Beam geometry, FS blur, ANode heel, Comp filters,Distortion pt1

X-ray Photon Production

X-ray photons are produced isotropically (in all directions). To direct photons toward the tube window, the target is placed on an angle. The target angle is determined by the beveled edge of the anode.

Focal Spot Size

Determined by two factors:

1) The length of the filament.

2) The angle of the target (anode angle). The angle of the target affects the path of the electron stream.

Line Focus Principle Definitions

Actual focal spot – area on the target that the electron stream strikes (determined by filament size).

Effective focal spot – the size of the focal spot projected perpendicular to the target onto the image receptor (IR).

Line Focus Principle Explanation

If the target is angled less than 90° from the electron stream, the effective focal spot is smaller than the actual focal spot. The effective focal spot size decreases as the anode angle decreases. This allows a small focal spot size without reducing the tube’s heat capacity.

Line Focus Principle Purpose

The effective focal spot size can be changed by adjusting the anode (target) angle. Typical anode angle: 12–15°. Decreasing the angle results in a smaller effective focal spot while maintaining a large actual focal spot for heat dissipation. This provides the detail of a small focal spot and the heat capacity of a large one.

Line Focus Principle Advantage

Decreasing the anode angle increases the tube’s ability for high-energy, short exposures. The principle allows better image detail and higher heat capacity simultaneously.

Photon Travel

X-ray photons travel in straight lines. EM energy from a point source travels linearly from that source. X-rays are produced across the entire target area (multiple points), leading to beam divergence.

Beam Divergence

The smaller the source area, the less beam divergence. Divergence refers to the spreading out of photons as they move away from the source.

Geometric (Focal Spot) Blur

Image blurring caused by the divergent quality of the x-ray beam. Also called penumbra or edge gradient. Dependent on: 1) Effective focal spot size (direct), 2) Source-to-Image Distance (SID, inverse), 3) Object-to-Image Distance (OID, direct). Formula: Blur ∝ Focal Spot Size × OID / SOD.

Effective Focal Spot

The width of the x-ray photon source (“focal spot projected onto the patient”). A smaller effective focal spot produces less edge gradient and sharper image detail.

Object-to-Image Receptor Distance (OID)

The linear distance between the object and the image receptor (IR). Increased OID increases beam divergence past the object’s edges and increases edge gradient (blur).

Source-to-Image Receptor Distance (SID)

The linear distance between the effective focal spot (target) and the IR. A longer SID reduces beam divergence and decreases edge gradient, improving image sharpness.