Resident Physics Lectures

Christensen, Chapter 8

GridsGrids

George DavidAssociate ProfessorDepartment of RadiologyMedical College of Georgia

PurposeDirectional filter for

photonsIdeal grid

passes all primary photons photons coming from

focal spotblocks all secondary

photons photons not coming from

focal spot Film

Patient

“Good” photon

“Bad” photon

GridX

FocalSpot

Grid ConstructionLead

~ .05“ thick upright strips (foil)

Interspacematerial between lead stripsmaintains lead orientationmaterials

fiber aluminum wood

LeadInterspace

Grid RatioRatio of interspace height to width

h

w

Grid ratio = h / w

LeadInterspace

Grid RatioExpressed as X:1Typical values

8:1 to 12:1 for general work3:1 to 5:1 for mammography

Grid function generally improves with higher ratios

h

w

Grid ratio = h / w

Lines per Inch# lead strips per inch grid widthTypical: 103

25.4Lines per inch = ------------ W + w

w = thickness of interspace (mm)

W = thickness of lead strips (mm)

w

W

Grid PatternsOrientation of lead strips as seen from

aboveTypes

LinearLinearCross hatchedCross 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

• Focused

Parallel Gridlead strips

paralleluseful only for

small field sizes

large source to image distances

Focused GridSlightly angled lead strips

Strip lines converge to a point in space called convergence convergence lineline

Focal distancedistance from convergence line to grid plane

Focal rangeworking distance range

width depends on grid ratio smaller ratio has greater range

Focal range

Focal distance

Ideal Gridpasses all primary radiation

Reality: lead strips block some primary

LeadInterspace

Ideal Gridblock all scattered radiation

Reality: lead strips permit some scatter to get through to film

LeadInterspace

Primary TransmissionFraction of a scatter-free beam

passed by gridIdeally 100% (never achieved)

LeadInterspace

Primary TransmissionTypical values: 55 - 75%Theoretic calculation: (fraction of grid

that is interspace)Tp (%)= 100 X W / (W+w) where

W = Interspace thicknessw = lead strip thickness

actual transmission < theoreticalprimary attenuated by

interspace material focusing imperfections

w

W

W+w

Bucky FactorRadiation 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 gridhigh ratio grids have higher bucky factors

Bucky FactorHigher bucky factor means

higher x-ray techniquehigher patient dose

typically 3-6

More Lines / inch at Same Ratio Means Less Lead Content & Less Contrast Improvementthinner lead & same

ratio less lead (less thickness, same

height)Same interspace dimensionssame contrast

improvement for 133 line 10:1 and 80 line 8:1 grids

h

d

Grid ratio = h / d

Grid Disadvantages

Increased patient dosePositioning critical

poor positioning results in grid cutoff loss of primary radiation because images of

lead strips projecte wider

Grid Cutofffocused grids used upside downlateral decentering (or angulation)focus- grid distance decenteringcombined lateral & focus-grid distance

decentering

Upside Down Focused Grid

Dark exposed band in center

Severe peripheral cutoff

Lateral Decenteringuniform loss of

radiation over entire film

uniformly light radiograph

no recognizable characteristic (dangerousdangerous)

Lateral Decenteringalso occurs when

grid tiltedboth result in

uniform loss of intensityno other clinical cluesmay be mistaken for technique

problemsCan be compensated for by

increasing patient exposure

Lateral DecenteringSignificant problem in portable

radiographyCompensate by over-exposing patient

exact centering not possibleminimizing lateral decentering

low ratio gridslong focal distances

Distance DecenteringGrid too close or too far from focal spotDarker centerAll parallel grids have some degree of

distance decenteringFocused to infinity

Far focus-grid decentering

•Near focus-grid decentering

•cutoff at periphery

•dark center•cutoff proportional to

•grid ratio•decentering distance

Minimizing Distance Decentering Cutoff

low grid ratio small fields

Combined lateral and focus-grid distance decentering

Easy to recognizeuneven exposurefilm light on one

side, dark on the other

Moving Grids

Motion starts with second trigger Grids move ~1- 3 inches

must be fast enough not to see grid lines for short exposures

Motion blurs out lead strip shadows

Moving Grid Disadvantages

$$$Vibration PotentialMay limit minimum exposure timeIncreases patient dose

lateral decentering from motion up to 20% loss of primary

evenly distributes radiation on film stationary grid makes interspace gaps darker for

same amount of radiation

Grid Tradeoff Advantage

cleanup / scatter rejectionDisadvantage

increased patient doseincreased exposure timeincrease tube loadingpositioning & centering more critical$$$

Grid Selectionuse low ratios for low kVp, high ratios for

high kVpbook recommends

8:1 below 90 kVp12:1 above 90 kVp

Air Gap Techniques

Principleradiation scatters uniformlydecrease in scatter (most

scatter misses film) air gap decreases angle of

capture; increases angle of escape

Negligible attenuation in air gap

Angles ofescape

Air Gapair gap very effective in removing scatter

originating closest to filmmuch of scatter nearest tube doesn’t reach

film

Much attenuationof scatter in the body

Air gap decreasescapture angle

Air Gap ApplicationsMagnification Radiography including

mammographygeometry causes air gap

Grid not used with air gap

Mammo Cellular Grid