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Also called: Ultrasound knobology, Ultrasound settings, Image optimisation

Ultrasound Knobology: The Settings Behind a Good Image

A handful of controls decide most of image quality. What each one does, the usual mistakes, and how to use them to judge a machine before you buy it.

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The core B-mode controls

Start with the right probe and preset. A preset loads the frequency, gain, dynamic range, processing and measurement package designed for one exam — abdomen, thyroid, early pregnancy, vascular, cardiac. Many systems also have a one-touch optimise button that sets gain and TGC automatically. Then adjust:

ControlWhat it doesCommon mistake
FrequencyHigher frequency gives finer detail; lower frequency reaches deeperStaying on a high-resolution setting when the target is deep and the image is grainy
DepthSets how far into the body the screen showsToo much depth, leaving the target small at the top of the screen
GainBrightens or darkens all echoes, like the volume on a radioToo much fills cysts and vessels with grey noise; too little hides weak echoes
TGC (time gain compensation)Sliders that adjust gain at each depth to make up for sound weakening as it travelsSliders left in odd positions, giving bright or dark bands across the image
FocusNarrows the beam where detail matters mostFocal marker left well above or below the area of interest
Dynamic rangeSets how many shades of grey are shownToo low looks harsh; too high looks flat and washed out
ZoomEnlarges an area; write zoom rescans it at higher detailEnlarging a frozen image (read zoom) when write zoom would show more

Read next Probe types and frequencies

Processing options

Tissue harmonic imaging

Builds the image from harmonic echoes generated in tissue. It reduces haze and clutter, especially in patients who are hard to scan, at some cost to penetration.

Compound imaging and speckle reduction

Combines several beam angles or smooths speckle for a cleaner image. It can also soften useful clues such as the shadow behind a stone, so know how to switch it off.

Freeze, cine and measurements

Freeze stops the image; the cine loop lets you scroll back through the last few seconds to find the best frame. Callipers and calculation packages turn that frame into measurements and a report.

Colour Doppler settings

  • Scale (PRF): lower it to show slow flow in veins and small vessels; raise it when colour wraps from red to blue (aliasing) in fast flow.
  • Colour gain: increase until speckles of noise appear in the tissue, then turn it down slightly.
  • Colour box: keep it as small and shallow as the question allows — a large box lowers the frame rate.
  • Angle: flow running straight across the beam shows little or no colour, so angle the probe or steer the box.
  • Spectral Doppler: keep the angle to flow at 60° or less when measuring velocities.
  • Wall filter: low for slow venous flow, higher for arterial work to remove wall-motion noise.

Read next Doppler modes and when you need them

Artefacts worth recognising

Acoustic shadowing

A dark band behind strong reflectors such as stones, bone or gas.

Posterior enhancement

A brighter area behind fluid-filled structures such as cysts and a full bladder.

Reverberation

Repeating bright lines between two strong reflectors — the basis of A-lines in lung ultrasound.

Mirror image

A duplicate of a structure appearing behind a strong curved reflector, such as the liver seen above the diaphragm.

Use knobology to judge a demo

  • Count how many touches it takes to get a good image from the preset.
  • Check that the controls you use most — depth, gain, freeze, measure — are physical or one tap away.
  • Test the auto-optimise button on a difficult patient, not only a slim volunteer.
  • Ask whether you can save your own presets and report templates.

Read next Choosing an ultrasound machinePOCUS training and equipment

Related guides

Point-of-Care Ultrasound (POCUS) EquipmentPOCUS answers focused clinical questions at the bedside. Here are the common exams, the equipment that suits them and how to start safely.Ultrasound for GP Clinics: A First-Machine GuideWhat a family-medicine practice should scan, which probes and format fit, and how to make the first ultrasound pay its way.Color Doppler Ultrasound Machine: Modes, Probes and UsesDoppler shows blood flow on top of the grey-scale image. Here is what each Doppler mode does, which exams need it and what to test before you buy.

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Frequently asked questions

What is ultrasound knobology?
Knobology is practical knowledge of the machine’s controls — preset, depth, gain, TGC, focus, frequency and Doppler settings — and how to use them to get a diagnostic image.
What does TGC do on an ultrasound machine?
Time gain compensation adjusts brightness at each depth separately, because sound weakens as it travels into the body. Set correctly, the image is evenly bright from top to bottom.
Why does simple fluid look grey instead of black?
Usually the overall gain is too high, or the frequency and focus need adjusting. Simple fluid such as urine in the bladder should look black on a well-set image.
Why does colour Doppler show no flow in a vessel I can see?
Common reasons are a scale (PRF) set too high for slow flow, colour gain set too low, or the vessel running at 90° to the beam. Lower the scale, raise the colour gain and angle the probe.

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