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Basics of x-ray tubes
Published in Gavin Poludniowski, Artur Omar, Pedro Andreo, Calculating X-ray Tube Spectra, 2022
Gavin Poludniowski, Artur Omar, Pedro Andreo
The drawback to employing a shallow anode angle is that it limits the useful angles of emission for an x-ray beam. There is a limit for the width of the beam defined by the grazing angle with the anode surface. In addition, because the x rays are emitted at depth in the target, as the emission angle approaches the limit the x rays must pass through an increased thickness of target material to escape. This reduces the fluence. The phenomenon is known as the anode heel effect. This is illustrated in fig. 2.3 for a tube potential of 80 kV and a total filtration of 2.5 mm of aluminium. The fluence (per mAs) at a plane 100 cm from the focus is plotted against the position, for anode angles of 12∘ and 24∘. The usable beam width is less for the lower anode angle. This comes, however, with a factor of 2 reduction in effective focal spot size ().
Kilovoltage X-Ray Units
Published in W. P. M. Mayles, A. E. Nahum, J.-C. Rosenwald, Handbook of Radiotherapy Physics, 2021
The inverse-square law and the amount of oblique filtration through attenuating materials in the beam path will govern the directional distribution of the beam. The beam profile perpendicular to the anode–cathode direction should be symmetric. This is unlikely to be the case in the anode–cathode direction, where x-rays will be differentially absorbed within the target depending on the angle where they emerge. This heel effect can result in increased or reduced beam intensity on the anode side of the beam axis compared with that on the cathode side (Klevenhagen et al. 2000). The magnitude of this effect depends on both the target angle and applied kV, and it is additionally constrained by the need for a large useful beam at relatively short SSDs. The target angle, defined as the angle between the incident electrons and the normal to the target, is typically 40° for superficial x-rays and 30° for orthovoltage equipment. For multi-energy equipment, the larger target angle is chosen so that the useful field size at the superficial energy is not significantly reduced by the heel effect.
Diagnostic Imaging Using X-rays
Published in Debbie Peet, Emma Chung, Practical Medical Physics, 2021
Debbie Peet, Richard Farley, Elizabeth Davies
One application of planar radiography is mammography which uses X-ray projection to image the breast (Figure 4.4b). The breast has very little inherent contrast as it is mainly formed of glandular tissue and fat. Imaging therefore needs to be optimised to enable detection of small tumours and cysts with good soft-tissue discrimination. To achieve this, X-rays are usually generated (and often filtered) using molybdenum instead of tungsten to achieve a much lower energy range. The breast is compressed to reduce attenuation and scatter, thereby reducing the dose required to provide a good image. The tube is also angled within its housing to take advantage of angular variations in the intensity of X-rays emitted from the anode (called the anode “heel effect”) to better image from chest wall to nipple. High-resolution display screens are used to view mammographic images in dim lighting conditions to help prevent small or subtle changes in the breast from being missed. More detail on the technical requirements of mammography compared to planar radiography can be found in Farr’s Physics for Medical Imaging (Allisy-Roberts and Williams 2007) and Dendy and Heaton (2012).
Use of CT simulation and 3-D radiation therapy treatment planning system to develop and validate a total-body irradiation technique for the New Zealand White rabbit
Published in International Journal of Radiation Biology, 2021
Yannick Poirier, Charlotte Prado, Karl Prado, Emily Draeger, Isabel L Jackson, Zeljko Vujaskovic
To the authors’ knowledge, this study represents the first detailed description of a total body irradiation technique for rabbit models since the foundational radiation biology papers published in the 1950s. To the authors’ knowledge, the only paper to show a similar level of details regarding radiation dosimetry in total body rabbit irradiations dates from 1952, and describes a 190 kVp x-ray beam filtered with 0.8 mm Cu + 1.0 mm Al added filtration (Lennox et al. 1952). As this was a low-energy kilovoltage x-ray technique, field homogeneity was limited by the lack of a flattening filter and the heel effect, which is why reported dose distributions ranged from 60 to 80% when not near the abdomen, and as high as 105% at the surface near the central axis. In contrast, our technique covers the entire animal with a dose ranging from 95 to 107%, with much of the volume receiving >105% being restricted to the extremities (e.g. head, feet) where there is lack of attenuation.
Radiation dosimetry in cell biology: comparison of calculated and measured absorbed dose for a range of culture vessels and clinical beam qualities
Published in International Journal of Radiation Biology, 2018
Elizabeth Claridge Mackonis, Lauren Hammond, Ana I. S. Esteves, Natalka Suchowerska
Conditions arising from the radiation source include the radiation beam energy and the beam profile. Exposure to a beam profile that is not flat, such as the kV beam, will result in a different dose being delivered to the inner and outer wells of a multi-well plate. In this study, this caused a systematic uncertainty of up to 16% for the 96 well plate in the 125 kVp and 280 kVp beams. Errors in dose as high as 23% (96-well plates, 125 and 280 kVp) can be introduced if an independent form of dosimetry such as film dosimetry is not performed. The need for high-throughput studies presents a big motivation for the use of 96-well culture vessels. They allow for a large number of samples to be irradiated simultaneously with little increase in cost or workload, but based on the assumption that all wells will receive the same dose and beam quality. The well-defined heel effect (Fuchs 1947; Geiger 1960), characterized by a gradient in beam intensity and in photon beam energy across the field, will also contribute to the large uncertainty observed. The intensity of radiation is reduced at the anode end of the field and at the edges of the field, where the distance from the target is the greatest. This effect explains the observed tilt in the beam profile for the kVp beams, with the right side of the graph indicating a lower dose than the left side. The difference in dose and beam quality across the field means that the wells across the 96-well plate will receive a different radiation dose for each exposure, making the above underlying assumption invalid. Should the use of 96-well plates be unavoidable, the outer most wells should be filled with buffer solution or similar, to maintain lateral radiation scatter conditions. As a last resort, all wells could be used provided the same wells are used for each arm of the experiment, based on knowledge gained from dosimetric measurements. For the MV beam, the beam energy and beam profile are far less dependent on position in the field than for the kV beam. The MV dose profiles are ‘flattened’ through the introduction of a flattening filter to improve dose uniformity, so the dose in each of the 96-well plate is likely to be very similar.