Topic 10: x-ray image quality

Question 1

Describe poor image quality

Answer 1

• Poor signal-noise ratio
• Poor spatial resolution
• Detract from extracting info

Question 2

Describe image geometry

Answer 2

• All radiographic images = larger than object X-rayed = magnification due to geometry
• Ideal =
  > Object parallel to X-ray bean + image receptor
  > Radiation beam at right angle to object
  > Long focus to receptor distance + small object to receptor distance
• Minimise distortion + magnification of unsharpness in image

Question 3

What is effect of object placement on image?

Answer 3

• As close to image receptor as possible
• Moving away from receptor = magnification increases = bigger image + magnifies unshapness
• Positioning patient = direct relationship to image quality

Question 4

Give formula for magnification in radiology

Answer 4

• M = magnification factor
• FRD = focus > receptor distance
• FOD = focus > object distance

Question 5

How much is the geometric unsharpness of an image?

Answer 5

• Less than 0.4 mm

Question 6

Give the formula for unsharpness

Answer 6

Ug = focal spot size x ORD / FRD
- Ug = unsharpness
- ORD = object > receptor distance
- FRD = focus > receptor distance

Question 7

What factors increase unsharpness?

Answer 7

• Movement
• Resolution of monitor

Question 8

Describe magnification + distortion

Answer 8

• If object not parallel to image receptor = magnified
• Different aspects = elongated/shortening
• Deliberately elongated = aid fracture diagnosis

Question 9

Describe the practical distances for imaging

Answer 9

• ORD = as short as possible = in contact if possible
• FRD = 100 cm when X-ray table + 180 cm when chest/cervical spine

Question 10

Describe signal-noise ratio

Answer 10

• Image quality assessed by ratio
• Signal = useful info imaged = derived from photoelectric interactions
• Noise = anything detracting from accessing image = derived from Compton scatter

Question 11

Describle signal-noise ratio for obtaining an image

Answer 11

• Image receptors = don’t have ability to determine originof scatter + have electrical noise
• High signal > noise = structures clear in image
• Signal =/< noise = structure disappear
• X-ray compromise between perfect signal + reducing noise

Question 12

What is a grid/bucky?

Answer 12

• Use it to enhance contrast = removes scattered photons before reaching detector
• Increased contrast = must be justified as grid = increase in dose

Question 13

What does an air gap do?

Answer 13

• If air gap between patient + receptor = less scattered X-ray intercepted
• Scattered X-ray from body = greater range of angles than transmitted photons

Question 14

Describe unsharpness

Answer 14

• Blurring = reduces image/diagnostic quality
• Sharpness characterized = modulation transfer function
• MTF = incorporate resolution + contrast

Question 15

How should practitioner determine quality of image?

Answer 15

• Depending on structures that need to be defined
• Image to determine bone position in cast = less resolution than original diagnostic = fine focus used
• If need short exposure time on patient likely to move = broad focus

Question 16

Describe spatial resolution

Answer 16

• Ability of imaging system = represent distinct anatomical features + distinguish neighbouring features
• Max SR = defined by pixel size + spacing

Question 17

Describe brightness

Answer 17

• Intensity of light + represents individual pixels of image
• Controlled by processing software + adjusted after processing
• Digital images = auto windowed = auto apply to data detected = best range of contrast + brightness
• But doesn’t check for over/underexposure = other checks necessary

Question 18

How is over/underexposure checked?

Answer 18

• Evaluation of exposure indicator
• Numbers allocated to image = in range of system used
• CR = 1700-2300
• DR = 200-800
• Evaluation of noise = underexposure
• Evaluation of burn through = overexposure
• If EI consistently higher = patient overexposed = very high dose

Question 19

Describe contrast

Answer 19

• Measure of relative brightness between 2 locations on image
• Beam quality + exposure important in film tech NOT in digital = 60/110 kV = similar contrast levels on monitor
• Natural contrast in body = varies depending on thickness of area
• Thorax = high contrast = large differences in density of structure
• Abdomen = low contrast = adjacent areas similar atomic mass

Question 20

What is the effect of scatter on contrast?

Answer 20

• Scattered > detector = reduces contrast of image
• Scatter doenst carry useful info + creates signal

Question 21

How to reduce production of scatter

Answer 21

• Close collimation to area of interest
• Displacement of body part

Question 22

How to prevent scatter reaching detector

Answer 22

• Using grid assembly
• Using air gaps