<P> X-ray detectors are devices used to measure the flux, spatial distribution, spectrum, and / or other properties of X-rays . </P> <P> Detectors can be divided into two major categories: imaging detectors (such as photographic plates and X-ray film (photographic film), now mostly replaced by various digitizing devices like image plates or flat panel detectors) and dose measurement devices (such as ionization chambers, Geiger counters, and dosimeters used to measure the local radiation exposure, dose, and / or dose rate, for example, for verifying that radiation protection equipment and procedures are effective on an ongoing basis). </P> <P> To obtain an image with any type of image detector the part of the patient to be X-rayed is placed between the X-ray source and the image receptor to produce a shadow of the internal structure of that particular part of the body . X-rays are partially blocked ("attenuated") by dense tissues such as bone, and pass more easily through soft tissues . Areas where the X-rays strike darken when developed, causing bones to appear lighter than the surrounding soft tissue . </P> <P> Contrast compounds containing barium or iodine, which are radiopaque, can be ingested in the gastrointestinal tract (barium) or injected in the artery or veins to highlight these vessels . The contrast compounds have high atomic numbered elements in them that (like bone) essentially block the X-rays and hence the once hollow organ or vessel can be more readily seen . In the pursuit of nontoxic contrast materials, many types of high atomic number elements were evaluated . Unfortunately, some elements chosen proved to be harmful--for example, thorium was once used as a contrast medium (Thorotrast)--which turned out to be toxic, causing a very high incidence of cancer decades after use . Modern contrast material has improved and, while there is no way to determine who may have a sensitivity to the contrast, the incidence of serious allergic reactions is low . </P>

Where is the image detector placed during an x-ray
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