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Ultrasound physics and Doppler
High frequency gives good resolution and shallow penetration, low frequency gives deep penetration and poor resolution; echoes arise at boundaries between different acoustic impedances, and Doppler measures the frequency shift of the echo to calculate flow velocity.
Why it matters in the exam
The standard question: what happens when the transducer is changed from 5 MHz to 2 MHz? Penetration increases and resolution falls. So a high-frequency linear transducer (7–18 MHz) is used for thyroid, breast, testes, tendons and superficial vessels, and a low-frequency curved transducer (2–5 MHz) for the abdomen and pelvis. The machine assumes a sound speed of 1540 m/s in soft tissue. Reflection depends on the difference in acoustic impedance between two tissues: at a boundary with air or bone almost the whole wave returns, so nothing is seen beyond it. TGC (time gain compensation) amplifies echoes from depth to compensate for attenuation; if echoes at 4–6 cm are weak — raise the TGC at that depth. Harmonic imaging improves resolution and reduces artefacts.
Artefacts are part of the diagnosis. Posterior acoustic shadowing behind a stone or calcification; posterior enhancement behind a fluid-filled cyst; "comet-tail" artefact from cholesterol crystals in gallbladder adenomyomatosis; "ring-down" from air bubbles; mirror image near the diaphragm; and twinkling on colour Doppler behind a stone. The WES sign (wall-echo-shadow) describes a gallbladder full of stones: wall, echo, then shadow.
Doppler: the frequency shift is proportional to velocity and to the cosine of the angle between beam and flow, so the angle should be 60° or less. Colour Doppler shows direction and mean velocity; power Doppler is more sensitive to slow flow and angle-independent but shows no direction; spectral Doppler measures velocities. Aliasing appears when the frequency shift exceeds half the PRF (the Nyquist limit) — raise the scale, lower the baseline or lower the frequency. Spectral broadening — filling of the "window" under the curve — indicates turbulent flow, for example beyond a stenosis. Resistive index RI = (PSV minus EDV) divided by PSV; in the kidney a value above 0.7 is considered high. Waveforms: internal carotid, renal and hepatic arteries — low resistance with diastolic flow; limb arteries at rest — triphasic with high resistance.
Key points
- High frequency — resolution; low — penetration.
- Reflection depends on the acoustic impedance difference.
- Shadow — stone; enhancement — fluid; comet tail — cholesterol crystals.
- Doppler angle 60° or less.
- Spectral broadening — turbulence.
Summary table
| Topic | Rule | Example | Typical question |
|---|---|---|---|
| Frequency | High → resolution; low → penetration | 5 MHz to 2 MHz: penetration up, resolution down | Choosing a transducer |
| TGC | Amplifies echoes from depth | Weak echoes at 4–6 cm → raise it there | What to do with weak echoes |
| Reflection | Depends on the acoustic impedance difference | Air and bone reflect almost everything | Why nothing is seen through gas |
| Artefacts | Shadow, enhancement, comet tail, mirror | Stone → shadow; cyst → enhancement | Recognising a finding |
| Doppler | Frequency shift depends on velocity and angle | Angle 60° or less; aliasing above Nyquist | Spectral broadening = turbulence |
A free exam question
TGC ( Time Gain Compensation )היא מערכת שמטרתה: (the question as examined, in Hebrew)
- א. לפצות על ההנחתה של האלומה בדימות על-קול
- ב. להקטין את ארטיפקט הצל האקוסטי
- ג. לשפר את ה ניגודיות של התמונה
- ד. ליצור אלומה בעלת מוקד משתנה
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Sources
- Bushberg et al., The Essential Physics of Medical Imaging, 4th edition (2020) — Ultrasound
- AIUM Practice Parameter for the Performance of Peripheral Arterial Ultrasound Examinations Using Color and Spectral Doppler Imaging, 2020 — Doppler technique
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.