2️⃣ Beam Geometry, Focal-Spot Blur, Anode Heel & Distortion

Geometric blur (edge gradient)

The loss of sharpness at the edges of structures on the image caused by the finite size of the focal spot and beam geometry; also called focal-spot blur or penumbra.

Line focus principle

By angling the anode target, you can use a larger actual focal spot for heat loading while creating a smaller effective focal spot to improve image sharpness.

How to change effective focal-spot size

Decreasing the anode (target) angle makes the effective focal spot smaller (better detail but more heel effect); increasing the angle makes the effective focal spot larger.

Anode heel effect – definition

Variation in x-ray beam intensity along the anode–cathode axis; intensity is higher on the cathode side and lower on the anode side because photons toward the anode travel through more target material and are more likely to be absorbed.

Anode heel effect – clinical use

Place the thicker or denser part of the body under the cathode side of the tube (where the beam is more intense) and the thinner part under the anode side to even out exposure; useful in exams like AP T-spine, femur, and foot.

Magnification – definition

Size distortion where the image of an object appears larger than its true size; caused by the divergence of the x-ray beam.

Magnification formula

Magnification factor M = SID ÷ SOD, where SOD is the distance from the source to the object (SOD = SID − OID).

How to reduce magnification

Use the largest practical SID and the smallest possible OID (get the part as close to the image receptor as you can).

Distortion – definition

Misrepresentation of the size or shape of an object on the image; includes size distortion (magnification) and shape distortion (elongation and foreshortening).

Elongation – what it is

Shape distortion in which the image of an object appears longer than it really is, usually caused by tube angulation or tilting the image receptor relative to the object.

How to correct elongation

Keep the tube, part, and image receptor properly aligned; avoid unnecessary tube or IR angulation unless it is part of the standard projection.

Foreshortening – what it is

Shape distortion in which the image of an object appears shorter than it really is, usually caused by angling or flexing the part while the tube and receptor are correctly aligned.

How to correct foreshortening

Position the part so that its long axis is parallel to the image receptor and align the central ray perpendicular to both the part and the IR.