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Pegasus Lectures, Inc. COPYRIGHT 2006 Volume I Companion Presentation Frank R. Miele Pegasus Lectures, Inc. Ultrasound Physics & Instrumentation 4 th Edition.

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Presentation on theme: "Pegasus Lectures, Inc. COPYRIGHT 2006 Volume I Companion Presentation Frank R. Miele Pegasus Lectures, Inc. Ultrasound Physics & Instrumentation 4 th Edition."— Presentation transcript:

1 Pegasus Lectures, Inc. COPYRIGHT 2006 Volume I Companion Presentation Frank R. Miele Pegasus Lectures, Inc. Ultrasound Physics & Instrumentation 4 th Edition

2 Pegasus Lectures, Inc. COPYRIGHT 2006 License Agreement This presentation is the sole property of Pegasus Lectures, Inc. No part of this presentation may be copied or used for any purpose other than as part of the partnership program as described in the license agreement. Materials within this presentation may not be used in any part or form outside of the partnership program. Failure to follow the license agreement is a violation of Federal Copyright Law. All Copyright Laws Apply.

3 Pegasus Lectures, Inc. COPYRIGHT 2006 Volume I Outline  Chapter 1: Mathematics  Chapter 2: Waves  Chapter 3: Attenuation  Chapter 4: Pulsed Wave  Chapter 5: Transducers  Level 1  Level 2  Chapter 6: System Operation

4 Pegasus Lectures, Inc. COPYRIGHT 2006 Chapter 5: Transducers - Level 1

5 Pegasus Lectures, Inc. COPYRIGHT 2006 Chapter 5: Transducers A transducer is any device which converts one form of energy to another form of energy.  nerves  lights  speakers  heaters  etc.

6 Pegasus Lectures, Inc. COPYRIGHT 2006 Ultrasound transducers use the piezoelectric effect to convert electrical energy into mechanical energy and mechanical energy back into electrical energy. Piezoelectric Effect PZT Electrical to acoustic transformation Acoustic to electrical transformation

7 Pegasus Lectures, Inc. COPYRIGHT 2006 Piezoelectric Effect Fig. 2a Expansion Fig. 2b Contraction Fig. 2c At Rest (Pg 235)

8 Pegasus Lectures, Inc. COPYRIGHT 2006 Block Analogy of Crystal Oscillation Fig. 3: (Pg 236) A B C D E A: At Rest B: Stretched C: Recoiled D: Oscillate E: At Rest This analogy is useful to illustrate the concept of the piezoelectric effect.

9 Pegasus Lectures, Inc. COPYRIGHT 2006 Crystal Thickness (t) and PW Operating Frequency A thicker crystal “vibrates” at a lower frequency when driven in a pulsed mode. There is therefore an inverse relationship between crystal thickness and operating frequency in a pulsed mode operation. Longer Period (Lower Frequency) Shorter Period (Higher Frequency) Thickness (t) Fig. 4: (Pg 238) Thickness (t)

10 Pegasus Lectures, Inc. COPYRIGHT 2006 PW Operating Frequency (Animation) (Pg 238)

11 Pegasus Lectures, Inc. COPYRIGHT 2006 PW Operating Frequency Equation

12 Pegasus Lectures, Inc. COPYRIGHT 2006 CW Operating Frequency Fig. 5: (Pg 239) In a CW mode of operation, the frequency at which the crystal vibrates is related to the frequency of the electrical drive signal (as visualized in the animation of the next slide). 5 MHz Voltage 5 MHz Acoustic 10 MHz Voltage 10 MHz Acoustic

13 Pegasus Lectures, Inc. COPYRIGHT 2006 CW Frequency (Animation) (Pg 239)

14 Pegasus Lectures, Inc. COPYRIGHT 2006 CW Operating Frequency Equation 5 MHz Voltage5 MHz Acoustic

15 Pegasus Lectures, Inc. COPYRIGHT 2006 Pulse Response Fig. 6: (Pg 240) 2 MHz4 MHz Like a bell when rung, a single pulse produces multiple cycles of ringing (the “impulse response”). The following two figures represent the impulse response for a 2 MHz and a 4 MHz transducer design.

16 Pegasus Lectures, Inc. COPYRIGHT 2006 Single Crystal Dimensions Fig. 7: (Pg 241) The diameter of a crystal affects the beamwidth and hence, the focus. The thickness of the crystal affects the operating frequency. These two parameters should not be confused. Diameter (D)PZT Crystal thickness (t)

17 Pegasus Lectures, Inc. COPYRIGHT 2006 CW Beamshape Fig. 8: (Pg 241) Since CW is continuously transmitting, the wave exists at all locations simultaneously producing a beam similar to that of a flashlight. (As visualized in the animation of the next slide) PZT Crystal Lateral Elevation Depth

18 Pegasus Lectures, Inc. COPYRIGHT 2006 CW Beamshape (Animation) (Pg 241)

19 Pegasus Lectures, Inc. COPYRIGHT 2006 PW Beamshape Fig. 9: (Pg 242)Unlike CW mode, in PW, the transmit is turned on and off. The beamshape is therefore a “description” of the shape of the path the sound wave travels over time (as visualized in the animation of the next slide). Lateral PZT CRYSTAL Lateral T1T1 Elevation Lateral T1T1 Elevation Lateral T1T1 Elevation Lateral T1T1 Elevation Lateral T1T1 Elevation Depth

20 Pegasus Lectures, Inc. COPYRIGHT 2006 PW Beamshape (Animation) (Pg 242)

21 Pegasus Lectures, Inc. COPYRIGHT 2006 Basic Beam Characteristics Fig. 10: (Pg 243) Although greatly simplified, the basic beamshape is helpful in roughly describing the beam parameters. Notice that the beam is approximately half as wide as the crystal diameter at the focus and the same width as the crystal diameter at the twice the focal depth. Natural Focus Fraunhoefer ZoneFresnel Zone D/2D PZT Crystal D1D1 NZL = D 2 f Near Zone Length

22 Pegasus Lectures, Inc. COPYRIGHT 2006 Crystal Diameter and Focus A larger crystal diameter results in a deeper focus for the same operating frequency. Fig. 11: (Pg 244) PZT Crystal Deeper Focus Shallower Focus D1D1 D2D2

23 Pegasus Lectures, Inc. COPYRIGHT 2006 Aperture Effects on Beam Diameter Notice that increasing the crystal aperture by a factor of 2 increases the focal depth by a factor of 4. D2D2 D1D1 D1D1 NZL 1 NZL 2 D 1 /2 D 2 /2D2D2

24 Pegasus Lectures, Inc. COPYRIGHT 2006 Quarter Wavelength Matching Layer Fig. 12: (Pg 246) The ideal matching layer thickness is one fourth the wavelength (quarter wavelength). With quarter wavelength thickness, the energy that reflects back from the front surface is 180 degrees out of phase with the reflection from the front surface, resulting in destructive interference. This is beneficial since reflections from the matching layer would otherwise obscure the actual desired image from the patient.

25 Pegasus Lectures, Inc. COPYRIGHT 2006 Axial Resolution The roundtrip effect helps separate by a factor of 2 the echoes returning in the time. Therefore, the resolution in the depth direction is better (less) than the spatial pulse length by a factor of 2. Fig. 13: (Pg 247)

26 Pegasus Lectures, Inc. COPYRIGHT 2006 Axial Resolution Further Defined *Higher frequencies have shorter wavelengths improving axial resolution.

27 Pegasus Lectures, Inc. COPYRIGHT 2006 Lateral Resolution and Beamwidth Fig. 14: (Pg 248) The lateral resolution equals the lateral beamwidth dimension. If the beam is wider than the distance between two structures, the echo from both structures will overlap, making it impossible to distinguish between the two structures laterally.

28 Pegasus Lectures, Inc. COPYRIGHT 2006 Lateral Resolution *Higher frequencies form narrower beams improving lateral resolution.

29 Pegasus Lectures, Inc. COPYRIGHT 2006 Techniques for Changing Focus There are four techniques which can be used to change the focus from the natural focus of a crystal design 1.Lenses 2.Curved elements 3.Electronic focusing 4.Mirrors

30 Pegasus Lectures, Inc. COPYRIGHT 2006 Use of Lenses for Focusing Fig. 15: (Pg 249) Lens PZT Crystal

31 Pegasus Lectures, Inc. COPYRIGHT 2006 Curved Surface for Focusing Fig. 16: (Pg 250) Concave Surface PZT Crystal

32 Pegasus Lectures, Inc. COPYRIGHT 2006 Techniques for Changing Focus Mirrors were rarely used and not used currently so will not be further discussed. Electronic focusing is discussed in level 2.

33 Pegasus Lectures, Inc. COPYRIGHT 2006 Simple Transducer Block Diagram Fig. 17: (Pg 250) Lens Matching Layer Piezoelectric Crystal Backing Material Wires _ + The simple block diagram is useful since it illustrates the principal transducer components. You should be able to describe each components purpose and function.

34 Pegasus Lectures, Inc. COPYRIGHT 2006 Notes:


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