3.10.4.2
A-scan and B-scan ultrasound
Medical Physics | AQA A-Level Physics
- A-scanAmplitude scan. A single ultrasound pulse is sent into the body and reflected pulses are displayed as spikes on a voltage-time graph. The time delay gives the depth of reflecting boundaries. Used for measuring distances, not for imaging. (amplitude scan): a single pulse is sent into the body and returning echoes are displayed as spikes on a voltage-time graph. The time between transmitted and received pulses gives the distance to the reflecting boundary: $d = \frac{ct}{2}$, where $c$ is the speed of sound in tissue and $t$ is the round-trip time. Used for measuring distances and thicknesses, not for producing images.
- B-scanBrightness scan. Multiple A-scans are taken from different angles or positions and combined by a computer to build a 2D (or 3D) image. The brightness of each dot corresponds to the strength of the echo. (brightness scan): many A-scan lines are taken from different positions or angles. The echoes are displayed as bright dots on a screen, with brightness proportional to echo strength. A computer combines these to form a 2D cross-sectional image. Modern B-scans use arrays of transducers to produce real-time images.
- Advantages of ultrasound: non-ionising (no radiation dose), safe for pregnant women and unborn babies, relatively cheap, portable, and gives real-time images.
- Disadvantages of ultrasound: lower resolution than X-ray or CT, cannot penetrate bone or gas-filled structures (lungs, bowel), operator-dependent image quality.