Download Crystal Bechmann

Crystal Bechmann
3-18-08
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.
Density is the overall blackening on the radiograph. Density’s controlling factors
are miliamperage and time which gives you mass which is the quantity of radiation
reaching the IR or film. As you increase the mAs you increase the quantity of radiation
which will therefore increase the mAs -density. As you decrease the mAs you decrease
the quantity of radiation which will in turn give you less density. Density has a direct
relationship with mAs. The influencing factors of density are kVp which as you increase
kVp you increase the quantity of the beam which will give you an increase in density. As
you decrease the kVp you decrease the quantity of the beam which will decrease the
density also by changing kVp you are using the 15% rule which is changing the kVp by
15% will have the same effect on the IR or film as just like doubling the mAs or reducing
the mAs by half. Another influencing factor of density is OID and SID. As you increase
the SID you need to increase the mAs which will decrease the density. As you decrease
the SID you need to decrease mAs which will increase the density. This is because of the
inverse square law which states that the intensity of the beam is inversely related
(proportional) to the square of the distance from the square another factor are grids which
absorb scatter before it reaches the IR or film. Also the use of grids will decrease the
density. Another factor is collimation. As the collimation increases-a smaller field- there
will be a decrease in density. As collimation decreases-a larger field size- the increase in
density you will have. Also another factor could be the reciprocity law which states that
the density produced on the IR or film will be equal for any combination of ma and time
as long as the product of mAs is equal. Another thing that can affect density is the
generator output-it all depends on the generator that the facility is using. Also another
factor is tube filtration-by adding filters (compensating) will produce a more uniform
density. With filters the mAs needs to be increased the compensating filters will decrease
the intensity of the beam. Another factor is the anode heel affect. Another factor is
noise. As mAs decreases reaching the IR (scatter) the increase in quantum mottle. An
increase is the number of photons reaching the IR will decrease the quantum mottle.
Those are the effects of density with film and digital.
Along with the above another effect of density with film is the films screen speed.
An increase in the film screen speed you will need to decrease the mAs to maintain
density. The greater the speed of the film the greater the amount of density. The lower
the speed of the film the less density. A formula you can use for relative speed is the
conversion formula. Also another factor that can affect density on a film is the
processing of the film. Such as the temperature or film chemistry or even the transport of
the film.
Things that also affect density on digital are the window level for the image
brightness. As you increase the window level you will decrease your density. As you
decrease window level you will increase density. With density and digital you are
provided with a wide dynamic range.
Contrast is the degree of difference between adjacent structures of density.
Contrast affects both film and digital. Contrast comes in two categories. Subject contrast
and film contrast. Subject contrast is a result of absorption characteristics of the
anatomic tissue radiographed and level of kVp used. Film contrast is a result of inherent
properties manufactured into the type of film and how it is radiographed along with
processing conditions. Contrast can also be described as low and high. Low contrast
means a radiograph with a large number of densities but with little differences among
them. Which is also known as having a long scale of contrast. High contrast is when a
radiograph with few densities but with great differences among them. Also known as
having a short scale of contrast.
The controlling factor of contrast is kVp. Which is the quality of penetrability
power of the x-ray beam. As you increase kVp you increase the power of the beam.
Which will decrease absorption on the tissue and more transmission which would give
you a long scale of contrast. Low kVp creates fewer densities but with great differences
among them resulting in a high/short scale of contrast. A higher kVp also produces more
scatter which only adds useless density on the radiograph. As kVp increases scatter
increases which will decrease contrast. As kVp decreases scatter production decreases
which will result in an increase in contrast. There are some influencing factors that affect
contrast such factors are grids, collimation, OID. Grids will absorb scatter that exits the
patient but before it reaches the IR which means if there is less scatter, there will be an
increase in contrast. As collimation increases (smaller field size) then the contrast will
increase because you are limiting the are being exposed and decreasing the chance of
scatter. As collimation decreases (larger field size) then there will be a decrease in
contrast because you are exposing more of the patient (tissue) then you need to which is
only producing more scatter. Which will decrease contras on the radiograph. OID can
produce an air gap which is created when the object is further away from the IR which
will prevent scatter from reaching the film, however the air gap will increase the contrast
but you will get magnification of the object. For digital it is the same with film but with
digital you can adjust the contrast on the computer monitor by the window width
selection. A wide width gives you low contrast or more shades of gray. A narrow
window width gives the image a higher scale of contrast with few shades of gray.