Prior to being hit by x-rays, the image on the plate is white. As x-rays hit the plate the image turns progressively darker. Tissues and materials absorb x-rays to varying degrees depending on their density, therefore influencing the number of x-rays reaching the plate beneath them and creating the x-ray image. Areas in which all x-rays are obstructed, and therefore do not reach the plate, remain white (known as radiopaque), whilst areas where a lot of x-rays pass through to the plate become black (known as radiolucent).
X-ray absorption
Metal (such as in implants) is seen as bright white, as it is even more dense than bone, and no x-rays can penetrate through it.
Bone is generally seen as white on a radiograph, as most of the x-rays are absorbed due to its density. It is radiopaque.
Soft tissue or fluid is normally grey on a radiograph, as it is less dense than bone, so more x-rays pass through.
Fat is normally seen as dark grey on a radiograph, as it is less dense again than fluid and soft tissue.
Gas has a very low density and as such appears black on a radiograph, as hardly any x-rays are absorbed. It is very radiolucent. This means that parts of the body with lots of gas (such as the lungs) often appear very dark on radiographs.
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Exposure
Overexposure. If the x-ray settings are set too high (kV and mA/s increased), there will be too much energy in the primary beam. As a result, little energy will be absorbed by the patient and more x-ray radiation will hit the detector. The detector starts off white and turns dark as radiation hits it, and as a result the image will become too dark and no longer be diagnostic.
Underexposure. If the x-ray settings are set too low, there won't be enough energy in the primary beam. A proportionally large amount of energy would be absorbed from the primary beam, meaning very little radiation would hit the x-ray detector and turn the image darker. This means there will be very little change from the detector's original white colour, and the overall image will be too light.
Remember! If the radiograph is too dark it is overexposed, and if it is too light it is underexposed. To help you remember, we can use the toast analogy. If we leave bread in the toaster too long (overexposure) it turns black, and if we don’t toast it for long enough (underexposure), it remains white.
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