08 Ultrasound

Duty factor

DF = PD × PRF. The fraction of time the transducer is transmitting.

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Simulation

Duty factor — Change the numbers; the scene follows.

Where it works

Ultrasound

Console

On the console — depth (13 µs/cm), PRF, frame rate, dynamic range, aliasing limit.

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Formula

\mathrm{DF}=\mathrm{PD}\times\mathrm{PRF}

Variables

PD · Pulse durationµsPRF · Pulse repetition frequencykHz

Results

DF
Duty factor
0.008
DF
Duty factor
0.8%

Explanation

\mathrm{DF}=\mathrm{PD}\times\mathrm{PRF}

What it means

A pulse of duration PD (µs) sent PRF times per second occupies DF of the timeline. B-mode DF is tiny (0.1–1%) because you wait for echoes; PW Doppler and colour raise DF (and heating). I_spta = I_sppa × DF, so duty factor is how a high instantaneous intensity becomes a moderate time-average — the number that drives the thermal index. This is a working relation in Ultrasound.

Where it is used

Clinically it sits on the Ultrasound — Console. On the console — depth (13 µs/cm), PRF, frame rate, dynamic range, aliasing limit. Ultrasound equations sit on the probe face and along the beam: impedance, Snell, Doppler, MI and TI. They explain why gel matters, why aliasing appears, and why a mechanical index is on the screen.

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How to use it

Enter PD in µs and PRF in kHz (or Hz — the unit is labelled kHz so 5 kHz = 5000 Hz). 2 µs × 4 kHz = 0.008 = 0.8%. Flag if DF > 1 (physically impossible). Change one input and watch the curve and the simulation follow.

Symbols

PDPulse duration2 µs
PRFPulse repetition frequency4 kHz

Worked example

A typical case from the default values: PD = 2 µs (Pulse duration); PRF = 4 kHz (Pulse repetition frequency). Substituting into the relation gives DF = 0.008; DF = 0.8 %. These are teaching numbers — align them with your machine.

Typical values give

DF = 0.008
DF = 0.8%

Where it comes from

The displayed formula is the working relation. DF = PD × PRF. The fraction of time the transducer is transmitting. Usual reference: Edelman / Kremkau. Derive it in the specialty lesson, then return here to pin the numbers.

Reference: Edelman / Kremkau

Assumptions & limits

Rectangular pulse, constant PRF. Colour-flow packets and multi-focus B-mode have a more complicated timeline (several pulses per line). Does not compute heating — see TI.

Pitfalls

Soft-tissue 1540 m/s is an assumption — not a measurement in that patient. Doppler angle 90° gives no shift. MI and TI are on-screen estimates, not absorbed dose. Rectangular pulse, constant PRF. Colour-flow packets and multi-focus B-mode have a more complicated timeline (several pulses per line). Does not compute heating — see TI.

Keep this

Gel, angle, and assumed speed of sound — get those three right before you trust a centimetre. Rectangular pulse, constant PRF. Colour-flow packets and multi-focus B-mode have a more complicated timeline (several pulses per line). Does not compute heating — see TI.

In this specialty

Ultrasound