Ultrasound Physics (Principles, Transducers, Doppler & Artefacts)

Diagnostic ultrasound is produced by a piezoelectric transducer that converts electrical energy into high-frequency sound and the returning echoes back into signals, using the pulse-echo principle in which sound is reflected at interfaces between tissues of differing acoustic impedance. The fundamental frequency trade-off is between resolution and penetration: a higher-frequency beam gives better axial resolution but attenuates more quickly so penetrates less deeply, whereas a lower-frequency beam penetrates deeper but with poorer resolution. That is why superficial musculoskeletal structures, such as tendons and the rotator cuff, are imaged with a high-frequency linear probe, and deeper structures, such as the hip, with a lower-frequency curvilinear probe. The anisotropy artefact is the classic musculoskeletal pitfall and follows from the physics: tendons and other fibrillar structures return strong echoes only when the beam strikes them perpendicularly, so if the probe is angled even a few degrees off perpendicular, the tendon returns fewer echoes and appears falsely hypoechoic, mimicking a tear or tendinopathy. It is the commonest cause of a false-positive diagnosis, and it is avoided by keeping the transducer perpendicular to the structure and by tilting, the heel-toe manoeuvre, to confirm that the apparent hypoechoic area fills in when the angle is corrected. I would also use the helpful artefacts deliberately - acoustic shadowing behind a focus confirms calcification, and posterior acoustic enhancement behind a lesion confirms it is fluid - while recognising reverberation and mirror-image artefacts. Because ultrasound is real-time, dynamic and radiation-free but highly operator-dependent, correct technique and machine settings are essential.