Grids

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Grids

RTEC 244

2 Dr. Gustave Bucky

Since Dr. Gustav Bucky built the first grid in 1913,

his original principle of lead foil strips standing on edge separated by x-ray transparent interspacers has remained one of the best-known techniques to trap the scatter

CROSSHATCH GRID

Potter-Bucky Diaphragm

Dr. Hollis Potter made improvements to the use of grids Realigned lead strips

to run in one direction Moved grid during

exposure to make lines invisible on image

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Creating the Image Transmission

Responsible for dark areas Absorption

Responsible for light areas Scatter

Creates fog Lowers contrast

Increases as kV increases Field size increases Thickness of part

increases Z# decreases

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Grid Selection Patient Dose Exam Detail required Part thickness Desired technique

(kVp) Equipment availability

Indications for Grid Use

• Part thickness > 10 cm• kVp > 60• EXCEPTIONS?

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Grid is placedbetween patient (behind table or upright bucky) & cassetteIf placed BACWARDS CAN CAUSE GRID ERRORS

6What are some factors that increase scatter radiation?

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8Grids “clean up” scatter radiation

A high quality grid can attenuate 80 –90 % of scatter radiation

3 factors contribute to an increase in scatter

Increased kVp Increased x-ray field size Increased patient

thickness

9Ideally, only those x-rays that do not interact with the patient should reach the IR….

However, scatter radiation is a factor that must be managed

Proper collimation has the PRIMARY effect of reducing patient dose by _________ ?

Proper collimation also improved image contrast by reducing radiographic noise or fog caused by scatter

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How does increasing kVp affect patient dose?

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Basic Grid Construction• Radiopaque lead strips• Separated by

radiolucent interspace material– Typically aluminum

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Grids Allow primary

radiation to reach the image receptor (IR)

Absorb most scattered radiation

Primary disadvantage of grid use Grid lines on film

13CASSETTES W/ GRID CAPSSTATIONARY GRIDS

Stationary gridsGrids that can be attached to a cassette for use or a specially designedGrid cassettes

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Grid Dimensions• h = the height of the

radiopaque strips• D = the distance

between the strips – the thickness of the

interspace material

Height of lead strips divided by thickness of interspacing material

Grid ratio = h/D

GRID RATI0 The distance between lead strips may remain

constant so the thickness of the grid must increase as grid ratios increase. It is possible to appreciate

the smaller angle of deflection of the x-ray photon that will pass through the 16:1 ratio grid.

Thus, high ratio grids usually "clean-up the beam," removing scatter radiation more effectively than low ratio grids.

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Grid Ratio Higher grid ratio

More efficient in removing scatter

Typical grid ratio range is 5:1 to 16:1

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Grid Ratio Example If a grid has an

interspace of 0.5mm and lead strips that are 3mm high, what is it's grid ratio?

GR = 3mm/0.5mm

GR = 6:1

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Grid Frequency The number of lead

strips per inch or cm Frequency range

60-200 lines/in 25-80 lines/cm

Typically higher frequency grids have thinner lead strips

Higher frequency with the same interspace distance reduces the grid effectiveness

18The 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%

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Digital Imaging Systems Very high-frequency grids

103-200 lines/in 41-80 lines/cm

Recommended for use with digital systems Minimizes grid line appearance

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Air-Gap Technique Or Air filtration Increase OID by 10 to 15 cm This reduces the amount of scatter reaching the

IR because some scatter will miss the IR. It is about the same as using an 8:1 grid mAs is increased 10% for every cm of air gap Increases magnification and reduction in detail.

Has some selective uses with chest imaging and cerebral angiography

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Air-Gap Technique

An alternative to grid use 10” air gap has similar

clean-up of 15:1 grid

Problems: Increased OID = increase in blur Must increase SID Motion due to lack of

contact to IR

AIR GAP. Drawing illustrates the tube, object, grid

and film relationship in conventional radiography and the use of an air gap to decrease the effect of scatter radiation. Note that an increase in the FFD tends to decrease the magnification of the image on the film.

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Grid Patterns Criss-cross or cross-hatched Linear

Parallel Focused

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Linear Grid Lead strips run the length of cassette Allows primary beam to be angled along the

long axis of grid without obtaining “cut-off”

24Focused Linear Grids Lead strips are angled

to match divergence of beam Primary beam will align

with interspace material Scatter absorbed by lead

strips

Convergence line Narrow positioning latitude

Improper centering results in peripheral cut-off

Only useful at preset SID distance

Higher ratio grids require careful alignment with tube

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Parallel Linear Grids All lead strips are

parallel to one another Absorb a large amount

of primary beam Resulting in some cut-

off

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Grid Use & Movement Potter-Bucky diaphragm

The Bucky Mounts a 17” x 19” grid above cassette Moves the grid during exposure

Reciprocating Motor drives grid back and forth during

exposure Oscillating

Electromagnet pulls grid to one side Releases it during exposure

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28Grids and Exposure Factors Whenever a grid is placed in beam to

remove scatter Density of radiograph will go down Exposure factors must be increased to

compensate for lack of density

Required increase in technique can be calculated Grid conversion (GCF) or Bucky factor

GCF = mAs with grid mAs without grid

29GRID CONVERSION FACTORSNO GRID 15:1 26:1 38:1 410/12:1 516:1 6

CHANGES IN MAS

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Give It a Try! Original: 20mAs with an 8:1 grid Find new mAs with a 12:1 grid

mAs2 = 20 mAs x 5 4 mAs2 = 100 4 mAs2 = 25

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

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Selectivity “K” factor Describes grid’s ability to allow primary

radiation to reach image receptor and prevent scatter

Grids are designed to absorb scatter Sometimes they do absorb primary radiation

Compares radiographic contrast of an image with a grid to radiographic contrast of an image without a grid

Typically ranges between 1.5 – 3.5

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Grid Errors

Off-level Off-center Off-focus Upside-down Moire effect

Proper alignment between x-ray tube and grid Very important

Improper alignment will result in cut-off

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Grid Problems Increased OID, especially with moving

grids The biggest problem with grids is

misalignment

GRID PROBLEMS RESULT IN:UNDEREXPOSED IMAGEOR UNDEREXPOSEDEDGES OF IMAGE

36Grid Problems – Off Level

CUT OFF MORE SEVERE ON ONE SIDE THAN THE

OTHER

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Grid Problems – Off Center

A problem with focused & crossed grids

38Grid Problems – Off Focus (wrong SID)

CUT OFF EQUAL ON BOTH SIDES

39Grid Problems – Upside-Down

A problem with focused grids = severe cut off on both edges

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STATIONARY GRID/NOT MOVINGGRID CUT OFF - EVENLY ACROSS

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GRIDS CAN

LEAVE LINES

ON THE IMAGE

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CR GRIDS

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REG GRID VS DR GRIDWhat is this called?

What causes this?

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Moire Effect Digital systems

When grid lines are parallel to scan lines

High frequency grids can prevent this phenomenon

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Moire Effect

PMT

Beam deflector

LaserSource

Light channeling guide

Plate translation: Sub-scan direction

Laser beam: Scan direction

Output Signal

Reference detec tor

Beam splitter

Cylindrical m irrorf-thetalens

Amplifier

ADC

To imageprocessor

When grid lines are parallel to scan lines

46DR USES HIGER FREQUENCY GRIDS

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