Heather Garen
Digital Image Qualities
A radiographer views an image to determine if it is acceptable photographically and
geometrically. To compensate for a quality they would have to determine what needs to be
changed to provide an adequate picture. The following factors may have to be altered to make
an image sufficient.
Contrast is the difference between adjacent densities on a radiograph. It is the
distinctness of black and white. As photons pass through an object they are absorbed, pass
through, or slightly pass through reaching the IR. In tissues where absorption differs
visualization of densities varies. An image with few densities with strong differences is one with
high contrast. This can also be defined as a long scale contrast. In conclusion it can be
described as long and low, or short and high. The anatomic tissue radiographed and kVp used
determines contrast. Low kVp used results in higher contrast. Thinner the patient the higher
the contrast, kVp would have to be adjusted to get that same contrast level on a larger patient.
Scatter radiation decreases contrast adding to unwanted density. When OID is
increased it creates an air gap causing low contrast. Collimation increased results in less scatter
reaching IR so high contrast. Tube filtration increases contrast by decreasing low energy
photons. Contrast can be altered by window width on the monitor after processing. The more
pixel depth and number the higher contrast. A wide dynamic range also helps have a high
contrast short scale of gray.
Spatial resolution con simply is defined as the blurring of an image, an edge’s degree of
sharpness on an image. It is determined by multiple factors. Including increasing the number
of pixels in the image matrix increases SP. Increasing OID decreases sp, object is close to the IR
making no magnification. Increase in SID would increase sharpness. Larger the FSS the worse
the SP. Add tube, patient, or IR angulations and you will decrease SP. Motion from the patient
ofxray equipment will cause poor SP. In the reader the smaller and faster the laser spot size
the better. The better the pixel depth the better the SP. The greater crystal size in the cassette
the less spatial resolution. CR angle increase decreases detail because it increases distortion.
Recorded detail is the accuracy of structural lines recorded.Optimal geometric quality is
achieved by maximizing the amount of recorded detail and decreasing distortion. As FSS
increases, unsharpness increases and recorded detail increases. So small FSS is chosen for
better recorded detail. Increasing the SID decreases the amount of unsharpness and increases
the amount of recorded detail in the image. Source of photons is closer so less scatter results in
less added density which inturn makes better recorded detail high contrast. Increasing the OID
is inversely proportional to recorded detail (degree of sharpness). The more OID causes an air
gap that scatter is made and diverging beams hit IR.
Density is the amount of brightness the image has. Picture must have sufficient density
to demonstrate all essential anatomy. Controlling factors would include mAs, influencing
factors are the following. The 15% rule states that changing the kVp by 15% will have the same
effect on density as doubling the mAs, or reducing the mAs by half. SID affects the quanity of
radiation reaching the IR. As SID increase density decrease. As SID increases the xray beam
intensity is spread over a larger area and more diverging beam hits IR. As SID increases the
density decreases as a result of the square of the distance. OID is increased less density results.
With this air gap less intensity of the beam and diverging beam will strike the receptor.
Increasing collimation makes field size smaller which decreases density, less scatter to add
density. Anode heel effect decrease the primary beam on the anode end of the tube making
density appears not uniformed on an image. A patient’s part thickness changes density. The
thicker the part the less radiation reaching IR so decrease in density. Generator must be
calibrated for consistent output or it will affect density. Less tube filtration adds density.Adding
a compensating filter to the primary beam produces more uniform density, but requires an
increase in mAs.