08 Ultrasound

Ispta from Isppa and duty factor

I_spta = I_sppa × DF. Spatial-peak pulse-average × duty factor = spatial-peak temporal-average.

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Simulation

Ispta from Isppa and duty factor — Change the numbers; the scene follows.

Where it works

Ultrasound

Tissue path

Along the beam in tissue — impedance mismatch, Doppler shift, MI/TI at the focus.

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Formula

I_{\mathrm{spta}}=I_{\mathrm{sppa}}\times\mathrm{DF}

Variables

I_sppa · Spatial-peak pulse-averageW/cm²DF · Duty factor

Results

I_spta
Spatial-peak temporal-average
0.8W/cm²
I_spta
Ispta
800mW/cm²

Ispta exceeds the 720 mW/cm² obstetric FDA cap (derated).

Explanation

I_{\mathrm{spta}}=I_{\mathrm{sppa}}\times\mathrm{DF}

What it means

Ultrasound intensities come in a family: spatia-peak or spatial-average, pulse-average or temporal-average. Isppa is the ‘instantaneous’ heating inside the pulse; Ispta is what a slow thermometer would see. FDA track-3 obstetric Ispta cap is 720 mW/cm² (derated). Because DF is ~1% in B-mode, a 100 W/cm² Isppa becomes 1 W/cm² Ispta. Doppler’s higher DF is why TI climbs when you switch mode. This is a working relation in Ultrasound.

Where it is used

Clinically it sits on the Ultrasound — Tissue path. Along the beam in tissue — impedance mismatch, Doppler shift, MI/TI at the focus. 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.

Ultrasound · Open this machine

How to use it

Enter Isppa (W/cm²) and DF (fraction). 80 W/cm² × 0.01 = 0.80 W/cm² = 800 mW/cm², just over the obstetric cap — flagged. Change one input and watch the curve and the simulation follow.

Symbols

I_sppaSpatial-peak pulse-average80 W/cm²
DFDuty factor0.01

Worked example

A typical case from the default values: I_sppa = 80 W/cm² (Spatial-peak pulse-average); DF = 0.01 (Duty factor). Substituting into the relation gives I_spta = 0.8 W/cm²; I_spta = 800 mW/cm². These are teaching numbers — align them with your machine.

Typical values give

I_spta = 0.8W/cm²
I_spta = 800mW/cm²

Where it comes from

The displayed formula is the working relation. I_spta = I_sppa × DF. Spatial-peak pulse-average × duty factor = spatial-peak temporal-average. Usual reference: IEC 62359 / FDA 510(k). Derive it in the specialty lesson, then return here to pin the numbers.

Reference: IEC 62359 / FDA 510(k)

Assumptions & limits

Derating (0.3 dB/cm/MHz) is assumed already applied if you paste a displayed value. Spatial-average (Isata) is smaller than spatial-peak by the beam-area factor. Not a TI, though they correlate.

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. Derating (0.3 dB/cm/MHz) is assumed already applied if you paste a displayed value. Spatial-average (Isata) is smaller than spatial-peak by the beam-area factor. Not a TI, though they correlate.

Keep this

Gel, angle, and assumed speed of sound — get those three right before you trust a centimetre. Derating (0.3 dB/cm/MHz) is assumed already applied if you paste a displayed value. Spatial-average (Isata) is smaller than spatial-peak by the beam-area factor. Not a TI, though they correlate.

In this specialty

Ultrasound