Radiography in the Digital Age - Anode Bevel and Focal Spot

Radiography in the Digital Age

The Anode Bevel and the Focal Spot

• Chapter 21

The Anode Bevel

• Refers to the angle of the target surface of the anode in relationship to a vertical line drawn perpendicular to the long axis of the tube.
• Affects both:
  • The size of the projected focal spot (line-focus principle).
  • The distribution of x-ray intensity within the beam (anode heel effect).

Line Focus Principle

• The size of the projected or effective focal spot is crucial to the sharpness of any radiographic image.
• It is controlled by:
  1. The width of the electron beam from the filament.
  2. The angle of the anode bevel.
• The smaller the projected focal spot, the greater the sharpness of details in the image.
• With an anode surface bevel of 45 degrees, the effective focal spot is the same size as the beam of electrons.
• At a smaller angle, making the surface bevel steeper, the effective FS is smaller than the actual focal spot.
• The true actual focal spot is measured along the anode surface.
• The actual focal spot is the area available for dispersion of heat from electron bombardment.
• Using a very small electron beam to achieve a small FS would concentrate heat and melt the anode surface.
• The line focus principle makes it possible to achieve a very small effective focal spot, while at the same time allowing sufficient area for heat dispersion at the actual focal spot.
• The goal is to achieve maximum sharpness while maintaining good heat dispersion.
• Standard x-ray tubes have an anode bevel in the range of 15 -17 degrees, producing a large FS of 1.0 to 1.2 mm and a small FS of 0.5 to 0.6 mm.
• Special procedures tubes such as are used for angiographic procedures have an anode bevel from 7- 10 degrees.
  • These produce fractional focal spots as small as 0.2 – 0.3 mm.
• The size of the projected focal spot changes according to the angle of projection toward the image receptor plate.
  • From the anode end of the IR, the FS appears smaller.
  • From the cathode end of the IR, the FS appears larger.
• This means that the anode end of the image is sharper than the cathode end of the image.
• This effect can be measured on large image receptor plates (35 X 43 cm).
• The effective focal spot is more accurately defined as the focal spot projected by the CR.

Considerations for Digital Radiography

• For digital radiography, image sharpness is limited by pixel size.

Anode Heel Effect

• A variation in the x-ray intensity along the longitudinal tube axis.
• Intensity falls off rapidly toward the anode end of the x-ray beam.
• Intensity also increases somewhat toward the cathode end of the beam.
• The anode heel is defined as the lower back corner of the anode disc.
• The anode material itself acts as a form of inherent filtration.
• From a particular point of origin, x-rays emitted toward the anode heel must pass through more filtration to escape the anode.
• X-rays emitted toward the cathode have the least material to pass through.
• The heel effect is worsened with:
  1. Longer field sizes
  2. Steeper (lesser) anode bevel angles
  3. Larger focal spots
  4. Shorter SIDs
• The heel effect is worsened with steeper (lesser) bevel angles.
• In turn, for angiographic and other special procedures, this can limit the length of the IR or field that can be used.
• For variable body parts, always place the thinnest end of the anatomy toward the anode end of the x-ray tube.
• Example Suggestions for positioning:
  • AP Chest: Patient’s head to left
  • AP T-Spine: Patient’s head to left
  • AP Femur: Patient’s head to right
  • AP Foot: Patient’s head to right
  • AP Humerus: Patient’s head to right
• As a percentage of intensity at the CR, experimental measurements show the intensity of the x-ray beam as low as 31% at 20 degrees toward the anode, and as high as 105% at 12 degrees toward the cathode.
• The anode heel effect is more concentrated at shorter SIDs.
• The anode heel effect is more extreme for larger IR sizes and field sizes.

Focal Spot Size

• When discussing effects on the image, all references to the FS refer specifically to the effective focal spot as “seen” at the center of the IR.
• The term focal spot derives from the focusing of the electron stream onto this area of the anode surface.

Effect on Sharpness of Detail

• The small focal spot resolves much smaller lines than the large FS.

Effect on Spatial Resolution

• Spatial resolution is the only image quality affected by the focal spot size
• The smaller the focal spot the sharper the detail, and the better the spatial resolution.
• Specifically, the size of the focal spot is inversely proportional to spatial resolution, so the rule is: “As focal spot size increases, spacial resolution decreases” and “As focal spot size decreases, spacial resolution increases”

Effect on Image Penumbra

• The size of the focal spot is directly proportional to penumbra.
• When the focal spot size is tripled, the spread of penumbra is also tripled.
• As penumbra grows, it spreads inward as well as outward, invading the umbra and causing it to shrink slightly.
• When the focal spot is larger than a projected object, it is possible for umbra to shrink to the point of disappearing entirely.
• This is important in angiography, where small embolisms or other pathology could fail to be demonstrated.
• Whether an x-ray is absorbed or not depends on its point of origin within the area of the focal spot.

The Nature of Penumbra

• Within this measurable width, there is a transition from total penetration to the total absorption the object is capable of.
• This “spreading” of the image edge is known as geometrical penumbra.

Effect on Image Penumbra

• Doubling the size of the focal spot doubles the spread of penumbra : Doubling the “blurriness” in the image Cutting spatial resolution precisely in half
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Regarding Magnification

• When increasing the focal spot size, from A to B, as penumbra expands both outward and inward, the umbra actually shrinks.
• Unless the umbra expands, no magnification is occurring – The focal spot is unrelated to magnification
• The human eye locates the edge of the image in the middle of the penumbra, and will measure both images at the same size.

Anode Heat Load

• Small focal spot sizes are only available at low mA stations.
• This is done to protect the x-ray tube anode from over-concentrated heat load when too many electrons are focused onto too small an area
• A common misconception is that smaller focal spots produce less radiation than large focal spots – this is false.
• The amount of radiation produced is the same at a given mA station and kVp setting regardless of which focal spot is used
• The focal spot is a geometrical factor, whereas the quantity of x-ray production is an electrical factor

Regarding Other Image Qualities

• Focal spot size is strictly a geometrical factor and does not affect any of the visibility components of the image: exposure level, subject contrast, or noise
• Image magnification is affected only by distances (the ratio of SID/SOD).
• Image distortion is affected only by alignment.
• Exposure level, subject contrast, noise, magnification and shape distortion are NOT affected by focal spot size!

Is FS size a “controlling factor”?

• Since the focal spot is the only technical factor that exclusively affects sharpness, it is considered to be the controlling factor for spatial resolution in the latent image reaching the IR, that is, during the initial projection
• However, in the digital age, the “controlling factor” for resolution of the final, electronically-displayed digital image is pixel size
• These are both “limiting factors” for spatial resolution: Whichever one limits the resolution most becomes the “controlling factor” for a particular projection