A-Scans and B-Scans
Everything so far has explained how a single ultrasound echo is made and how strong it is. This lesson turns that single echo into something useful: a depth measurement, or a full two-dimensional image — and asks how ultrasound stacks up against the other imaging methods you'll meet later in this topic.
A-scans: amplitude against time
The simplest way to use pulse-echo ultrasound is the A-scan (amplitude scan). A single pulse is sent from the transducer along one line into the body. As that pulse encounters boundaries between tissues of different acoustic impedance, part of its energy reflects back at each one, producing a series of separate echoes returning to the transducer at different times. An A-scan displays these echoes as a graph of signal amplitude (echo strength) against time, so each reflecting boundary along the pulse's path shows up as a distinct spike.
Since the speed of ultrasound in tissue is known (taking a standard assumed value, or measured for the tissue in question), the time delay of each spike directly gives the depth of the boundary that produced it. If a pulse takes time $t$ to travel down to a boundary and its echo travel back again, and the speed of sound in the tissue is $c$, then the total distance travelled by the pulse (there and back) is $ct$ — so the depth of the boundary is half that distance:
The factor of 2 is essential and easy to forget: the measured time $t$ covers the pulse's full round trip, to the boundary and back, not just the one-way journey.
Because an A-scan only reports depth along a single line, it is best suited to situations where a precise one-dimensional measurement is all that's needed — for example, measuring the thickness or dimensions of a specific internal structure — rather than cases where a full picture of an area is required.
B-scans: building a 2D image
The B-scan (brightness scan) extends the same basic pulse-echo idea into a genuine two-dimensional cross-sectional image. Instead of sending a single pulse along one line, many A-scan-style lines are taken across a range of adjacent positions or angles — achieved either by mechanically sweeping a single transducer element, or, more commonly in modern systems, by using an array of many transducer elements arranged side by side.
Rather than displaying each line's echoes as a spike on an amplitude-time graph, a B-scan converts the strength of each echo into a brightness value: a strong echo is shown as a bright dot, a weaker echo as a dimmer one, all positioned at the correct depth along that line. Assembling many of these brightness-coded lines together, side by side, builds up a full two-dimensional cross-sectional image showing the internal structures the pulses have passed through. This brightness-coded, multi-line approach is the basis of standard real-time diagnostic ultrasound imaging, including familiar applications such as obstetric (pregnancy) scanning.
Ultrasound compared with other imaging modalities
Ultrasound has a distinctive set of strengths and weaknesses relative to the other imaging techniques covered elsewhere in this topic and this module, and comparing modalities in this way is a common style of extended-response question throughout.
| Aspect | Ultrasound |
|---|---|
| Safety | No ionising radiation — safe for repeated use and during pregnancy, unlike X-ray or radionuclide-based methods |
| Real-time imaging | Yes — can show motion live, e.g. a beating heart or a moving fetus |
| Cost | Relatively low-cost and widely available equipment |
| Spatial resolution | Generally lower than CT or MRI |
| Operator dependence | Image quality depends significantly on the skill of the operator |
| Imaging through bone/gas | Poor — strong reflection/attenuation at these boundaries obscures structures beyond them |
The safety advantage is particularly significant: because ultrasound uses no ionising radiation at all, it can be used repeatedly and safely on the same patient, including during pregnancy, in situations where X-ray or radionuclide imaging would raise real dose concerns. Its real-time capability — showing genuine motion as it happens rather than a single static snapshot — is also distinctive among the imaging methods in this module. Against these advantages, ultrasound typically offers lower spatial resolution than modalities such as CT or MRI, its image quality is more dependent on operator skill than some other methods, and it performs poorly wherever the pulse's path crosses bone or gas-filled structures, since these produce very strong reflection and attenuation that obscures whatever lies beyond them.
This safety/resolution/convenience style of comparison across imaging modalities — weighing ionising radiation risk, spatial resolution, cost, and real-time capability against each other — recurs explicitly at the very end of this whole module, in the lesson "Comparing Imaging Techniques," where ultrasound, X-ray, CT, radionuclide imaging, and MRI are all set side by side.
Key takeaways
- An A-scan sends a single pulse along one line and displays returning echoes as spikes of amplitude against time.
- The round-trip time delay $t$ of an echo gives the depth of the reflecting boundary via $\text{depth} = \dfrac{ct}{2}$, where $c$ is the speed of ultrasound in the tissue.
- A-scans suit precise one-dimensional depth or thickness measurements rather than full imaging.
- A B-scan combines many A-scan-style lines across a range of positions, converting echo strength into brightness rather than spike height.
- Assembling many brightness-coded lines side by side builds a full 2D cross-sectional image — the basis of standard diagnostic ultrasound, e.g. obstetric scanning.
- Ultrasound's advantages include no ionising radiation, real-time imaging, and relatively low cost.
- Its drawbacks include lower spatial resolution than CT/MRI, operator-dependent image quality, and poor imaging through bone or gas.
- This modality comparison is developed fully across every imaging technique in this module's closing lesson, "Comparing Imaging Techniques."