Gastrointestinal and Hepatobiliary · Imaging physics

Abdominal Doppler and Elastography Physics Essentials

Protocol-first review of abdominal Doppler physics, artefact avoidance and quantitative elastography, covering insonation angle, aliasing, waveform criteria and Baveno-aligned liver stiffness thresholds.

This package covers the practical physics and protocol essentials of abdominal Doppler ultrasound and hepatic elastography: greyscale optimisation, insonation angle, the Nyquist limit and aliasing, colour versus power Doppler, spectral analysis, normal portal and hepatic venous waveforms, TIPS surveillance ranges, and strain versus shear-wave elastography with reliability and interpretation criteria.

Examination sequence
Perform greyscale imaging first, then colour Doppler, then spectral Doppler; optimise gain, depth, focus and probe selection before measuring any velocity.Velocity measurements made on poorly optimised settings are a recurring viva pitfall.
Doppler angle
Keep the insonation angle at or below 60 degrees; the Doppler frequency shift is proportional to the cosine of the angle, and at 90 degrees no shift is detected.Small angle errors above 60 degrees produce large velocity errors.
Nyquist limit and aliasing
The maximum detectable Doppler shift is half the pulse repetition frequency; aliasing appears as colour or spectral wraparound once this limit is exceeded.Corrections include raising the PRF, shifting the baseline, lowering the transmit frequency, using a shallower gate or switching to continuous-wave Doppler.
Colour Doppler display
Colour Doppler displays mean velocity and variance estimated by autocorrelation; it is angle-dependent and is not a quantitative measure of stenosis severity.Confirm any suspected stenosis with spectral Doppler.
Power Doppler
Power Doppler displays integrated Doppler amplitude; it is angle-independent, does not alias and is more sensitive to slow flow, but provides no velocity or direction information.It is vulnerable to flash artefact from motion; useful for renal cortical, testicular and transplant perfusion.
Spectral window and broadening
Laminar flow produces a clear spectral window; spectral broadening suggests disturbed flow or technical error.Exclude excessive gain, an oversized sample volume or a gate partly outside the lumen before diagnosing stenosis.
Sample volume placement
Use a small sample volume, typically 2-3 mm, positioned within the highest-velocity stream, and align the angle correction cursor parallel to flow.An oversized gate averages velocities and falsely broadens the spectrum.
Wall filter setting
The wall filter removes low-frequency high-amplitude signals from vessel wall motion; if set too high it erases low-velocity diastolic or venous flow.Use a low wall filter when assessing portal or renal venous flow.
Colour gain and persistence
Set colour gain just below the threshold at which random noise appears, and choose persistence and colour priority appropriate to the organ and flow speed.Excess colour gain simulates flow outside the lumen and mimics arteriovenous fistulae.
Resistive and pulsatility indices
Resistive index equals peak systolic velocity minus end-diastolic velocity divided by peak systolic velocity; pulsatility index additionally divides by the mean velocity.Normal renal interlobar resistive index is approximately 0.6-0.7; interpret against clinical context and local reference values.
Normal portal venous waveform
Portal venous flow is hepatopetal with gentle respiratory and cardiac phasic variation; the portal vein usually measures up to about 13 mm during quiet inspiration.Pulsatile portal flow suggests raised right-sided pressures or tricuspid regurgitation.
Normal hepatic venous waveform
Hepatic veins show a triphasic hepatofugal pattern with systolic and diastolic forward components and a small reversed atrial wave.A reversed or ventricularised waveform should prompt cardiac assessment.
TIPS Doppler surveillance
Commonly accepted main-stent peak systolic velocities lie roughly between 90 and 190 cm/s; values below about 50-60 cm/s, or a change of more than 50 cm/s from baseline, raise concern for shunt dysfunction.Thresholds vary by institution and guideline; correlate with portosystemic gradient data and clinical status.
Strain versus shear-wave elastography
Strain elastography measures relative tissue deformation and is qualitative or semi-quantitative; shear-wave elastography tracks shear-wave propagation to provide quantitative stiffness in metres per second or kilopascals.Stiffer tissue conducts shear waves faster; Young's modulus approximates to three times tissue density times shear-wave speed squared.
Liver shear-wave acquisition
Acquire in the supine or right lateral decubitus position through a right intercostal approach, at least 1 cm below the capsule and away from vessels, collecting at least 10 valid measurements.A common reliability target is an interquartile-range-to-median ratio of 30 percent or less; ascites and narrow intercostal spaces degrade acquisition.
Liver stiffness interpretation framework
Under the Baveno VII framework, liver stiffness below 10 kPa argues against compensated advanced chronic liver disease, 10-15 kPa is indeterminate, and above 15 kPa supports clinically significant portal hypertension.Cut-offs are aetiology- and device-specific; always state the device, failure rate and interquartile range.
Spleen stiffness and variceal risk
In patients with liver stiffness below 15 kPa, spleen stiffness below 21 kPa rules out high-risk varices, while spleen stiffness above 55 kPa rules them in.Baveno VII positions spleen stiffness as a triage adjunct, not a standalone diagnostic test.
Elastography pitfalls and safety
Falsely high stiffness occurs with congestion, acute inflammation, cholestasis, recent food intake and subcapsular measurements; apply ALARA principles and monitor thermal and mechanical indices.Report the device, probe, number of valid measurements, median value and interquartile range.

Caution

Doppler velocities and elastography thresholds are device-, vendor- and population-specific. Baveno VII cut-offs apply to chronic liver disease pathways and must not be extrapolated to acute hepatitis or acute portal vein thrombosis. Follow institutional protocols and current society guidance for surveillance intervals.

Exam pearl

High-yield viva pairs: the Nyquist limit with four aliasing corrections; the resistive index formula with causes of a raised renal resistive index; strain versus shear-wave elastography; and the Baveno VII liver stiffness thresholds. Always state units, the device used and reliability criteria when quoting stiffness values.

Viva questions

Define the Nyquist limit and list four ways to correct aliasing on pulsed-wave Doppler.
The Nyquist limit is half the pulse repetition frequency and represents the maximum Doppler shift that can be measured without aliasing. Corrections include raising the PRF, shifting the baseline, lowering the transmit frequency, using a shallower sample gate, or switching to continuous-wave Doppler.
Compare colour Doppler and power Doppler.
Colour Doppler encodes mean velocity and variance, is angle-dependent and can alias. Power Doppler encodes signal amplitude, is angle-independent, does not alias and is more sensitive to slow flow, but provides no velocity or direction information and is prone to flash artefact.
What makes a liver shear-wave elastography acquisition reliable?
At least ten valid measurements taken from a right-lobe region at least one centimetre below the capsule, avoiding vessels and artefacts, with a small interquartile range relative to the median, commonly thirty percent or less. The device and failure rate should be reported.
How do Baveno VII liver stiffness thresholds guide assessment of compensated advanced chronic liver disease?
Liver stiffness below 10 kilopascals argues against compensated advanced chronic liver disease, 10 to 15 kilopascals is indeterminate, and above 15 kilopascals supports clinically significant portal hypertension and triggers decisions such as variceal screening, always interpreted with the clinical context and device used.
Give four causes of falsely high liver stiffness.
Common causes are hepatic congestion, acute inflammation or hepatitis, cholestasis, recent food intake, and technical factors such as a narrow intercostal window or measurements taken too close to the capsule.
Differentiate strain elastography from shear-wave elastography.
Strain elastography measures relative tissue deformation under compression and is qualitative or semi-quantitative, while shear-wave elastography measures the speed of an induced shear wave and provides absolute stiffness in metres per second or kilopascals, allowing serial comparison on the same platform.

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