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Resolution
Crystal Bechmann
Resolution is the ability of the imaging system to resolve or distinguish between
two adjacent structures. The two subcategories of resolution are spatial and contrast
resolution. Spatial resolution is the smallest detail that can be seen on an image.
Contrast resolution is the ability of the imaging system to distinguish between small
objects having about the same contrast. Basically resolution is the sharpness of the
image on film and on digital. Sharpness is the structural lines that can be seen on the
image. You are always going to have some degree in unsharpness, no matter what you
do. Unsharpness is the result of the relationship among the size of the FSS, SID, OID. By
changing any of these items can result in some unsharpness to your image.
FSS(focal spot size) is a selection that you can make on the control panel and all
that FSS is where the electrons are being made-for x-ray production. You can either pick
a large or a small FSS. As the focal spot size increases then the unsharpness of the
image will increase, therefore the recorded detail (resolution) will decrease. However if
the FSS decreases then the unsharpness of the image will decrease, which will increase
the recoded detail. Ideally the small focal spot size should be used, but you cannot
always do that due to the particular body part being radiographed. All body parts have
a different FSS that you must use to get the image to turn out the way you want it to.
Also another reason that you cannot always use the small focal spot size is because it
could burn out, because of the high ma that you are applying to the filament for x-ray
production. That is usually why there are two focal spot so you can choose the right
one, that you need for the particular exam that you are doing, and also for the technical
factors that you are going to be using.
Distance is another factor that affects resolution. For example increasing SID-the
distance between the x-ray tube and the source (IR)-will decrease the amount of
unsharpness which will give you an increase in the recorded detail in the image. And
when the SID is decreased the unsharpness will increase and the recorded detail will
decrease. The reason why increasing the SID increases detail is because when the x-ray
beam exits the patient it continues to diverge with an increase in SID, the exit beam
does not diverge as much because of the distance between the tube, patient, and image
receptor. Another example is OID-the object to distance. Increasing the OID will
increase the amount of unsharpness and decreases the recorded detail. And decreasing
the OID decreases the amount of unsharpness and increases the recorded detail. You
should always make sure that the object being radiographed is as close to the IR as
possible. You are always going to have some distance between the IR and the object,
because of the cassette (the thickness) or the Bucky board, there is not much that you
can do about that, but you can help the object stay as close as it possibly can.
All the items listed above affect both digital and film. But there are some
differences between digital and film regarding resolution. For example, in film, the film
screen combination can have an effect of resolution such as the material that it is made
of and the film screen contact. For digital (spatial) resolution, a controlling factor is the
pixel size, which a pixel is the smallest component of the matrix. The matrix is an image
displayed as a combination of rows and columns of pixels. Depending on the monitor
that the imaging system has will affect the spatial resolution. The greater the number
the pixels the better the spatial resolution. Flat panel detectors in digital are said to
have improved spatial resolution cause of the quicker access to the image because with
digital you don’t have to process the film and when you process the film there is always
some information that is lost during the process. But with the digital it eliminates
processing of the plain film processing step. Which would improve your spatial
resolution.