1. Pegasus Lectures, Inc. Volume II Companion Presentation Frank Miele Pegasus Lectures, Inc. Ultrasound Physics & Instrumentation 4 th Edition

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3. Pegasus Lectures, Inc. Volume II Outline  Chapter 7: Doppler  Chapter 8: Artifacts  Chapter 9: Bioeffects  Level 2  Chapter 10: Contrast and Harmonics  Chapter 11: Quality Assurance  Chapter 12: Fluid Dynamics  Chapter 13: Hemodynamics

4. Pegasus Lectures, Inc. Bioeffects There are two principle mechanisms for bioeffects: Thermal bioeffects (temperature rise related to):  temporal average intensity  duty factor  scan time  scanned vs. non-scanned modalities Mechanical bioeffects (cavitation related to:)  peak rarefactional pressure

5. Pegasus Lectures, Inc. Intensity Recall the equation for intensity as derived in Chapter 3: However, there are many different ways of making an intensity measurement. For example, the measurement can be made where the beam reaches its peak in space, its average over space, looking over the shorter duration during which the transmit is on ( the pulse duration), or during the longer period of time referred to as the PRP. Since each of these approaches to measuring the intensity lead to a potentially different indications for the risk of bioeffects, we will need to look at each approach.

6. Pegasus Lectures, Inc. Common Intensities: Four Concepts Understanding the common intensities is facilitated by understanding four basic concepts. These four concepts are detailed in the next 7 slides Concept 1: A peak is always greater than or equal to an average.

7. Pegasus Lectures, Inc. Concept 1: Peak versus Average A peak is always greater than or equal to an average. Therefore an SP is greater than an SA and a TP is greater than a TA.

8. Pegasus Lectures, Inc. Concept 2: The First Letter (S) The first letter (S) refers to how the beam is distributed over physical space. Fig. 2: (Pg 628)

9. Pegasus Lectures, Inc. Fig. 1: (Pg 627) Temporal: Pulsed Wave Timing Diagram DF The foundational drawing for PW is helpful in understanding concepts 3 and 4 (the temporal aspects) of bioeffects.

10. Pegasus Lectures, Inc. Fig. 3: (Pg 628) Concept 3: Pulse Measurements (PP and PA) The pulse intensity measurements are made during the shorter period of time referred to as the pulse duration (PD). The pulse peak is the highest intensity that occurs during the pulse duration (PD). The pulse average is the average intensity over the PD.

11. Pegasus Lectures, Inc. Fig. 4: (Pg 629) Concept 4: Temporal (PRP) Measurements The temporal intensity measurements are made during the longer period of time referred to as the pulse repetition period (PRP). Notice that the pulse peak and the temporal peak are the same measurement. Therefore, the pulse peak is not used.

12. Pegasus Lectures, Inc. Fig. 5: (Pg 630) Combining Temporal Intensity Measurements This diagram shows the three different temporal intensity measurements and the relative amplitudes. The highest intensity is clearly the TP, followed by the PA, and the TA as the lowest intensity. Note that the ratio of the TA to the PA is equivalent to the duty factor.

13. Pegasus Lectures, Inc. Review: Spatial Distribution vs. Time Distribution Spatial Distribution: refers to how the beam energy is distributed over physical space in the body. The beam parameters principally determine the spatial distribution of the beam. Temporal Distribution: refers to how the energy is distributed over time. For intensity measurements, the first set of two letters refers to the spatial distribution of energy and the second set of two letters refers to the spatial distribution of energy:

14. Pegasus Lectures, Inc. Fig. 5: (Pg 630) Spatial Distribution and the BUF As suggested by the name, the beam uniformity factor is a measure of beam uniformity. A completely uniform beam would have a BUF equal to 1. Normally, the BUF is greater than one, since the spatial peak intensity is usually greater than the average spatial intensity.

15. Pegasus Lectures, Inc. Thermal Bioeffects Non-scanned modalities have a greater risk of thermal bioeffects than scanned modalities. Scanned modalities:  2-D (B-Mode)  Color Doppler Non-scanned modalities:  CW Doppler  PW Doppler  M-mode  A-mode

16. Pegasus Lectures, Inc. Scanned vs. Non-Scanned Non-scanned modalities transmit repeatedly in the same direction, allowing for heat to build up, increasing the risk of thermal bioeffects. For a scanned modality, there is time between transmits being repeated in the same direction, allowing time for the heat to dissipate. Scanned B-ModePW Non-scanned

17. Pegasus Lectures, Inc. Typical Transmit Voltages Fig. 7 (Pg 635)

18. Pegasus Lectures, Inc. Two ways to overflow the bucket: A High Intensity for a Short time Mechanical Bioeffects A Low Intensity for a Long Time Thermal Bioeffects Understanding Intensity Restrictions

19. Pegasus Lectures, Inc. Intensity Restrictions (I) “Highest Duty Factor” “Lowest Duty Factor” “Lowest Transmit Voltage “Highest Transmit Voltage” B-Mode PW (larger gate) CW

20. Pegasus Lectures, Inc. Intensity Restrictions (II) “Highest Risk of Thermal” “Lowest Risk of Thermal” “Lowest Risk of Mechanical” “Greatest Risk of Mechanical” B-Mode PW (larger gate) CW

21. Pegasus Lectures, Inc. Fig. 8: (Pg 637) Acoustic Power Measurements Acoustic Power measurements are made in complex systems as pictured below. A specialized ultrasound transducer referred to as a hydrophone is placed in the water tank and aligned to the beam to make the various intensity measurements.

22. Pegasus Lectures, Inc. Membrane Hydrophone Fig. 9a: (Pg 638) The hydrophone pictured below is called a “membrane” hydrophone.

23. Pegasus Lectures, Inc. Fig. 9b: (Pg 638) Needle Hydrophones The four hydrophones pictured below are called “needle” hydrophones.

24. Pegasus Lectures, Inc. Transducer Flaw and Abnormal Beam (from Animation CD) Schlieren Image of a focused beam with a bad element. (Pg 640 A)

25. Pegasus Lectures, Inc. (Pg 640 B) Schlieren Images: Visualizing Beams (from Animation CD) Schlieren Image of a normal and abnormal beam at 3.5 MHz.

26. Pegasus Lectures, Inc. (Pg 640 C) Schlieren Image of a Pulsed Wave (from Animation CD) Schlieren Image of a 5 cycle pulsed wave

27. Pegasus Lectures, Inc. Pulsed Wave Schlieren Image (Animation) Notice the beam shape that occurs over time as the 5 cycle pulse travels. (Pg 640 D)

28. Pegasus Lectures, Inc. Mechanical Index (MI) With the new output display standard, the best indicator for the risk of mechanical bioeffects (cavitation) is the mechanical Index (MI).

29. Pegasus Lectures, Inc. Thermal Indices (TIS, TIB, TIC) With the new output display standard, the best indicator for the risk of thermal bioeffects was replaced with three different thermal indices: The TI values indicate the model predicted worst case temperature rise (in Celsius) based on the imaging parameters in use (preset, depth, transmit power, focus, etc.)

30. Pegasus Lectures, Inc. Ultrasound Safety (AIUM Statement) The official AIUM statement on the safety of diagnostic ultrasound basically states:  diagnostic ultrasound has been in use since the 1950’s  there are clear clinical benefits and recognized efficacy for medical diagnosis including during pregnancy  to date there have been no confirmed bioeffects on either the patient or the instrument operator using currently accepted power levels and intensities  although there is a risk that bioeffects will be identified in the future, current studies indicate that the benefits of prudent application of diagnostic ultrasound outweigh those risks, if there are any

31. Pegasus Lectures, Inc. Prudent Use & ALARA Principle Prudent use implies that steps should be taken to minimize the risk of bioeffects to patients, including:  not performing needless scans  not extending scan times needlessly  using the minimum amount of power to achieve good clinical results (ALARA Principle: As Low As Reasonably Achievable)  if there is a risk of missing clinically relevant information by using a lower transmit power, increasing the transmit power is warranted. (Recall that the benefits currently outweigh the risks)

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