ICU · equipment-physics
Ultrasound & Imaging Physics — Comprehensive (Piezoelectric Effect, Frequency Versus Penetration, Axial and Lateral Resolution, B-Mode and M-Mode, Doppler, Artefacts, Probe Types)
Also known as Ultrasound physics · Piezoelectric effect · Pulse-echo principle · Acoustic impedance · Axial resolution · Lateral resolution · Spatial pulse length · B-mode · M-mode · Doppler ultrasound · Nyquist limit · Aliasing · Ultrasound artefacts · Acoustic shadowing · Reverberation · Ring-down artefact · Curvilinear probe · Phased array probe
Ultrasound and imaging physics for the ICU First Part: the PIEZOELECTRIC effect (crystals convert electrical energy to mechanical sound energy on transmission and back to electrical energy on reception), the pulse-echo principle (time-gated depth from the 1540 m/s soft-tissue speed of sound), the central trade-off between FREQUENCY and PENETRATION (higher frequency gives better resolution but penetrates less deeply because attenuation rises with frequency, so deep/cardiac use 2-5 MHz and superficial/vascular use 10-15 MHz), acoustic impedance and reflection (why air and bone obstruct and why coupling gel is used), AXIAL resolution (the ability to distinguish two objects along the beam — equals half the spatial pulse length, improved by higher frequency and shorter pulse length), LATERAL resolution (the ability to distinguish two objects side by side — improved by focusing the beam and narrowing its width), imaging modes (B-mode greyscale, M-mode motion over time, colour Doppler for direction and speed, pulsed-wave and continuous-wave spectral Doppler), the Nyquist aliasing limit, the common ARTEFACTS (acoustic shadowing behind calcified structures like gallstones and bone, acoustic enhancement behind fluid, reverberation, mirror image, and ring-down from gas), and the probe types (curvilinear for abdominal and POCUS, linear for vascular, phased array for cardiac).
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Target exams
Red flags
- Air and bone reflect almost all ultrasound — the aerated lung and gas-filled bowel cannot be imaged through, and bone casts a dense acoustic shadow; choose acoustic windows that avoid bowel gas and assess the lung by its artefactual patterns rather than parenchyma
- Doppler is angle-dependent and is BLIND at 90 degrees — the shift is proportional to the cosine of the beam-to-flow angle, maximal when aligned with flow and zero at 90 degrees; misalignment underestimates velocity (a common source of error in estimating pulmonary pressures from tricuspid regurgitation)
- Pulsed-wave Doppler aliases above the Nyquist limit (half the pulse repetition frequency) and underestimates peak velocity — use continuous-wave Doppler for high-velocity jets such as aortic stenosis, or raise the PRF and align the beam
- Higher frequency improves resolution but PENETRATES LESS — using a high-frequency probe on a deep target (e.g. a 10 MHz linear probe on an abdominal aorta) yields a beautifully resolved near-field but a dark, uninformative deep field
- Acoustic shadowing can MIMIC pathology — a gallstone or rib shadow can hide a structure behind it, so scan from multiple angles and windows rather than accepting a single shadowed view
- Ring-down and reverberation from bowel gas can masquerade as lung B-lines or obscure the abdomen — recognise the artefact, reposition the probe, and do not over-call pathology from a single artefact pattern
Meet the patient
A registrar images the abdominal aorta with a 10 MHz linear probe and gets a beautifully resolved near-field over a dark, empty deep field. On the gallbladder there is bright enhancement behind a fluid-filled structure and a dense clean shadow behind an echogenic focus within it. In the dependent lung zones, vertical ring-down artefacts stream from the pleural line to the bottom of the screen.[1][4]
That one vignette plants the whole topic: why the deep field went black, what the two gallbladder artefacts are, and what a B-line actually is. Frequency-versus-penetration, shadowing with enhancement, and ring-down — own those three and you own ultrasound physics for the First Part.[1][2]
References4ShowHide
- [1]Feldman MK, Katyal S, Blackwood MS US artifacts Radiographics, 2009.PMID 19605664
- [2]Volpicelli G, Elbarbary M, Blaivas M, et al. International evidence-based recommendations for point-of-care lung ultrasound Intensive Care Med, 2012.PMID 22392031
- [3]Bicalho RC, Galvão KN, Guard CL, et al. Optimizing the accuracy of detecting a functional corpus luteum in dairy cows Theriogenology, 2008.PMID 18468674
- [4]Sims MW, Margolis DJ, Localio AR, et al. Impact of pulmonary artery pressure on exercise function in severe COPD Chest, 2009.PMID 19318664