Ten Commandments of Intensity

1) Intensities may be reported in various ways with respect to time and space

2) The different measurements of intensities are important in the study of bioeffects

SPTA (spatial peak, temporal avg) is the most relevant intensity with respect to tissue heating

3) The units for all intensities are watts/cm²

4) A number called the beam uniformity coefficient (also called SP/SA factor) describes the spread of a beam in space. The SP/SA factor is unitless, with a value of 1 or greater.

5) The duty factor describes the relationship of beam intensities with time. The duty factor is a unitless number with a value between 0 and 1.

6) For continuous wave ultrasound, the beam is always “on” and the pulse average and temporal average intensities are the same. Thus, SPTA = SPPA and SATA = SAPA.

7) When pulsed and continuous wave sound beams have the same SPTP intensities, the continuous wave beam has the higher SPTA intensity.

8) When pulsed and continuous wave sound beams have the same SATP intensities, the continuous wave beam has the higher SATA intensity.

9) Temporal considerations:

• Temporal peak intensity (Itp) is maximum intensity in time.

• Im intensity is averaged over the most intense half-cycle

• Pulse average intensity (Ipa) is averaged only during the pulse duration (“on” time only)

• Temporal average intensity (Ita) is averaged during the PRP (both the “on” and “off” times)

10) Spatial Considerations:

• Spatial peak intensity (Isp) is the maximum in space.

• Spiral average intensity, (Isa) is averaged over the cross-sectional area of the beam

The rank of intensities from largest to smallest:

SPTP → Im → SPPA → SPTA → SATA