5️⃣ Scatter Control, Collimators, Compression & Grids

Scatter – what it is

Radiation that has changed direction after interacting with matter; in diagnostic radiography most scatter is produced by Compton interactions in the patient.

Scatter vs secondary radiation

Scatter refers to photons that have changed direction; secondary radiation is characteristic radiation produced when atoms are ionized (for example after photoelectric absorption). In practice both add unwanted exposure if they reach the detector or staff.

Effect of scatter on image contrast

Scatter that reaches the receptor adds uniform unwanted exposure (fog), which reduces contrast resolution and makes the image look more gray.

Effect of scatter on occupational dose

Scatter is the main source of radiation dose to radiographers and other staff around the x-ray room.

Main factors that increase scatter production

Higher kVp, larger field size, and thicker or denser body parts all increase the amount of scatter produced in the patient.

Ways to reduce scatter production

Collimate to the smallest practical field size, use lower kVp when appropriate, and apply tissue compression to reduce part thickness.

Ways to reduce scatter reaching the receptor

Use beam restriction (collimators), use a grid when appropriate, or use an air-gap technique so scattered photons miss the receptor.

Collimator – definition

A beam-restricting device with adjustable lead shutters (light-localizing variable-aperture collimator) that limits the x-ray field size to the area of interest.

Why collimation is important

Reducing field size decreases the volume of tissue irradiated, which reduces scatter production, reduces patient dose, and improves image contrast.

Tissue compression – definition and purpose

Applying pressure to a body part (for example in mammography) to reduce its thickness and motion; this lowers patient dose, improves spatial and contrast resolution, and decreases scatter production.

Grid – what it does

A device placed between the patient and the image receptor that preferentially absorbs scatter radiation while allowing most primary radiation to pass, improving image contrast.

Grid construction

Thin lead strips aligned with the primary beam, separated by radiolucent interspace material (such as aluminum or plastic/fiber), all enclosed in a protective cover.

Grid ratio – definition

Grid ratio = h / D, where h is the height of the lead strips and D is the distance between them (interspace width); higher ratios remove more scatter but require more mAs and have less positioning latitude.

Grid factor (Bucky factor) – definition

The ratio of radiation incident on the grid to the radiation transmitted through the grid; it indicates how much you must increase mAs when using a grid to maintain the same receptor exposure.

Using Bucky factors to change grids

When changing grids, use mAs₂ = mAs₁ × (B₂ / B₁), where B₁ and B₂ are the Bucky factors for the starting and new grids.

Types of grids – basic

Common grid types include parallel and focused grids, and stationary grids (which may show lines) versus moving Bucky grids (which blur the lines during exposure).

Grid cutoff – definition

Unwanted loss of primary radiation to the receptor due to improper grid use or design, resulting in reduced image density, often more severe at the edges.

Common causes of grid cutoff

Off-level (tilted) grid, off-center grid relative to the central ray, off-focus use of a focused grid at the wrong SID, and placing a focused grid upside-down.

Air-gap technique – definition

A scatter-reduction method in which the patient is moved away from the receptor to create a large OID so that many scattered photons miss the receptor; often combined with increased SID to control magnification.