·
Scattered radiation detected by
a detector device be it digital or film-screen cause darkening (fog), but does
not add information content to the image.
·
Grids are used to combat the
effects of scatter on the image and thus improve contrast.
·
Disadvantage is that they
increase patient exposure/ dose
·
Placed between patient and the
detector and uses geometry to reduce the amount of scatter reaching the
detector
o
Primary radiation is orientated
in the same axis as the lead strips and passes through
o
Scatter is multidirectional and
is absorbed by the strips (fig 8-1 C:99; fig 6-25 BB:168)
·
2 basic types of grid pattern
(Orientation of the lead strips)
o
Linear
o
Crossed
o
The above may be focused
(converging) or parallel
·
Grid ratio = Height (mm)/
Interspaces (mm) – this is the single most important parameter affecting
performance
·
Grid frequency refers to the
number bars per unit length, but does not influence the scatter cleanup – but
is important in digital radiography as it may create aliasing problems
·
Parameters used to describe
grid properties:
|
Primary transmisssion
|
Fraction of primary photons that pass through the
grid; (I – intensity):
Tp=I with grid/I without
grid
Calc. Tp = Interspace (mm)/ Interspace (mm) + Strip (mm)
|
|
Bucky factor
High B – increases film quality;
High B – increases exposure and dose
|
Ratio of entrance exposure to the patient without and
with grid:
B = Incident rad. (without grid)/ transmitted rad. (with grid)
|
|
Contrast improvement factor
|
Contrast with grid vs. contrast without the grid:
K = contrast with/ contrast without
Depends on kVp; Thickness; Field size
|
|
Lead content
|
Measure of amount of lead in the grid;
May indicate ability to absorb scatter:
mg/cm2
|
·
Artifacts – grid cut-off
o
Focused grid upside down
o
Lateral decentering (off
center) and tilting (angulation)
o
Focus grid distance decentering
(off focus)
§ Near > Far
o
Combined lateral and focus-grid
distance decentering
·
Bucky grid:
o
Moves to blur out the shadows
cast by lead strips
o
Reciprocate – move back and
forth continuously throughout exposure
o
Important to move it very fast
o
Important to synchronize
transverse motion with pulses of the generator
