Grids - Radiology - Lecture Slides, Slides of Radiology

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Grids

Purpose

 Directional filter for photons

 Ideal grid

 passes all primary photons

 photons coming from focal

spot

 blocks all secondary photons

 photons not coming from focal

spot

Film

Patient

“Good” photon

“Bad” photon X Grid

Focal Spot

Grid Ratio

 Ratio of interspace height to width

h

w

Grid ratio = h / w

Lead Interspace

Grid Ratio

 Expressed as X:

 Typical values

8:1 to 12:1 for general work

3:1 to 5:1 for mammography

 Grid function generally improves with higher

ratios

h

w

Grid ratio = h / w

Grid Structure

Grid Patterns

 Orientation of lead strips as seen from above

 Types

 Linear

 Cross hatched

 2 stacked linear grids

 ratio is sum of ratios of two linear grids

 very sensitive to positioning & tilting

 Rare; only found in specials

Parallel Grid

 lead strips parallel

 useful only for

 small field sizes

 large source to

image distances

Focused Grid

 Slightly angled lead strips

 Strip lines converge to a point in

space called convergence line

 Focal distance

 distance from convergence line to grid plane

 Focal range

 working distance range

 width depends on grid ratio  smaller ratio has greater range

Focal range

Focal distance

Ideal Grid

 block all scattered radiation

 Reality: lead strips permit some scatter to get

through to film

Lead

Interspace

Primary Transmission

 Fraction of a scatter-free beam passed by

grid

 Ideally 100% (never achieved)

Lead

Interspace

Bucky Factor

Radiation incident on grid

transmitted radiation

 indicates actual increase in exposure because

of grid’s presence

 due to attenuation of both primary & secondary

radiation

Bucky Factor

 Measures fraction of radiation absorbed by grid

 high ratio grids have higher bucky factors

More Lines / inch at Same Ratio Means

Less Lead Content & Less Contrast

Improvement

 thinner lead & same

ratio

 less lead (less thickness,

same height)

 Same interspace dimensions

 same contrast

improvement for 133 line

10:1 and 80 line 8:1 grids

h

d

Grid ratio = h / d

Grid Disadvantages

 Increased patient dose

 Positioning critical

 poor positioning results in grid cutoff

 loss of primary radiation because images of lead strips

projecte wider