Chapter 12 Ultrasound safety
There has been a considerable and rapid growth in the use of ultrasound as a diagnostic imaging tool over the past 50 years since it was first used to image the unborn fetus. This rapid growth has resulted in millions of people worldwide being scanned every year. Changes and advances in technology are leading to increasing power levels being used to obtain diagnostic information. The interaction of ultrasound with biological tissue can result in effects which may cause heating, interfere with the normal functioning of cells, and cause structural damage. The potential damaging nature of ultrasound leads to concern over its safety. All operators therefore need to have an understanding of the safety implications in order to practice safely.
WHY ARE WE INTERESTED?
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.
Amplitude, power, and intensity are parameters used to describe the strength of an ultrasound beam.
Amplitude is a measure of a wave’s magnitude of oscillation, that is, the magnitude of the maximum disturbance in the medium, and in ultrasound often refers to the maximum variation (see Fig. 12.1). Amplitude is measured in units of pressure: MPa (megapascals).
Power in ultrasound describes the rate at which energy is generated and transferred by the acoustic wave per unit of time. Power is measured in watts (W) and milliwatts (mW), one milliwatt being one thousandth of a watt. Power is proportional to the square of the amplitude, i.e. if the power is doubled then the amplitude is quadrupled.
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.
Fig. 12.2 Diagram showing a typical ultrasound beam profile and a cross-sectional area. Cross-sectional area varies with depth and is smallest at the focus where the intensity is the highest
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:
Fig. 12.3 Demonstrating the variation of intensity across an ultrasound beam. The spatial intensity across the beam is at its highest at the center, tailing off towards the edges
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||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.
OPERATING MODES AND THEIR POTENTIAL RISK
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.
Uses the lowest output power and intensities and is generally considered safe in all applications.
The longer pulses, higher powers, and pulse repetition rates typically used in pulsed Doppler result in higher average intensities compared to B-mode imaging and therefore there is an increased potential of producing a biological effect, particularly from ultrasound-induced heating.
Dwell time, i.e. the length of time that the ultrasound beam is fixed on a specific tissue area, is an important factor when considering the potential for heating. This is particularly relevant for spectral pulsed Doppler where the beam is held in a fixed position during an investigation; this leads to a further increase in temporal average intensity and therefore increased risk of causing a temperature rise.
Color Flow Mapping and Power Doppler Imaging Modes
These modes involve some beam scanning, and so generally have a heating potential that is somewhere between that of B-mode and that of spectral pulsed Doppler.
Table 12.1 summarizes data collected from a survey conducted by Henderson in 1997. Here you can see that the highest intensities are produced when using pulsed Doppler modes.