Ultrasound Physics (Principles, Transducers, Doppler & Artefacts)

The transducer. A piezoelectric transducer converts electrical energy into high-frequency sound, and converts the returning echoes back into electrical signals. It both makes the sound and receives it.

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The pulse-echo principle. Sound is reflected at interfaces between tissues of differing acoustic impedance. The machine times each returning echo, its time-of-flight, to localise the structure and place the pixel, and from these echoes builds a real-time image with no ionising radiation.

Acoustic impedance. Impedance is tissue density multiplied by the speed of sound, Z = ρ × c, and the proportion of sound reflected at an interface depends on the mismatch in Z. A large mismatch, soft tissue against bone or gas, reflects almost all the sound: a strong echo, with shadowing beyond. A small mismatch, soft tissue against soft tissue, reflects a little and transmits the rest, which is what allows imaging.

The assumed speed of sound. To convert time-of-flight into depth, the machine assumes a fixed soft-tissue speed of about 1540 m/s. Real tissues differ, fat slower and bone far faster, and the difference is a source of some artefacts.

Attenuation. Sound weakens with depth through absorption, scatter and reflection, by roughly 0.5-1 dB/cm per MHz, so higher frequencies attenuate more; that is the physics behind their poorer penetration. Time-gain compensation (TGC) corrects for this depth-dependent loss.

The trade-off. This is the fundamental rule of the modality. Higher frequency gives better axial resolution but attenuates faster and so penetrates less; lower frequency penetrates deeper at the cost of resolution. Choose the probe to match the depth:

• High-frequency linear probes for superficial musculoskeletal structures
• Lower-frequency curvilinear probes for deeper structures, such as the hip

The two resolutions. Axial resolution, along the beam, improves with higher frequency, because the pulse is shorter. Lateral resolution, across the beam, improves with focusing and is best at the focal zone.

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Artefacts divide into those that help the diagnosis and those that cause misdiagnosis. Shadowing and posterior enhancement are used deliberately, anisotropy and mirror image mislead, and reverberation is recognised so that it is not mistaken for pathology.

Anisotropy. This is the most important practical pitfall in musculoskeletal ultrasound. Tendons and other fibrillar structures are highly reflective only when the beam strikes them perpendicularly; angled even a few degrees off, the structure returns fewer echoes and appears artefactually hypoechoic, exactly mimicking a tear or tendinopathy. It is the classic source of false-positive diagnoses.

Avoiding it. Keep the transducer perpendicular to the structure being examined. Then tilt it, the heel-toe manoeuvre, to confirm that an apparent hypoechoic area fills in when the angle is corrected.

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The helpful artefacts. Use them deliberately. Acoustic shadowing behind a focus confirms calcification, as in calcific tendinopathy; posterior enhancement behind a lesion confirms that it is fluid-filled or cystic, as in a ganglion.

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Reverberation and mirror image. These are recognised so that they are not mistaken for pathology. A comet-tail indicates metal, gas or a foreign body, and a mirror image is a false duplicate.

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Technique. Good musculoskeletal ultrasound depends on the right probe and frequency, machine settings (depth, focus, gain and TGC), positioning and standoff pads, all of which limit artefacts. Limiting and correctly interpreting artefacts, especially anisotropy, is what avoids diagnosing pathology where none exists. Ultrasound is real-time, dynamic, cheap and radiation-free, but highly operator-dependent, and correct technique is what separates a reliable study from a misleading one.

"Operator-dependent" is the stock criticism of ultrasound, and it is repeated so often that candidates assume the modality is second-rate. The accuracy data say otherwise for the commonest musculoskeletal indication.

Ultrasound against MRI for cuff tears. A meta-analysis of 65 studies, all requiring an open or arthroscopic surgical reference standard, found no significant difference in either sensitivity or specificity between MRI and ultrasound for partial-thickness or full-thickness tears (P greater than 0.05). The area under the summary ROC curve was in fact numerically higher for ultrasound (0.889) than for MRI (0.878), though that pairwise difference was not significant.

MR arthrography beat both. It was significantly more sensitive and specific than either (P less than 0.05), with the highest area under the curve at 0.935. The price is that it is invasive.

In the viva. Do not concede that ultrasound is inferior. For cuff tears it performs comparably to MRI at lower cost, in real time, with dynamic assessment and no ionising radiation, and MR arthrography is the most accurate if the added invasiveness is justified.

Where operator dependence bites. These are pooled published studies, largely from centres with a declared interest in musculoskeletal ultrasound, so the figures describe the modality in experienced hands. That is precisely what operator dependence means: the technique can match MRI, but only if the person holding the probe knows the anatomy, keeps the beam perpendicular and recognises the artefacts. The physics does not transfer with the machine.

What is not established. The figures are for the rotator cuff and should not be assumed to hold for every tendon, nerve or joint. No comparable pooled accuracy figure exists for most other musculoskeletal sites, and no published study measures how often anisotropy specifically produces a false-positive report.