Chapter05_level_1_printable

Download Report

Transcript Chapter05_level_1_printable

Ultrasound Physics & Instrumentation

4 th Edition

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

Pegasus Lectures, Inc.

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.

Pegasus Lectures, Inc.

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 Pegasus Lectures, Inc.

Chapter 5: Transducers - Level 1

Pegasus Lectures, Inc.

Chapter 5: Transducers

A transducer is any device which converts one form of energy to another form of energy.

 nerves  lights  speakers  heaters  etc.

Pegasus Lectures, Inc.

Piezoelectric Effect

Ultrasound transducers use the piezoelectric effect to convert electrical energy into mechanical energy and mechanical energy back into electrical energy.

Electrical to acoustic transformation Acoustic to electrical transformation Pegasus Lectures, Inc.

Piezoelectric Effect

Fig. 2a Expansion Fig. 2b Contraction Fig. 2c At Rest

(Pg 235)

Pegasus Lectures, Inc.

Block Analogy of Crystal Oscillation

This analogy is useful to illustrate the concept of the piezoelectric effect. A: At Rest B: Stretched C: Recoiled D: Oscillate E: At Rest B A C Fig. 3:

(Pg 236)

Pegasus Lectures, Inc.

E D

Crystal Thickness (t) and PW Operating Frequency Longer Period (Lower 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) Shorter Period (Higher Frequency)

f o

 1

thickness

Thickness (t)

Fig. 4:

(Pg 238)

Pegasus Lectures, Inc.

PW Operating Frequency (Animation)

( Pg 238)

Pegasus Lectures, Inc.

PW Operating Frequency Equation

 

thickness

Pegasus Lectures, Inc.

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 5 MHz Acoustic

Fig. 5:

(Pg 239)

10 MHz Voltage 10 MHz Acoustic

f o

 Pegasus Lectures, Inc.

CW Frequency (Animation)

(Pg 239)

Pegasus Lectures, Inc.

CW Operating Frequency Equation

f o



5 MHz Voltage 5 MHz Acoustic

 Pegasus Lectures, Inc.

Pulse Response

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.

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.

thickness (t) Fig. 7:

(Pg 241)

Pegasus Lectures, Inc.

CW Beamshape

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) Depth Fig. 8:

(Pg 241)

Pegasus Lectures, Inc.

CW Beamshape (Animation)

(Pg 241)

Pegasus Lectures, Inc.

Fig. 9:

(Pg 242)

PW Beamshape

Depth Depth Depth 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).

Depth Depth Depth Pegasus Lectures, Inc.

PW Beamshape (Animation)

(Pg 242)

Pegasus Lectures, Inc.

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 D/2 D NZL = D 2 • f 0 6 2 • Near Zone Length

Fig. 10:

(Pg 243)

Pegasus Lectures, Inc.

Crystal Diameter and Focus

A larger crystal diameter results in a deeper focus for the same operating frequency.

Deeper Focus Shallower Focus

Fig. 11:

(Pg 244)

Pegasus Lectures, Inc.

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.

D 2 /2 D 2 D 1 D 2 D 1 /2 D 1 NZL 2 NZL 1 Pegasus Lectures, Inc.

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.

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)

Pegasus Lectures, Inc.

Axial Resolution Further Defined

*Higher frequencies have shorter wavelengths improving axial resolution.

Pegasus Lectures, Inc.

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. 

Beamwidth

Fig. 14:

(Pg 248)

Pegasus Lectures, Inc.

Lateral Resolution

*Higher frequencies form narrower beams improving lateral resolution.

Pegasus Lectures, Inc.

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 Pegasus Lectures, Inc.

Use of Lenses for Focusing

Lens Fig. 15:

(Pg 249)

Pegasus Lectures, Inc.

Curved Surface for Focusing

Concave Surface Fig. 16:

(Pg 250)

Pegasus Lectures, Inc.

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.

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.

_ Wires + Lens Matching Layer Piezoelectric Crystal Backing Material Fig. 17:

(Pg 250)

Pegasus Lectures, Inc.

Notes:

Pegasus Lectures, Inc.