5.2 Compression Paddles, Grids, and System Geometry

Compression is an equipment function, not a finishing touch

Compression is built into the unit: a motorized drive, force and thickness displays, and interchangeable paddles. Adequate compression reduces compressed thickness, which shortens the path the beam must travel, lowers scatter generated in the breast, cuts mean glandular dose for a given detector exposure, immobilizes the breast so motion blur drops, and produces a more uniform thickness so automatic exposure control and the detector are not fighting a huge chest-wall-to-nipple gradient. Spreading overlapping fibroglandular tissue also improves the chance that a mass or calcification cluster is projected without superimposed density.

The Mammography Quality Standards Act (MQSA) equipment rule in 21 CFR 900.12 requires a compression device on every unit. Published FDA numbers — not unpublished ARRT trivia — are that the system must provide at least 111 N (25 lb) of force, and the maximum force for the initial power drive must fall between 111 N and 200 N (25–45 lb). Power-driven compression must be hands-free from both sides of the patient, with fine manual adjustment also available from both sides. How a medical physicist tests force, thickness readout, and paddle deflection belongs in the quality-control chapter. Here the point is what the hardware is supposed to do: apply a known, limited power drive, then allow the technologist to fine-tune without abandoning immobilization.

Clinical force is usually chosen for adequate immobilization and even thickness, not for hitting the 45 lb ceiling on every patient. Under-compression leaves thickness, scatter, and motion on the image; over-compression without a clinical reason is a patient-care failure, not “better physics.”

Paddle designs named on the content specifications

The ARRT Mammography specifications list fixed, flexed, curved, spot, and implant paddles. FDA additionally requires paddles that match each full-field receptor size. Screen-film and some digital systems provide 18 x 24 cm and 24 x 30 cm receptors; many full-field digital units use one detector with matching full-size paddles. Special-purpose paddles smaller than the receptor are allowed.

Fixed paddles are rigid and remain flat and parallel to the breast support. Under load they must not deflect from parallel by more than 1.0 cm at any point (FDA). They produce a uniform slab of tissue and a thickness readout that AEC and dose estimates can trust.

Flexed (flex or tilt) paddles are designed not to stay rigidly parallel. A spring or hinge at the anterior aspect lets the paddle follow the natural slope of the breast so the chest wall is not crushed while the anterior breast still receives adequate compression. FDA carves these out: equipment intended not to be flat and parallel must meet the manufacturer’s design specifications. Flex paddles often improve comfort and evenness. They can also change apparent thickness compared with a rigid paddle, which matters when you compare serial measurements or AEC behavior.

Curved paddles use a contour rather than a flat plate to match breast shape. Like flexed paddles, they are specialty geometry, not a “broken” fixed paddle. Use them only as the manufacturer and protocol intend.

Spot compression paddles are small — often only several centimeters across. They apply force over a limited region so adjacent tissue can bulge out of the way. Local thickness drops, superimposed densities separate, and geometric unsharpness over that spot decreases. That is why spot compression is a diagnostic workup tool rather than a screening default. FDA treats them as special-purpose paddles: they need not meet the full-field chest-wall edge rules (straight, parallel to the receptor, and not visible on the image) that apply to full-size paddles. The 1.0 cm parallel-deflection limit is the fixed-paddle rule; flexed and curved paddles follow the manufacturer's design instead.

Implant paddles are used when the implant is displaced (implant-displaced / Eklund-type views) or when a smaller footprint is needed so the implant can be held posterior while anterior native tissue is compressed. Match the paddle to the view. A full-size rigid paddle is not a substitute if the protocol calls for the implant paddle, and implant-displaced compression is not the same job as compressing a breast without an implant.

All full-field paddles share chest-wall design rules. The chest-wall edge must be straight and parallel to the receptor. It may be bent upward for comfort but must not appear on the image. A right-angle lip at the chest wall — a commonly specified design feature, often described as about 3 cm high in equipment literature — keeps axillary and abdominal tissue from draping into the field. If the lip prints over posterior breast, the paddle or receptor alignment has failed, not the patient.

Grids: typically on for contact, off for magnification

A compressed breast still produces scatter, especially as thickness and kVp rise. Contact imaging therefore typically uses a moving (reciprocating) grid in the Bucky so grid lines do not print as artifacts. Mammography grids are low ratio compared with general radiography — about 4:1 to 5:1 is typical equipment design — with high line frequency and low-attenuation interspace (carbon fiber or similar) so the already-soft beam is not wasted in the grid itself. A moving grid is an FDA requirement for screen-film receptors of each size; digital contact systems retain a grid for the same scatter reason.

Magnification mammography typically removes the grid. Raising the breast on a magnification platform creates an air gap. Scattered photons diverge and miss the detector, so the gap replaces the grid’s cleanup. Leaving a grid in during magnification would increase dose and exposure time on a heat-limited small focal spot without a proportional scatter benefit. If an item says “a grid is always used in mammography,” the magnification exception is the discriminator. If an item says “remove the grid on every contact CC to save dose,” that is the opposite error: contact geometry has little air gap, so scatter reaches the detector unless the grid is in.

System geometry: SID, OID, and magnification factor

Dedicated units use a short source-to-image distance (SID) compared with general radiography. About 65–66 cm is typical C-arm geometry; some digital units sit in a similar 65–70 cm band. Cite that as typical equipment design, not as a secret ARRT constant. General radiographic SIDs of 100 cm (table Bucky) or 180 cm (upright chest) would starve the receptor of photons at ~28 kVp and would make a compact rotating gantry impractical. The short SID is a compromise among output, geometric blur, and covering a 24 x 30 cm field with a small anode angle.

Object-to-image distance (OID) is small in contact imaging: the breast rests on the Bucky, with only the support, grid, and cover between tissue and detector. Magnification is produced by increasing OID — the breast is placed on a platform — while SID stays essentially fixed. Magnification factor is:

M = SID / SOD = SID / (SID − OID)

Typical platforms produce about 1.5× to 2.0× enlargement. As a geometry example, not an ARRT constant: SID 65 cm with SOD ~35 cm gives M ≈ 1.86. Because OID is large, geometric blur would explode with the large focal spot, so the small focal spot (~0.1 mm) is mandatory. The air gap provides scatter cleanup. Spatial resolution of microcalcifications and mass margins improves if blur and motion are controlled. Motion risk rises because small-spot mA is low and time is long — another reason compression and patient coaching still matter on magnification views.

The C-arm keeps the tube and receptor opposed as you rotate into craniocaudal and mediolateral oblique projections, so SID and chest-wall alignment do not change with gantry angle. Collimation is matched to the image receptor, not trimmed to the skin line; the field should include the chest-wall edge of the detector.

Exam scenario: a magnification view of calcifications is blurry while the contact craniocaudal image is sharp. Check that the small focal spot was selected and that the breast was actually on the mag stand (OID increased), not merely collimated tightly while still sitting on the Bucky. A second scenario: a contact image looks gray and low-contrast on a thick breast with the grid accidentally left out — restore scatter control; do not “fix” it by jumping to 70 kVp.