Imrc 181 Lo1 Density

8/8/2019 Imrc 181 Lo1 Density

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Density


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Attenuation

Defined as the reduction in the total number

of x-ray photons remaining in the beam after

passing through a given thickness of material Amount of Attenuation determined by

Type of irradiated material (atomic #)

Thickness of irradiated material

Density of irradiated material (quantity of matter perunit of volume measured kg/m3)


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Attenuation

Summarized Table

atomic # attenuation thickness attenuation

density attenuation


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Patient Relationship to Image

Quality

The patient impacts all properties affecting

radiographic quality : density, contrast,

recorded detail and distortion The relationship between these factors and

the patient are termed subject density,

subject contrast, subject detail and subject

distortion


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Quality

Subject density radiographic density will be

altered by changes in the amount or type of

tissue being irradiated Thicker and denser body parts absorb more radiation

therefore producing radiographic density (vise versa)


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Quality

Subject Contrast degree of differential

absorption resulting from the differing

absorption characteristics of the tissue in thebody If there is little difference in the absorption

characteristics of the given body tissues within the part

being examined, subject contrast will be low


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Quality

Subject Detail one of the primary factors

that affects the sharpness or detail of an

image is the distance between the structureof interest and the film The larger the patient, the greater the distance

between the anatomical structures and the film, results

in less sharpness in recorded detail


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Quality

Subject Distortion the misrepresentation of

the size and shape of the structure of interest The way certain structures lie within the body can

cause an inaccurate representation on the film


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Aspiration Pneumonia


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Bacterial Pneumonia


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Pathology

Destructive Conditions disease causes the

affected body tissue to decrease in thickness,

effective atomic # or density - there will be less

attenuation - radiographic density For examples refer to page 248 C&A


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Emphysema


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Emphysema


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Assessing Density

Density degree of overall blackening from the

silver deposited in the emulsion

Density is the easiest to evaluate and adjust

Evaluating density takes practice and knowledge

of evaluation criteria from Merrils


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Assessing Density

A dark radiograph has received too many

photons (has received too much info) can

sometimes be rectified with bright light, ifdigital images have been used adjustment

can be made digitally to lighten the film

Optical density range visible by humans is

0.25 2.50


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mAs Controlling Factor

As you mAs - exposure s to a point

Density is primarily determined by the

amount of exposure a film receives Exposure is directly proportional to mAs

Therefore mAs is the controlling factor of

density


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Reciprocity Law states that density on a

film should remain unchanged as long as the

intensity and duration of exposure remainsthe same

In english: as long as mAs is constant any

combination of mA and time (s) will create the

same density


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Minimum change necessary to cause a

visible shift in density is 30% of mAs or any

other influencing factor that would equal thischange


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Influencing Factors of Density

kVp alters the intensity of the beam in 2 ways 1. controls the average energy of the x-ray photons

produced at the anode - therefore change in kV alters the

intensity of the beam when mAs and other factors remainunchanged

2. affects the production of scatter

kV - scatter - density


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15% rule a 15% increase in kV causes a

doubling of exposure to the film a 15%

decrease in kV causes a halving of exposureto the film This rule will ALWAYS change the contrast


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Distance

SID alters the intensity of the beam according to

the inverse square law the affects to exposure

are inversely proportional to the square of the

distance I1 = D2

I2 = D1

as distance - radiation intensity & radiographicdensity

Eg. If you increase your distance from 40 to 72

(approx. double) your intensity would decrease appox.

4 times


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Density Maintenance Formula mAs1 = D1

mAs2 = D2

This formula can be used to determine your new mAs

when you are changing yourSID


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Distance

OID as OID - density

BUT in most instance OID variations are insufficient to

cause visible density changes


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Generators

The total # of high energy photons in the x-ray

tube emission spectrum is controlled by the

amount of ripple in the waveform

1 2P wave form has a lower average photon

energy that 3 12P, resulting in less density

Refer to conversion chart on page 407 in Carltonand Adler to convert mAs for different generators


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Focal Spot Size

If the x-ray unit is properly calibrated the

difference between large focal spot and small

focal spot is negligible


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Anode Heel Effect

Alters the intensity of radiation, therefore the

density between the anode and the cathode ends

of the tube

Image density is greater at the cathode end

More pronounced when wide collimation is used

More pronounced with extremely small angle anodes

(12 or less)

Can be used to our advantage with varying

subject densities (cathode end over the thick part,

anode end over the thin part)


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Filtration

Alters the beam intensity which affects density

All types of filtration alter density As filtration - density


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Collimation

As collimation (decrease field size) scatter

radiation - therefore overall image density

Compensation of technical factors for collimation only

needs to occur in the following circumstances

Large anatomical part

High kV

Low grid ratio Non grid examinations


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Grids

Grids absorb scatter that would otherwise add

density to the film

As grid ratio - density

Formula to maintain density while changing grid

ratio mAs

1

= GCF1

mAs2 = GCF2

Because the primary purpose of a grid is to

improve contrast using kVp to compensate for

density is not recommended


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Film/Screen Combinations

Films and intensifying screens can alter density

As relative speed - density Density can be maintained with the following

formula mAs1 = RS2

mAs2 = RS1


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Anatomical Part

As tissue thickness, average atomic # of tissue or

tissue density - image density Things to consider

If contrast (air or barium) have been used (changed

atomic #)

Pathology (additive or destructive)

Casts (adjust for thickness) Compression - compression - part thickness -

density


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Contrast Media

Radiopaque (positive) Contrast Agent - Density Eg. Barium, iodinated (water soluble, oil based)

Radiolucent (negative) Contrast Agent - Density Eg. Air


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Film Processing

Condition of solution can dramatically alter density As developer temp - density

As immersion time in dev. - density

As replenishment rate - density

Contamination of dev. density


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Factors Affecting

Density Summary mAs Density

kVp Density

# of pulses in gen. waveform Density

SID Density

Filtration Density Collimation (smaller field) Density

Part thickness Density

Grid Ratio Density

Film/Screen Speed Density

Processing time, temp, rep DensityRadiolucent contrast (air) Density

Radiopaque contrast (barium) Density

Additive Pathology Density

Destructive Pathology Density


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Definition of Collimation

Collimation (as per Dorlands Dictionary) - the

elimination of the peripheral (more divergent)

portion of an x-ray beam by means of metal

tubes, cones, or diaphragms interposed in

the path of the beam.

Therefore increase collimation is the increase

of the elimination of the peripheral portion ofan x-ray beam

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Imrc 181 Lo1 Density