Marla Holman
Spatial Resolution, Density, Contrast
Concept mapping
3-18-08
Spatial resolution in regards to digital imaging includes a factor called recorded detail. Recorded
detail is the degree of sharpness of the structural lines recorded in the image. (AKA definition,
sharpness, detail.) Recorded detail is measured by resolution which is the ability to distinguish between
two adjacent structures. There are two types of resolution: spatial resolution and contrast resolution.
Spatial resolution is the smallest detail that can be detected in an image. Spatial resolution has
three components which are: geometric unsharpness, receptor unsharpness, and motion unsharpness.
Geometric unsharpness is the relationship between focal spot size, Source-Image-Distance, and ObjectImage-Distance. (It’s the ‘fuzziness’ of an image.) When the focal spot size increases, the fuzziness
increases and the recorded detail decreases. When the Source-Image-Distance increases, the fuzziness
decreases, and the recorded detail increases. And then the Object-Image-Distance increases, the
fuzziness increases, and recorded detail decreases. Receptor unsharpness is the relationship between
matrix and pixel size, bit depth, and pixel pitch. A bigger matrix brings better resolution. When pixel size
is smaller creates a sharper image. Pixel pitch is the distance between two pixels and the smaller the
distance the better the resolution. Bit depth is the number of shades of grey assigned to a pixel. More
bit depth means a sharper image. Scanning frequency is also a factor because it is how fast the laser
moves back and forth which the higher the frequency the sharper the image. Motion unsharpness is the
last factor that affects spatial resolution. Motion decreases the recorded detail, decreases the contrast
and increases the noise. It blurs the image giving it a greyer appearance, thereby decreasing the
contrast. Ways to control the motion from a radiographer’s standpoint are to communicate instructions
to the patient, stabilize the part being examined, and/or shorten the exposure time. Another way is
stabilize the image receptor and equipment so it doesn’t move during the exam. Laser spot size is one
more item in spatial resolution. A smaller laser spot size equals better resolution. (How thick is the
laser.)
Contrast resolution is the ability to display objects in 2-Dimension viewing. This is controlled by
the matrix, the image receptor and/or the monitor. The larger the matrix means smaller pixels giving the
image better detail. The type of digital image receptor gives better detail as mentioned in the previous
paragraph relating to pixel pitch, bit depth, and pixel size. A smaller image plate means scanning it more
gets more data and better resolution. Sampling frequency is proportionate to spatial resolution: lower
frequency means less resolution. The clarity of the monitor that the radiologist has to view the images
on plays a role in determining the image detail.
The only difference with film imaging qualities is the receptor unsharpness. Adjusting the speed
of the film, or the film/screen contact, or using single versus duplitized film are ways that affect spatial
resolution. A faster speed film creates a less sharp image and gives poor resolution. The larger the
tabular grains in the emulsion in the film that is associated with faster speed film. (The smaller the grains
the better the image resolution.) Larger tabular grains are more light sensitive giving poor resolution. A
thicker emulsion also is more light sensitive giving poor resolution. For film/screen contact to be useful
the film sits between two intensifying screens. Light is emitted from the intensifying screen and the
phosphor crystals spread out the light to the film and causes structural lines to be fuzzy. Duplitized film
has emulsion on both sides of the film. It’s a faster speed which gives less detail and poor resolution.
Single film has one sided emulsion and is slower speed which gives better detail. The duplitized film has
two phosphor layers screens and the back screen is thicker and faster than the front. A thin layer screen
gives better detail and if there is dye added it will improve sharpness which will also give better detail.
With both film and digital the size of the image receptor (cassette) is also a factor. If you use a
big cassette for a smaller part, the image will be very small and hard to read. Using the correct size
cassette will help with the making the image have a nice spatial resolution. The further the distance
between the apertures for a beam restrictor means better resolution: less penumbra. When you angle
the tube or the part you create shape distortion that contributes to poor spatial resolution. The
divergence of the beam as it comes from the focal spot is attributed to the apertures because of the
areas of fuzziness surrounding the part or region of interest.
In digital imaging the brightness is controlled by window leveling. Window leveling is the
blackness on the image after processing. This could be taken as density, but it’s not really called density
in digital imaging…window leveling. When you increase the window level you will increase the image
brightness and decreasing the window level will obviously decrease the image brightness on the
monitor. Window leveling picks the middle range of pixels. AEC produces consistent levels of density
when used correctly because of the controlled levels of radiation reaching the image receptor and the
computer adjustment.
For film there are many things that affect density. To increase density you could either increase
kVp or mAs, but not at the same time. You could decrease the Object-Image-Distance to increase the
density. Using the anode heel affect will put the thicker part under the cathode where the beam is more
intense giving a more uniform density over the longer axis. Decreasing collimation gives less scatter and
will increase density. As film screen speed increases so does the density and poor resolution.
Decreasing the grid ratio will increase density. (Grid ratio is the height of the lead divided by the width
of the interspace material.) Also when you decrease your Source-Image-Distance (the intensity of the
beam) you will increase your density. Some processing chemicals create densities (chemical fog) where
there should not be due to unexposed silver halide.
However, patient thickness is a factor to decrease density because much of the beam will get
scattered or absorbed before it reaches the image receptor. The thickness could be reduced by using a
compression paddle to decrease that thickness and reduce the scatter. Another factor is the central ray
angle because it spread the beam over a larger area part (increasing the distance and increasing the
patient part) and decreasing the density. Compensating filters absorb the beam and decrease the
density. The examples in the previous paragraph could also be termed vice versa as well.
Contrast on the digital image is the amount of shades of grey on an image. It’s controlled by window
width. Increasing the window width (making it wider) will decrease the contrast making the image have
more shades of grey. And of course decreasing the window width (making it narrow) will increase the
contrast making the image have fewer shades of grey. There is a dynamic range that relates to the
image contrast. The higher the range the more shades of grey available to display. Increasing pixel
depth will of course increase contrast resolution. Also increasing contrast is the decrease in scatter and
decreasing the patient’s body part thickness. (Basically, decrease the part thickness, decreases the
scatter which will increase the contrast.) This is due to the fact that digital imaging is very sensitive to
scatter. The more scatter that occurs the more noise also occurs on the image. Contrast has a direct
relationship to collimation and grid ratio. When you increase these factors they will, in turn, increase the
contrast (making it higher/shorter scale—black/white image). When you collimate the amount of scatter
is lessened and the contrast increases. When grid ratio is higher it’s cleaning up the scatter and
increasing the contrast.
As for film the more shades of grey means narrow range of density and many shades of grey
wider range density which is the same for digital. Contrast has a direct relationship to collimation and
grid ratio. In film, increasing the kVp will decrease the contrast. Increasing Object-Image-Distance will
increase contrast and decrease the scatter that reaches the image receptor. As in digital, with the
patient’s part increases this will increase the scatter and degrade the contrast. Contrast is related to the
speed of the film and the exposure latitude. Wider latitude indicates lesser contrast (more grey) and a
narrow latitude gives more contrast (less grey). Automatic Exposure Control doesn’t affect the contrast
of the film.
Whether using digital or film, tube filtration decreases the contrast because of the increase in
technique and, in turn, scatter. If you’re using digital or film, grids are a good way to improve contrast.
They absorb scatter from the patient before it reaches the image receptor. Make certain that grid lines
are in same direction and beam and grids are placed correctly on the image receptor.