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grids

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

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