Optimizing the image Flashcards
Radiographic quality
Refers to the ease with which details can be perceived on a radiograph
Dependent on 3 things:
Radiographic density
Contrast
Geometric unsharpness
Radiographic density
Defined as the degree of blackness or “darkness” of a radiograph
Dependent on the number of x-rays reaching the film/cassette
A radiograph that consists of many black areas and is very dark when viewed has a high density
Factors affecting radiographic density
mAs – more x-rays are produced so more x-rays reach the patient and the film
kVp x-rays of higher penetrating power are produced so are able to push through the patient and hit the film rather than being absorbed by the patient
Tissue Thickness and Type – thicker body parts absorbs more x-rays so less reach the film resulting in a lighter image on radiograph (ie: less density)
Developing Time – if recommended developing times are extended the x-ray film becomes overdeveloped and the density is increased
Temperature of the Developer – if temperatures increase over those recommended, density increases due to overdevelopment
Different types of tissue
Air (black on film)
Fat
Soft tissue
Bone (white on film)
Higher the tissue density = lower the radiographic density
Due to absorption of x-rays by tissues
Radiographic density is inversely proportional to tissue density
Density of tissue doubles = number of x-rays reaching film is halved = radiographic density is halved
Contrast
Contrast divided into two separate categories:
Radiographic Contrast
Density difference between two adjacent areas on radiograph
Subject Contrast
Difference in density and mass between two adjacent anatomic structures
Radiographic contrast
When density difference is great – radiograph said to have high contrast
Fewer shades of gray
Radiograph which exhibits small density difference between two adjacent areas has low contrast
Many shades of gray
Increased number of gray tones between the white and black on a radiograph constitute a long scale of contrast
Takes a long time to get from black to white
Radiographic contrast is influenced by
kVp
mAs
Distances
Film/screen combinations
Grid/filters
Collimation
Anode heel effect
Scatter radiation
Film fog/processing parameters
Subject contrast
Subject contrast is dependent on the thickness and density of anatomic part
High subject contrast increases radiographic contrast and low subject contrast decreases radiographic contrast
Exposure factors mAs
Affects contrast only when too low or too high
mAs too low = insufficient number of x-rays reaching film
Entire image lacks density even though dense structures have been penetrated
Entire radiographic density is reduced
mAs too high = overall radiograph density is increased
Exposure factor kVp
Affects both contrast and density
Increased kVp = increased penetration
Radiographic contrast altered due to scatter radiation
Decreased kVp = decreased penetration
Anatomic image imperceptible
“soot and whitewash” appearance
Scatter radiation
Biggest single factor contributing to decreased film quality
Present in every single radiographic examination
Effect is to produce generalized radiographic fog on film
Reduces contrast between adjacent areas
Primarily originates from patient
Materials such as table and film tray also act as source of scatter
Radiation arising from sources behind x-ray film may be scattered back to image – called backscatter
Most cassettes contain lead backing to prevent this
Limiting size of x ray beam is most effective way to reduce scatter radiation
Scatter radiation intensity is affected by
Patient density
Patient size
Total volume of tissue irradiated
kVp of primary beam
The grid
Device composed of vertical alternating strips of lead and spacer material
Designed to absorb non-image forming x-rays (scatter radiation)
Is put in motion
Lead strips are aligned with primary x-ray beam so desirable x-rays reach film
Vertically to axis of table
Placed between patient and film
Grid may be placed directly on top of cassette, built into the cassette or placed under the table
Grids generally 2 cm larger than cassette
Grid ratios
Grid ratio is the height of the lead strips to the distance between the lead strips
Height : Distance between
This ratio should be stamped on the grid
Required to make a technique chart or adjust settings
Line pair is one vertical strip and one interspace strip
Expressed as line pairs/cm (lp/cm)
If too far apart will show up on film
In veterinary medicine we generally use a 8:1 or 10:1 grid ratio with 41 lp/cm
These two are known as grid efficiency
Focused vs parallel grids
Focused grid is made of lead strips which are angled to the same angle as the x-ray beam as it leaves the tube
These grids must be used at a specified SID in order to absorb scatter radiation without absorbing the primary beam
Parallel grid
The lead strips are all parallel on cross section rather than angled
Can only be used with very small x-ray fields or with long focal - grid distances
Grid focus
The lead strips of the grid may vary in size and angle but each grid has a center point called the focal point
The central x-ray must be centered on this point
The distance from the x-ray tube to the grid is called the grid focus and is specified on the grid by the manufacturer
Assumed to be 86-112 cm (equal to SID)
If the grid is used outside this range, grid cutoff may occur
Grid cutoff
Cutoff is a progressive decrease in x-ray intensity near the edge of the grid due to absorption of the primary x-rays by the lead strips within the grid
Image appears lighter, with distinct white lines over the underexposed areas of the film
There are 4 main reasons for grid cutoff:
Incorrect food distance
Improper centering
Angle of cassette not parallel to tube
Grid upside down
or any combination of above
Buck diaphragm
A mechanism that moves the grid across the x-ray beam at a uniform speed
The diaphragm holds a focused grid mounted under the table with the lead strips running parallel to the length of the table
Travels back and forth during exposure
This filters scatter radiation while eliminating grid lines from the finished radiograph
Care of grid
Grids are delicate and expensive
If a grid is dropped it can be damaged permanently
Once the lead strips have been warped a permanent artifact will be seen with that grid
Buckys tend to be well protected and need little care other than mechanical breakdown
Geometric unsharpness can be due to
Motion
Patient motion is the most common artifact in veterinary radiography
Distortion
Magnification
Large focal spot size
Always use smallest focal spot
Improper SID
Screen-film combinations
Motion
Resolution is the ability to differentiate fine details within an x-ray image
Resolution is destroyed by motion
Motion can be reduced by
Sedation
Waiting a few extra seconds while the patient relaxes
Proper restraint techniques
Proper exposure times
Distortion
The foreshortening or elongation of a body part because of angulation
Not being parallel to the cassette, x-ray tube not perpendicular etc.
Elongation will occur if the primary beam is centered on one end rather than in the middle of the object
A longer or stretched out image is produced
Foreshortening will occur if one end of the object is close to the cassette and the other end is farther away
A shorter or stubby image is produced
Useful distortion is used in dental radiography
Magnification
If the region of interest is raised above the image receptor it will cause magnification
The further away from the cassette the more magnified the image becomes and the more indistinct the detail due to blurring of the edges
To prevent magnification, the subject must be close to the cassette/image receptor
