Analyzing the ImageAnalyzing the Image

DensityDensity

DensityDensity

Overall blackening of the image

Assessing DensityAssessing Density

Anatomical area of interest– OD 0.25-2.50

Darker image preferable to light image

Factors Affecting DensityFactors Affecting DensitymAsmAs

Controlling Factor

In the straight line portion of the sensitometric curve, density proportional to log relative exposure.

mAs can be adjusted to compensate for changes made in other technical factors.

DensityDensitymAs as Controlling FactormAs as Controlling Factor

Reciprocity Law– Density should remain the same as long as the

same mAs is used regardless of the mA and time combination

Reciprocity Law failure– Occurs at short exposure times (less than .01)

or long exposure times (several seconds)

DensityDensitymAs as Controlling FactormAs as Controlling Factor

Minimum change 30% in exposure– Either 30% change in mAs or other factors that equal 30%

change in exposure

For mAs changes, adjust in increments of 2 X or ½ mAs

BEWAREBEWARE

The “Step in Time”

DensityDensityKilovoltageKilovoltage

15% increase in kVp doubles the exposure to the film

15% decrease in kVp halves the exposure to the film– In the lower kVp range (30-50), a change of 4-5 % in kVp may be detectable; in the middle

range (50-90), a change of 8-9% may be necessary; in the higher range (90-130), 10-12% is necessary to detect change in density.

Density Density kVp – 15% RulekVp – 15% Rule

May vary up to 25% rule at high kVp

15% change in kVp will always change

Contrast!

DensityDensityInfluencing FactorsInfluencing Factors

Focal Spot Size – Properly calibrated equipment should not

exhibit change in density with a change in focal spot size.

DensityDensityInfluencing FactorsInfluencing Factors

Anode Heel Effect– The x-ray beam intensity may vary up to 45% from the

cathode to anode end of the beam– More noticeable

With small angle anodes With the collimator open wide

– Advantage Place thicker or denser body part under cathode end of tube

DensityDensityAnode Heel EffectAnode Heel Effect

Examples– Femur– Lower leg– Humerus– Forearm– Thoracic Spine– Lumbar spine

DensityDensityDistanceDistance

SID – Inverse Square Law– Intensity of radiation is inversely proportional

to the square of the distance from the source– Increase SID, decrease Density

I1 D 22

______ = ______

I 2 D 1 2

Density MaintenanceDensity MaintenanceFormula Formula

mAs1 D12

_____ = _____

mAs2 D22

DensityDensity

Filtration – Increase in added filtration decreases density

DensityDensityBeam RestrictionBeam Restriction

– Reduction of beam size reduces the production of scattered radiation, therefore reducing the amount of radiation reaching the film and reducing density.

– Dependent upon: Amount of scatter produced Efficiency of grid Amount of reduction

DensityDensityBeam RestrictionBeam Restriction

Usually not necessary to compensate for a restriction in beam size because the change in density is

less than 30% and therefore not noticeable.Compensation may be required:

Large patientHigh Kilovoltage techniquelow grid efficiency (or no grid)

DensityDensityAnatomical PartAnatomical Part

Greater tissue thickness, the less density. Radiopaque contrast media decreases density. Radiolucent contrast media increases density. Destructive pathology increases density. Additive pathology decreases density. Angled techniques decrease density

– Variable (depends on degree of angulation)

DensityDensityAnatomical PartAnatomical Part

Angled techniques decrease density– Variable (depends on degree of angulation)

DensityDensityGridsGrids

Grids absorb scattered radiation, therefore decreasing density.

Loss of density compensated for by increasing mAs.

Primary purpose of grid is to increase contrast

DensityDensityGridsGrids

Grid ConversionsNo grid to:GFC

– 5:1 2 MAS1 GCF1

– 6:1 3 -------- = -------

– 8:1 4 MAS2 GCF2

– 12:1 5– 16:1 6

DensityDensityFilm/Screen CombinationFilm/Screen Combination

As intensifying screen speed increases, density increases; to compensate mAs is decreased.

MAS1 RS2

----------- = -----------

MAS2 RS1

MAS2

DensityDensity

Film Processing should not affect density as long as processing parameters are maintained.

DensityDensity

Increase mAs + Decrease mAs -

Increase kVp + Decrease kVp -

Increase generator + Decrease generator –

Focal Spot Size 0 Anode Heel Effect 0

Increase SID - Decrease SID +

Increase filtration - Decrease filtration +

Inc. Beam restrict - Dec beam restrict +

Inc. part thickness - Dec. part thickness-

DensityDensity

Radiolucent CM + Radiopaque CM -

Additive pathology -Destructive pathology +

Increase grid ratio - Decrease grid ratio +

Increase F/S RS + Decrease F/S RD -

Increase Processing + Decrease Processing -

Increase CR angle - Decrease CR angle +