1/6/2023 0 Comments Compton scatteringBut we can compress breast to achieve smaller thickness of this part of body, so x-rays will be passing through the tissue. Thickness of the patient: We usually don’t have a lot of control over the patient’s thickness. If it increases that means we have a larger volume of the patient that has been covered with x-rays, and therefore there’s going to be a higher chance of scattered x-rays ending up measured on the detector. However, these grids do block more of the photons from hitting the detector and therefore the x-ray intensity needs to be increased.Ĭollimation is the area of the x-rays exposed at one time. The contrast enhancement factor will generally be higher for higher grid ratio grids, and the average k for antiscatter grids is ~1.5-2.5. It is the ratio of image contrast with the grid present to image contrast without the grid. The contrast enhancement factor (k) measures how impactful the grid is at improving the contrast. The main purpose for using the scatter grid is to increase the image contrast by reducing the background fog due to scatter. Therefore, in clinical practice several x-ray grids are used, each of which has a range of distances that it operates best.Īlso, moving grids are used to reduce image artifacts as the effect of the grid will be averaged out during the acquisition. Therefore, in order to block fewer primary photons it is best to have a focused grid where the septa are directed toward the x-ray source.įrom this focused grid figure it is clear that there should be more focusing of the grid as the source to detector distance is smaller and less focusing as the source to detector distance is larger. The primary x-rays all travel in straight lines from the x-ray tube to the detector. The x-rays all come out from there in what is called a divergent geometry. In x-ray imaging the x-rays are emitted from the anode of the x-ray tube. Another parameter of anti-scatter grids is the grid frequency which is how often the grid repeats and it is defined as 1/(D+t). There is also a desire to have the grid septa width (t) to be as thin as reasonably possible so that fewer primary photons are blocked before reaching the detector. Common grid ratios used clinically are 4:1, 6:1, 10:1 or 12:1. If the width of the grid plates is left constant then higher grid plates will block more scatter. The grid ratio is the ratio of the height of the grid plate, often called grid septa, to the separation width (grid Ratio = h/D). The grid ratio is the ratio of the height of the grid plate, often called grid septa, to the grid width. So, primary photons will pass through grid plates while scatter x-rays will more likely be stopped by the plates of the grid. Under some conditions, it is possible to mistakenly identify the Compton scatter artifact as approximately 1% of an element that is not present.For anti-scatter grids an x-ray attenuating material, such as lead, is commonly used. Though small, the artifact is not inconsequential. However, with the advent of silicon drift detectors and their utility for trace element analysis, we anticipate that more people will observe it and possibly misidentify it. It seems likely that this artifact has not previously been reported because it only occurs under specific conditions and represents a relatively small signal. We have identified a new sample-dependent artifact that we attribute to Compton scattering of energetic X-rays generated in a small feature and subsequently scattered from a low atomic number matrix. Others, such as secondary fluorescence peaks and scatter peaks, can be traced to the sample. Many artifacts, such as the Si escape peak, absorption edges, and coincidence peaks, can be traced to the detector. Peaks that result from nonideal behavior in the detector or sample can fool even an experienced microanalyst into believing that they have trace amounts of an element that is not present. Artifacts are the nemesis of trace element analysis in electron-excited energy dispersive X-ray spectrometry.
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