Image Formation Notes

Image Formation

Objectives

• Identify the steps of image acquisition.
• Discuss the types of attenuation recognized in radiographic imaging, including the photoelectric effect and Compton effect.
• Define the term ionization.
• State the composition of exit radiation.
• Describe the process of creating the latent image on an Image Receptor (IR).
• Identify the attributes of a quality radiographic image.
• Explain the importance of displayed brightness and contrast to image quality.
• Differentiate between displayed high-contrast and low-contrast images.
• Explain the importance of temporal and spatial resolution, size, and shape distortion to displayed image quality.
• State the effects of quantum noise, scatter, and image artifacts on displayed image quality.
• Explain the importance of radiographic opacities and contrast to image interpretation.
• Differentiate radiographic imaging from dynamic imaging.

Attenuation

• Two components of attenuation need to be understood.
• Differential absorption occurs when X-ray photons are absorbed differently by various tissues.
• It happens due to variations in tissue composition and density.

Differential Absorption

• Primary x-ray photons interact with the patient.
• Some photons are absorbed by the part.
• Some photons become scatter radiation.
• Photons that penetrate the part help form the image.

X-ray Beam Absorption: The Photoelectric Effect

• An incoming photon has sufficient energy to eject an inner-shell electron and is completely absorbed.
• An electron from an upper-level shell fills the electron hole or vacancy.
• A secondary photon is created because of the difference in the electrons’ binding energies.
• The probability of this effect depends on the energy of the incoming x-ray photon and the composition of the anatomic tissue.
• Fewer photon interactions occur at a higher kVp, but of those interactions, a smaller percentage are photoelectric interactions.

Scattering: Compton Scatter

• The Compton effect: an incoming photon hits an electron, losing some but not all of its energy to the ejected electron, then changes its direction.
• Can occur within all diagnostic x-ray energies.
• Is dependent only on the energy of the incoming photon, not the atomic number of the tissue.
• Higher kVp reduces the number of interactions overall, but the number of Compton interactions increases in comparison to the number of photoelectric interactions.

Coherent (Classical) Scattering

• The incoming photon interacts with the atom as a whole but does not enter.
• It causes the whole atom to become excited.
• The x-ray does not lose energy, but it changes direction.

Factors Affecting Beam Attenuation

• Tissue thickness
  • X-rays are attenuated exponentially and generally reduced by ~50% for each 4 to 5 cm (1.6 to 2 in) of tissue thickness.
• Type of tissue
  • Tissues composed of a higher effective atomic number will increase beam attenuation.
• Tissue density
  • Increasing the compactness of the atomic particles will increase beam attenuation.
• X-ray beam quality
  • Higher kVp increases the energy of the x-ray beam and will decrease beam attenuation.

Exit Radiation

• Transmitted, remnant, or exit radiation is composed of transmitted and scattered radiation.
• The varying amounts of transmitted and absorbed radiation create an image that structurally represents the anatomic area of interest in the Differential Absorption process.
• This remnant radiation will ultimately produce an electronic data set in a digital image receptor (IR).
• Scatter radiation reaching the image receptor creates unwanted exposure called fog.
• Anatomic tissue that absorbs incoming x-ray photons is considered radiopaque and creates the light areas (increased brightness) on the displayed image.
• The areas within the anatomic tissues that transmit the incoming photons are considered radiolucent and create dark areas (decreased brightness) on the displayed image.
• Anatomic tissues that vary in absorption and transmission range between radiopaque and radiolucent to create a range of dark and light areas (shades of gray).

Electronic Data Set

• When the exit or remnant radiation interacts with the digital IR, it is converted to electronic signal values through the capture, couple, collect process.
• The strength (intensity) of the electrical signal and the differences in signal values result from x-ray beam attenuation caused by differential absorption.
• The electronic data set is computer processed to produce a visible image displayed on a monitor.

Radiographic Quality

• A quality radiographic image accurately represents the anatomic area of interest, and its information is well visualized for diagnosis.
• Visibility of anatomic structures is determined by:
  • Brightness
  • Density/intensity
  • Contrast
  • Contrast resolution
• Accuracy of structural lines (sharpness) is determined by:
  • Spatial resolution
  • Distortion
    • Motion
    • Elongation
    • Foreshortening
    • Blooming

Displayed Image Contrast

• The digital image must exhibit differences in the brightness levels (image contrast) to differentiate among anatomic tissues.
• The range of brightness levels displayed is, in part, a result of the tissues' differential absorption of the x-ray photons.

Subject Contrast

• Displayed image contrast is the result of multiple factors associated with the anatomic structure, radiation quality, image-receptor characteristics, computer processing, and display monitor.
• Subject contrast refers to the absorption characteristics of the anatomic tissue imaged and the quality of the x-ray beam.
• Anatomic tissues that attenuate the beam similarly have low subject contrast.
• Anatomic tissues that attenuate the beam very differently have high subject contrast.
• The ability to distinguish among types of tissues is determined by the differences in brightness levels in the image, or contrast.
• Contrast resolution describes the ability of an imaging receptor to distinguish between objects having similar subject contrast and is dependent on the gray scale.
• Gray scale: number of different shades of gray that can be stored and displayed in a digital image.
• Determined by the system bit depth.

Displayed Image Contrast

• Displayed image contrast is controlled by both the subject contrast and the contrast resolution of the digital image receptor.
• In addition, contrast can be altered with computer processing before and after the image is displayed (leveling).

Contrast Resolution

• Ability to distinguish structures similar in subject contrast.
• Difficult to quantify.
• Improved:
  • At lower kVp
  • With grid use
  • Increased bit depth (longer gray scale)
  • Low noise
  • No quantum mottle
  • Limited scatter

Spatial Resolution

• Anatomic structures must be displayed accurately and with the greatest amount of sharpness.
• Spatial resolution refers to the smallest object that can be detected in a digital image.
• Will never be perfect.
• Improves with:
  • Smaller pixel (larger matrix) size
  • Smaller focal spot
• Controlled by geometric factors:
  • SID (Source-to-Image Distance)
  • OID (Object-to-Image Distance)
  • SOD (Source-to-Object Distance)
• Affects:
  • Magnification: Increase in the size of the part.
    • Occurs with short SID or long OID.
    • $MF = \frac{image \ size}{object \ size} \ or \ \frac{SID}{SOD}$
  • Distortion: Unequal magnification across a part.
    • When IR or parts are not perpendicular or parallel to the CR.
    • Can be affected by object:
      • Shape
      • Position
      • Thickness
  • Focal Spot Blur:
    • Because X-rays don’t originate at a pin-point.
    • Larger focal spot, more blur.
    • Anything that increases magnification will also increase blur.

The Anode Heel Effect

• Due to the anode heel angle, photons traveling toward the anode side are more likely to be attenuated than those traveling away from the anode.
• Can also cause focal spot blur.
• The cathode side of the image will have a greater degree of blur and poorer spatial resolution.

Image Receptor Speed

• Faster speed IR’s are more sensitive (require lower technique).
• Less dose to the patient.
• Lower quality image.
• More noise.
• Poorer resolution.

Motion Blur

• Caused by patient motion during exposure.
• Should be repeated (unless intentional).
• Avoid by using high mA and low time.
• Not a common problem now with high functioning generators and high-speed receptors.

Temporal Resolution

• Temporal resolution (TR) is the inherent resolution on an image as a function of image acquisition time.
• Increasing the time of exposure during image acquisition can increase motion unsharpness and therefore decrease temporal resolution, even if it is not visible on the displayed image.
• More often used to describe fluoroscopy.

Scatter

• Unwanted exposure to the image receptor resulting in fog.
• A result of Compton interactions.
• Provides no useful information.
• Does provide density.
• Scatter or fog decreases image contrast.
• Considered noise.
• Covers the desired brightness of the image and changes the adjacent brightness levels.
• Does not change spatial resolution, but hides it.
• Reduced with lower kVp and a grid.

Quantum Noise

• Visible as graininess in the image.
• Caused by not enough transmitted beam to provide information.
• Provides no useful information.
• Cannot be fully compensated for with window and level.

Image Artifact

• An artifact is any unwanted brightness level on a radiographic image that is not part of the patient’s anatomy.
• Artifacts may obscure anatomic information.
• Will affect the image histogram.
• Examples:
  • Anatomic
  • Removable (necklace, retainer, earring, etc.)
  • Non-removable (prosthetic, implant, pacemaker)

Dynamic Imaging: Fluoroscopy

• Fluoroscopy uses a continuous or pulsed beam of x-rays to create images of moving internal anatomic structures.
• Dynamic imaging of internal anatomic structures can be visualized with the use of a flat panel detector or image intensifier.
• The exit radiation interacts with the acquisition device, is processed or converted, and then transmitted to the display monitor for viewing.

Summary

• Extensive knowledge is required to optimize image quality.
• Understanding the physics of x-ray generation to differential absorption in the body, interactions at the image receptor, and ultimately image processing is key to optimizing image quality.
• Specific terminology must be utilized in discussing image quality to ensure thorough and accurate communication.