal axis. A flexible attachment at the very edge of the crystal resulted in a much more uniform field which could be further improved by rapidly sweeping the frequency of the signal used to excite the piezoelectric crystal. A heater in which the rear electrode of the piezoelectric crystal is sectioned into a series of concentric rings has also been developed. This heater can deposit more acoustical energy at the edges of a treatment volume than at the center, and the diameter of the ultrasound beam can also be adjusted. The results demonstrate that (i) the acoustical fields of ultrasound heaters must be accurately measured before using the heaters clinically, and that (ii) by careful design, both uniform or controlled nonuniform acoustical fields can be generated.

Hot Temperature↗
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↗