Sound Waves

For Medical Students

What It Shows

An ultrasound probe emits a pulse of sound and builds the image from the echoes that come back, timing each one to place it at a depth. 5.2.8 covers reflection.

Sound travels through soft tissue at a nearly constant speed, which is the assumption the depth scale rests on. 5.2.9 covers refraction.

It matters because sound is absorbed as it travels, so the useful depth is limited and the knee's deep structures are out of reach. 5.2.7 covers the frequency selection.

How It Is Acquired

The probe both transmits and receives, and the machine converts echo delay into depth. 5.2.8 covers reflection.

Attenuation increases with depth and with frequency, which is the trade the next page is about. 5.2.7 covers the frequency selection.

Bone reflects almost all the sound at its surface, so nothing beyond a cortical surface is imaged. 5.2.8 covers reflection.

Gain and depth settings are adjusted live, so the operator is part of the acquisition in a way no other modality requires. 5.4.1 covers the anterior knee.

The image is formed in real time, which is what allows a structure to be imaged while it moves. 5.4.15 covers dynamic instability.

MRCS Level

How It Is Acquired

In a viva the expected answer explains why ultrasound is excellent for the patellar tendon and useless for the cruciate.

The expected points are the echo timing, the attenuation with depth, the bone surface, the live settings, and real time imaging.

Say that the cruciate ligaments sit behind bone and cannot be reached. 5.4.1 covers the anterior knee.

Say that the superficial tendons are where ultrasound outperforms magnetic resonance imaging for resolution. 5.4.6 covers the patellar tendon.

Say that real time scanning answers dynamic questions no static study answers. 5.4.14 covers patellar tracking.

How Good It Is

Ultrasound is the highest resolution modality for a superficial knee structure and blind to anything deep or behind bone.

Its best contribution is the superficial tendon and the dynamic question, and its resolution there exceeds any magnetic resonance sequence in routine use. 5.4.6 covers the patellar tendon.

It is limited by depth, by bone, and by being operator dependent in a way that makes a negative study hard to interpret without knowing who did it. 5.4.1 covers the anterior knee.

No figures appear for accuracy on this page, under decision 028, and no sound speed or attenuation value appears either: those are tabulated physical quantities and this environment cannot check them.

FRCS Level

How Good It Is

At decision level ultrasound is chosen for a specific superficial question or a dynamic one, and for nothing else about the knee.

A tendon question at the front of the knee is answered faster and at higher resolution here than anywhere. 5.4.6 covers the patellar tendon.

A dynamic question, such as a tendon that snaps or a patella that tracks abnormally, has no static equivalent. 5.4.13 covers snapping tendons.

An intervention is guided better here than by any other modality, because the needle and the target are seen together in real time. 5.4.16 covers the knee injection.

An intra-articular question is the wrong question for ultrasound, and asking it produces a confidently negative study. 5.1.3 covers choosing the correct investigation.

Where It Misleads

The commonest error is requesting ultrasound for an intra-articular structure. 5.1.3 covers choosing the correct investigation.

A negative study is read as excluding a lesion the probe could not reach. 5.1.6 covers imaging pitfalls.

Acoustic shadowing behind a calcified structure is read as a lesion. 5.2.19 covers artefacts.

The depth scale is trusted where the sound speed assumption fails. 5.2.9 covers refraction.

The operator's experience is not recorded, so a negative cannot be weighted. 5.20.1 covers structured reporting.

Fellowship Level

Where It Misleads

At fellowship level ultrasound's dependence on the operator is its defining property and the one least captured by any report.

The study is a live examination recorded as a few images, so the reader of the report cannot review what was seen the way they can review a magnetic resonance study. 5.20.1 covers structured reporting.

That makes a negative ultrasound a statement about a particular examination rather than about the knee, and the distinction is rarely made explicit. 5.1.6 covers imaging pitfalls.

Published accuracy for knee ultrasound therefore reflects the operators in the study as much as the technique, which is why figures transfer poorly between centres. 5.21.1 covers the meniscal imaging literature.

The dynamic examination is its unique contribution and the hardest to standardise, so the thing it does best is the thing least comparable between departments. 5.4.15 covers dynamic instability.

The practical answer is to treat a knee ultrasound as a consultation with the person holding the probe, and to ask them the question directly. 5.4.1 covers the anterior knee.