Image Contrast

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Introduction

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

Image contrast is the difference between adjacent densities/IR exposures

The differences can range from clear white through shades of grey to black

Any change in overall density/IR exposure will affect contrast

Assessing Contrast

 

Describing Contrast

1–Great differences between adjacent densities/IR exposures describes high contrast

•Fewer discernible shades of grey

•Short scale contrast

2–Minimal differences between adjacent densities/IR exposures describes low contrast

•More discernible shades of grey

•Long scale contrast

Low Contrast Image

•Provides more diagnostic information

•More information is visible because more densities are present, therefore there are more possibilities for contrast differences

Scale of Contrast

Defined as the number of useful visible density or shades of grey

–Short scale contrast will demonstrate considerable difference between densities and has a minimal total number of densities

–Long scale contrast will demonstrate minimal difference between densities and has a maximum total number of densities

Manipulating Contrast

The image receptor has the ability to record a wide range of densities; many are not visible by human eye.

–The recorded densities can be expanded or compressed to form a range of visible density by:

•Changes in D log E curve

•Adjustments in kVp

•Digital window width adjustments

Image Contrast

Image contrast is the total contrast from both the image receptor and anatomical part (subject contrast)

1.Image receptor contrast

–Range of densities that the IR is capable of recording

2.Subject contrast

–Range of intensity of x-ray beam after it has been attenuated by the patient

Image Contrast = IR Contrast X Subject Contrast

Image Receptor Contrast

Film contrast – range of densities that a film is capable of recording

-Represented by D log E curve

Film contrast depends on:

1.Intensifying screens

2.Film density/IR exposure

3.The D log E curve

4.Processing

1.Intensifying screens

–Using intensifying screens inherently creates a high contrast image compared to an image created only by exposure to x-rays

2.Film density/IR exposure

–An optimal range of densities/IR exposures permit maximum visualization of contrast

–Excessive or inadequate density/IR exposure decreases contrast

3.The D log E curve

–As the slope of the D log E curve becomes steeper, contrast is increased

–With steeper curves the visible density range is compressed into a narrower exposure range

4.Processing

–Increasing film developer time, temperature or replenishment rate will increase chemical fog and decrease contrast

–The slope of the curve decreases, especially in the toe region

–Any factors that change base plus fog levels will affect contrast

Interpretation of Control Charts

Subject Contrast

Range of differences in the intensity of the beam after it has been attenuated by tissues of the body

Subject contrast is dependent on:

1.Kilovoltage

2.Amount of irradiated tissue

3.Type of irradiated tissue

1.Kilovoltage (kVp)

–Primary controlling factor of subject contrast

–As kVp increases; the range of photon energies exposing the image receptor increases, which results in overall lower contrast

–As long as the kVp is adequate to penetrate the part being examined:

»Low kVp will produce high subject contrast

»High kVp will produce low subject contrast

–Another aspect of kVp that must be considered is it’s effect on the production of Compton scatter

»Compton scatter causes radiation fog which has a significant effect on contrast

–As kVp increases, the percentage of Compton interactions versus photoelectric absorption increases. This results in more scatter radiation reaching the image receptor, decreasing contrast

2.Amount of irradiated tissue

–The thickness of the body part and the size of the collimated field both affect the number of photons that reach the image receptor

»As the body part thickness increases, x-ray attenuation increases; a direct relationship

–The difference in absorption of adjacent tissues influences subject contrast

»When the thickness difference between adjacent body parts is great, subject contrast is increased

–As the overall thickness of the body part increases and as field size increases, the amount of scatter radiation produced increases; the outcome is a decrease in subject contrast; an inverse relationship.

3.Type of irradiated tissue

–The atomic number of tissue as well as the tissue density both affect the number of photons that reach the image receptor

»As the atomic number of body tissues increase (introduction of barium or iodine contrast media) the percentage of x-ray photons attenuated increases; a direct relationship

–The difference in absorption of adjacent tissues influences subject contrast

»When the average atomic number between adjacent body parts is great, subject contrast is increased.

–As the tissue density increases (how tightly the atoms are packed together) the percentage of x-ray photons attenuated increases; a direct relationship

–The difference in absorption of adjacent tissues influences subject contrast

»When the density differences between adjacent tissues is great, subject contrast is increased.

Evaluating Contrast

 

•This is one of the most difficult skills for a MRT to develop

•The image must have sufficient density/IR exposure, within the visibility range, throughout the anatomical part of interest

•More information is recorded on an image than can be seen; an important consideration is to discern density differences (shades of grey), throughout the anatomical part of interest, to assess the contrast

Contrast mask

–Are used to simplify the evaluation of contrast

–The mask is placed over the area of interest therefore blocking surrounding areas that may be either too light or too dark making contrast evaluation difficult

Selecting the appropriate kVp

Experience and knowledge will enable a MRT to determine which images require a change in contrast

•Carlton and Adler P. 405 Figure 27-1

–Illustrate the effect of contrast changes with a fixed density /IR exposure

•Carlton and Adler P. 414 Figure 27-12

–Illustrate the effect of kVp settings on contrast

•Carlton and Adler P. 415 Figure 27-13

–Illustrates the effect of image contrast when the kVp remains the same but tissue thickness is varied.

When an image is determined to be undiagnostic, that is, outside the acceptable limit, it must be repeated

TABLE 27-2Changes necessary to produce visible contrast differences

Level Change Necessary to Produce   Visible Change Change equal to Percent Change
30 – 50 kVp 4 – 5 percent 1 – 3 kVp
50 – 90 kVp 8 – 9 percent 4 – 8 kVp
90 – 130 kVp 10 – 12 % 9 – 16 kVp

–“The rule for contrast changes is to make adjustments in increments of 15 or 8 percent.”

•You will recall our study of density/IR exposure a 15% change in kVp is used when repeating an image

–The same 15% rule applies when repeating an image to change contrast

•To obtain a higher contrast image and maintain density/IR exposure; decrease the kVp and increase the mAs 38.

–I have often said, “Select your kVp according to part thickness. Thin body parts require lower kVp settings than thick body parts.”

–This is true to a certain degree; however a MRT cannot continue to increase kVp without also considering the production of Compton scatter versus photoelectric absorption.

•Recall that the number of photoelectric absorption interactions is directly related to contrast

•Factors are classified as either controlling factors or influencing factors

•The controlling factor will have the greatest effect on contrast and therefore should be considered first to change the image contrast

kVp as the controlling factor

–As kVp increases, image contrast decreases; as kVp decreases, image contrast increases

•As kVp increases, beam penetration increases; attenuation of the beam decreases; contrast decreases

•As kVp increases; the production of radiation fog (scatter) increases; contrast decreases

–The MRT must select an appropriate kVp for the part to be examined; consider body habitus, pathology, equipment (generator)

Original image has optimum density/ IR exposure…

  Affecting Contrast (P.6)   kVp IR exposure mAs
Required to decrease   image contrast Increase by 15% IR exposure doubles with   a 15% Increase in kVp; therefore…. Reduce mAs by 50% (by   halve) to maintain IR exposure
Required to increase   image contrast Reduce kVp by 15% IR exposure is halved   with a 15% decrease in kVp; therefore…. Increase mAs by 100%   (double) to maintain IR exposure

Factors Affecting Contrast (continued)

EXAMPLE of the 15% rule

•An abdomen x-ray was taken using 25 mAs and 90 kVp. The resulting image has low image contrast and high density/IR exposure. What change in factor(s) should be made to increase image contrast and improve image density/IR exposure?

EXAMPLE of the 15% rule

•An abdomen x-ray was taken using 25 mAs and 90 kVp. The resulting image has low image contrast and optimum density/IR exposure. What change in factors should be made to increase image contrast and maintain image density/IR exposure?

EXAMPLE of the 15% rule

•A chest x-ray was taken using 5.5 mAs and 85 kVp. The resulting image has high image contrast and optimum density/IR exposure. What change in factors should be made to reduce image contrast and maintain image density/IR exposure?

Influencing Factors

Milliampere-Seconds (mAs)

•mAs is the controlling factor for density/IR exposure

•Without adequate density; contrast cannot be assessed or is absent

•Both underexposed and overexposed will result in a decrease in image contrast (Carlton P. 408 FIGURE 27-5)

Focal Spot Size

•Negligible effect on image contrast

Anode Heel Effect

•Negligible effect on image contrast

Distance

•Distance is an influencing factor for density/IR exposure and contrast

•Large changes in SID without mAs compensation (DSL) will affect the density/IR exposure and indirectly compromise contrast

•OID is an important consideration; as OID increases; the radiation fog from scatter decreases; image contrast increases

>Air-gap technique

Filtration

•Filtration is an influencing factor for density/IR exposure and contrast

•Filtration increases the average photon energy and decreases the beam intensity

•Increasing the photon energy causes an increase in Compton scatter which will decrease image contrast

–Beam Restriction

•Beam restriction is an influencing factor for density/IR exposure and contrast

•When reducing the field size, the MRT is reducing the number of x-ray photons that will interact with the tissue

•The outcome is reduced scatter production and increased contrast.

–Anatomical Part

•Anatomical part is an influencing factor for density/IR exposure and contrast

•An increase in part thickness and/or tissue density will result in greater Compton scatter production; the outcome is decreased contrast

•A positive contrast media will alter the average atomic number of tissue resulting in more photoelectric absorption; the outcome is increased contrast

•Positive contrast media alters the average atomic number of tissue resulting in more photoelectric absorption; the outcome is increased contrast

•Certain diseases can change the effective atomic number or density of the tissue; which in turn will affect the attenuation of the beam

–Destructive and additive pathological conditions alter image contrast.

–Grid Construction

•Grid construction is an influencing factor for density/IR exposure and contrast

•The purpose of grids is to improve contrast; the contrast improvement factor (K between 1.5 and 3.5) is an indication of a grids ability to remove scatter and therefore increase contrast

•Higher grid ratios remove more scatter and therefore have greater contrast improvement factors.

–Film/Screen Combination

•The slope of the D log E curve is determined by the physical composition of the film

•The use of intensifying screens inherently create an image of higher contrast when compared to an image exposed only to x-rays

•As slope (average gradient/gamma) of the D log E curve increases; contrast increases

–A developer solution that is operating in suboptimal conditions will result in a decrease in the slope of the D log E curve and therefore contrast is decreased

 

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