Beam geometry, FS blur, ANode heel, Comp filters,Distortion mine

Photons Emission

X-ray photons are emitted in all directions (isotropic emission). For efficiency, the anode target is placed at an angle to direct photons toward the tube window.

Focal Spot Definition

The focal spot is determined by two factors: 1) the length of the filament, and 2) the angle of the target (anode angle).

Anode Angle Definition

The anode angle is the angle of the target surface relative to the path of the electron stream.

Actual Focal Spot

The area on the anode target that the electron stream strikes. Determined by filament size (large or small focus selected at the console).

Effective Focal Spot

The size of the x-ray beam that exits the tube and is directed toward the patient. The effective focal spot is smaller than the actual focal spot.

Line Focus Principle

States that as long as the anode angle is less than 90°, the effective focal spot will be smaller than the actual focal spot. This allows for improved spatial resolution and reduced heat buildup.

Purpose of the Line Focus Principle

Allows use of a small effective focal spot (better image detail) while maintaining a larger actual focal spot (better heat dissipation). Typical anode angles are 12–15°.

Beam Divergence

X-ray photons are produced at the target and travel in straight lines, spreading out as they move away from the source. The smaller the focal spot (source), the less beam divergence occurs.

Geometric (Focal Spot) Blur

Also known as penumbra or edge gradient. It is the blurring at the edges of the image due to beam divergence. Blur increases with larger effective focal spot, shorter SID, and greater OID.

Effective Focal Spot and Blur

The effective focal spot is the width of the x-ray photon source as projected onto the patient. A smaller effective focal spot reduces edge gradient and improves image sharpness.

Object-to-Image Distance (OID)

The linear distance between the object being imaged and the image receptor (IR). Increasing OID increases magnification and blur due to more beam divergence.

Source-to-Image Distance (SID)

The linear distance between the effective focal spot (x-ray source) and the IR. Increasing SID decreases magnification and blur by reducing beam divergence.

Relationship Between SID and OID

To maintain image sharpness, increase SID when OID must be increased. Increasing SID allows for better collimation and less geometric distortion.