What an ultrasound can and cannot see

Ultrasound uses sound waves rather than radiation. It is safe, quick, comparatively inexpensive and available in the clinic — but it is not a general-purpose scanner, and it is worth understanding what it is actually good at.

What it sees well

• Fluid-filled and solid soft tissue structures — kidneys, gallbladder, liver, thyroid, ovaries, testicles, bladder
• Blood flow, in real time. This is why it is the first test for a suspected clot in the leg.
• Whether a lump is solid or a fluid-filled cyst — a question it answers better than almost anything else
• Things that move — a beating heart, a moving tendon, a joint under load
• Guiding a needle accurately during a procedure

What it struggles with

• Anything behind bone. Sound does not pass through the skull or the spine, which is why brain imaging is CT or MRI.
• Anything behind gas. Bowel gas blocks the beam, which is why the stomach and intestines are poorly assessed and why some studies need fasting.
• Lungs, for the same reason.
• Deep structures in larger patients — sound loses strength with depth.
• Small early cancers in most organs. Ultrasound is not a cancer screening test except in specific situations.

Why preparation matters

It is not bureaucracy. A gallbladder study needs fasting because eating empties the gallbladder and there is then nothing to look at. A pelvic study needs a full bladder because the fluid gives the sound a clear window through to the organs behind. An unprepared scan frequently has to be repeated, which costs you a second trip.

Why it is done in the clinic

Because the alternative is a referral to an imaging centre, a wait, a separate appointment, and a report that arrives at your provider a week later. Having it in the same building usually means the provider who asked the question is the one looking at the answer, and you hear it in days rather than weeks.