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Principles of Imaging Science II (RAD 120)

Radiographic Grids

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Image-Forming X-Rays

Four X-ray paths • a. X-rays interact with patient and

scatter away from the receptor

• b. X-rays interact and are absorbed (photoelectric absorption) within patient

• c. X-rays are transmitted through patient without interaction and strike receptor

• d. X-rays interact with patient (Compton scatter) and scatter towards

• C and D are referred to the image- forming x-ray photons

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Radiographic Grid

• Used to reduce scatter radiation from reaching the image receptor (IR) through absorption

• Cleans up scatter radiation

• Inherent part of bucky, placed between the patient and IR

• Table or upright bucky usage

– >60 kVp, 10 cm tissue

• When primary x-rays interact

with the patient, x-rays are scattered from the patient in all directions.

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Image Contrast • Difference in optical

density between adjacent structures

• High vs Low contrast

– Skeletal anatomy

– Abdomen, Chest

High

Medium

Low

Radiographs of a cross section of long bone. A, High

contrast would result from the use of only transmitted,

unattenuated x-rays. B, No contrast would result from the

use of only scattered x-rays. C, Moderate contrast results

from the use of both transmitted and scattered x-rays. 4

Grid Design

• Radiolucent interspace material with alternating radiopaque strips – Aluminum, plastic or carbon fiber

for interspace

– Lead, tungsten, platinum, gold strips

• Transmits x-rays traveling in a straight line, oblique x-rays absorbed by strips

The only x-rays transmitted through a grid

are those that travel in the direction of the

interspace. X-rays scattered obliquely

through the interspace are absorbed.

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% Grid Surface X-ray Absorption

• Formula applied to determine the percentage of x-rays exiting the body that will be absorbed

• Based upon grid design – Lead strip width and interspace width

– Higher % yields > absorption

Surface area

of grid 6

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Application

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Grid Ratio

• Grid Ratio is the height of the grid strip (h) divided by the thickness of the interspace material (D). T = strip width. – Grid Ratio = h/D

• Affected by changing – Height of lead strips

– Thickness of strips

– Width of interspace

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Grid Ratio

• High ratio grids absorb more scatter yet require higher mAs or kVp

– mAs is factor of choice

• 5:1, 6:1, 8:1, 10:1, 12:1, 16:1 ratio designs

High-ratio grids are more effective than

low-ratio grids because the angle of

deviation is smaller.

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Grid Ratio Application

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Grid Frequency

• Number of strips or grid lines per inch or cm – 25 – 45 lines/cm, 60 – 110

lines/in – 25 – 80 lines/cm, 60 – 200

lines/in

• Higher grid frequency requires higher technique – Less grid lines appear in image – Often used in mammography

• 80 lines/cm, 200 lines/in

• Typically higher frequency grids have thinner lead strips

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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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Lead Content of Grid

• Lead content

– Most important factor in grid’s efficiency

– Measured in mass per unit area

• g/cm2

– High ratio grids tend to have highest lead content

– As lead content increases, removal of scatter increases and therefore contrast increases

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Application

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Grid Performance

• Contrast Improvement Factor (k)

– Comparison of image contrast with a grid to image contrast without a grid

– k is higher for higher ratio grids

• K = Radiographic contrast with grid

Radiographic contrast without grid

• Measured at 100 kVp using a step wedge

• Manufacturer Avg 1.5 – 2.5 – Use of a grid approximately doubles the contrast

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Grid Performance • Bucky Factor

– Higher technique required with grid usage

• mAs X bucky factor avg

– Measurement of technical factor and patient dose increase based upon penetration of primary & scatter radiation through the grid

– Bucky factor increases with increased grid ratio and increased kVp

Grid Ratio

70 kVp

90 kVp

120 kVp

Avg

Non-Grid

1 1 1 1

5:1 2 2.5 3 2

8:1 3 3.5 4 4

12:1 3.5 4 5 5

16:1 4 5 6 6

Approximate Bucky Factor Values

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Approximate Exposure Factor Changes Necessary

for Standard Grids

Grid Ratio mAs Increase kVp Increase

Non-Grid 1X 0

5:1 2X 8 – 10

6:1 3X 11 – 12

8:1/10:1 4X 13 – 15

12:1 5X 20 – 25

16:1 6X 30 – 40

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Grid Types Linear Parallel Linear Focused

• Linear/Parallel – Vertical lead strips do not

coincide with the primary beam

– Absorption of 10 beam (Grid cut-off) occurs with:

• Short SID

• Large IR

A parallel grid is constructed with parallel

grid strips. At a short source-to-image

receptor distance (SID), some grid cutoff

may occur.

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Parallel Grid

With a parallel grid, optical density

(OD) decreases toward the edge of the

image receptor. The distance to grid

cutoff is the source-to-image receptor

distance (SID) divided by the grid ratio. 19

Grid Types

• Linear/Focused

– Angled lead strips to coincide with primary beam divergence

– Focal distance set to SID usage to minimize grid cut-off

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Comparison of Transmitted Photons Parallel & Focused Grids

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Grid Focus

Convergent Line.

Imaginary lines drawn above a

linear focused grid from each

lead strip meet to form a

convergent point. The points

form a convergent line along the

length of the grid.

Convergent Point.

The convergent line or point of a

focused grid falls within a focal

range.

Grid Types

• Crossed (Criss-Cross, Cross-Hatched)

– 2 parallel grids perpendicular

– Not common

– High Grid cut-off if off-centered to CR

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Grid Types

• Moving grid mechanism – Upright or table bucky activation

– Eliminate grid lines from image

• High frequency = less grid lines possible

• Low frequency = more grid lines possible

– Reciprocating

• 2 cm movement transversely

• Motor drives grid back and forth during exposure

– Oscillating

• Circular movement 2-3 cm movement

• Electromagnet pulls grid to one side

• Releases it during exposure

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Grid Types

• Grid Cap

– Permanently mounted grid, IR slides into device

– Used with a variety of IR sizes

• Grid Cassette

– Permanently mounted grid

– Specific grid sizes

• Wafer Grid

– Non-permanent grid mount, must be secured

– Specific grid size

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Long vs. Short Dimension Grids

• Orientation of lead strips for a long- and short-dimension grid.

Grid Types

• Air-Gap (Air Filtration) – Common on dedicated

Chest X-ray units

– Part is @10-15 cm from IR ( 4” - 6”)

– Similar to 8:1ratio grid

• 10” air gap equivalent to 15:1 ratio grid

– mAs increased 10% per cm gap

– Magnification results unless SID is increased

6”

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Grid Cutoff

• A decrease in the number of transmitted photons that reach the image receptor because of some misalignment of the grid

• Grid Errors

– Off-level grid

– Off-center grid

– Off-focus grid

– Upside-down focused grid

Grid Errors • Off-level

– Parallel & focused

– Decreased density across image

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Grid Errors

• Off-Center (lateral decentering)

– Focused Grid

– Decreased density across image

– Most common error

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Grid Errors • Off-focus

– Incorrect SID use

– Decreased density at edges of image • Direct relationship

– More critical with high ratio grid

• Upside-down

– Mobile radiography

– CR not directed to tube side

– Marked decreased density at edges of image and points lateral to CR

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Grid Cutoff – Off Level

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Grid Cutoff – Off Center

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Grid Cutoff – Off Focus

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Grid Cutoff – Upside Down Focused

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Grid Cutoff

Upside-Down Focused Grid Cutoff.

Radiograph produced with an

upside-down focused grid

Off-Center Grid Cutoff.

Radiograph demonstrating grid cutoff

caused by off-centering.

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Grid Errors - Moire Effect • Zebra pattern

• Caused by

– Similar grid frequency to laser scanning frequency in CR processing

– Using a grid cassette in the bucky tray

• Correct by

– Selecting a high grid frequency

– Use a moving grid mechanism

– Do not use two grids

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Grid Selection • Patient dose

• Kvp usage

• Scatter absorption

– <90 kVp 8:1 satisfactory

– >90 kvp >8:1 grids used

As grid ratio increases, transmission of scatter radiation decreases

faster than transmission of primary radiation. Therefore, cleanup of

scatter radiation increases. 38

Clinical Consideration in Grid Selection

Grid Degree of Scatter Removal

Off-Center latitude

Off-focus latitude

kVp

Comments

5:1 + Very Wide Very Wide Up to 80 Low cost; easy to use

6:1 + Very Wide Very Wide Up to 80 Low cost; mobile radiography

8:1/10:1 +/+++ Wide/Wide Wide/Wide Up to 100 General stationary exams

12:1 ++++ Narrow Narrow Over 110 Precise centering; usually fixed mount

16:1 +++++ Narrow Narrow Over 100 Precise centering; usually fixed mount

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Summary

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