Dose reduction in mammography - what reduction? Which dose?
Explore the source record for details and available documents.
Female↗
Biomedical subjects
D Gur
Publications and source records attributed to D Gur.
Photon contamination in 8-20-MeV electron beams from a linear accelerator.
The amount of x-ray contamination near the surface of a phantom irradiated with electron beams was measured directly. Measurements were done to ascertain if photon contamination in the beam contributes a higher dose to the more superficial layers of an irradiated medium than indicated by conventional methods. A 1.4-kG magnetic field was used to deflect the electron beams generated by a Philips SL/75-20 linear accelerator. The electron energies studied were 8, 10, 12, 14, 17, and 20 Mev. After sweeping the electron beam, a significant amount of photon contamination was measured in all cases. The characteristic qualities of the photon contamination were measured directly in a water tank. They were found to agree with those of bremsstrahlung spectra generated in a thin target with a virtual source at the location of the scattering foil.
Electrons↗
Relative measurements of fast neutron contamination in 18-MV photon beams from two linear accelerators and a betatron.
Fast neutron contamination in photon beams in the 20 MV range have been reported in recent years. In order to determine if the variations were due mainly to differences in measurement procedures, or inherent in the design of the accelerators, three different 18-MV (BJR) photon beams were compared using identical analytical techniques. The units studied were a Philips SL/75-20 and a Siemens Mevatron-20 linear accelerators and a Schimadzu betatron. Gamma spectroscopy of an activated aluminum foil was the method used. By comparing the relative amounts of neutron contamination, errors associated with absolute measurements such as detector efficiency and differences in activation foils were eliminated. Fast neutron contaminations per rad of x rays in a ratio of 6.7:3.7:1 were found for the Philips, Schimadzu and Siemens accelerators, respectively.
Fast Neutrons↗
Dosimetric evaluation of a variable field electron beam applicator.
A commercially available variable electron beam applicator designed for the Phillips SL-75/20 linear accelerator was tested for possible clinical use. The basic design of this collimator consists of four movable L-shaped stainless steel blades forming the corners. In addition, four smaller fixed blades of the same material and thickness fill in the sides. Film placed in polystyrene phantoms were exposed in all experiments. Field size, field flatness, beam homogeneity and penumbra measurements, all indicated that the applicator is acceptable for clinical use. However, for use with large field sizes and/or high energy electron beams some minor modifications are suggested.
Electrons↗
Simultaneous mass spectrometry and thermoconductivity measurements of end-tidal xenon concentrations: a comparison.
Studies in which the rate of local cerebral blood flow is determined by xenon-enhanced computerized tomography require measurement of end-tidal gas. A mass spectrometer and a thermoconductivity detector were compared in measuring the concentration of xenon in end-tidal gas both in humans and in a clinically simulated ("breathing bag") system. The experiments showed that the two instruments provided virtually identical results up to a rate of 18 breaths/min, at which rate recorded concentrations showed marginal degradation with the thermoconductivity detector. Given the absence of a significant difference in the ability of the mass spectrometer and the thermoconductivity detector to measure end-tidal xenon concentration, the thermoconductivity detector appears to be a satisfactory alternative in studies measuring the rate of local cerebral blood flow by enhanced computerized tomography.
Cerebrovascular Circulation↗
The development of a xenon/computed tomography cerebral blood flow quality assurance phantom.
A simple, easy to use, quality assurance and performance test phantom was developed for the xenon/computed tomography (CT) cerebral blood flow method. The phantom combines an inhalation system which allows for the simulation of xenon buildup or washout in the arterial blood as well as a multisection translatable cylinder in which several sections can be scanned during a preselected protocol to simulate the CT enhancement in brain tissue during a study. The phantom and scanning protocol are described and their use is demonstrated. The results compare favorably to the theoretically expected fast, intermediate, and slow "flow" values designed into the phantom.
Cerebrovascular Circulation↗
Errors in cerebral blood flow determinations by xenon-enhanced computed tomography due to estimation of arterial xenon concentrations.
Errors in the determination of xenon concentrations in arterial blood during inhalation of xenon-oxygen mixtures are used to assess errors in the derivation of regional cerebral blood flow by the xenon-enhanced computed tomography (CT) method. The results of this study indicate that approximating the arterial buildup by a single exponential introduces relatively small errors in estimated flow values. The most significant systematic error is introduced by errors in estimation of the xenon arrival time to the brain in relationship to sequential (CT) scanning times.
Cerebrovascular Circulation↗
The effect of computed tomography noise and tissue heterogeneity on cerebral blood flow determination by xenon-enhanced computed tomography.
The errors associated with derivation of cerebral blood flow values by the xenon-enhanced computed tomography (CT) method have been evaluated as a function of tissue heterogeneity and CT noise. The results of this study indicate that CT noise introduces large errors in the derived flow value when data for a single, unprocessed voxel are used for this purpose. CT noise increases the derived flow values in a systematic way. Tissue heterogeneity results in a systematic error which lowers the derived flow values. Errors due to both parameters are computed for typical and extreme conditions.
Brain↗
The use of storage phosphors for portal imaging in radiation therapy: therapists' perception of image quality.
Two drawbacks in quality of portal radiographs in radiation therapy are their low contrast and low spatial resolution. These are due to the low differential absorption of body tissues at therapeutic energies and to a relatively large radiation source. We used an experimental, high-contrast sensitivity storage phosphor imaging system (Eastman Kodak Co.) to produce portal images. The system consists of a storage phosphor detector, a high-contrast sensitivity laser scanner (12 bit), an image processing module, and a laser printer (12 bit). Patients undergoing radiation therapy treatments had both a conventional portal image and a storage phosphor image taken. Both were displayed side-by-side and were evaluated independently by three radiotherapists according to quality of information to verify the treatment field. Each of the three radiotherapists rated the storage phosphor images to be better (p less than 0.001) than the conventional images. However, rated improvements of low-contrast storage phosphor images of the pelvis and abdomen (40) were significantly lower than those of high-contrast (head, neck, and chest) images (53).
Humans↗
Evaluation of analog contrast enhancement and digital unsharp masking in low-contrast portal images.
An experimental high-contrast sensitivity storage phosphor imaging system was used to produce double-exposure localization portal images of abdominal and pelvic treatment fields. The images were contrast enhanced by using an analog windowing technique and edge enhanced with a digital unsharp masking routine. A laser printer was used to print the storage phosphor images onto film. Conventional images were obtained by placing film in the cassette with the storage phosphor plates prior to exposure. Four radiation oncologists rated the storage phosphor and conventional films for perceptibility of anatomical detail needed to verify the placement of the treatment field. Contrast enhancement alone did not result in a significant improvement in perceptibility over unprocessed conventional film (p greater than 0.20). However, the combination of contrast and edge enhancement did result in a significant improvement over conventional film (p less than 0.05).
Humans↗
Xenon and iodine enhanced cerebral CT: A closer look.
Xenon and iodine enhanced dynamic computerized tomography (CT) have been used experimentally to obtain both qualitative and quantitative information on local cerebral blood flow in both normal and infarcted tissue. Direct comparisons between Xenon enhancement, iodine enhancement and pathological findings demonstrate significant differences between results derived from each of the 2 in vivo techniques. While iodine enhanced dynamic CT yields valuable information concerning the patency and density of vasculature, xenon enhanced studies can provide highly focal information on cerebral tissue perfusion.
Animals↗
Performance factors of CT scanners.
Explore the source record for details and available documents.
Cerebrovascular Circulation↗
Progress in cerebrovascular disease: local cerebral blood flow by xenon enhanced CT.
A noninvasive technique for measuring local cerebral blood flow (LCBF) by xenon enhanced x-ray transmission computed tomography (CT) has been developed an reported quite extensively in recent years. In this method, nonradioactive xenon gas in inhaled and the temporal changes in radiographic enhancement produced by the inhalation are measured by sequential computed tomography. Time dependent xenon concentrations within various tissue segments in the brain are used to derive both local partition coefficient (lambda) and LCBF. An assessment of this method reveals that although it provides functional mapping of blood flow with excellent anatomic specificity, there are distinct limitations. The assumptions underlying this methodology are examined and problems associated with various potential applications of this technique are discussed.
Animals↗
Clinical experience with the use of xenon-enhanced CT blood flow mapping in cerebral vascular disease.
Cerebral blood flow mapping with the xenon-enhanced/CT method has become a useful clinical tool in the management of patients with occlusive cerebral vascular disease. Studies involving 4-5 minutes of inhaling a xenon/oxygen mixture (less than or equal to 35%) can now be performed routinely with acceptable patient tolerance and compliance. Four cases with acute and chronic ischemic injuries are reported here to illustrate the manner in which this method has been used to characterize flow pattern in such patients and the relevance of this flow information to clinical patient management.
Adult↗
Measurement of cerebral blood flow during xenon inhalation as measured by the microspheres method.
Measurements of cerebral blood flow (CBF) were performed using the microsphere technique in non-human primates (baboons) to assess the effect of non-radioactive xenon gas inhalation on CBF. Blood flows in small tissue volumes (approximately 1 cm3) were directly measured before and during the inhalation of xenon/oxygen gas mixtures. The results of these studies demonstrated that when inhaled in relatively high concentrations, xenon gas does increase CBF, but the changes are more global than tissue-specific. The problems and limitations of such evaluations are discussed.
Animals↗
Xenon-enhanced CT of the brain: effect of flow activation on derived cerebral blood flow measurements.
The errors associated with derivation of cerebral blood flow values by the xenon-enhanced CT method have been evaluated through computer simulations as a function of flow-activation patterns and different scanning protocols. The results of this study indicate that actual flow activation during inhalation increases the derived flow values in a systematic way. Compared with the errors introduced by CT noise and/or variations in scanning protocols, flow activation introduces relatively small errors in the derived flow value when the washin technique is used.
Administration, Inhalation↗
Stable xenon CT cerebral blood flow imaging: rationale for and role in clinical decision making.
The stable xenon CT method of measuring cerebral blood flow has been investigated in research studies for over 10 years. Recently, it has been gaining clinical acceptance, primarily owing to a combination of several unique advantages it holds over other cerebral blood flow measurement techniques. The accuracy of this technique in quantifying low cerebral blood flow gives it a unique application in cases of brain death and acute stroke and it can be repeated after an interval of 20 min. making it possible to evaluate autoregulation and cerebrovascular reserve. Furthermore, cerebral blood flow information is directly coupled to CT anatomy. Although it is more difficult to administer than a standard CT scan, careful monitoring can ensure patient safety during the examination. In this article we review the physiologic and technical bases for the clinical application of xenon CT-derived quantitative cerebral blood flow information and discuss the advantages and disadvantages of the technique. We also describe its current clinical applications, including its usefulness in the evaluation of acute stroke, occlusive vascular disease, carotid occlusion testing, vasospasm, arteriovenous malformations, and head trauma management.
Cerebrovascular Circulation↗
The Three Mile Island Population Registry.
Shortly after the March 28, 1979, accident at the Three Mile Island (TMI) nuclear plant outside Harrisburg, Pa., the Pennsylvania Department of Health, in conjunction with the Centers for Disease Control and the U.S. Bureau of the Census, conducted a census of the 35,930 persons residing within 5 miles of the plant. With the help of 150 enumerators, demographic and health-related information was collected on each person to provide baseline data for future short- and long-term epidemiologic studies of the effects of the accident. Individual radiation doses were estimated on the basis of residential location and the amount of time each person spent in the 5-mile area during the 10 days after the accident. Health and behavioral resurveys of the population will be conducted approximately every 5 years. Population-mobility, morbidity, and mortality will be studied yearly by matching the TMI Population Registry with postal records, cancer registry records, and death certificate data. Because the radiation dose from TMI was extremely small, any increase in morbidity or mortality attributable to the accident would be so small as not to be measurable by present methods; however, adverse health effects as a result of psychological stress may occur. Also, a temporary increase in reporting of disease could occur because of increased surveillance and attention to health.
Accidents↗