Overview
This lecture covers the concept of intensity in ultrasound, including its spatial and temporal variations, measurement methods, and its significance for patient safety and bio effects.
Intensity Basics
- Intensity describes the strength of a wave and is proportional to power and amplitude squared, and inversely related to area.
- The unit of intensity is watts per centimeter squared (W/cm²) in ultrasound.
- In diagnostic ultrasound, intensity ranges from 0.1 to 100 W/cm².
- The ultrasound beam is most intense at its center and weaker at the edges, similar to a flashlight beam.
Intensity and Area
- Intensity increases when energy is concentrated in a smaller area and decreases as the area grows.
- The ultrasound beam narrows to a focus (most intense) and then widens again, making intensity vary across space.
- Spatial peak intensity is at the center of the beam; spatial average is the mathematical middle.
- The Beam Uniformity Ratio (BUR), also called SPSA, BUC, or BUF, compares spatial peak to spatial average intensity (typically >1).
Intensity and Time
- Pulsed ultrasound has short bursts of energy with long off periods; duty factor describes the fraction of "on" time.
- Duty factor varies by ultrasound mode: <1% for 2D imaging, up to 10% for Doppler, and 100% for continuous wave.
- Temporal peak intensity is the strongest within a pulse; pulse average is the average during the "on" time; temporal average considers both "on" and "off" time.
- For continuous wave, pulse average equals temporal average due to no off time.
Measuring Intensity
- Hydrophones (needle or disc types) measure beam intensity from ultrasound transducers.
- Measurements should specify spatial (peak/average) and temporal (peak/average/pulse) points.
- There are six combined intensity measurements: SPTP, SATP, SPPA, SAPA, SPTA (most relevant for safety), and SATA (weakest).
Bio Effects and Safety
- SPTA (spatial peak temporal average) is used for assessing ultrasound's bio effects on tissue.
- Mechanical bio effects (cavitation) are more likely with strong 2D pulses; measured using SPPA.
- Thermal bio effects (heating) arise from longer exposure, especially with Doppler modes; SPTA is key for monitoring.
- Machines display MI (mechanical index) and TI (thermal index) values to indicate risk levels.
- FDA limits SPTA intensity to a maximum of 720 mW/cm² for clinical ultrasound devices.
Key Terms & Definitions
- Intensity — Power per unit area of the ultrasound beam (W/cm²).
- Spatial Peak (SP) — Highest intensity at the center of the beam.
- Spatial Average (SA) — Average intensity across the entire beam cross-section.
- Temporal Peak (TP) — Maximum intensity during the pulse's peak.
- Temporal Average (TA) — Average intensity over both "on" and "off" periods.
- Pulse Average (PA) — Average intensity during "on" time only.
- Beam Uniformity Ratio (BUR/SPSA/BUC/BUF) — Ratio of spatial peak to spatial average intensity.
- Duty Factor — Percentage of time the ultrasound pulse is "on".
- Hydrophone — Device for measuring ultrasound intensity in water.
- Bio effects — Biological effects of ultrasound energy, including mechanical (cavitation) and thermal (heating).
Action Items / Next Steps
- Review MI and TI indicators on clinical ultrasound machines.
- Study the six combined intensity measurements and their clinical relevance.
- Prepare for upcoming lessons on ultrasound bio effects.