Infographics / Ultrasound Physics

Intensity measures (ISPTA, ISPPA, spatial/temporal averaging)

Two plots. Left: intensity across the beam, with a bell-shaped profile whose spatial peak (SP) sits at the focus, above a lower spatial-average (SA) line. Right: intensity over time for a pulsed beam, three narrow pulses with a high temporal-peak (TP) line, a lower pulse-average (PA) line and a low temporal-average (TA) line because gaps are included. Below, three named indices sharing the spatial-peak location are ranked highest to lowest: ISPTP (temporal peak), ISPPA (pulse average), ISPTA (temporal average). ISPTA equals duty factor times ISPPA, is the lowest, and is highlighted as the key thermal-safety index. Notes cover pulsed Doppler high duty factor, hydrophone derating, and regulatory limits.

Ultrasound intensity measures: spatial peak vs average and temporal peak vs average, defining ISPTA, ISPPA and ISPTP Two plots. Left: intensity across the beam, with a bell-shaped profile whose spatial peak (SP) sits at the focus, above a lower spatial-average (SA) line. Right: intensity over time for a pulsed beam, three narrow pulses with a high temporal-peak (TP) line, a lower pulse-average (PA) line and a low temporal-average (TA) line because gaps are included. Below, three named indices sharing the spatial-peak location are ranked highest to lowest: ISPTP (temporal peak), ISPPA (pulse average), ISPTA (temporal average). ISPTA equals duty factor times ISPPA, is the lowest, and is highlighted as the key thermal-safety index. Notes cover pulsed Doppler high duty factor, hydrophone derating, and regulatory limits. Ultrasound intensity: peak vs average, across the beam and over time Across the beam — spatial intensity position across beam spatial peak (SP) — at the focus spatial average (SA) Over time — pulsed (low duty factor) intensity time temporal peak pulse average temporal average Named indices — spatial peak, three time-averages same spatial-peak location; only the time-averaging differs ISPTP temporal peak · highest ISPPA pulse average · middle ISPTA temporal average · lowest ISPTA = duty factor × ISPPA, so the highest measurable is ISPTP (or ISPPA), never ISPTA. ISPTA (spatial-peak temporal-average) is the key thermal-safety index. · pulsed Doppler uses a high duty factor, so its ISPTA is the highest of all scanning modes. · output is measured with a hydrophone in water, then derated (~0.3 dB/cm/MHz) for in-situ tissue intensity. · limits are set on derated ISPTA (e.g. fetal ~720 mW/cm²), not a flat 100 mW/cm² cap.
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