Beam geometry, FS blur, ANode heel, Comp filters,Distortion pt 2

Distortion

Defined as any misrepresentation of the true size or shape of an object as it appears on a radiographic image. There are two categories: size distortion (magnification) and shape distortion (foreshortening and elongation).

Size Distortion (Magnification)

Due to the divergence of the x-ray beam, all objects will appear magnified on the image. The amount of magnification depends on geometric factors such as SID (inverse relationship) and OID (direct relationship).

Magnification Factor Definition

The magnification factor is the ratio that quantifies how much an object is magnified on the image.

Magnification Factor Method 1 (Object and Image Size)

MF = Image size ÷ Object size. Example: An object is 2 cm wide and appears 2.8 cm on the image. MF = 2.8 ÷ 2 = 1.4.

Magnification Factor Method 2 (Using Geometry)

MF = SID ÷ SOD. Object size = Image size ÷ MF. Example: SID = 102 cm, SOD = 70 cm → MF = 102 ÷ 70 = 1.46. If image width = 2.5 cm, object width = 2.5 ÷ 1.46 = 1.71 cm.

Shape Distortion

Occurs when the shape of the object is misrepresented on the image. Two types: foreshortening and elongation.

Foreshortening

Occurs when the object appears shorter than its true size. Caused by misalignment of the part in relation to the image receptor (IR).

Elongation

Occurs when the object appears longer than its true size. Caused by misalignment of the central ray (CR) with the image receptor (IR), typically due to CR angulation.

Anode Heel Effect

An effect that causes a decrease in beam intensity toward the anode side of the x-ray tube. This happens because photons are produced at different depths within the anode target; photons produced deep within the target are more attenuated by the anode material.

Anode Heel Effect Positioning

To take advantage of the anode heel effect, position the x-ray tube so that the cathode side is aligned with the thicker part of the patient’s anatomy. Examples: AP foot (heel under cathode side), decubitus chest or abdomen, and femur (hip under cathode side).