Radiographic Quality

Chapter 5

Radiographic quality

Refers to how easily details can be perceived on a radiograph. Need to obtain as much diagnostic

information as possible about the internal structures of the patient.

Quality depends on

Radiographic Density

Contrast

Geometric Factors that affect detail

Radiographic Density

Defined as the degree of blackness or darkness on a radiograph.

Black areas on a developed radiograph are produced by deposits of metallic silver in the film emulsion that result from exposure to x-rays and their subsequent processing.

Density continued ….

X-rays make radiographic film black Degree of blackness on a radiograph

depends on the amount of x-rays reaching the film.

Density is influenced by the quantity and quality of the x-ray beam, as well as the type and thickness of the tissue under examination.

Factors Affecting Radiographic Density.

Greater radiographic density may be produced by increasing: Total # of x-rays that reach the film The penetrating power of the x-rays The developing time The temperature of the developer

Let’s Review

mAs- number of x-rays leaving the x-ray tube in a set period of time.

When kVp increases, the penetrating power increases as well. This means more x-rays will reach the film causing a darker radiograph.

Other Density Influences

Thickness and type of tissue being radiographed. Increase in thickness, means that the

patient can absorb more x-rays which will results in a lighter image.

Large animals absorb more x-rays, so less x-rays reach the film, so the lighter the image. To compensate, we increase the kVp.

Type of Tissue

Type of tissue affects density as well. Higher density tissues will cause less x-rays to reach film, therefore have lighter areas on the film at that place.

Gray film, lacks good density

Contrast

Defined as visible difference between two adjacent radiographic densities

Is divided into Radiographic Contrast Subject Contrast

Radiographic Contrast

Density between two adjacent areas on a radiograph. Many black and white areas means you have

contrast. If a radiograph has many grays and a small density

difference between 2 adjacent areas, then it has low contrast.

Need right amount of contrast. Not desirable to have to high or too low of contrast. Need it to be just right. Want grays, blacks and whites so eye can easily see detail.

Contrast is influenced by:

kVp level Subject contrast Scatter radiation Film type Film fog

Subject Contrast

Defined as the difference in density and mass between two adjacent anatomic structures

Depends on: Thickness of the anatomic part. Density of the anatomic part.

Contrast Continued

Bone will have more white on the film than soft tissue.

Bone -> high contrast -> low kVp

Soft tissue -> low contrast -> high kVp

Exposure factors

Poor contrast is due to inappropriate exposure factors.

mAs- affects contrast when too little or too much is used. Primarily affects density so should not affect

contrast if proper kVp is used. If mAs is insufficient, contrast is reduced because

overall density of the radiograph is reduced. If quantity of x-rays reaching the film is too low, film

will be pale. If too much mAs is applied, overall film will be

blacker but less effect on contrast.

How should mAs be adjusted?

Kilovoltage

Affects both contrast and density Increase in kVp = Increase in

penetrating power. Increase in kVp = shorter wavelengths

are produced. As penetrating power increases, so does

scatter radiation. This can alter radiographic contrast.

kVp

If too low, will have gray and white appearance and image will be imperceptible.

Will have low density because unable to reach film.

Causes difficulty in distinguishing anatomic organs

Scatter Radiation

Radiation that does not form an image and is scattered in all directions.

Contrast is decreased because inappropriate areas of the film are being exposed.

Sources of Scatter Radiation

From the Patient Table Film Tray

Backscatter

Backscatter- radiation arising from sources behind the image plane that are scattered back to the image.

Avoid backscatter by limiting the size of the x-ray beam so that the field does not exceed the image receptor.

Cassettes contain lead- foil backing to prevent backscatter from reaching the film.

Grids

When thick body parts are being radiographed, you want to minimize scatter radiation, you do this by using a grid.

Grid- device placed between the patient and the radiographic film designed to absorb non-image forming x-rays.

Composed of alternating strips of lead and spacer material.

Grids Continued

Spacer material usually consists of fiber, alumininum, or plastic because they have low x-ray absorption

Grids may be Placed on top of the cassette. Built into the cassette. Placed under the table between the patient

and the cassette.

Grid Pattern

Linear Grid- most table top machines are equipped with a linear grid- Lead strips parallel in their longitudinal axis.

Crossed Grid- 2 superimposed linear grids absorbs maximum amount of scatter radiation

Grids continued

Generally grids now are a part of the cassette or the x-ray table.

Potter Bucky Diaphragm- usually found in large animal situations.

Grids must be adequately cared for, can not be dropped because will result in permanent damage.

Radiographic Detail and Definition Terms used to describe image

sharpness, clarity, distinctness, and perceptibility.

Lack of detail factors 1. Geometric unsharpness- loss of detail

due to geometric distortion Large focal spot size Decreased SID Motion Screens and films

2. Geometric Distortion and Magnification- variation in size and shape of anatomic structures due to their position in relation to the x-ray source and film. Important to keep areas being

radiographed parallel to image receptor to avoid distortion.

Distortions include: Magnification, elongation, foreshortening.