AQA · A-Level · Physics

    Course version: 7408 · Version from Sept 2016

    Ultrasound imaging (A-level only): A-scans and B-scans.

    An A-scan is a one-dimensional amplitude–time display: a pulse is emitted, and echoes from boundaries are shown as vertical spikes whose height gives echo amplitude and whose horizontal position…

    Checked against the AQA 7408 specification (opens in a new tab) on .

    • Point 4 of 5
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    An A-scan is a one-dimensional amplitude–time display: a pulse is emitted, and echoes from boundaries are shown as vertical spikes whose height gives echo amplitude and whose horizontal position gives time, hence depth via distance = speed × time ÷ 2. A B-scan builds a two-dimensional cross-section by moving the transducer and combining many A-scan lines; echo brightness, not spike height, represents amplitude, so a B-scan image shows tissue boundaries and structures. For example, an A-scan can measure eye axial length from a corneal and retinal echo, while a B-scan can show a foetal head outline. Both use the same pulse–echo principle and the same speed of sound in tissue.

    What earns the marks

    • Describes an A-scan as a one-dimensional amplitude–time graph in which echo spikes are plotted against time.
    • Explains that in an A-scan the vertical spike height represents echo amplitude and the horizontal position represents time, allowing depth to be calculated using distance = speed × time ÷ 2.
    • Describes a B-scan as a two-dimensional image formed by moving the transducer and combining many A-scan lines.
    • Explains that in a B-scan the brightness of each point represents echo amplitude, so boundaries between tissues of different acoustic impedance appear as bright lines.
    • Applies the pulse–echo principle to both scans, including the need for a coupling medium and the use of the speed of sound in tissue.
    • Compares the information provided: A-scans give precise depth measurements along one line, while B-scans give spatial structure across a plane.

    Common mistakes and how to put them right

    • Confusing the axes of an A-scan.

      Put it right: The horizontal axis is time, and depth is calculated from time using distance = speed × time ÷ 2.

    • Stating that a B-scan uses spike height to show echo amplitude.

      Put it right: A B-scan uses brightness to represent echo amplitude.

    • Forgetting the factor of two in depth calculations.

      Put it right: The pulse travels to the boundary and back, so distance = speed × time ÷ 2.

    Exam tips

    • When describing an A-scan, always state what the vertical and horizontal axes represent before interpreting a trace.
    • When calculating depth from an echo time, write the full equation distance = speed × time ÷ 2 and substitute values with units.
    • When comparing A-scans and B-scans, use the terms one-dimensional and two-dimensional explicitly and link each to its display method.

    Try the lesson

    The start of the Ultrasound imaging (A-level only): A-scans and B-scans. lesson, made from checked teaching notes for this specification point.

    Have a go first

    In ultrasound imaging, which type of scan displays the reflected pulses as a one-dimensional graph of signal amplitude against time?

    Listen

    An A-scan is a one-dimensional amplitude–time display: a pulse is emitted, and echoes from boundaries are shown as vertical spikes whose height gives echo amplitude and whose horizontal position gives time, hence depth via distance = speed × time ÷ 2. A B-scan builds a two-dimensional cross-section by moving the transducer and combining many A-scan lines; echo brightness, not spike height, represents amplitude, so a B-scan image shows tissue boundaries and structures. For example, an A-scan can measure eye axial length from a corneal and retinal echo, while a B-scan can show a foetal head outline. Both use the same pulse–echo principle and the same speed of sound in tissue. A strong answer earns marks for these points. Describes an A-scan as a one-dimensional amplitude–time graph in which echo spikes are plotted against time. Explains that in an A-scan the vertical spike height represents echo amplitude and the horizontal position represents time, allowing depth to be calculated using distance = speed × time ÷ 2. Describes a B-scan as a two-dimensional image formed by moving the transducer and combining many A-scan lines. Explains that in a B-scan the brightness of each point represents echo amplitude, so boundaries between tissues of different acoustic impedance appear as bright lines. A common slip: Confusing the axes of an A-scan: the error is treating the horizontal axis as depth directly. Instead, the horizontal axis is time, and depth is calculated from time using distance = speed × time ÷ 2. Exam tip: When describing an A-scan, always state what the vertical and horizontal axes represent before interpreting a trace.

    See it

    A top-down flowchart. It starts with a node labelled 'Pulse emitted', which leads to 'Echoes from boundaries'. This leads to 'A-scan: one-dimensional amplitude–time display'. From that A-scan node, three branches emerge. The first goes to 'Vertical spike height = echo amplitude'. The second goes to 'Horizontal position = time', which then leads to 'Depth from distance = speed × time ÷ 2'. The third goes to 'Move transducer and combine many A-scan lines', which leads to 'B-scan: two-dimensional cross-section'. That leads to 'Brightness = echo amplitude', which finally leads to 'Tissue boundaries and structures shown'.
    It shows how the pulse–echo principle produces an A-scan and how combining many A-scan lines builds a B-scan.

    Key words

    Say what each word means before you turn the card.

    A-scan

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    B-scan

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    Pulse–echo principle

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    Depth calculation from echo time

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    See every definition
    A-scan
    A one-dimensional amplitude–time display in which a pulse is emitted and echoes from boundaries are shown as vertical spikes.
    B-scan
    A two-dimensional cross-section built by moving the transducer and combining many A-scan lines, with echo brightness representing amplitude.
    Pulse–echo principle
    A pulse is emitted and echoes from boundaries are detected, used by both A-scans and B-scans with the same speed of sound in tissue.
    Depth calculation from echo time
    Depth is found from the echo time using distance = speed × time ÷ 2, since the pulse travels to the boundary and back.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. It starts with a node labelled 'Pulse emitted', which leads to 'Echoes from boundaries'. This leads to 'A-scan: one-dimensional amplitude–time display'. From that A-scan node, three branches emerge. The first goes to 'Vertical spike height = echo amplitude'. The second goes to 'Horizontal position = time', which then leads to 'Depth from distance = speed × time ÷ 2'. The third goes to 'Move transducer and combine many A-scan lines', which leads to 'B-scan: two-dimensional cross-section'. That leads to 'Brightness = echo amplitude', which finally leads to 'Tissue boundaries and structures shown'.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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