Billie Suttle
Cmap Narrative – Film
March 18, 2008
Film Image Qualities
Film image qualities consist of photographic and geometric properties. Photographic
properties consist of Density and Contrast. Geometric properties consist of Recorded Detail and
Distortion.
Density is defined as the amount of blackness on the film. It is affected by a lot of items.
Grid use, pathology, generator, collimation, OID, filtration, screen speed, technique, SID, scatter,
anode heel affect, and AEC all affect density. Improper use of a grid will cause an overall loss
of density. Additive pathologies decrease density and destructive pathologies increase density.
Using a single phase generator will decrease density and using a high frequency generator will
increase density. Increasing collimation, OID, and filtration will all decrease density. Increasing
screen speed, increasing kVp & mAs, increasing SID, increasing scatter, and fog will all increase
density. There are four types of fog. Chemical fog is from the film processor, safelight fog,
white light fog, and radiation fog which is scatter.
The anode heel affect is when extreme ends of the x-ray field will show a visible
difference in density on film. The thicker end of the patient should be placed under the cathode
end of the x-ray tube to help avoid this. If AEC is used, it will provide more consistent film
densities. However, it is crucial that the patient is centered correctly and the correct sensors are
selected for this to happen.
Density represents exposure intensities, over or underexposures, selective absorption, and
optical densities. Optical densities are measured using a densitometer. The useful range of
densities are 0.25 – 2.5 OD. Optical density is controlled by the exposure latitude. This is the
range of exposures that will produce optical densities in the useful diagnostic range.
There are two types on contrast. High or short-scale contrast and low or long-scale
contrast. Contrast is affected by many different items. Technique, kVp only, is indirectly
proportional to contrast. Increasing kVp decreases contrast. The generator used also has an
effect. High frequency generators decrease contrast and single phase generators increase
contrast. Correct grid use also increases contrast. Exposure latitude affects contrast too.
Increased exposure latitude decreases contrast. Collimation and OID also directly affect
contrast. Increased collimation and increased OID show an increase in contrast.
There is also image receptor/film contrast, subject contrast, and display contrast. The
image receptor/film contrast is determined by the films emulsion and the inherent contrast level
in the IR. Subject contrast is determined by a number of items. Wavelength & frequency of xrays, differential absorption, tissue mass density, atomic #, patient shape & size, and high or low
kVp. Always use optimal kVp for the procedure to assure an adequate scale of contrast. Display
contrast consists of mainly the lighting used to display the image. This can be affected by the
lighting on the view box or the ambient lighting in the room/department.
Contrast can also be degraded by quantum mottle, scatter, noise, or fog. Quantum mottle
is known as “white noise.” Noise consists of background radiation that reaches the film. This is
known as “black noise.” Increased quantum mottle and increased noise both decrease contrast.
Fog also degrades contrast. There are four types of fog. They are radiation fog (scatter),
chemical fog (the processor), safelight and white light fog.
Recorded Detail is the degree of sharpness of structural lines on a radiograph. It is also
called definition, spatial resolution, sharpness, and detail. It consists of penumbra, umbra,
contrast resolution and spatial resolution. Penumbra is an area of unsharpness and umbra is the
actual shadow. Contrast resolution is the ability to distinguish between structures of similar
subject contrast. Spatial resolution is determined by the type of IP, thickness of phosphors, and
crystal size in film. This is measured by spatial frequency (lp/mm) or modulation transfer
function.
There are three influencing factors on spatial resolution. They are motion, receptor, and
geometric unsharpness. Motion unsharpness is caused by voluntary, involuntary, or equipment
motion. These can be prevented with immobilization devices, good communication with patient,
short exposure times, and ensuring that all equipment is in tact and working properly.
Decreasing motion increases detail. Receptor unsharpness includes speed, single vs. duplitized
film, and film/screen contact. Slower speeds, single film, and better film/screen contact all
increase spatial resolution. Duplitized film causes a crossover effect that causes a loss in detail.
Geometric unsharpness includes FSS, SID, and OID. Increased SID, decreased OID, and smaller
FSS all increase spatial resolution. Also, recorded detail is degraded by scatter.
Distortion consists of shape and size distortion. Shape distortion includes foreshortening
due to the part angle and elongation from IR angle. Size distortion consists of magnification
from increasing OID and minification from using an IR that is too large.