Contrast RADIOGRAPHIC IMAGING

 Contrast RADIOGRAPHIC IMAGING

A high overall radiographic density or a low overall radiographic density also holds true for contrast an image can offer only little differences in density between different structures and be known as a low contrast image on the other hand a great difference in density between different structures can exist and this is known as a high contrast image the reason why a low contrast image gives little difference in densities is because there are so many densities present on a low contrast image ranging from light to dark this long range of densities gives the low contrast image the term long-scale contrast, on the other hand, a great difference in densities exists on a high contrast image because only a few densities are recorded on these images this is why high contrast images are also known as short-scale contrast images it might be tempting to assume that high contrast radiographic images are good and low contrast images are bad.

 A high contrast image allows the viewer to easily make out differences between structures on the image low contrast images offer up more anatomical and pathological information due to the long-range of densities present on the image neither type of contrast is ultimately bad the trick is to find a balance between an image that is a long scale enough to provide sufficient diagnostic information and one that is short scale enough to make the differences appreciable that takes us to the factors that determine whether or not an image would have a higher low radiographic contrast the first factor is the subject contrast now this might seem a bit tricky at first but is actually really easy let us discuss in clear terms before moving on to more standard definitions.

we already know that the radiographic contrast is the difference between densities on an image the subject contrast is the difference between density or thickness of structures within the anatomy that is being imaged this difference in thickness determines how the x-ray photons act when they pass through different structures you see the photons easily pass through thin structures but are absorbed by thicker structures let us go into the more standard definitions subject contrast is the difference in intensity of photons emerging from the anatomical part under examination this difference occurs because the photon is attenuated or reduced in different ways when it passes through different structures for example if the chest is being radiographed photons that would pass through and emerge from the area of the ribs would be greatly reduced compared to photons that would pass through and emerge from the area of the lungs which would be much less reduced as the lungs are not nearly as thick as the bony tissue of the ribs let us explain further using this illustration let us imagine this is our x-ray too and this on the other hand is our image receptor this time let's place anatomy on the image receptor picture this as a part with structures of similar thickness a low subject contrast part then here's our x-ray beam let's place three hypothetical photons in our x-ray beam just for simplicity photons normally travel in millions what happens when our three photons pass through this structure with low subject contrast they go through receive the same degree of attenuation and get to the image receptor looking alike thus the image produced will have similar densities across the image producing a low radiographic contrast this explains why plane radiographs of the abdomen is of low radiographic contrast the abdomen contains mainly soft tissue of similar thickness.

 Contrast media now what would happen to watch what happens when our three photons try to pass through this anatomical part you would observe that only two out of the three photons make it to the image receptor one is absorbed completely by the thicker structure implies that certain areas of the image receptor would have fewer photons than other areas due to these differences in photon attenuation this produces an image with higher radiographic contrast that takes us to the next factor affecting radiographic contrast the kilovoltage

 we learned how the kV selected determines the speed and energy at which photons travel in turn the speed and energy of a photon determines how well a photon can penetrate tissues a high energy photon would pass through most structures easily while a low energy photon will be attenuated by many structures this means that if the kv is too high the photons would have sufficient energy to penetrate all structures under examination no matter how thick they are thus there would be little or no difference in attenuation of the photons and the image would have low radiographic contrast when the kv is reduced photons in the direction of thick structures would be greatly attenuated while those moving in the direction of thinner or no structures at all would make it to the image receptor with less attenuation this is known as differential attenuation and will produce an image with higher radiographic contrast from all that has been said so far it is safe to say that as kv increases the subject contrast and radiographic contrast both decrease and vice versa this is why the kv is known as the major controlling factor for radiographic contrast