Anode Heel Effect And Line Focus Principle

Question 1

Line focus principle states that

Answer 1

• by angling the face of the anode target, a large ACTUAL focal spot size can be maintained, and a small EFFECTIVE focal spot can be created
• relationship between the actual and effective focal spot

Question 2

Line focus principle

Answer 2

• allows for a LARGER area for heating while maintaining a small focal spot (allows for good image quality, bad for equipment)
  
  • achieved by angling the target

Question 3

Focal spot

Answer 3

• the area of the target where x-rays are emitted

- actual x-ray source

Question 4

Actual focal spot

Answer 4

• area of the target that is ACTUALLY bombarded with filament electrons
• size of electron stream is determined by the filament size selected
• ideal for this area to be as large as possible for better heat distribution
  
  • the smaller the stream, the greater the heat produced in a small area

Question 5

Actual and effective focal spot size

Answer 5

• DIRECT relationship
• the SMALLER the actual focal spot, the SMALLER the effective focal spot
• the LARGER the actual focal spot, the LARGER the effective focal spot size
• large actual focal spot will have less heat on the anode, than a small focal spot (same quantity of photons over a larger area vs a smaller area)

Question 6

Effective focal spot

Answer 6

• origin of the x ray beam
• x-ray beam area as seen from the patients perspective
• when specifying focal spot sizes, this is the focal spot that is being identified
• the smaller the area, the shaper the image will be
• for best image quality, it is desirable to keep this as SMALL as possible
• controlled by the actual focal spot

Question 7

Actual focal spot is controlled by

Answer 7

• filament length
• target angle
• charge on the focusing cup
• depth of filament in focusing cup
• dimensions and shape of focusing cup

Question 8

Target angles

Answer 8

• typically between 5-20 degrees for a general purpose x-ray room
• the SMALLER the anode angle, the SMALLER the effective focal spot while maintaining a large actual focal spot
• results in a larger surface area for heat dissipation and a smaller effective focal spot to improve image quality

Question 9

So the target angle should be as small as possible right?

Answer 9

No! If the target angle is too small the x-ray beam area (field size) may be too small to effectively image all required anatomy
-useful beam will be too narrow

Question 10

Anode heel effect is caused by?

Answer 10

• line focus principle

- intensity of radiation on the cathode side of the x-ray field is greater than that on the anode side

Question 11

Anode heel effect

Answer 11

• the “heel” of the target is in the path of the beam
• x-rays on the anode side must first penetrate a portion of the target before exiting the tube
• results in loss of x-ray photons and loss of energy
• intensity of x-rays that are emitted through the “heel” are reduced due to: longer path to travel and increased absorption

Question 12

Radiation intensity

Answer 12

• as a result of the anode heel effect, the useful beam will have a variation of radiation intensity
• central ray is imaginary centre most x-ray in the beam designated at 100% radiation intensity
• cathode side can be as high as 120%
• anode side may be as low as 75%

Question 13

Working with the anode heel effect

Answer 13

• when imaging areas that have great differences in thickness or mass density, need to be conscious of AHE
• in general: position the cathode over the thicker part of the anatomy to produce more uniform exposure on the image receptor ex femur or thoracic spine
• designated head end of the table

Question 14

Anode heel effect is most apparent with

Answer 14

• large IR size

- short SID (x-ray source)