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Results for “Scattering, Radiation”

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At least 37 records · Page 2Linked to original sources

PubMed1994

Scattered radiation from linear accelerator and cobalt-60 collimator jaws.

PURPOSE: Solid state diodes and/or thermoluminescent dosimeters (TLDs) are often used to measure scattered radiation doses to critical organs immediately adjacent to radiation field sites. The energy-dependent response of these commonly used in vivo dosimeters sometimes makes the interpretation of measured values uncertain. This study investigates scattered radiation arising from the collimator jaws of linear accelerators and the treatment head of a cobalt-60 teletherapy unit. METHODS AND MATERIALS: A thin window Markus-type parallel-plate ionization chamber placed in a polystyrene phantom was employed to document the magnitude, energy composition, and sources of scattered radiation at surfaces near radiation fields. Measurements were taken both with and without additional phantom material covering the ionization chamber, as well as with various distances between the ionization chamber and edges of the radiation fields tested. RESULTS: Data was collected, analyzed and compared for treatment units produced by different manufacturers. It was found that the magnitude of scattered radiation to surfaces immediately adjacent to radiation fields ranged from 1% to 15% of the maximum dose along the beam central axis. These values showed a strong dependence upon distance from the edge of the radiation field, beam energy, collimator setting (field size), and the presence of externally mounted accessories. Teletherapy unit differences due to manufacturing firm origins were found to only slightly affect scattered radiation magnitude, while the orientation of upper and lower collimator jaws had absolutely no effect. CONCLUSIONS: Percent depth dose curves of scattered radiation were obtained and analyzed. The shapes of these depth dose curves suggest the presence of complex energy spectra from secondary electrons and scattered x-rays. Because of the presence of these complex energy spectra in areas immediately adjacent to radiation fields, caution should be observed when interpreting patient doses near radiation fields, if dose values have been measured in vivo using thermoluminescent dosimeters (TLDs) or solid state diodes. Many of these on-patient dosimetry devices are strongly energy dependent and may demonstrate large over- or under-responses in areas dominated by scattered radiation. The results of this study, thus, suggest that ionization chambers are preferred for determination of scattered radiation doses in such regions.

Humans↗

PubMedDate not supplied

An analytical model of the scattered radiation distribution in diagnostic radiology.

A simple scatter model is used to analytically derive the point spread function (PSF) for scattered radiation in diagnostic radiology. The resulting equation is a function of four physical parameters; object thickness, object-to-detector distance (air gap), and the linear attenuation coefficients for both primary and scatter radiation. Though the model is based upon single scattering, it is shown that by reducing the scatter attenuation coefficient the analytic model compares well to the multiple scattering PSF determined using Monte Carlo analysis.

Biophysical Phenomena↗

PubMed1985

Quantitative digital subtraction angiography: two scanning techniques for correction of scattered radiation and veiling glare. Work in progress.

Scattered radiation and veiling glare in digital subtraction angiography degrade contrast signals in a nonlinear and nonuniform way. This effect prohibits direct use of image data for accurate iodine measurement or energy subtraction imaging. Two techniques based on the use of scanning lead bars were proposed to measure the spatial distribution of scattered radiation and veiling glare in the unprocessed image data. With either technique, signals behind the lead bars were used to estimate the scatter-glare component and remove it from the image data. The scanning-lead-bar techniques can be used to reduce the scatter-glare component by a factor of up to 10. The scatter-glare correction leads to the recovery of degraded contrast signals and achieves reasonable linearity, uniformity, and consistency in the contrast measurement.

Angiography↗

PubMed1998

Distribution of scattered radiation during intraoral radiography with the patient in supine position.

OBJECTIVE: The purpose of this study was to investigate the spatial distribution of scattered radiation during intraoral radiography with the patient in a supine position to determine the exposure to an operator without a suitable barrier. STUDY DESIGN: A phantom was placed in the supine position on a dental chair with the occlusal plane perpendicular to the floor, and four intraoral periapical radiographic examinations in the anterior and posterior regions of the maxilla and mandible were performed. The scattered radiation was measured with an ionization chamber at distances ranging from 25 to 200 cm at 25 cm increments and at intervals of 45 degrees. Measurements were made at two different heights: level with the occlusal plane and 30 cm below it. The exposure was converted to an air-kerma/absorbed dose in air. RESULTS: The distribution of scattered radiation was symmetric during examinations of the anterior region. Circular iso-exposure curves of up to 0.5 microGy were observed at the level of the occlusal plane, and curves of up to 0.1 microGy were observed 30 cm below the plane. The lowest exposures were measured 30 cm below the occlusal plane and behind the chair. The distribution of scattered radiation was not symmetric during examinations of the posterior region. Iso-exposure curves of up to 0.25 microGy were observed at the level of the occlusal plane during exposure of the maxilla, and curves of up to 0.5 microGy were observed during exposure of the mandible. Circular iso-exposure curves of up to 0.1 microGy were observed 30 cm below the occlusal plane. Lower values for scattered radiation were observed 30 cm below the occlusal plane between 135 and 180 degrees behind the phantom. CONCLUSION: The spatial distribution of scattered radiation with the patient in a supine position was not the same in eight directions around the chair. Although the preferred position for the operator is behind a suitable barrier, the preferred position in the absence of a barrier is 200 cm behind the patient.

Dental Auxiliaries↗

PubMedDate not supplied

Scatter radiation from abdominal CT examinations.

Several water phantoms were scanned under a number of conditions and radiation exposure due to scatter was measured at the center of one end. The decrease of the scatter radiation exposure is nearly exponential with distance from the slice plane. The half-value distance is a function of the slice cross-sectional area. An empirical equation is presented to relate exposure to the pertinent variables.

Humans↗

PubMed1979

Scattered radiation from beam modifiers used with megavoltage therapy units.

The magnitude and distribution of scattered radiation produced by scatterers inserted into megavoltage therapy beams, including the beams from an 8MV medical linear accelerator and a 60Co teletherapy unit, were investigated. The intensity distribution of scatter depends on the distance from the scatterer to the measurement plane (retraction distance) and also, to a lesser extent, on the atomic number of the scattering material. The effective energy of the scattered radiation was determined by depth dose measurements in tissue equivalent material using thermoluminescent dosimeters and was found to increase with photon beam energy.

Cobalt Radioisotopes↗

PubMed2004

[Measurement of scatter radiation on MDCT equipment using an OSL dosimeter].

The recent introduction of multi-detector row computed tomography (MDCT) has made it possible to scan the entire abdomen within approximately 10 sec in procedures such as interventional radiology computed tomography (IVRCT), which are associated with operator exposure. Therefore, anxious patients and patients who are not able to remain still can be examined with an assistant. In the present study, radiation exposure to the assistant was estimated, and the distribution of scattered radiation near the gantry was measured using an optically stimulated luminescence (OSL) dosimeter. Simultaneous measurements were obtained using a direction storage (DIS) dosimeter for reference. The maximum value of 1.47 mSv per examination was obtained at the point closest to the gantry's center (50 cm from the center at a height of 150 cm above the floor) . In addition, scattered radiation decreased as the measurement point was moved further from the gantry's center, falling below the limit of detection (0.1 mSv or less) at 200 cm and at the sides of the gantry. OSL dosimeters are also employed as personal dosimeters, permitting reliable values to be obtained easily. They were found to be an effective tool for the measurement of scattered radiation, as in the present study, helping to provide better understanding of the distribution of scattered radiation within the CT room.

Facility Design and Construction↗

PubMed1983

Correction for scattered radiation in a ring detector positron camera by integral transformation of the projections.

A "deconvolution" algorithm for the determination of the scatter contribution in positron emission tomography is described. The projected distributions of scattered radiation measured with a line source at different positions in water phantoms are described analytically. It is shown that an integral transformation of the observed projections with a slightly modified analytical function gives an adequate description of the scattered radiation. The scatter distribution from any composite object can thus be calculated and subsequently subtracted. The algorithm is tested on different objects. The result shows that the level of scattered radiation can be reduced from 25 to 1% of the total count rate in the center of the projection from a homogeneous phantom.

Models, Theoretical↗

PubMed1984

Energy imparted to fluorescent screens from primary and scattered radiation. Variations with atomic composition and screen thickness.

The effects of thickness and atomic composition of fluorescent screens on the impartation of primary and scattered radiation is analysed. Different fluorescent screens give rise to different quotients epsilon p/epsilon s of energy imparted from primary and scattered radiation respectively, due to their variation in atomic composition and thickness. The image contrast in a given radiation field therefore varies with the choice of screen. Energy imparted by primary and scattered radiation in different fluorescent screens has been calculated as a function of tube voltage using a simple analytical model. Comparison of the results for fluorescent screens of different thicknesses with hypothetical screens with both negligible and total absorption has been made. From the results it is evident that the image contrast increases with increased detector thickness if scattered radiation is present. The trends found in the calculations have been confirmed by measurements of the energy imparted from scattered and primary radiation for a number of fluorescent screens including both commercial and specially made screens. Differences in the ratio epsilon p/epsilon s of up to 50% have been measured, and differences reported by other authors are discussed. Their use of detectors with different response to scattered and primary radiation may explain the discrepancies found.

Calcium Compounds↗

PubMed1999

Reducing scatter radiation to the contralateral breast with a mobile, conformal shield during breast cancer radiotherapy.

During a standard course of breast radiotherapy, the contralateral breast generally receives approximately 2.5 to 6.0 Gy of scattered radiation. Although most studies have not found an overall increase in metachronous contralateral breast cancers in patients undergoing radiotherapy, a cohort of younger women may be genetically more susceptible to radiation-induced breast cancers and may thus be adversely affected by the scattered radiation. We are attempting to develop a simple, convenient, effective mechanism for minimizing the scattered radiation to the contralateral breast during the process of clinical breast radiotherapy. We therefore designed a conformal, platform-based breast shield consisting of 2.5 cm of molded lead in a mobile counterweighted polystyrene casing. This shield was intended to serve as a physical barrier to prevent both low and high energy scattered photons from the medial and lateral tangential fields. We conducted a prospective trial of 20 women, each woman serving as her own control. Each woman received breast radiotherapy with and without shield, and an array of thermoluminescent dosimeters was positioned across the contralateral breast to evaluate the in vivo dosimetry and the impact of the breast shield on surface absorption of scattered radiation. We found that the use of the breast shield reduced the median dose of scattered radiation by approximately 60% (p < 0.0001). This represented a median dose reduction of approximately 300 cGy at the nipple. The shield was easily positioned and added < 1 min to daily setup time. We conclude that the use of this sort of surface barrier shielding technique was feasible, effective, and practical for clinical use. The degree of scatter reduction accomplished through the use of this breast shield may be biologically significant, especially for those patients with biologic or epidemiologic risk factors that may predispose them to the development of radiogenic breast cancers.

Breast Neoplasms↗

PubMedDate not supplied

Influence of scattered radiation on heterogeneity correction in three-dimensional treatment planning.

The first and multiple scattered radiations of 60Co gamma rays and 10 MV X-rays were measured using an ionization chamber, and the values obtained were compared with the calculated values. The experimental values of the angle distributions of the first scattered photons from tissue-equivalent materials in free space agreed well with the theoretical values calculated from the Klein-Nishina probability in the compton scattering process. However, the scattered radiation in phantoms was not represented by only the first scatter components. The results indicate that the weighting factor of all scattered radiations in the calculated three-dimensional dose distribution can be set by the inverse square correction for the distance from the scattering point to the reference point within an accuracy required in clinical dosimetry. The present method provides a simple and practical approach to obtaining the computation speed in three-dimensional treatment planning.

Cobalt Radioisotopes↗

PubMed2001

Scattered radiation during fixation of hip fractures. Is distance alone enough protection?

We measured the scattered radiation received by theatre staff, using high-sensitivity electronic personal dosimeters, during fixation of extracapsular fractures of the neck of the femur by dynamic hip screw. The dose received was correlated with that received by the patient, and the distance from the source of radiation. A scintillation detector and a water-filled model were used to define a map of the dose rate of scattered radiation in a standard operating theatre during surgery. Beyond two metres from the source of radiation, the scattered dose received was consistently low, while within the operating distance that received by staff was significant for both lateral and posteroanterior (PA) projections. The routine use of lead aprons outside the 2 m zone may be unnecessary. Within that zone it is recommended that lead aprons be worn and that thyroid shields are available for the surgeon and nursing assistants.

Anesthesiology↗

PubMed1976

Exposure of radiologists to scattered radiation during radiodiagnostic examinations.

The exposure to scattered radiation was measured on both hands, the forehead and gonads of six radiologists by means of thermoluminescent dosimeters in two fluoroscopy rooms. Exposure of the forehead was found not to exceed approximately 1% of the maximum permissible exposure (MPD) (3% pursuant to NCRP (recommendations). During examinations of the colon, the exposure of the right hand proved to be significantly higher compared to the left. There was only slight exposure to the gonads as measured behind the apron.

Environmental Exposure↗

PubMed1988

Blackening of unprotected dental X-ray films due to scattered radiation.

Unexposed films awaiting exposure as well as exposed films awaiting processing are subjected to scattered radiation if kept unprotected in the dental X-ray clinic. The purpose of the present study was to analyze the influence of various storing principles, film speed, and distance from X-ray source upon the degree of film blackening. Test films were subjected to scattered radiation from 150 exposures. Maximum additional blackening (0.27 D) was recorded for type E films not protected by lead foil at the shortest distance studied (55 cm). At a distance of 200 cm blackening was reduced to 0.02 D and could be further reduced by utilizing the inherent protective effect of the lead foil. It is concluded that if dental X-ray films not in use are kept at a distance of 200 cm from the X-ray source and protected by lead foil additional blackening due to scattered radiation is negligible and further protective precautions are unnecessary.

Lead↗

PubMed1989

Scattered radiation doses during per operative screening.

Measurements of scattered radiation doses during per operative screening indicate maximum mean dose rates to the eye are unlikely to exceed 56 microgray per minute for lateral projections and 14 microgray per minute for PA projections. The potential radiation doses to staff can be further reduced by the implementation of some simple protective measures.

Environmental Exposure↗

PubMed2006

Study of scattered radiation for in-air calibration by a multiple-distance method using ionization chambers and an HDR 192Ir brachytherapy source.

The aim of this study is to estimate the room-scatter correction when measuring air kerma rate of an HDR 192Ir brachytherapy source by in-air calibration. The variation in scattered radiation due to the specially designed jig and from the room walls was also studied. Two therapy ion chambers of volume 0.1 cm3 and 0.6 cm3 were used in the present study. Air kerma was measured by placing the source at several distances between 10 cm and 20 cm from the chamber. The scatter radiation was determined by superimposing the theoretically derived model curve of known scatter (based on the inverse square law) over the plot of measured air kerma strength values. The scatter radiation was estimated in terms of percentage of the primary radiation at 10 cm measurement distance. The scatter estimated by the 0.6 cm3 chamber at two positions was 0.33% and 0.59%, respectively. Similarly the scatter estimated at two other positions by the 0.1 cm3 chamber was 0.58% and 1.11%. This variation in scatter with position as well as with the chamber was due to the varying scatter contribution from components of the measurement set-up. The scatter radiation becomes constant at a distance greater than 100 cm from the walls of the room. We conclude that a fixed chamber with changing source positions should be used in multiple-distance measurement of air kerma rate when using a measurement jig.

Brachytherapy↗

PubMedDate not supplied

On the significance of very small angle scattered radiation to radiographic imaging at low energies.

We have studied the angular distribution of scattered radiation at low energies for angles between 2 degrees and 15 degrees from the outward normal to the exit surface of several phantoms, with 1 degree resolution. A cryogenically cooled germanium detector was used to measure the spectra of the scattered radiation. The differential scattering fluences, or numbers of photons per unit solid angle per unit surface area, exhibit distinct peaks at angles in the vicinity of 5 degrees, with the angular position being only slightly energy dependent but very material dependent. The scattered spectra show large changes as a function of angle, in some cases actually becoming harder than the exit unscattered beam. The significance of this behavior relative to the imaging of targets in mammography examinations is discussed.

Humans↗

PubMedDate not supplied

Analysis of radiation scatter during angiographic procedures: evaluation of a phantom model and a modified radiation protection system.

PURPOSE: To study the radiation scattering associated with the digital subtraction angiography (DSA) unit in angiographic procedures and to design an effective radiation protection shield based on these data. MATERIALS AND METHODS: The number of scattered photons was measured at three points relative to the operator's position. Anteroposterior abdominal and lateral cranial fluoroscopy were evaluated. As protective devices, a lead curtain, sliding shields, and a brim-shaped image intensifier (II) hood were designed. RESULTS: In abdominal fluoroscopy, radiation was found to scatter to the operator's lower limbs from the underside of the catheter table, to the abdomen from the side of the patient's body, and to the head and neck from the table surface adjacent to the patient. The use of protective devices reduced exposure from 2.89 to 0.058 mR/min for the operator's lower limbs, from 0.987 to 0.069 mR/min for the operator's abdomen, and from 0.696 to 0.139 mR/ min for the operator's head and neck area. With lateral cranial fluoroscopy, radiation was detected to scatter to the operator's lower limbs from the underside of the catheter table, to the abdomen from the patient's temporal area, and to the head and neck from the patient's face. The use of protective devices reduced exposure from 0.248 to 0.010 mR/min for the operator's lower limbs, from 0.129 to 0.010 mR/min for the operator's abdomen, and from 0.162 to 0.018 mR/min for the operator's head and neck area. CONCLUSIONS: The characteristic directions of scattering to the operator were identified. An effective modified radiation protection system was designed based on this information.

Abdomen↗