A-scan (Amplitude scan)
Medical Imaging - OCR A-Level Physics
- A-scanAmplitude scan — a 1D ultrasound display showing pulse amplitude vs time, used to measure distances. (AmplitudeThe maximum displacement of a point on a wave from its equilibrium (rest) position. Measured in metres (m). scan): displays reflected pulse amplitudeThe maximum displacement of a point on a wave from its equilibrium (rest) position. Measured in metres (m). against time on an oscilloscope. The time delay between pulses gives the depth of the boundary: $d = ct/2 (divide$ by 2 because the pulse travels there and back).
- A-scans are used for precise distance measurementsIn ultrasound: calculated from the time delay between transmitted and reflected pulses using d = vt/2., e.g. measuring the length of an eyeball.
- B-scanBrightness scan — a 2D ultrasound image built from many A-scans, showing a cross-section of the body. (Brightness scan): the transducer is moved across the surface (or uses an array). Each reflection is displayed as a bright dot on screen, with brightness proportional to reflected intensityThe powerThe rate of energy transfer. Measured in watts (W). transmitted per unit area perpendicular to the wave direction. Measured in W m⁻². Proportional to amplitude squared.. This builds up a 2D cross-sectional imageA flat slice through the body, showing internal structures at one depth — as produced by B-scan ultrasound or a single CT slice..
- B-scans are used for imaging (e.g. fetal imaging during pregnancy).
- Real-time B-scanning uses an array of transducers to produce live images - useful for monitoring moving structures (heart valves, fetal movement).
Examiner Tips and Tricks
- In calculations, always remember that the time measured is for a round trip: the pulse travels to the boundary and back.
- Use d = ct/2, not $d = ct$.