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Ultrasound Intensity and Safety

Sep 28, 2025

Overview

This lecture explains ultrasound intensity, how it is measured and described both spatially and temporally, and its importance for understanding bio effects and ensuring patient safety in diagnostic medical ultrasound.

Intensity Basics

  • Intensity is a measure of a wave's strength and is one of the seven key parameters used to describe ultrasound.
  • It is directly proportional to the power of the wave and to the square of its amplitude, and inversely proportional to the area over which the energy is spread.
  • The formula for intensity shows that as area increases, intensity decreases, and as power increases, intensity increases.
  • The unit for intensity is watts per centimeter squared (W/cm²). In diagnostic ultrasound, intensity typically ranges from 0.1 to 100 W/cm².
  • The ultrasound beam is most intense at its center and becomes weaker toward the edges, similar to how a flashlight beam is brightest in the middle and dimmer at the edges due to energy spreading over a larger area.

Intensity and Area

  • When the ultrasound beam is concentrated in a small area, intensity increases; when spread over a larger area, intensity decreases.
  • The beam is narrowest and most intense at the focus, where the area is smallest.
  • As the beam leaves the transducer, it starts at the width of the transducer, narrows at the focus, and then diverges and widens beyond the focus, causing intensity to change along the beam.
  • Spatial variations in intensity occur both across the cross-section of the beam and along its length.
  • The spatial peak intensity is found at the very center of the beam, where intensity is highest.
  • The spatial average intensity is the mathematical mean of all intensities across the beam's area.
  • The beam uniformity ratio (BUR), also called SPSA factor, BUC (beam uniformity coefficient), or BUF (beam uniformity factor), is the ratio of spatial peak to spatial average intensity. It indicates how uniform the beam is: a value close to 1 means the beam is more uniform, while a higher value means greater variation between center and edges.
  • Real-world analogy: Like sitting near a campfire, the closer you are (smaller area), the more intense the heat; farther away (larger area), the heat is less intense.

Temporal Aspects of Intensity

  • The duty factor describes the fraction of time the ultrasound is "on" during pulsed ultrasound. It ranges from 0 (off) to 1 (continuous).
    • In 2D imaging, the duty factor is usually less than 1%, meaning the sound is on for a very short time and off most of the time.
    • In Doppler modes, the duty factor can increase to 1–10%, but most of the time is still off.
    • In continuous wave ultrasound, the duty factor is 100% (always on).
  • Within a pulse, intensity is not uniform: it is usually strongest in the center of the pulse (temporal peak) and weaker at the beginning and end.
  • Key temporal intensity measurements:
    • Temporal Peak (TP): Highest intensity at any instant during a pulse.
    • Pulse Average (PA): Mean intensity during the pulse's on-time.
    • Temporal Average (TA): Mean intensity over both on and off times; includes periods when the machine is listening (off), so it is much lower than TP or PA.
  • In continuous wave ultrasound, there is no off-time, so the pulse average and temporal average are equal.
  • The temporal average is calculated by multiplying the duty factor by the pulse average.

Measuring Intensity

  • Intensity is measured using hydrophones (microprobes), which are placed in water baths and connected to oscilloscopes.
    • Needle hydrophones can be moved through the beam; disc hydrophones capture information from the whole beam.
  • Hydrophones measure parameters such as pulse repetition period, pulse duration, and wave period, allowing calculation of amplitude, power, and intensity.
  • The location (spatial peak or average) and timing (temporal peak, average, or pulse average) of the measurement must be specified.
  • There are six possible combinations of spatial and temporal intensity measurements:
    • SPTP: Spatial Peak, Temporal Peak (strongest intensity)
    • SAPT: Spatial Average, Temporal Peak
    • SPPA: Spatial Peak, Pulse Average
    • SAPA: Spatial Average, Pulse Average
    • SPTA: Spatial Peak, Temporal Average (most important for bio effects)
    • SATA: Spatial Average, Temporal Average (weakest intensity)
  • SPTA (spatial peak temporal average) is the key measurement for assessing patient safety and bio effects, as it best represents the actual exposure experienced by tissue.

Bio Effects and Safety

  • Mechanical bio effects (cavitation): Strong ultrasound pulses can cause the formation and collapse of tiny gas bubbles in tissue, potentially leading to cell damage. This is more likely with high-intensity, short pulses (e.g., in 2D imaging).
    • The risk of cavitation is closely related to the SPPA (spatial peak pulse average) intensity.
  • Thermal bio effects: Prolonged exposure to ultrasound can cause tissue heating, as some sound energy is converted to heat. This is more likely in Doppler modes, where the same area is repeatedly exposed to longer pulses.
    • The risk of heating is monitored using SPTA (spatial peak temporal average) intensity.
    • An increase of 2°C in tissue temperature can cause harm.
  • SPTA intensity is regulated to a maximum of 720 mW/cm² to ensure patient safety.
  • Ultrasound machines display the Mechanical Index (MI) and Thermal Index (TI) to help monitor potential bio effects:
    • MI: Indicates the likelihood of cavitation (mechanical effects).
    • TI: Indicates the potential temperature increase (thermal effects).
    • Higher MI means greater risk of cavitation; higher TI means greater risk of heating.
  • The FDA regulates ultrasound machines, restricting their use to medical professionals and ensuring safety standards are met.

Key Terms & Definitions

  • Intensity: Power per unit area (W/cm²); describes the strength of the ultrasound wave.
  • Spatial Peak (SP): Maximum intensity at the center of the beam.
  • Spatial Average (SA): Mean of all intensities across the beam's area.
  • Temporal Peak (TP): Highest intensity at any instant during a pulse.
  • Pulse Average (PA): Mean intensity during the pulse on-time.
  • Temporal Average (TA): Mean intensity over both on and off times.
  • Beam Uniformity Ratio (BUR/SPSA/BUF/BUC): Ratio of spatial peak to spatial average intensity; indicates beam uniformity.
  • Duty Factor: Proportion of time the pulse is on versus off.
  • Cavitation: Formation and collapse of gas bubbles due to mechanical effects of ultrasound.
  • Mechanical Index (MI): Indicates the likelihood of cavitation.
  • Thermal Index (TI): Indicates the potential temperature increase in tissue.

Action Items / Next Steps

  • Review how hydrophones are used to measure ultrasound intensity and understand the six possible intensity combinations.
  • Learn to identify SPTA, MI, and TI values on ultrasound machine displays for monitoring patient safety.
  • Prepare to study more about ultrasound bio effects and safety considerations in the next section of the course.

Full transcript