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=objectsizeimagesizeorSODSID
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.