1725 Contrast

What is contrast?

• One of the 2 properties that compromises visibility of detail

• It is the difference between adjacent densities/IR exposures

• These differences can range from clear white to various shades of grey to black

Hight Contrast

• Difference between adjacent densities are great

• Short scale of contrast

• Short/Narrow dynamic range

• Fewer shades of grey

• Increased Contrast

• Low kVp

Low Contrast

• Differences between adjacent densities are minimal

• Long scale of contrast

• Large/Wide dynamic range

• More shades of grey

• Decreased contrast

• High kVp

“Good” Contrast

• Without contrast there would be no image

  • Densities on the image would appear identical

• High contrast is not necessarily the most desirable

• High contrast images may be more pleasing to the eye

• Low Contrast images often demonstrate more information

• Appropriate or “Good” contrast is dependent upon the exam

Scale of Contrast

Is defined by the number of useful visible densities or shades of grey

Short Scale

• Refers to an image that demonstrates maximum differences between densities

• Has minimal total number of densities

• High contrast - Can easily see light vs. dark

Long Scale

• Refers to an image that demonstrates minimal differences between. densities

• Maximum total number of densities

• Low contrast - Harder to see differences in color (many shades of grey)

Physical Contrast

• Total range of density/IR exposure values recorded by the image receptor

• Max contrast possible and the most accurate representation of the varying intensities present in the x-ray beam after it passed through the subject

Visible Contrast

• Total range of density/IR exposure values that can be perceived by the human eye in a single image

• Portion of physical contrast

• Radiographer manipulate the physical contrast available into the visible range to produce the diagnostic image

Manipulating Contrast

• Recorded densities can be compressed or expanded

• Accomplished by:

  • Adjustments in kVp

  • Digital window width adjustments

Digital Image Receptor Contrast

• Primary means is by adjusting window width

• Digital images undergo post-processing

Subject Contrast

• Range of differences in the intensity of the x-ray beam after it has been attenuated by the subject

• Result of differential attenuation by the tissues in the body

• Dependent on kVp and irradiated material

kVp

• This is the primary controller of subject contrast

• As kVp increases, a wider range of photon energies is produced, which in return increases the ability of the photons to penetrate the body tissues

• Subject contrast is decreased because more uniform penetration occurs between thick and thin parts

• This ultimately leads to wider range of exposures which will lower contrast

• As long as kVp is adequate to penetrate the body part being examined, low kVp will produce high subject contrast area

• If kVp is too low, the photons do not reach the image receptor and is absorbed by patient

• Low kVp produces higher subject contrast - low energy photons are absorbed by thicker parts while penetrate the thin part

High vs. Low kVp

• High kVp

  • Chest x-rays use high kVp - wide range of densities, contrast is decreased (as looking at whole picture)

• Low kVp

  • Rib x-rays use low kVp - low range of densities, contrast is increased (as looking at whole picture)

Fog

• Fog can be caused by

  • Scatter radiation

  • Low level ionizing radiation

    • If it’s low they are going to get absorbed in the body probably won’t even make it to the part and create fog on the image

• Increasing fog will decrease contrast

Factors Affecting Contrast

• Controlling Factor: kVp

• Influencing Factor: mAs

• Focal Spot

• Anode Heel Effect

• Distance

• Filtration

• Beam Restriction

• Anatomical Part

  • Tissue Type

  • Tissue Thickness

  • Contrast Media

  • Pathology

• Grid Construction

  • Ratio

  • Frequency

• Image Receptor

  • Digital systems

  • Film/Screen Systems

kVp as the controlling factor

• As kVp increases, contrast decreases

• Increasing kVp, increases scatter, which decreases contrast

• Increasing kVp, increases fog, which decreases contrast

mAs effect on contrast

• Alters image receptor exposure

• If exposure causes density/IR exposure differences out of range, such as under/overexposed - Contrast is decreased

• mAs does not inherently affect contrast. If you severely change the number of photons it will change the receptor exposure enough to change contrast

Focal Spot Size

• The possibility of focal spot size altering contrast enough to be visibly noticeable is extremely unlikely

Anode Heel Effect

• Remember - Radiation is greater on the cathode end

• Difference in contrast only if collimation was opened up and with a small anode target angle of less than 12 degrees

• Has very little effect on contrast

Distance

• SID alters intensity of the beam reaching image receptor according to inverse square law

• Greater distances decrease density/IR exposure, less distances increase it

• As distance alters exposure, it can change the contrast exactly as does mAs

OID

• Air gap technique increases OID, permits scatter to avoid image receptor

• This increases contrast by removing scatter

Filtration

• Affects contrast by changing the average photon energy and decreasing beam intensity

• Increasing photon energy will increase compton interactions, therefore decreasing contrast

• Decreasing beam intensity will decrease densities/IR exposure therefore decreasing contrast

• Increased filtration will decrease contrast

Beam Restriction

• Beam restriction will reduce the number of photons available

• By reducing the photons available, it reduces scatter, and increases contrast

Anatomical Part

• As part increases in size, scatter increases, therefore decreasing contrast

• As atomic number increases, contrast is increased

Grid Construction

• Primary Function: Clean up scatter, increase contrast

• Contrast improvement factor (K) is the best measure of how well a grid accomplishes this function

Contrast Improvement Factor

• Based on the amount of scatter produced, which is controlled by kVp and the amount and type of tissue being irradiated

• As scatter radiation which reaches the image receptor is increased, the lower the contrast and the contrast improvement factor

K

• K = average gradient with the grid/average gradient without a grid

• If K = 1 no improvement in contrast has occurred

• Most grid have K between 1.5 and 3.5

• The higher the K factor the greater the contrast improvement