6. FACTORS AFFECTING SCATTER
PRODUCTION WITHIN THE PATIENT
• KILOVOLTAGE
• FIELD SIZE
• THICKNESS OF IRRADIATED TISSUE
SCATTER IS PRODUCED WITHIN THE PATIENT
THROUGH COMPTON INTERACTION
7. GRID RADIOGRAPHY IS RECOMMENDED
FOR:
• ANATOMICAL PARTS > 10 cm
• WITH HIGH kVp ( NOT ALWAYS—MAMMO)
• SOFT TSSUE STRUCTURES TO INCREASE
CONTRAST
• STRUCTURES AFFECTED BY PATHOLOGICAL
CONDITION THAT WOULD INCRESE SCATTER
PRODUCTION
11. COMPARISON BETWEEN TWO TYPES
OF INTERSPACE MATERIALS
ALUMINUM ORGANIC
1. STRUCTURALLY STRONGER
2. REMOVES MORE SECONDARY
RADIATION- MORE IMAGE
CONTRAST
3. REMOVES MORE PRIMARY
RADIATION- MORE PATIENT
EXPOSURE
1. STRUCTURALLY WEAKER
2. REMOVES LESS SECONDARY
RADIATION
3. REMOVES LESS PRIMARY
RADIATION
12. IDEAL GRID
REMOVES 100% OF SCATTER RADIATION
PASSES ALL PRIMARY RADIATION
IN REALITY THERE IS NO SUCH IDEAL GRID
13. GRID RATIO
• HEIGHT OF LEAD STRIPS(h) DIVIDED BY
DISTANCE BETWEEN EACH LEAD
STRIPS(D)
GR =h/D
HIGHER THE RATIO-
a. INCREASED IMAGE CONTRAST
b. MORE EXPOSURE 2 PATIENT
c. MORE EFFECT OF GRID CUT-OFF
15. GRID PATTERN
ORIENTATION OF LEAD STRIPS IN THEIR
LONGITUDINAL AXIS
a. LINEAR b. CROSSED
GRID RATIO OF CROSSED GRIDS ═
SUM OF THE RATIOS OF THE TWO
LINEAR GRIDS
16. GRID STYLES
FOCUSSED GRID- LEAD STRIPS ARE ANGLED
SLIGHTLY; THEY FOCUS IN SPACE
• CANTING- TILTING OF THE LEAD STRIPS TO
CREATE FOCUSSED GRID
• MOST GRIDS ARE FOCUSSED
• CONVERGENT LINE
• CONVERGENT POINT
• FOCAL DISTANCE
17. • FOCUSSING RANGE- WIDE FOR LOW-RATIO
GRID AND NARROW FOR A HIGH RATIO GRID
PARALLEL GRID- LEAD STRIPS ARE PARALLEL
• USED WITH VERY SMALL X-RAY FIELD OR
LONG TARGET-GRID DISTANCE
18. GRID FREQUENCY
MEASURED BY LINES PER INCH
Typical: 103 (NORMAL RANGE- 60 TO 200)
25.4
Lines per inch = ------------
W + w
w = thickness of interspace (mm)
W = thickness of lead strips (mm)
w
W
19. LEAD CONTENT
• MORE LINES/INCH AT CONSTANT GRID RATIO
MEANS LESS LEAD CONTENT AND LESS
CONTRAST IMPROVEMENT
21. PRIMARY TRANSMISSION
MEASUREMENT OF THE PERCENTAGE OF
PRIMARY RADIATION TRANSMITTED
THROUGH THE GRID
P.T = (INTENSITY WITH GRID/
INTENSITY WITHOUT GRID)×100
P.T-IDEALLY 100%
22. PRIMARY TRANSMISSION
• Typical values: 55 - 75%
• Theoretic calculation: (fraction of grid that is
interspace)
Tp (%)= 100 X W / (W+w) where
W = Interspace thickness
w = lead strip thickness
• actual transmission < theoretical
– primary attenuated by
interspace material
– focusing imperfections
w
W
W+w
23. BUCKY FACTOR
RATIO OF THE INCIDENT RADIATION FALLING
ON THE GRID TO THE TRANSMITTED
RADIATION PASSING THROUGH THE GRID
B.F= INCIDENT RADIATION/
TRANSMITTED RADIATION
24. • INDICATES ACTUAL INCREASE IN EXPOSURE
DUE TO GRID’S PRESENCE
• DUE TO ATTENUATION OF BOTH PRIMARY
AND SECONDARY RADIATION
• HIGHER GRID RATIO = HIGHER BUCKY FACTOR
• TYPICAL VALUE : 3-6
25. CONTRAST IMPROVEMENT FACTOR
ULTIMATE TEST OF GRID’S
PERFORMANCE
C.I.F= CONTRAST WITH A GRID/
CONTRAST WITHOUT A GRID
HIGHER THE GRID RATIO, HIGHER THE
C.I FACTOR
26. GRID CUT OFF
LOSS OF PRIMARY RADIATION THAT
OCCURS WHEN THE IMAGES OF THE
LEAD STRIPS ARE PROJECTED WIDER
THAN THEY WOULD BE WITH
ORDINARY MAGNIFICATION
27. TYPES
FOCUSSED GRIDS USED UPSIDE DOWN
LATERAL DECENTERING
DISTANCE DECENTERING
COMBINED DECENTERING
28. FOCUSSED GRID UPSIDE
DOWN
DARK BAND OF CENTRAL
EXPOSURE
SEVERE CUT-OFF AT
PERIPHERY
. CROSSED GRID- SMALL
SQUARE AT THE
CENTRE IS EXPOSED
29. LATERAL DECENTERING
RESULTS FROM THE X-RAY TUBE BEING
POSITIONED LATERAL TO THE
CONVERGENT LINE BUT AT THE
CORRECT FOCAL DISTANCE
PROBABLY MOST COMMON TYPE OF
GRID CUT-OFF
30. LATERAL DECENTERING
uniform loss of
radiation over entire
film
uniformly light
radiograph
no recognizable
characteristic
(dangerous)
31. LATERAL DECENTERING
• also occurs when grid
tilted
• Magnitude depends upon-
grid ratio
focal distance
amount of decentering
32. LATERAL DECENTERING
• Significant problem in portable
radiography
• Because exact centering not possible
• minimizing lateral decentering
– low ratio grids
– long focal distances
33. DISTANCE DECENTERING
• TARGET OF THE X-RAY TUBE IS
CORRECTLY CENTERED TO THE GRID
BUT POSITIONED ABOVE OR BELOW
THE CONVERGENT LINE
• FAR AND NEAR DECENTERING-
CUTOFF GREATER IN NEAR
• ALL PARALLEL GRIDS HAVE SOME
DEGREE OF DISTANCE DECENTERING
34. • Far focus-grid
decentering
•Near focus-grid
decentering
•cutoff at
periphery
•dark center
•cutoff
proportional to
•grid ratio
•decentering
distance
35. COMBINED LATERAL AND FOCUS
DECENTERING
MOST COMMONLY
RECOGNIZED
• UNEVEN
EXPOSURE-
FILM IS LIGHT ON
ONE AND DARK ON
OTHER SIDE
36. MOVING GRIDS
• INVENTED BY DR. HOLLIS E. POTTER IN 1920
• MOVE TO BLUR OUT SHADOWS CAST BY LEAD
STRIPS
• MOVES 1 TO 3 CMS BACK
FORTH THROUGHOUT THE
EXPOSURE
38. PRECAUTIONS
• GRID MUST MOVE FAST ENOUGH TO
BLUR ITS LEAD STRIPS
• TRANSVERSE MOTION OF THE GRID
SHOULD BE SYNCHRONOUS WITH THE
PULSES OF THE X-RAY GENERATOR
