Image Production · Grids

Grid ratio and grid cutoff: cleanup, dose & alignment errors

A grid cleans scatter to improve contrast, but it also absorbs some primary beam—so technique (and patient dose) must rise. Grid ratio sets how selective the channels are. Misalignment produces grid cutoff. This diagram shows both the trade and the failure modes.

Diagram of a focused grid showing strip height, interspace width, and primary-beam paths through the channels

Grid ratio

Grid ratio = strip height ÷ interspace width

Higher ratio → narrower acceptance angle → better scatter cleanup and less tolerance for positioning error.

Bucky factor / dose trade

Because the grid removes scatter and some primary, mAs must increase to keep receptor exposure. That multiplier (Bucky factor / GCF) is also how much patient dose rises. Higher ratio and higher kVp generally mean a larger factor.

Grid conversion on the ARRT formula sheet →

Grid cutoff (break it on purpose)

Focused grids show little cutoff when level, centered, right-side up, and used at focal distance. Classic errors: off-level, off-center, off-focus (wrong SID), upside-down—each fails differently. Learn the pattern, not just the name.

Frequently asked questions

What is grid ratio in radiography?

The height of the lead strips divided by the width of the interspace. Higher ratios clean scatter more aggressively and demand more precise alignment.

Why does using a grid increase patient dose?

The grid absorbs scatter and some primary radiation, so mAs must increase to maintain receptor exposure. That mAs increase is what raises dose.

What causes grid cutoff?

Misalignment of a focused grid—commonly off-level, off-center, off-focus (wrong SID for the grid), or upside-down—so primary rays hit strips instead of interspaces.

Interactive diagram

Free layer covers ratio, Bucky factor, and cutoff types. Unlock full access to scrub alignment errors and linked practice.

Correct alignment · 8:1 grid · 70 kVp · no adjustment · No cutoff

Nothing cut off — this is the reference: The grid is level, centered to the central ray, the right way up, and used at its focal distance. Every primary ray runs down its own interspace, so nothing is cut off — a focused grid, properly positioned, exhibits no grid cutoff at all.
An 8:1 grid accepts rays up to 7.1° off: Grid ratio is the height of the strip divided by the width of the interspace. That single number sets how far a ray may deviate and still get down the channel — the taller and narrower the channel, the smaller the angle it accepts. It is why a high-ratio grid cleans up scatter better, and, in the same breath, why it cuts off more readily when you misalign it. Cleanup figures are published only for the ends of the range, so none is stated here.
Bucky factor 3× at 8:1 and 70 kVp: A grid absorbs scatter, but it absorbs some primary radiation too, so the technique has to go up to put the same exposure on the receptor. The Bucky factor is how much — and it is also how much the patient dose goes up. It climbs with grid ratio, and it climbs with kVp, because higher kVp makes more scatter and that scatter has a harder time getting through. Raising the ratio therefore buys contrast and costs dose, which is the trade the exam asks you to reason about.

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