Ultrasound safety

Chapter 12 Ultrasound safety


Ultrasound is a mechanical form of energy which interacts with the biological tissue through which it travels. Millions of women have their pregnancy routinely scanned with ultrasound and any adverse effects are more likely to damage the rapidly developing cells found in embryo, fetus, and neonate.

New research is continually being carried out, but variable parameters of ultrasound exposure and mechanisms of interaction with tissue lead to problems in determining when safe levels are being breached.

Important Parameters

Amplitude, power, and intensity are parameters used to describe the strength of an ultrasound beam.

Intensity (I)

Intensity is the rate at which energy passes through the unit area and is an important quantity when discussing bioeffects and safety.

The average intensity is equal to the power of an ultrasound beam, normally expressed in mW, divided by the cross-sectional area of the beam, expressed in cm2. Units of intensity for diagnostic ultrasound are typically expressed in mWcm−2.

From the equation in Figure 12.2 we can identify that the intensity of an ultrasound beam is directly proportional to its power, i.e. if beam power increases, then intensity increases and, conversely, if beam power decreases, the intensity decreases.

In addition, we can see that intensity is also inversely proportional to the beam area, i.e. if the beam area decreases, then the beam intensity increases and, conversely, if the beam area increases, the beam intensity decreases.

The maximum intensity along an ultrasound beam lies at the focus (the narrowest part of the beam)where all the power is concentrated into a small cross-sectional area.

The intensity within an ultrasound beam also varies from point to point across the beam (spatial considerations), as demonstrated in Figure 12.3. Two values of intensity can de defined:

Because of the pulsed nature of ultrasound, the intensity of the ultrasound beam also varies over time (temporal considerations). Figure 12.4 shows the relevant times for three intensities:

These spatial and temporal variations within the ultrasound beam result in the fact that there are a number of ways of defining intensity, as detailed below:

Highest intensity image ISPTP – Spatial peak-temporal peak (SPTP)
ISATP – Spatial average-temporal peak (SATP)
ISPPA – Spatial peak-pulse average (SPPA)
ISAPA – Spatial average-pulse average (SAPA)
ISPTA – Spatial peak-temporal average (SPTA)
Lowest intensity ISATA – Spatial average-temporal average (SATA)

ISPTA is the measure most associated with temperature rises.


For any operating mode there is a large variation of output powers and intensities. The output data for all modern day ultrasound machines can be sourced from the operator’s manual.

Exposures used in Doppler modes such as spectral pulsed Doppler and color flow imaging are higher than for B and M modes.

Mar 10, 2016 | Posted by in ULTRASONOGRAPHY | Comments Off on Ultrasound safety
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