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DMS 110 Chapter 5 Notes: Intensities in Ultrasound (Page-by-Page)

Page 1

  • Course: DMS 110 - Physics I, Chapter 5
  • Context: Introduction to ultrasound beam intensities and how they are measured and reported.

Page 2

  • Helpful Video: "That Sounds Intense" (context for intensities in ultrasound)

Page 3

Five Key Terms for Intensities of Pulsed Waves

  • Spatial: relates to location or space. Ultrasound beam does not have the same intensity at different locations.
    • Beam intensity at the location of its maximum is called spatial peak intensity, denoted as Isp (also seen as Isptp in combined notation).
    • Average intensity across the beam’s entire cross-sectional area is called spatial average intensity, denoted as Isa.
  • Temporal: relates to time (transmit/pulse duration and receive). The ultrasound does not have the same intensity at different times.
  • Pulsed: refers to transmit/pulse duration only. The beam is not continuously on; it has “on” and “off” times.
    • Pulse average intensity is the average during the pulse duration.
  • Peak: maximum value during measurement.
  • Average: mathematical middle value over a specified interval.

Page 4

Measuring & Reporting Sound Beam Intensity

  • The intensity of the ultrasound beam varies over time.
  • The intensity also varies at different depths and at different lateral positions within the beam.

Page 5

Spatial Characteristics

  • Location or space matters: beam intensity varies by location.
  • Isp (Spatial Peak Intensity) = the intensity at the location of maximum intensity.
  • Isa (Spatial Average Intensity) = the average intensity across the beam’s entire cross-sectional area.
  • Note: Diagram labels reference outer edge measurements (conceptual illustration of spatial distribution).

Page 6

Beam Uniformity Ratio (BUR)

  • BUR =
    \nBUR = \frac{\text{Spatial Peak}}{\text{Spatial Average}} = \frac{I{sp}}{I{sa}}
  • BUR is also called:
    • SP/SA Factor
    • Beam Uniformity Coefficient (BUC)
    • Beam Uniformity Factor (BUF)
  • Meaning: Describes how uniform or consistent intensities are across the beam.
  • Typical value: greater than 1 (SP is stronger than SA). In practice, SP > SA, so BUR > 1.

Page 7

Temporal Characteristics (Basics)

  • Temporal refers to time.
  • For pulsed ultrasound, the transducer is usually listening/receiving and briefly transmitting/talking.
  • Four different methods exist for measuring intensity that account for pulsing characteristics (named and defined in subsequent pages):
    • Itp: Temporal Peak intensity at the instant of maximum value.
    • Imax: Average intensity during the most intense half-cycle.
    • Ipa: Pulse Average intensity, average during the pulse duration.
    • Ita: Temporal Average intensity, average over the entire pulse repetition period (transmit + receive).

Page 8

Temporal Peak (Itp) and Maximum (Imax)

  • Itp (Temporal Peak Intensity): the intensity at the instant in time of the maximum value.
    • Usually occurs near the middle of the pulse cycle.
  • Imax (Maximum during the most intense half-cycle): the average intensity during the most intense half-cycle of the pulse.

Page 9

Pulse vs Temporal Averages

  • Ipa (Pulse Average Intensity): average intensity during the pulse duration (transmit time).
  • Ita (Temporal Average Intensity): average intensity over the entire pulse repetition period (both transmit and receive times).
  • Relationship:
    I{ta} = I{pa} \times \text{Duty Factor}
  • The Duty Factor (DF) is the fraction of time the beam is on within a PRP (pulse repetition period).
  • DF is unitless and ranges between 0 and 1: 0 \le DF \le 1

Page 10

Temporal Intensities: Ordering

  • Temporal intensities from highest to lowest (typical):
    • Itp > Imax > Ipa > Ita
  • Rationale:
    • Itp is the instantaneous maximum.
    • Imax is the average over the most intense half-cycle (slightly less than Itp).
    • Ipa averages over the pulse duration (transmit only), thus lower than Imax.
    • Ita averages over the entire PRP (including off time), thus the lowest.

Page 11

Combining Spatial and Temporal Intensities (Bioeffects context)

  • To study bioeffects, intensities must reflect both spatial and temporal characteristics.
  • Defined intensities at locations where the beam is most intense, with temporal averaging appropriate to the measure:
    • Isptp: Spatial Peak Temporal Peak intensity — at the spatial peak location, at the instant of maximum value.
    • Isppa: Spatial Peak Pulse-Average intensity — at the spatial peak location, averaged over the transmit (pulse) time.
    • Ispta: Spatial Peak Time-Average intensity — at the spatial peak location, averaged over all time (transmit + receive).
    • Isata: Spatial Average Time-Average intensity — averaged across the entire cross-sectional beam and over all time.

Page 12

SPTP (Strongest) and Visualization

  • SPTP stands for Spatial Peak Temporal Peak intensity.
  • In the diagram, SPTP is labeled as the strongest intensity.

Page 13

SATP

  • SATP stands for Spatial Average Temporal Peak intensity.
  • It is shown as a next tier after SPTP in the visualization.

Page 14

SPPA

  • SPPA stands for Spatial Peak Pulse-Average intensity.
  • Placed after SATP in the visualization.

Page 15

SAPA

  • SAPA stands for Spatial Average Pulse-Average intensity.
  • Labeled as a distinct category in the diagram (between SPPA and SPTA in the visual ordering).

Page 16

SPTA (Bioeffects focus)

  • SPTA stands for Spatial Peak Temporal Average intensity.
  • The diagram notes a bioeffects emphasis (SPTA) for heating considerations.

Page 17

SATA

  • SATA stands for Spatial Average Temporal Average intensity.
  • Labeled as the weakest intensity in the diagram.

Page 18

“Ten Commandments” of Intensity

  • Intensities are reported in various ways with respect to time and space.
  • SPTA is the most relevant intensity for tissue heating (thermal bioeffects).
  • Unit for all intensities is watts per square centimeter,
    \text{Unit} = \text{W}/\text{cm}^2
  • Because peak measurements are higher than average measurements, the largest intensity is SPTP and the smallest is SATA:
    • In order (largest to smallest) among the common measures: \text{SPTP} > \text{Imax} > \text{SPPA} > \text{SPTA}^* > \text{SATA}
    • The star next to SPTA indicates a special note: SPTA is the true representation for tissue heating in many diagnostic contexts.
  • The Beam Uniformity Coefficient (BUR) describes spatial beam spread and is unitless with a value of 1 or greater:
    BUR = \frac{I{sp}}{I{sa}}
  • Duty Factor (DF) describes the time relationship of beam activity, is unitless, and lies between 0 and 1: 0 \le DF \le 1
  • If the ultrasound is continuous wave (no pulsing), then\
    SPTA = SPPA \quad\text{and}\quad SATA = SAPA
  • When comparing pulsed and continuous beams with the same SPTP intensities, the continuous beam has the higher SPTA intensity.
  • Practical reminders:
    • Remember temporal considerations (Itp, Imax, Ipa, Ita) and spatial considerations (Isp, Isa) when evaluating bioeffects and safety.
    • The FDA regulates diagnostic ultrasound to keep SPTA below specific limits:
    • Focused beams: \text{SPTA} \le 1000\ \text{mW}/\text{cm}^2
    • Unfocused beams: \text{SPTA} \le 100\ \text{mW}/\text{cm}^2
  • There are two main bioeffects to consider:
    • Mechanical (MI) — cavitation: microbubbles expanding/contracting, potentially bursting; gray-scale imaging is associated with SPPA.
    • Thermal (TI) — heating from sound energy attenuation; Doppler imaging raises concerns due to potential temperature rise (e.g., ~2°C).
  • Note: SPTA is the most relevant measure for thermal bioeffects; ultrasound safety is regulated accordingly.
  • Summary takeaways:
    • Measured intensities vary in space and time; use SP/SA and peak/average distinctions to describe them.
    • For heating-related effects, focus on Ispta/SPTA and related measures; for mechanical effects, SPPA/SPTP and Imax/Ia considerations matter.
    • Always report intensity in units of \text{W}/\text{cm}^2 and consider BUR to understand beam uniformity.

Page 19

Intensities and Bioeffects (Application Context)

  • Intensity determines potential bioeffects on tissue.
  • With appropriate diagnostic use, no known adverse bioeffects have been documented.
  • Two main bioeffect categories to assess:
    • Mechanical (MI): cavitation risks, microbubble dynamics; higher risk with gray-scale imaging (SPPA-related) and pulsed characteristics.
    • Thermal (TI): tissue heating due to attenuation; higher risk with increased TI, especially in Doppler imaging.
  • Key intensity contributor to heating is the SPTA, as it reflects energy delivery over time across the beam.
  • Regulatory emphasis (FDA) ensures exposure stays within safe limits for diagnostic use at typical depths and exposure times.

Page 20

Recap: Essential Concepts and Connections

  • Ten Commandments distilled:
    1) Intensities are reported with respect to space and time.
    2) Isptp, Isppa, Ispta, Isata are the principal combined-spatial/temporal intensities used in bioeffects discussions.
    3) SPTA is the most relevant for thermal effects.
    4) Units are consistently in \text{W}/\text{cm}^2.
    5) SP/SA (BUR) describes beam uniformity.
    6) Duty factor ties time-domain characteristics to intensity values.
    7) For continuous wave (CW), SPTA = SPPA and SATA = SAPA.
    8) When pulsed and CW beams share the same SPTP intensity, CW yields higher SPTA.
    9) Always consider both temporal and spatial factors when evaluating exposure.
    10) Be mindful of the real-world implications: tissue heating and cavitation risks depend on the chosen intensity metric and exposure pattern.

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What’s Next

  • Review Chapter 5 and Chapter 5 Quiz
  • Chapter 6 Review
  • Midterm Review and Midterm Exam