Line Focus Principle and Anode Heel Effect

Line Focus Principle

• Angling the anode target face maintains a large actual focal spot while creating a small effective focal spot.
• The principle enhances heat distribution while improving image quality.

Focal Spot

• The focal spot is the area from which X-rays are emitted, serving as the actual x-ray source.

Actual Focal Spot

• Represents the area bombarded by filament electrons; larger size enhances heat distribution.
• Smaller electron streams generate more concentrated heat, necessitating better heat spread over a larger area.

Effective Focal Spot

• Acts as the origin of the X-ray beam, influencing image quality.
• Visualized from the patient's perspective; defined by the controlled actual focal spot size.
• Smaller effective focal spots yield sharper images.

Controls of Actual Focal Spot

• Influenced by various factors: filament length, target angle, charge on the focusing cup, filament depth, and focusing cup dimensions.

Purpose of Line Focus Principle

• Allows for a larger heating area on the anode while retaining a smaller effective focal spot for better image clarity.

Direct Relationship

• Smaller actual focal spots correlate directly with smaller effective focal spots, and vice versa.

Large Actual Focal Spot

• Distributes heat more effectively than smaller focal spots due to the larger area over which X-rays are emitted.

Target Angle

• Optimal between 5-20 degrees for general X-ray use; affects effective focal spot size.
• Smaller angles result in improved heat dissipation and enhanced image quality.

Limitation Factor on Target Angle

• Ensures adequate X-ray coverage of the largest field size used, typically 35cm x 43cm image receptor.

Bi-angle Target

• Features two target angles, requiring two vertically-aligned filaments; adjusting the angle alters effective focal spot size.

Limitations to Line Focus Principle

• Extremely small target angles may result in inadequate X-ray beam area, limiting anatomy coverage and producing a narrower beam.

Advantage of Line Focus Principle

• Enhances both spatial resolution and the heat capacity of the X-ray system.

Anode Heel Effect

• Results from the line focus principle, reducing X-ray intensity on the anode side due to a longer travel path and increased absorption.
• Cathode side emits greater intensity compared to the anode side; effect magnifies with smaller anode angles.

How Anode Heel Effect Happens

• X-ray production involves electron bombardment leading to emission at angles between 45-90 degrees.
• Electrons on the anode side encounter more material before leaving the tube, causing absorption and energy loss.

Radiation Intensity

• Central ray intensity is at 100%, while the anode side may decrease to 75%, and the cathode side can reach up to 120%.

Central Ray

• Represents an imaginary line that defines the useful beam's center; it is marked by 100% intensity output.

Purpose of Anode Heel Effect

• Positioning the cathode side over denser anatomy ensures uniform radiation exposure across the image receptor.

Anode Heel Effect is Most Apparent

• This effect is pronounced with larger image receptor sizes and shorter source-to-image distances (SID).