FRCR Physics Ultrasound Revision Guide

How to think about this heading

Ultrasound currently contributes six stems. The heading links wave physics, transducer design, pulse-echo imaging, beam formation, resolution, Doppler, artefacts and biological safety.

The most effective approach is to keep the wave, pulse and image separate. Frequency and wavelength describe the wave; pulse duration and spatial pulse length determine axial resolution; aperture, focusing and depth shape the beam and lateral resolution.

Concept map

Wave propagation

Ultrasound is a mechanical pressure wave. In a given medium, speed equals frequency multiplied by wavelength. Diagnostic systems assume an average soft-tissue speed for distance calculation.

Acoustic impedance and interfaces

Acoustic impedance is density multiplied by sound speed. Reflection depends on the impedance mismatch and angle at an interface; most energy may still transmit when the mismatch is small.

Transducers

Piezoelectric elements convert electrical energy to ultrasound and returning pressure waves to electrical signals. Backing, matching layers, damping and array geometry affect bandwidth, pulse length and beam formation.

Pulse-echo localisation

Depth is inferred from round-trip travel time using the assumed propagation speed. Pulse repetition frequency is limited by the time required for echoes from the chosen depth to return.

Resolution

Axial resolution depends primarily on spatial pulse length; lateral resolution depends on beam width, focus and depth. Elevational resolution is governed by slice thickness in the out-of-plane direction.

Doppler and safety

Doppler shift depends on transmitted frequency, target velocity and the cosine of the insonation angle. Thermal index and mechanical index are displayed safety indicators rather than direct measurements of tissue injury.

High-value relationships and trade-offs

Frequency, wavelength and penetration

At a fixed sound speed, higher frequency gives shorter wavelength and can improve spatial resolution, but attenuation rises with frequency and useful penetration falls.

Axial resolution

Axial resolution is approximately half the spatial pulse length. Fewer cycles and shorter wavelength improve it; damping helps shorten the pulse at the cost of reduced transducer sensitivity.

Attenuation

Attenuation increases with path length and frequency in soft tissue. Gain can amplify returning signals but cannot restore information that was never received.

Doppler angle

Measured Doppler shift is proportional to cosθ. It approaches zero near 90 degrees, and small angle-estimation errors become increasingly important at large insonation angles.

Aliasing

Pulsed-wave Doppler aliases when the Doppler shift exceeds the Nyquist limit of half the pulse repetition frequency. Raising PRF or scale, lowering transmitted frequency or shifting the baseline can improve display, each with limitations.

Depth and PRF

Greater imaging depth requires more listening time and therefore limits maximum PRF. This links deep targets to a lower Nyquist limit in pulsed Doppler.

Common True/False traps

Saying higher frequency penetrates more deeply because it carries more energy
Confusing axial resolution with beam width or lateral resolution with pulse length
Treating gain as a way to improve true SNR or recover absent echoes
Assuming no Doppler shift at 90 degrees means there is no flow
Calling every duplicated structure mirror artefact without locating a strong reflector
Treating thermal index or mechanical index as exact patient temperature rise or injury probability

A practical revision framework

Draw one pulse-echo cycle

Mark transmission, propagation, interface reflection, return, transducer conversion, amplification and display. Attach each assumption and artefact to the relevant stage.

Separate three dimensions of resolution

Create a table for axial, lateral and elevational resolution: physical determinant, best depth and the parameter that improves or worsens it.

Learn artefacts by violated assumption

Reverberation, mirror, refraction, speed error, range ambiguity, shadowing and enhancement become easier when you identify which pulse-echo assumption failed.

Practise Doppler as a system

Work through angle, PRF, depth, frequency, wall filter, sample volume, spectral broadening and aliasing together rather than memorising isolated fixes.

Readiness checklist

Answer these aloud without notes. They are concept prompts, not copied examination or question-bank items.

I can use c = fλ and predict what changes in a fixed medium.
I can explain acoustic impedance, reflection and transmission at an interface.
I can describe how a piezoelectric array transmits, receives and focuses ultrasound.
I can distinguish axial, lateral and elevational resolution.
I can explain attenuation, time-gain compensation and why gain does not restore lost information.
I can derive the direction of Doppler changes from frequency, velocity and angle.
I can recognise the main pulse-echo and Doppler artefact mechanisms.
I can explain thermal and mechanical indices without treating them as direct injury measurements.

Frequently asked questions

Which ultrasound relationships are most important?

Prioritise c = fλ, acoustic impedance, attenuation with frequency and depth, spatial pulse length and axial resolution, Doppler angle dependence, PRF and the Nyquist limit.

Do I need to memorise the speed of sound in every tissue?

Know the standard soft-tissue assumption and understand that different true propagation speeds create localisation and refraction errors. An exhaustive table is less useful than the principle.

How should I revise ultrasound artefacts?

Classify each by the assumption that failed—single reflection, straight-line travel, constant speed, correct range or adequate sampling—then link it to appearance and a practical control.

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Ultrasound for FRCR Part 1 Physics