Detail And Distortion

Question 1

What are the four factors that affect radiographic image quality?

Answer 1

Photographic
-(optical) density
-contrast
Geometric
-detail
-distortion

Question 2

Resolution

Answer 2

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

Question 3

Line pairs

Answer 3

Consist of the line and the space

-line pair= line+space

Question 4

Detail

Answer 4

Controlled by three factors:

• geometric unsharpness
• motion unsharpness
• image receptor unsharpness

Question 5

Factors that affect geometric unsharpness

Answer 5

Focal spot size
Distance
-SID
-OID

Question 6

Resolution

Answer 6

Resolution test pattern

• device to record and measure line pairs
• the more line pairs visualized, the more resolution and recorded detail present

Question 7

Focal spot size

Answer 7

The only thing that focal spot size affects is recorded detail on the image

Question 8

Focal spot blur

Answer 8

• 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

Question 9

Anode heel effect

Answer 9

• 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

Question 10

SID

Answer 10

• 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

Question 11

OID

Answer 11

• 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

Question 12

Geometric unsharpness

Answer 12

• 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)

Question 13

Image receptor

Answer 13

⬆️ in speed = ⬆️ in unsharpness

Question 14

Film/screen system

Answer 14

• factors that affect recorded detail:
• screen properties
  
  • phosphor size
  • phosphor layer thickness
  • phosphor concentration
• film screen contact

Question 15

Phosphor size

Answer 15

• large phosphor size results in the information being spread out and will decrease recorded detail

Question 16

Film screen contact

Answer 16

(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

Question 17

Distortion

Answer 17

Results from radiographic misinterpretation of an object

• either size or shape
  
  • size distortion (magnification)
  • shape distortion (elongation and foreshortening)

Question 18

Distortion and technologists

Answer 18

Distortion is directly related to positioning. We can minimize distortion with careful attention to:

• distances
• central ray
• patient position
• IR position

Question 19

Size distortion

Answer 19

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

Question 20

OID

Answer 20

• 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

Question 21

Magnification factor

Answer 21

How we calculate size distortion:
MF= (SID/SOD)

(SOD=SID-OID)

OR

MF= (image size/object size)

Question 22

Shape distortion

Answer 22

• 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

Question 23

Shape distortion continued

Answer 23

• 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

Question 24

Minimizing shape distortion

Answer 24

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)