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grids

grids
STUDY
PLAY
GRIDS
reduce scatter from getting to the IR, series of lead strips and radiolucent interspace material can absord 80-90 % of scatter
SHORT SCALE
high contrast
black and white image
lower kVp
major differences
LONG SCALE
low contrast
gray image
higher kVp
slight differences
WHEN TO USE A GRID
when body thickness is above 10 cm
techniques above 70 kVp
HOW A GRID WORKS
designed to transmit xrays in the direction of a straight line, scatter does not travel straight, are absorbed by the grid
GRID ABSORPTION FORMULA
T
___ T= thickness of lead
T + D x10 D= thickness of interspace
GRID RATIO
the height of the grid to the width of the interspaces in the grid
GRID RATIO FORMULA
h
-------
D
COMMON GRID RATIOS
8:1, 10:1, 12:1 common in general xray
5:1 mammo
higher the grid ratio, more clean up of scatter, increases pt dose
GRID FREQUENCY
The # of grid strips per centimeter or inch, higher frequency, less lines.
MOST COMMON GRID FREQUENCIES
25-45 lines per cm. 60-110 lines per inch
GRID FREQUENCY FORMULA
10,000
-----------------
T + D um / line pairs
CONTRAST IMPROVEMENT FACTOR
ratio of radiographic contrast with a grid to that without a grid
formula k= image contrast with a grid
image contrast without a grid
BUCKY FACTOR
using a grid requires increase in technique
higher grid ratio--higher BF
BF increases with increasing kVp
COMMON BUCKY FACTORS
non grid..........1 8:1...............4
3:1, 4:1...........2 10:1..............5
5:1, 6:1...........3 12:1, 16:1.......6
BUCKY FACTOR FORMULA
new mAs = new BF
------------------------------------------
old mAs = old BF
SELECTIVITY
ratio of primary radiation to scatter radiation transmitted through the grid. lead content the primary influence
GRIDS WITH HIGH RATIO
high contrast improvement factor
HIGH FREQUENCY GRIDS
low contrast improvement factor
PARALLEL GRIDS
simplest, has problems with grid cut off, clean up scatter in only one direction
CROSSED GRID
has strips that run parallel with long axis and short axs, centering is critical, cleans up a lot of scatter
FOCUSED GRID
designed to minimize grid cutoff , has a specific focal range
MOVING GRID
removes grid lines , blurring, uses focus grids2 types reciprocating and oscillating
OFF LEVEL GRID
grid must be per to CR, usually occurs from improperly positioned tubes, may occur when grid tilts during portables. If Cr is anglled grid cutoff will occur across the entire film
OFF CENTER GRID
grid per to IR, shifts laterally, grid cutoff occurs across the entire film
OFF FOCUS GRID
at the wrong focal distance, the farther the distance,the worse the cuttoff. Cut off is more severe toward edges of film. This is more important with high grid ratios
UPSIDE DOWN GRIDS
shows extreme cutoff on either side of the CR
GRID CUT OFF FORMULA
SID
-------------------------------------
GRID RATIO
AIR GAP TECHNIQUE
IR moved 10-15 cm from the pt, some of the scatter never reaches the IR. mAs increased 10% for every cm of air gap.not effective with high kVp tech, disadvantage: magnification