Spatial vs Temporal concepts

• Question 1 in class: two variables of ultrasound intensity. Tara answered: spatial and temporal. Teacher confirmed this is correct. Core idea: spatial relates to space, temporal relates to time.
• Key definitions:
  • Spatial: refers to space (space within the beam). Example: spatial pulse length; spatial peak vs spatial average.
  • Temporal: refers to time (how the beam varies over time).

Spatial beam perspective (drawing and references)

• When looking down the ultrasound beam (beam cross-section), picture should be a circle.
• Spatial peak should be in the center (like a bull’s-eye or plus sign).
• Spatial average could be represented by a dotted line spanning inside the circle to indicate the average intensity over the beam’s cross-section.
• Takeaways:
  • Spatial peak (SP) vs spatial average (SA) are two reference points for the beam’s intensity distribution in space.
  • The discussion emphasized using the PowerPoint visuals to reference SP and SA.

BUR and BUC (spatial intensity concepts)

• BUR stands for Beam Uniformity Ratio (also discussed as a beam uniformity concept).
• BUC stands for Spatial Peak / Spatial Average factor (often written as SP/SA). It’s a ratio that compares the peak intensity to the average intensity across space.
• Practical note from the talk:
  • The SP/SA factor generally is greater than 1 (ISP > ISA), i.e., the peak intensity is higher than the average.
• You may need to look up exact definitions in your text, but the classroom emphasis was:
  • BUR = Beam Uniformity Ratio (or a related uniformity coefficient).
  • BUC = SP/SA (spatial peak divided by spatial average).

Temporal perspective: intensities over time

• Temporal concepts: peak vs average across time, and how those relate to the pulse.
  • Temporal peak intensity (I_TP): the maximum instantaneous intensity during the pulse.
  • Pulse average intensity (I_PA): the average intensity over the pulse duration (the “on” time).
  • Temporal average intensity (I_TA): the average intensity over the entire cycle (on + off).
• Important relationships:
  • Duty factor (DF) links on-time to the full cycle: DF = \frac{t{on}}{t{on} + t_{off}}.
  • There is a formula noted in class: the temporal average equals the pulse average multiplied by the duty factor: I{TA} = I{PA} \cdot DF.
• On-time vs off-time interpretation:
  • On-time: when the transducer is actively emitting.
  • Off-time: when the transducer is listening for the return signal.
  • Even during off-time, sound may still be present in tissue (reflection, scattering, absorption), contributing to bioeffects.
• Visual cue:
  • Pulsed ultrasound is “on” and “off.” The temporal peak occurs during the on-time, while the temporal average accounts for the entire cycle.

Ranking temporal intensities (highest to lowest)

• For temporal-only intensities (not spatial), the instructor noted a rank order commonly tested:
  • Temporal Peak (TP) is typically the highest.
  • Pulse Average (PA) is next.
  • Temporal Average (TA) is lowest.
• The instructor warned that quiz order may differ from midterm order, so memorize both possibilities and be prepared to adapt.
• Practical tip: when studying, rank the three as TP > PA > TA and be ready to justify why based on how much of the cycle the beam is “on” and how intensity is averaged.

Cheat sheets, chapter cues, and study strategy

• Kaylee was encouraged to put Chapter 5 items on her cheat sheet to ensure key concepts are written down.
• The talk highlighted that many items are straightforward if you’ve reviewed the PowerPoint, the video, and the lecture notes.
• The instructor emphasized using the worksheet as a study guide and recalling the exact terms (SP, SA, TP, PA, TA, DF) and their relationships.

Ten commandments of intensity (and test strategy)

• There is a set of “10 commandments of intensity” introduced for study. Most are common-sense, but two are not.
• The two non-obvious items are related to the relationship between continuous-wave and pulsed-wave intensities; these are the ones to star on the worksheet.
• Expect board-style questions that require ranking or explaining why certain intensity measures are more relevant for bio-effects (especially in pulseds vs continuous modes).

Basic formula for intensity and key relationships

• Core idea: intensity depends on more than just gross power; the spatial distribution and temporal behavior matter.
• A two-parameter view (as discussed):
  • Intensity is directly related to power and to amplitude; intensity is indirectly related to area.
  • In simple terms: higher power or larger amplitude increases intensity; spreading the beam over a larger area reduces intensity at a point.
• Standard proportional relationships mentioned:
  • I \propto \frac{Power}{Area}
  • I \propto \text{Amplitude}^2 (and more generally, intensity scales with pressure amplitude squared for acoustic waves)
• Summary phrasing used in class: two parameters (power and amplitude) directly increase intensity, while the beam area inversely affects intensity at a point.

Bioeffects: mechanical vs thermal effects

• Two primary bioeffects mechanisms discussed:
  • Mechanical index (MI): relates to mechanical effects such as microbubble vibration and cavitation; microbubbles can contract/expand with ultrasound.
  • Thermal index (TI): relates to heating of tissue; generally, heating effects are not evident in clinical practice except under extreme exposure.
• Key empirical notes:
  • Heating: robust evidence in animals (e.g., mice) shows heating after very long exposure (e.g., ~50 hours continuous ultrasound).
  • In obstetric (OB) ultrasound, a notable heating effect has been observed at interfaces where bone is near soft tissue, leading to increased TI readings at these interfaces.
  • Overall axiom: ultrasound should not be used for entertainment or non-clinical purposes; avoid scanning babies repeatedly for fun.
• Important bioeffects ranking (in the context of intensities):
  • The spatial peak temporal average (SPTA) is the most relevant for bioeffects (it reflects tissue exposure over time in regions of peak intensity).
  • The hierarchy used in class (for intuition and exams) runs from strongest to weakest as: SPTP > (next strongest) SAP? > SPPA > SPAPA? > SPTA. The exact names vary by notation, but the key takeaway is that spatial peak and temporal peak together yield the strongest instantaneous exposure, and temporal average (with spatial averaging) yields the weakest exposure, with SPTA being central for heating risk.
• Board exam emphasis:
  • Be prepared to explain why SPTA is the most relevant metric for bioeffects heating and how off-time still contributes to exposure during the total duty cycle.
• OB Doppler vs M-mode considerations:
  • Doppler can produce higher instantaneous intensity at the heart region; M-mode is often preferred for measuring cardiac motion or heart rate because it provides sound with potentially lower peak intensity at a small region compared to Doppler.
  • The video/text referenced indicates M-mode as the preferred safest method for cardiac measurements while Doppler is higher in intensity.

Practical notes from the instructor

• In OB ultrasound sessions, the heartbeat is often discussed; Doppler vs M-mode distinctions were highlighted.
• The instructor warned that Doppler can produce higher intensities at tiny, rapidly moving targets (like a fetal heart) and suggested M-mode as the safer measurement approach for routine heart-rate estimation.
• A common-sense reminder: avoid entertainment scanning and unnecessary exposure; safety guidelines emphasize minimizing exposure, especially in sensitive scenarios (e.g., fetal imaging).

Study and class logistics (upcoming quizzes and schedule)

• Quiz: Thursday for Chapter 5, with a calculator provided.
• Two-week window for Chapters 5 and 6; the instructor may open the quiz earlier or later but with the same due date.
• David will be absent next week; still keep up with material and review.
• Tomorrow: introduction to Chapters 5 and 6; opportunity to open the quiz early for those who want to start.
• The plan is to reinforce the same core concepts (spatial vs temporal, intensities, and bioeffects) across chapters, with emphasis on the 10 commandments and the ranking of intensities.

Quick-reference glossary (from lecture cues)

• Spatial: space, position within the beam.
• Temporal: time, duration within pulse cycles.
• SP: Spatial Peak
• SA: Spatial Average
• TP: Temporal Peak
• TA: Temporal Average
• PA: Pulse Average
• DF: Duty Factor, defined as DF = \frac{t{on}}{t{on} + t_{off}}
• ISPTP, ISPTA, ISAPA, ISATA: standard intensity descriptors (Spatial Peak Temporal Peak, Spatial Peak Temporal Average, Spatial Average Temporal Peak, Spatial Average Temporal Average)
• I_SPPA: Spatial Peak Pulse Average (alternative/less common descriptor used in some references)
• MI: Mechanical Index (mechanical bioeffects, microbubble dynamics)
• TI: Thermal Index (tissue heating)
• BUR/BUC: Beam Uniformity Ratio / Spatial Peak to Spatial Average factor (SP/SA) to characterize beam uniformity and exposure potential

Notes and reminders:

• Make sure you can identify SP, SA, TP, PA, TA on figures (e.g., the beam cross-section with a center peak and surrounding intensity gradient).
• Be comfortable explaining how the pulse on-time and off-time relate to duty factor and to temporal averages.
• Understand why SPTA is used as a primary indicator for heating risk in bioeffects discussions, and how off-time contributes to exposure even though the transducer is not actively emitting.
• Review the video examples cited in lecture slides for visual intuition on intensity distribution and the pulse-on/pulse-off cycle.
• Prepare to justify ranking of temporal intensities and the board-exam emphasis on the two non-obvious commandments related to continuous vs pulsed wave intensities.