temporal intensity

Introduction

  • Speaker: Chris Harrington

  • Topic: Temporal Intensity in Ultrasound Physics

Overview of Temporal Intensity

  • Caveat Regarding X-Axis:

    • For amplitude power and spatial intensity, the x-axis can be ignored.

    • Temporal intensity requires consideration of the x-axis:

    • Temporal intensity deals with time, which is represented on the x-axis.

    • Important components include Pulse Repetition Frequency (PRF) and Duty Factor, directly affecting temporal intensity.

Definition of Temporal Intensity

  • Definition: Temporal intensity refers to the measurement of intensity occurring in time. It is distinguished from spatial intensity, which measures intensity in space.

  • Variability of Intensity in Pulsed Ultrasound:

    • Pulsed ultrasound entails variations in intensity based on whether sound is actively being emitted (on) or not (off).

    • Higher intensity occurs when sound is on and much lower when off.

Continuous Wave vs. Pulsed Ultrasound

  • Continuous Wave (CW) Ultrasound:

    • Characterized by sound that is continuously on, leading to constant temporal intensity.

    • Example: Audible representation shows constant temporal intensity without variations.

  • Pulsed Ultrasound:

    • Sound intensity varies over time and with each pulse, illustrating higher intensities when on and diminishing when off.

    • Example: Pulsed waveform representation shows clear variation in strength, with peaks during pulse transmission and low intensities during off periods.

Temporal Intensities in Pulsed Ultrasound

  • Three types of temporal intensities:

    1. Pulse Average Intensity (PA)

    • Definition: The average temporal intensity during the time the sound is on (during the pulse duration).

    • Abbreviation: PA

    • Mechanism: Higher power settings increase PA.

    1. Temporal Average Intensity (TA)

    • Definition: Average intensity during the entire pulsing cycle (PRP).

    • Abbreviation: TA

    • Characteristics: Lower than PA due to longer off time in PRP.

    • Calculation: TA = PA × Duty Factor.

    • Example: If PA is 200 mW/cm² and duty factor is 0.001, TA is then 0.2 mW/cm².

    1. Temporal Peak Intensity (TP)

    • Definition: The instantaneous highest intensity during a pulse.

    • Abbreviation: TP

    • Characteristics: Higher than both PA and TA, represents an instantaneous measure.

    • Peak occurs during a short period in time relative to the pulse duration.

Hierarchical Relationships of Temporal Intensities

  • Ordering of Intensities by Magnitude:

    • TP > PA > TA

  • Factors Influencing TA:

    • Primarily the Duty Factor, which affects the average intensity value due to the proportion of ‘on’ time to ‘off’ time in a cycle.

Practical Implications of Power and PRF Increase

  • Increasing Power:

    • Raises PA intensity directly, leading to a corresponding increase in the TA.

  • Increasing PRF:

    • Results in more frequent pulsing, increasing temporal average intensity closer to continuous wave.

Summary of Temporal Intensities

  • TP (Temporal Peak): Absolute highest, instantaneous.

  • PA (Pulse Average): Average during pulse duration, high but less than TP.

  • TA (Temporal Average): Lowest intensity overall, averaged over entire PRP including off time.

Additional Considerations

  • Influence on Patient Exposure:

    • Increased temporal intensities can lead to greater patient exposure. Continuous wave shows highest exposure due to constant sound emission.

  • Duty Factor:

    • A critical factor influencing both PA and TA measurements. Higher duty factors raise the temporal averages significantly.

Combined Intensity Values

  • Combination of Spatial and Temporal Intensities:

    • Six possible combinations using spatial and temporal values to accurately describe ultrasound intensity:

    • SPTP: Spatial Peak Temporal Peak (highest value).

    • SPTA: Spatial Peak Temporal Average.

    • SATP: Spatial Average Temporal Peak.

    • SAPA: Spatial Average Pulse Average.

    • SPTA: Spatial Peak Temporal Average.

    • SATA: Spatial Average Temporal Average (lowest value).

  • Bio Effects Discussion:

    • Importance of identifying intensity types, specifically SPTA for bioeffects research.

Summary of Key Units and Values

  • Units for Intensity:

    • Absolute unit: milliwatt per centimeter squared (mW/cm²).

  • Typical Values:

    • Output Power: ~10 mW.

    • Intensity: 100 mW/cm² below which no known bioeffects occur.

    • Duty Factor for Pulsed Ultrasound: usually around 0.001.

    • For CW ultrasound: Duty Factor = 1.

Questions Review

  • Various practice questions and exercises provided for reinforcement, discussing relationships between power, duty factor, and intensity in ultrasound physics.

Conclusion

  • Emphasis on understanding the relationships between types of intensities and the implications for clinical practice and patient safety.