11.2 Grids

Grid Purpose and Construction

A grid is a device of thin lead strips separated by radiolucent interspace material (aluminum or fiber), placed between the patient and the image receptor. Its job is to absorb scattered radiation before it reaches the receptor, thereby preserving contrast. Scattered photons travel at oblique angles after Compton interactions; the tall, narrow lead strips stop them while allowing the aligned primary beam through. Gustav Bucky introduced the grid in 1913, which is why the moving-grid mechanism is still called the Bucky. Grids are the most effective scatter-cleanup tool for thick, high-kVp anatomy.

A grid is generally indicated when the body part exceeds about 10 cm in thickness or the technique exceeds about 60 kVp, because both conditions generate substantial scatter. Below those thresholds a grid adds dose without meaningful contrast benefit.

Grid Ratio and Frequency

Grid ratio is defined as the height of the lead strips divided by the distance (width of the interspace) between them — ratio = h/D. Common ratios are 5:1, 6:1, 8:1, 10:1, 12:1, and 16:1. A higher grid ratio removes more scatter, raising contrast, but it is less forgiving of positioning errors and requires more mAs (more patient dose). Grid frequency is the number of lead strips per unit length, expressed in lines per inch or lines per centimeter (typically 60-200 lines/inch, or about 25-80 lines/cm). Higher frequency means thinner strips and less visible grid lines, but for a given lead content, higher frequency can lower cleanup efficiency unless ratio is maintained.

The Bucky (Grid Conversion) Factor

Because a grid absorbs some primary beam along with scatter, you must increase mAs to maintain receptor exposure when you add or change a grid. The Bucky factor, also called the grid conversion factor (GCF), quantifies that mAs increase. It rises with both grid ratio and kVp. Approximate values (near 80-100 kVp) are shown below; convert technique with mAs2 = mAs1 x (GCF2 / GCF1).

Worked example: a non-grid technique used 4 mAs (GCF 1). Converting to a 12:1 grid (GCF 5): mAs2 = 4 x (5/1) = 20 mAs. That five-fold increase is the dose price paid for the contrast improvement — one reason collimation (Section 11.3) is preferred first when possible.

Grid Types

• Parallel (linear) grid: lead strips run parallel to one another. Because the strips do not match beam divergence, a parallel grid shows grid cutoff toward the lateral edges, worst at short SID and with wide collimation.
• Focused grid: strips are progressively angled to coincide with the divergence of the x-ray beam, converging at a focal point. It works only within a specified focal range (grid radius) of SIDs and must be centered and level.
• Crossed (cross-hatch) grid: two linear grids superimposed at right angles for maximum cleanup; requires perpendicular, centered beam (no tube angulation).
• Moving grid (reciprocating Bucky): the grid oscillates during exposure so the grid lines blur out and are not imaged. Stationary grids leave fine visible lines.

Choosing and Handling a Grid

Select grid ratio for the scatter conditions: low-ratio grids (5:1-8:1) are used at lower kVp and for mobile work where centering is hard to control, while high-ratio grids (12:1-16:1) are reserved for high-kVp, high-scatter exams because they clean up more scatter but tolerate almost no centering or tube-angle error. Focused grids also have a short dimension and a long dimension: the lead strips run along the long axis, so the tube may be safely angled along the strips but never across them. Many mobile grids are labeled with the focal range and a tube-side marker; ignoring these leads directly to the cutoff errors below. Modern DR units may apply virtual (software) grid processing that estimates and subtracts scatter without a physical grid, sparing the mAs penalty, but a physical grid remains the reference standard for thick anatomy.

Grid Errors and the Cutoff Each Produces

Grid cutoff is the unwanted absorption of primary photons by the lead strips, causing loss of receptor exposure. Memorize which error gives uniform cutoff versus peripheral cutoff:

Off-level is the most common error with tabletop and mobile exposures because the grid is not flat or the tube is angled across the strips. Off-center and off-level both darken/lighten the whole field uniformly, whereas off-focus and upside-down grids preserve the center and cut off the periphery. An inverted focused grid is the most dramatic: only a thin central band is properly exposed while both edges are severely cut off.

Grid-Line (Moire) Artifact

With computed radiography (CR), a stationary grid whose frequency interferes with the laser scan frequency produces evenly spaced light/dark bands called the moire (aliasing) artifact. Aligning the grid lines perpendicular to the scan direction or using a higher-frequency/moving grid prevents it.

Alternative: The Air-Gap Technique

Scatter can also be reduced without a grid using an air gap: increasing OID lets obliquely traveling scattered photons diverge past the receptor instead of striking it. The trade-off is magnification, so a longer SID is used to compensate (as in some lateral cervical-spine and magnification studies).