Ultrasound: echogenicity, probes, Doppler and artefacts
- Modality
Ultrasound sends sound into the body and listens for echoes, building a live picture without radiation.
What it is
An ultrasound probe holds piezoelectric crystals that turn electricity into short pulses of high-frequency sound and turn the returning echoes back into electricity. The machine assumes sound travels through soft tissue at about 1540 m/s, so the time each echo takes to return gives its depth, and its strength gives its brightness. Echoes form where two tissues of different acoustic impedance meet: small differences give useful grey shades, while soft tissue against gas or bone reflects almost everything, which is why gel is needed and why bowel gas and bone hide what lies behind them. Ultrasound is real-time, portable, cheap and free of ionising radiation, but the picture depends on the operator, the window and the patient's build.
Key facts
- Anechoic (black): simple fluid such as urine, bile, a simple cyst or fresh free fluid.
- Hyperechoic (white): bone surfaces, stones, calcification, gas, fat interfaces and the diaphragm.
- Posterior acoustic shadowing: a dark band behind a stone or bone, because almost nothing passes through.
- Posterior acoustic enhancement: a bright band behind fluid, because fluid weakens the sound less than tissue.
- Reverberation: repeated echoes between two strong reflectors, such as A-lines behind the pleura.
- Mirror image: a copy of the liver or spleen appears above the diaphragm, a normal sign of aerated lung.
- Doppler shift depends on the angle: flow at 90° to the beam gives no signal; keep the angle at 60° or less for speed measurements.
- Colour Doppler convention BART: blue away from the probe, red towards it; the colour shows direction, not artery or vein.
How the image is made
- Echoes come back from interfaces between tissues; the bigger the impedance difference, the brighter the echo.
- Depth is calculated from the return time of each echo, assuming a fixed speed of sound in soft tissue.
- Higher frequency gives finer detail but less depth; lower frequency reaches deeper with coarser detail.
- Linear probe (high frequency, flat): vessels, thyroid, pleura, tendons, nerves, superficial lumps.
- Curvilinear probe (low frequency, wide fan): abdomen, pelvis, obstetrics, FAST.
- Phased-array probe (low frequency, small footprint): the heart, scanning between ribs.
- Gain brightens or darkens the whole image; depth and focus should be set to the structure of interest.
What it is good for
- Fluid questions: free fluid, effusions, bladder, gallbladder, cysts versus solid lumps.
- Bedside answers in emergencies: FAST, lung, focused cardiac and leg-vein scans.
- Obstetrics, gynaecology, children and anyone in whom radiation should be avoided.
- Guiding needles: vascular access, drainage, biopsy and nerve blocks.
- Doppler: blood flow, its direction and speed, stenosis and thrombosis.
| Term | What it looks like | Typical examples |
|---|---|---|
| Anechoic | Black, no echoes | Urine, bile, simple cyst, fresh blood or free fluid |
| Hypoechoic | Darker than the tissue around it | Lymph node, many solid tumours, acute thrombus that is not fully black |
| Isoechoic | Same grey as the tissue around it | A lesion that only shows by its edge or mass effect |
| Hyperechoic | Brighter than the tissue around it | Bone surface, stone, calcification, fat, gas |
Limitations and risks
- Operator-dependent: the result is only as good as the scanner's skill and the views obtained.
- Gas and bone block the beam: bowel gas, obesity, dressings and surgical emphysema limit the view.
- A normal focused scan answers only the question asked; it does not exclude disease elsewhere.
- Artefacts can mimic or hide disease: shadowing can hide a mass, and enhancement can make the tissue behind a cyst look abnormally bright.
- Aliasing: when flow is faster than the machine can sample, colour wraps around and speeds are underestimated.
- No ionising radiation, but heat and mechanical effects exist: keep exposure as low as reasonably achievable, especially with spectral Doppler in early pregnancy.
Next step
- Choose the probe for the depth: linear for superficial, curvilinear for abdomen, phased array for the heart.
- If the window is poor, change the patient's position or the window before concluding the scan is normal.
- Record the views obtained and the question answered; save clips for review.
- Refer for a formal (comprehensive) ultrasound when the focused scan is unclear or a new finding needs characterising.
- Ask the radiologist when an artefact may be hiding or mimicking disease, or when CT or MRI would answer better.
Test yourself
Think of the answer, then tap the question to see it.
Which probe would you choose to look at a superficial vein, and why?
A high-frequency linear probe: high frequency gives fine detail, and the vein is near the surface, so the reduced depth does not matter.
What artefact appears behind a gallstone, and what appears behind the fluid-filled gallbladder?
Posterior acoustic shadowing behind the stone; posterior acoustic enhancement behind the gallbladder.
Why can a vessel show no colour Doppler signal even though blood is flowing?
If the beam is perpendicular to the flow the Doppler shift is near zero; tilting the probe to a smaller angle restores the signal.
On colour Doppler, does red mean an artery?
No. Colour shows direction relative to the probe (BART: blue away, red towards), not whether the vessel is an artery or a vein.
Connections
Educational content; it does not replace a radiologist's report and is not a clinical instruction. Each study is ordered on the patient's assessment and your institution's protocols.
Sources: Ultrasound Physics and Instrumentation (StatPearls (NCBI Bookshelf), 2023); Diagnostic Ultrasound Imaging: Physics Principles and Clinical Applications (StatPearls (NCBI Bookshelf), 2026); Doppler Ultrasonography (StatPearls (NCBI Bookshelf), 2023).
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Representative model, for educational purposes only.