14.1 Mediolateral (ML) and Lateromedial (LM)

Why a True Lateral Exists

Screening mammography is built on two views: craniocaudal (CC) and mediolateral oblique (MLO). Those two images are not 90° apart. The MLO is angled — commonly about 40°–60° — so the detector can follow the pectoralis muscle and include the axilla, the tail of the breast, and the inframammary fold in one exposure. That angle is a strength for tissue coverage and a weakness for geometry. A finding seen on only one of the screening pair still does not have a true orthogonal location, and calcium sitting in fluid will not reliably layer along a dependent wall.

The 90° lateral supplies that missing plane. ARRT lists it among the Procedures additional views as either mediolateral (ML) or lateromedial (LM). Both are true laterals: the C-arm is at 90°, the beam is horizontal, and superior/inferior relationships are measured from the nipple without the MLO's oblique distortion. Your exam job is to decide when to add one, which one to add, and how a finding should move when you compare it with the MLO.

A 90° lateral is also the mammographic projection that uses a true horizontal beam. That is the geometry that lets milk of calcium sediment in a cyst and present as a teacup or crescent. An MLO, no matter how steep, is still an oblique and can leave layering incomplete or misleading.

Beam Path, Detector Side, and OID

Projection names follow the central-ray path, not which way the patient is facing.

Geometric rule: put the lesion as close to the detector as you can. Object-to-image distance (OID) magnifies and unsharpens a finding. A lateral cluster imaged on LM sits far from the IR; the same cluster on ML sits against the detector and is sharper. Reverse that for a medial finding: choose LM so the medial breast is on the IR.

When the CC has not yet shown whether the finding is medial or lateral, many departments default to LM. The lateral margin is the mobile border, so you can pull more tissue onto the detector; the contralateral breast does not block the paddle; and offsetting the IR slightly across the sternum captures deep medial tissue. If the CC already shows a clearly lateral finding, switch to ML. Do not pick ML for a far-medial lesion just because "lateral" sounds like the default third view.

Triangulation: Muffins Rise, Lead Sinks

When a finding is visible on MLO but not on CC — or you need to predict the CC clock-face before you take another image — add a 90° ML and compare height relative to the nipple.

• Medial lesions move up on the true lateral compared with the MLO ("muffins rise").
• Lateral lesions move down ("lead sinks").
• Central lesions stay at nearly the same height.

The physical reason is the MLO angle. Oblique positioning lifts lateral and axillary tissue and drops medial tissue relative to a true lateral. Straightening the beam to 90° reverses that offset. The mnemonic applies only when one image is a true 90° lateral. Comparing two MLOs, or an MLO and a 60° "almost lateral," does not count.

Hanging-protocol method: align the nipples and order the images from steepest angle to flattest — 90° lateral, then MLO, then CC. A straight line through the finding on the lateral and the MLO points to where it should sit on the CC. If that line misses the CC field entirely, the finding may be so far medial or lateral that you need an exaggerated CC or a cleavage view rather than another straight lateral.

Worked example: A spiculated density sits 4 cm above the nipple on the MLO. On the 90° ML it sits 2 cm below the nipple. The finding fell, so it is lateral. Look in the outer breast on the CC (or add an XCCL if that tissue was not on the original CC). If the same MLO density had risen on ML, you would hunt the inner breast.

Milk of Calcium

Layering calcium in microcysts is a benign process, but only a true lateral proves it. On CC, milk of calcium looks smudgy, round, or amorphous — like looking down into a teacup of sediment. On a 90° lateral, particles settle along the dependent wall and form crescentic, concave-up teacups. An MLO may show partial layering and can be mistaken for coarse heterogeneous or amorphous calcifications that still need workup. If the radiologist's order is "prove milk of calcium," the correct add-on is ML or LM with a horizontal beam, not a steeper MLO and not a rolled CC.

Both ML and LM can show layering. Choose the one that also places any associated cyst or cluster closest to the detector.

Pre-wire Localization and Replacing the MLO

Needle or wire localization of a nonpalpable finding uses two views 90° apart, almost always CC plus a 90° lateral. The MLO is the wrong second view for this job because it is not orthogonal to CC; depth along the superior–inferior axis will be wrong. Choose ML or LM so the lesion is closest to the detector and the needle path from skin to lesion is the shortest. After the wire is placed, a pair of orthogonal views documents the hook relative to the finding before the patient leaves for the operating room.

A 90° lateral can replace the MLO when the patient cannot be positioned for an oblique: wheelchair or stretcher, severe kyphosis, pectus deformity, limited shoulder range of motion, or encapsulated implants that will not displace. Document that substitution as a limited study. The true lateral does not include as much posterior and axillary tissue as a well-positioned MLO. It is a poor choice for recovering posterolateral tissue that was missed on MLO — that job belongs to a better MLO or an axillary-tail view, not ML. ML can open an inframammary fold that folded on MLO, because the 90° angle and the absence of pectoralis in the field allow tighter anterior compression.

Labeling

MQSA-required identification still applies: patient name, a second identifier, exam date, facility (city, state, ZIP), technologist ID, and standardized laterality plus view. Label RML / LML or RLM / LLM — not "lat," not "90," and not MLO. Place the laterality marker toward the axilla (superior aspect) so superior and inferior are obvious at a glance. If the view replaced an MLO, say so in the annotation; filing an ML under an MLO hanging slot is a labeling error, not a positioning success.

Exam traps: choosing ML for a medial lesion (extra OID and blur); using MLO to "call" milk of calcium; treating a 90° lateral as equivalent posterior coverage to MLO; triangulating from two obliques instead of from a true lateral.

Exam Tip: If the question stem gives you a finding's height on MLO and then on ML, ignore the clock-face until you apply muffins rise / lead sinks. Movement tells you medial versus lateral; only then do you pick the next CC-family view.