# Volume I Companion Presentation Frank R. Miele Pegasus Lectures, Inc.

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

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. Pegasus Lectures, Inc. COPYRIGHT 2006

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

Chapter 5: Transducers - Level 1

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

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

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

Block Analogy of Crystal Oscillation
This analogy is useful to illustrate the concept of the piezoelectric effect. A B C D E A: At Rest B: Stretched C: Recoiled D: Oscillate E: At Rest Fig. 3: (Pg 236) 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. Thickness (t) Fig. 4: (Pg 238) (Lower Frequency) Longer Period (Higher Frequency) Shorter Period Pegasus Lectures, Inc. COPYRIGHT 2006

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

PW Operating Frequency Equation

CW Operating Frequency
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 MHz Acoustic 10 MHz Voltage MHz Acoustic Fig. 5: (Pg 239) Pegasus Lectures, Inc. COPYRIGHT 2006

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

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

Pulse Response Pegasus Lectures, Inc.
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. 2 MHz 4 MHz Fig. 6: (Pg 240) Pegasus Lectures, Inc. COPYRIGHT 2006

Single Crystal Dimensions
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) Fig. 7: (Pg 241) Pegasus Lectures, Inc. COPYRIGHT 2006

CW Beamshape Pegasus Lectures, Inc.
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) Lateral Elevation Depth PZT Crystal Fig. 8: (Pg 241) Pegasus Lectures, Inc. COPYRIGHT 2006

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

PW Beamshape Pegasus Lectures, Inc. 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 Elevation Depth PZT CRYSTAL T1 Lateral Elevation Depth PZT CRYSTAL T1 Lateral Elevation Depth PZT CRYSTAL T1 Lateral Elevation Depth PZT CRYSTAL T1 Lateral Elevation Depth PZT CRYSTAL T1 Depth PZT CRYSTAL Lateral Pegasus Lectures, Inc. COPYRIGHT 2006

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

Basic Beam Characteristics
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. Fresnel Zone Fraunhoefer Zone Natural Focus PZT Crystal D1 D/2 D NZL = D2 • f0 6 2 • Near Zone Length Fig. 10: (Pg 243) 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 D1 D2 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. D2 D1 NZL1 NZL2 D1/2 D2/2 Pegasus Lectures, Inc. COPYRIGHT 2006

Quarter Wavelength Matching Layer
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. Fig. 12: (Pg 246) Pegasus Lectures, Inc. COPYRIGHT 2006

Axial Resolution Pegasus Lectures, Inc.
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) Pegasus Lectures, Inc. COPYRIGHT 2006

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

Lateral Resolution and Beamwidth
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. Fig. 14: (Pg 248) Pegasus Lectures, Inc. COPYRIGHT 2006

Lateral Resolution Pegasus Lectures, Inc.
*Higher frequencies form narrower beams improving lateral resolution. 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 Lenses Curved elements Electronic focusing Mirrors Pegasus Lectures, Inc. COPYRIGHT 2006

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

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

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

Notes: Pegasus Lectures, Inc. COPYRIGHT 2006

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