Lecture 18 – Beam Geometry final

Big 13 (course concept)

The nickname for the main technical and geometric factors that influence the x-ray beam and image: examples include mA, exposure time, kVp, SID, OID, focal-spot size, filtration, collimation, anode heel effect, grid use, patient thickness, tissue density/atomic number, and receptor/speed system (check your class handout for the exact 13 used in your course).

Isotropic emission

X-rays leave the focal spot and travel in all directions from the source; they diverge from the focal spot like light from a bare bulb.

Actual focal spot

The physical area on the anode target struck by the electron stream from the filament.

Effective focal spot

The size of the focal spot projected onto the patient/IR, measured perpendicular to the beam. This is the focal spot size that actually affects image sharpness.

Line focus principle

By angling the anode target, you can use a large actual focal spot (for heat loading) while producing a smaller effective focal spot (for better spatial resolution).

Effect of anode angle on effective focal spot

Decreasing the anode angle makes the effective focal spot smaller (better sharpness) but increases the anode heel effect and limits field size at a given SID.

Anode heel effect

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 traveling toward the anode are more likely to be absorbed within the anode.

Clinical use of the anode heel effect

Place the thicker or denser part of the body under the cathode side (more intense beam) and the thinner part under the anode side (less intense beam) to even out exposure.

Geometric blur (focal-spot blur, penumbra)

Loss of sharpness at the edges of structures caused by the finite size of the focal spot and beam geometry; produces a blurry edge called penumbra.

Geometric unsharpness formula

Unsharpness (U) = focal-spot size (F) × OID ÷ SOD, where SOD is source-to-object distance (SID − OID).

Effect of focal-spot size on blur

Larger focal-spot size increases geometric blur and decreases spatial resolution; a smaller focal spot decreases blur and improves detail.

Effect of OID on blur and magnification

Increasing OID increases geometric blur and magnification; decreasing OID improves sharpness and reduces magnification.

Effect of SID on blur and magnification

Increasing SID decreases blur and magnification (better sharpness); decreasing SID increases blur and magnification.

Size distortion (magnification)

Any change in the size of the imaged object compared to its true size. Magnification factor M = SID ÷ SOD.

Shape distortion

Misrepresentation of the shape of an object caused by misalignment of the tube, part, or IR; includes foreshortening and elongation.

Foreshortening

Shape distortion where the object appears shorter than it really is, usually caused by part angulation relative to the IR while the tube and IR are properly aligned.

Elongation

Shape distortion where the object appears longer than it really is, usually caused by tube angulation or IR tilt relative to the object.

Compensating filter

A filter placed in the primary beam to even out exposure when part thickness/density varies across the field (e.g., wedge or trough filter). It attenuates the thin side more so the receptor sees more uniform exposure.