Fundamentals of Imaging This course will provide you with the principles involved in the formation and recording of the radiologic image in both conventional and digital imaging systems as well as the principles of image quality assessment. Control of Scatter Radiation

1. FUNDAMENTAL
S OF IMAGING
RAD 206
Control of Scatter Radiation

3. Contrast changes with the use of a grid
Less scatter radiation & less radiographic
noise – shorter scale = “better contrast”
With Grid No Grid

4. GRIDS

5. CONTRAST &
CONTRAST
RESOLUTION
Two devices are used to reduce Compton effect beam-
restricting devices and radiographic grids
Beam-restricting devices effects what reaches the patient.
Grids effect the remnant beam

6. CONTRAST &
CONTRAST
RESOLUTION
Contrast = the comparison of areas of light, dark and shades
of gray on the image
Contrast Resolution = the ability to image adjacent similar
tissues

7. BEAM-RESTRICTING
DEVICES
Are helpful to improve contrast resolution however the
inherent problem is they are placed between the source and
the patient. Even under the most favorable conditions, most
if the remnant x-rays are scattered.

8. EFFECTS OF SCATTER
RADIATION ON IMAGE
CONTRAST
Contrast is the degree of difference in OD between areas of
an image
If you could only capture transmitted, unscattered x-rays, the
image would be very sharp
The corresponding bone-soft tissue interface, would be very
abrupt, and therefore the image contrast would be high

9. GRIDS
Are very effective device for reducing scatter radiation
The grid is a series of sections of radiopaque material (grid
strips) alternating with sections of radiolucent material
(interspace material)
The grid is designed to transmit only x-rays that are traveling
in a straight line from the source to the IR

10. GRIDS “CLEAN UP” SCATTER
RADIATION
A high quality
grid can attenuate 80 –
90 percent of scatter
radiation

11. GRID STRIPS
Should be very thin and have high photon absorption
properties
Lead is most common
Tungsten, platinum, gold, and uranium have been tried but
Pb is still most desirable

12. INTERSPACE
MATERIAL
Used to maintain precise separation between the delicate
lead strips
Aluminum or Plastic Fiber
Grid Casing = covered completely by thin aluminum to
provide rigidity and to seal out moisture. Yuck!

14. GRID RATIO
3 important dimensions on a grid: The thickness of the grid
strips, the width of the interspace material, and the height of
the grid
The grid ratio is the HEIGHT of the grid divided by the
INTERSPACE WIDTH:
Grid ratio = h
D

15. H = HEIGHT OF THE GRID,
T = THICKNESS OF THE GRID STRIP,
D = WIDTH OF THE INTERSPACE MATERIAL

16. GRID RATIO
High-ratio grids are more effective in cleaning up scatter
radiation than low-ratio grids
The angle of deviation is smaller for high-ratio grids. (the
photon must be traveling in a straighter line to make it
through the grid)
However, the higher the ratio the more radiation exposure
necessary to get a sufficient number of x-rays through the
grid to the IR

17. THE HIGHER THE RATIO THE STRAIGHTER THE
PHOTON MUST TRAVEL TO REACH THE IR
Grid ratios range
from 5:1 to 16:1
Most common
8:1 to 10:1
A 5:1 grid will
clean up 85%
16:1 clean up 97%

18. GRID FREQUENCY
The number of grid strips or grid lines per inch or centimeter
The higher the frequency the more strips and less interspace
material and the higher the grid ratio
As grid frequency increases, patient does is increase
because more scatter will be absorbed

19. GRID FREQUENCY
Some grids reduce the thickness of the strips to reduce the
exposure to the patient, this over all reduces the grid clean
up
Grids have frequencies in the range of 25 to 45 lines per
centimeter (60 to 110 lines per inch)

20. HIGHER FREQUENCY WITH THE SAME
INTERSPACE DISTANCE REDUCES THE GRID
EFFECTIVENESS

21. GRID PERFORMANCE
The principal function of a grid is to improve
image contrast
Contrast Improvement Factor (k) = the ratio of
the contrast of a radiograph made with a grid
to the contrast of the radiograph made without
a grid. A contrast improvement factor of 1
indicates no improvements
The higher the grid ratio & frequency the
higher the k

22. Using grids require more patient dose. Why is this?
When a grid is used technique must be increased to maintain
OD
The amount of increase is given by the Bucky factor (B) or
grid factor
BUCKY FACTOR OR GRID FACTOR

23. BUCKY FACTOR OR GRID FACTOR
The higher the grid ratio or frequency the higher the bucky
factor
The Bucky factor increases with increasing kVp
Pg 235: We will use the average values for calculations.

24. Selectivity or ability to “clean up”
the heavier the grid the more Pb it contains

25. GRID CUTOFF
Distance to
cutoff
SID
Grid ratio
With decreasing
SID more
potential
for grid cutoff
IR size will also

26. GRID TYPES
PARALLEL GRID
simplest type of grid
All the lead strips are parallel
Only clean up scatter in one direction (along the axis of the
grid)
Inexpensive, Easy to make, however can cause noticeable
grid cutoff with short SID’s (the greater the SID the lesser
artifact seen) .

27. GRID CUTOFF FROM PARALLEL GRID
THE HIGHER-RATIO THE MORE CUTOFF POTENTIAL

28. FOCUSED GRID
Designed to minimize grid cutoff
Lead strips are aligned with the divergence of the x-ray beam
Each focused grid must be identified with the appropriate
SID
Wrong SID = Grid cutoff

29. Focused grid have a little SID latitude or tolerance
(e.g. 100cm Focused grid would produce artifacts
if used at less than 90cm – or more than 110cm)

30. CROSSED GRID
AKA crosshatch
Have lead strips running along the long and short axes of the
grid
Made by placing two parallel grid on top of each other

31. CROSSED GRID
Have twice the grid ratio as Parallel (linear) grids
However, CR vs grid placement
is critical. The CR must align
with the center of the grid and
the grid and CR must be exactly
parallel or grid cutoff will occur
e.g.

32. MOVING GRIDS
All stationary grids will give you grid lines on your
radiograph. Thinner Pb strips will give you less noticeable
lines. However, thinner strips have less Pb content not
“cleaning up” as well
Grid Lines are made when primary x-rays are absorbed in the
grid strips.

33. FOCUSED GRIDS ARE USUALLY
USED AS MOVING GRIDS
The grid is placed in a holding mechanism that begins
moving just before the x-ray exposure and continues moving
after the exposure ends
2 types of movement Reciprocating & Oscillating
Reciprocating: moving back and forth about 2 cm
Oscillating: moving in a circular motion 2-3 cm

34. GRID MOTION
Reciprocating = moves several times about 2cm back and
forth during the exposure
Oscillating = moves several times about 2 – 3 cm in a circular
pattern
Most grids are moving. Except for portable imaging

35. GRID PROBLEMS
Increased OID, especially with moving grids
The biggest problem with grids is misalignment
GRID PROBLEMS
RESULT IN:
UNDEREXPOSED IMAGE
OR UNDEREXPOSED
EDGES OF IMAGE
https://www.youtube.com/watch?
v=mZPD_gLs5Dw

36. GRID PROBLEMS – OFF LEVEL

37. GRID PROBLEMS – OFF CENTER
A problem with focused & crossed grids

38. GRID PROBLEMS – OFF FOCUS
(WRONG SID)

39. GRID PROBLEMS – UPSIDE-DOWN
A problem with focused & crossed grids

40. GRID SELECTION
Patient Dose
• Pg 241 – mAs changes
Exam
Detail required
Part thickness
Desired technique (kVp)
Equipment availability

41. Contrast Improvement by Using an Air Gap

44. QUESTIONS….?