Density/Image Receptor Exposure in Radiography

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Density/Image Receptor Exposure

INTRODUCTION

-Radiographic images require sufficient density/IR exposure (and contrast) to permit visibility of structural details

-Proper densities should be visualized throughout the anatomical area of interest

-The amount of black metallic silver remaining on a film after processing

-Digital images allow for post-processing; adjusting window level, will change monitor brightness

-Photographic property of density is termed density/IR exposure

ASSESSING DENSITY/IR EXPOSURE

Effects on image appearance

-The effects of technical factors and other influencing factors on density/IR exposure is not an exact science due to the multitude of variables in imaging systems

  • Consider film/screen technology compared to digital and their respective response curves

-Challenge is to determine how to change technical factors to produce a visible change in density/IR exposure that still falls within acceptable limits

Factors affecting density/IR exposure

Milliampere-Seconds as the Controlling Factor

-There is a direct relationship between mAs and IR exposure

*As mAs increases; x-ray exposure (intensity) increases and image density increases

– Recall the D log E curve of film; film density is the log10 of opacity and opacity is the relationship of incident light over transmitted light

*Along the straight-line portion, density is ~ proportional to log relative exposure; therefore a doubling of exposure will result in a doubling of ____________.

An increase in mAs does not mean an increase in penetration

An increase in mAs = increase in exposure = increase in film density; a linear relationship

A minimum 25 – 35% change in mAs is necessary to cause a noticeable change in film density however…

Generally mAs is changed (re-exposing the patient) by either halving or doubling the original value

-Direct square law (density maintenance formula)

*As distance increases, intensity decreases resulting in a decrease in density

Direct square law

Factors affecting density/IR exposure

Kilovoltage as an Influencing Factor

-An increase in kV = an increase in beam intensity and an increase in beam quality

-An increase in kV = increase in film density (not a linear relationship)

*4-5% change in kVp in 30 – 50 kV range

*8-9% change in kVp in 50 – 90 kV range

*10-12% change in kVp in 90 – 130 kV range

-Also, an increase in kV = increase scatter production which increases image density caused by fog

-The 15% rule is a rough guide to adjust and compensate for density/IR exposure changes

-“The 15% rule is somewhat accurate within the range of 60 to 100 kVp.”

*A 15% increase in kV causes a doubling of exposure to the IR therefore an increase in OD

*A 15% decrease in kV causes a halving of exposure to the IR therefore a decrease in OD

EXAMPLE of the 15% rule

A knee radiograph was obtained using 6 mAs and 75 kVp. What kVp is required to halve the exposure to the IR?

75 kVp x 15%

75 kVp x 0.15 = 11.25

75 – 11.25 = 63.75 → 64 kVp

-Will the contrast of the radiograph be affected? Explain.

FACTORS AFFECTING DENSITY/IR EXPOSURE

Other influencing factors

Focal spot size

-Focal spot blooming

*Focal spot size increases in direct proportion to the tube current

-Negligible when equipment is properly calibrated

Anode heel effect

-Alters the intensity of radiation between cathode and anode

*Thicker anatomy placed at cathode

Carlton P. 394 TABLE 26-2 Projections that may use the Anode Heel Effect to Advantage

Distance (SID and OID)

SID alters the intensity of the beam reaching the IR; Inverse Square Law (ISL)

mAs must be changed to compensate for a change in distance; Direct Square Law (DSL)

EXAMPLE of the DSL

A diagnostic image was produced using 4.5 mAs at 180 cm; what mAs will be required to maintain image quality at 100 cm?

mAs2 =   à  mAs2 = 1.4

Other influencing factors

Distance (SID and OID)

Variations in OID generally do not cause visible changes in density/IR exposure

 

Filtration

-Filtration and its ability to alter beam intensity affect density/IR exposure

-All types of beam filtration alter density/IR exposure; i.e. added filtration

-Density/IR exposure decreases when filtration is increased

Other influencing factors

Beam restriction

-Reducing the primary field size will reduce the total number of photons available

-The production of scattered radiation is reduced therefore the overall density/IR exposure is also reduced

-Generally, technical factors need to be altered, due to changes in density/IR exposure, under the following circumstances;

*Large anatomical part

*High kVp

*Low ratio grid

*Non-grid examinations

Other influencing factors

Anatomical part

-The patient is the primary beam attenuator and therefore has a great influence on density/IR exposure

-Amount of attenuation is dependent on; 1.Thickness of tissue

2.Tissue type: average atomic number and mass density

3.Presence of contrast media

4.Advanced pathology

*Depending on the type of contrast media (positive or negative) or the type of pathology (additive or destructive), an inverse or a direct relationship exists

5.Severe tube angles (> 15°)

*Special consideration when using severe tube angles, resulting in significant differences in tissue thickness across the beam

6.Cast material

*Adjustments for cast material should only be made if it increases the thickness of the body part when compared to the non-casted part

Other influencing factors

Grid construction

-The purpose of a grid is to absorb scatter; scatter radiation contributes to the density/IR exposure

-The following grid characteristics will reduce density/IR exposure;

*High ratio, low frequency, and dense interspace material

*Moving grids

*Improperly used grids

-When changing grid ratios, it is recommended to increase or decrease the mAs

-Note: Do not change the kVp because the benefit of the grid (improved image contrast) may be compromised

Grid conversion formula

 

GCF = Grid conversion factor

 

Example of grid conversion formula

A diagnostic abdomen radiograph was obtained using a 12:1 grid, 40 mAs, and 82 kVp. A second image is requisition using a 16:1 grid. What mAs is required to produce a satisfactory image?

mAs2 =   à mAs2 = 49

Other influencing factors

Image Receptors

-Film/screen systems are classified according to speed known as Relative Speed (RS)

-As RS increases, the amount of exposure required to maintain the same density/IR exposure decreases

-Film processing

*Density will increase with;

1.Increase in developer temperature

2.Increase in developer processing time / immersion time

3.Increase in developer replenishment rates

 

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