Detail And Distortion Flashcards
What are the four factors that affect radiographic image quality?
Photographic -(optical) density -contrast Geometric -detail -distortion
Resolution
Defined as: the ability of an imaging system to resolve or distinguish between two adjacent structures
- how recorded detail is measured and expressed
- is the combination of spatial and contrast resolution
- spatial resolution
- the ability to image small objects that have high subject contrast
- smallest object that can be seen/detected
- contrast resolution
- ability to distinguish between two objects with similar subject contrast
Line pairs
Consist of the line and the space
-line pair= line+space
Detail
Controlled by three factors:
- geometric unsharpness
- motion unsharpness
- image receptor unsharpness
Factors that affect geometric unsharpness
Focal spot size
Distance
-SID
-OID
Resolution
Resolution test pattern
- device to record and measure line pairs
- the more line pairs visualized, the more resolution and recorded detail present
Focal spot size
The only thing that focal spot size affects is recorded detail on the image
Focal spot blur
- occurs because the effective focal spot is not a single point
- rectangular source
- blurred region on radiograph is a result of the focal spot having size, not just a point source
- aka penumbra
- undesirable
- most important factor for spatial resolution
- High contrast objects that are smaller than the focal spot blur cannot be imaged
- geometric relationships that govern magnification influence blur
- if conditions are altered to increase magnification on the image, focal spot blur will also increase
- to decrease the effects of focal spot blur, and increase recorded detail:
- smallest focal spot possible
- SID as large as possible
- OID as small as possible
Anode heel effect
- variation in the size of the effective focal spot leads to variation in the focal spot blur
- caused by the attenuation of x-rays in the heel of the anode
- significant when x-ray tubes with small target angles are used at short SIDs
SID
- when SID is increased, image unsharpness decreases which results in: increased image detail
- with a longer SID, the diverging rays become more perpendicular to the object, that is what decreases the unsharpness
OID
- also has an effect on the unsharpness of the image
- when exit beam leaves the patient, it continues to diverge
- with more distance to diverge, more unsharpness will occur
- the most optimal OID would be 0 but this is impossible in diagnostic imaging
- distance between area of interest and image receptor (OID) have the greatest effect on the amount of geometric unsharpness recorded
- more than SID or focal spot size
Geometric unsharpness
- minimizing is important, but as a tech you must consider the effects of these variables
- small focal spot (not suitable for many exams, especially large parts)
- large SID (not practical for many exams)
- minimal OID (difficult to achieve, however, if there is extra OID present, can compensate slightly by increasing SID)
Image receptor
⬆️ in speed = ⬆️ in unsharpness
Film/screen system
- factors that affect recorded detail:
- screen properties
- phosphor size
- phosphor layer thickness
- phosphor concentration
- film screen contact
Phosphor size
- large phosphor size results in the information being spread out and will decrease recorded detail
Film screen contact
(Has to do with cassettes)
- film is sandwiched between two intensifying screens
- if there is a space, it creates an area of unsharpness
- wire mesh test
Distortion
Results from radiographic misinterpretation of an object
- either size or shape
- size distortion (magnification)
- shape distortion (elongation and foreshortening)
Distortion and technologists
Distortion is directly related to positioning. We can minimize distortion with careful attention to:
- distances
- central ray
- patient position
- IR position
Size distortion
Defined as: an increase in the size of an objects image as compared to its actual size
- images are always magnified in comparison to their true size (due to OID)
- SID and OID play a huge role in affecting size distortion
OID
- directly related to magnification
- has the greatest effect on magnification of all the factors
- some parts of the object will have more OID than other parts
Magnification factor
How we calculate size distortion:
MF= (SID/SOD)
(SOD=SID-OID)
OR
MF= (image size/object size)
Shape distortion
- when a radiograph misrepresents an object with distortion of its shape
- two forms:
1. Elongation (image appears longer than it actually is)
2. Foreshortening (image appears shorter than it actually is) - can occur from inaccurate central ray alignment of the tube
- inaccurate alignment of the part
- inaccurate alignment of the image receptor
- any misalignment of those factors will result in distortion
- can be used to our advantage with scaphoid
Shape distortion continued
- object thickness
- OID changes across the object
- more distortion with a thick object than a thin one
- more magnification over areas with more OID
- angling the tube could “throw” anatomy differently on the image
- could be advantageous
- the more irregular the shape, the more distortion will occur
- due to beam divergence
- to reduce superimposition of anatomy
Minimizing shape distortion
To minimize shape distortion, ensure that there is proper alignment of:
- x-ray tube
- part
- image receptor
- entry/exit point of the CR (central ray)
