Lesson 02: Sound Wave Production. This lesson contains 26 slides plus 11 multiple-choice questions.

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1 Lesson 02: Sound Wave Production This lesson contains 26 slides plus 11 multiple-choice questions. This lesson was derived from pages 2 through 7 in the textbook:

2 ULTRASOUND imaging: a diagnostic ultrasound classification in which graphic displays of the patient s internal structures are produced Ultrasound scanners provide images of soft tissue for abdominal, obstetric, gynecologic, ophthalmic, musculoskeletal, cardiac, and vascular evaluations. They are frequently used alone or to complement other diagnostic imaging modalities. Ultrasound has many advantages over other diagnostic imaging modalities. Ultrasound is generally noninvasive, and unlike radiographic and radionuclide diagnostic imaging techniques, it utilizes no radiation.

3 Sound Wave Production

4 SOUND

5 CATEGORIES OF SOUND INFRASOUND = below 20 Hz AUDIBLE SOUND = 20 Hz to 20 khz ULTRASOUND = above 20 khz frequency: the number of vibrations per second of a waveform of energy cycle: a complete variation of an acoustic variable, containing one condensation and one rarefaction hertz (Hz): one cycle per second Sound waves are mechanical vibratory disturbances resulting from molecular condensations (compressions) and rarefactions (decompressions). The frequency of the sound, or number of cycles per second, determines its category. The basic unit of frequency is the hertz. Sound categories include infrasound (frequencies below 20 Hz), audible sound (frequencies between 20 Hz and 20,000 Hz), and ultrasound (frequencies above 20,000 Hz).

6 MEDICAL DIAGNOSTIC ULTRASOUND ABOVE 1 MHz megahertz (MHz): one million hertz matter: anything that takes up space Ultrasound systems that are used for medical diagnostic applications operate at frequencies above one megahertz (1 MHz or 1,000,000 Hz). Sound waves propagate through matter by causing molecules to vibrate successively along the sound path. Sound waves carry energy, not matter, from one point to another.

7 SOUND VELOCITY STIFFNESS (velocity increases with stiffness) DENSITY density: density: the amount of mass in a substance for a given volume (velocity decreases with density) Stiffness and density properties of a material determine the velocity (speed of sound propagation), which in a patient typically averages 1540 meters per second. Although an increase in density results in a slight decrease in the velocity, the velocity of sound is significantly higher in materials that are very stiff. Examples of stiff materials that could significantly affect sound velocity in a patient are bones, gallstones, and hard plaque. The velocity of sound is not affected by frequency.

8 SOUND VELOCITIES Material Meters per second Air 330 Pure Water 1430 Fat 1450 Soft Tissue 1540 Muscle 1585 Bone 4080

9 SOUND VELOCITIES STIFFNESS OF MEDIUM DENSITY OF MEDIUM SOUND VELOCITY Increase Increase Decrease Decrease Increase Decrease Decrease Increase

10 PIEZOELECTRIC EFFECT TRANSMIT electrical energy to mechanical energy RECEIVE mechanical energy to electrical energy piezoelectric: refers to the conversion of electrical energy (voltage) into mechanical (sound) energy or the conversion of mechanical energy into electrical energy Ultrasound is produced by a transducer, which is a device that converts one form of energy into another. The transducer s main component is a vibrating piezoelectric element, or crystal, which can be energized by electrical energy to produce mechanical ultrasound energy, or acoustic pressure. The electrical energy typically applied to the element is in the range of 100 volts. Pressure is the amount of force per unit area and is often measured in Pascals.

11 RESONANT FREQUENCY The fundamental frequency of a transducer resonant: the ability of an object to either operate at or generate energy of a specific frequency only continuous mode: transducer excitation by repeating cycles of alternating driving voltage Transducers may be energized to produce ultrasound pulses within the range of the element s nominal, or resonant frequency. Some transducers may be operated to produce ultrasound pulses equal to one of two or three selected frequencies (multi-hertz) within the range of the resonant frequency of a broadband transducer. The continuous mode results in a non-pulsed wave with cycles that repeat indefinitely at the transducer s resonant frequency. The resonant frequency of a transducer is mainly related to the piezoelectric element s thickness. The thinner the element, the higher the resonant frequency of the transducer.

12 RESONANT FREQUENCY PIEZOELECTRIC ELEMENT THICKNESS RESONANT FREQUENCY Increase Decrease Decrease Increase

13 PIEZOELECTRIC CERAMICS lead zirconate titanate barium titanate lead metaniobate lead titanate Many piezoelectric elements are ceramic. Typical ceramic materials are lead zirconate titanate, barium titanate, lead metaniobate, and lead titanate. Lead zirconate titanate is the most commonly used material. The ceramic material in a transducer has piezoelectric characteristics because it was polarized while at a temperature above its Curie point. If a transducer is sterilized by heating it above the Curie point, depolarization may result.

14 PIEZOELECTRIC EFFECT Electrical energy Mechanical energy array: a transducer assembly containing several piezoelectric elements A transducer may transmit sound from a single piezoelectric element or an array.

15 PIEZOELECTRIC EFFECT Electrical energy Mechanical energy Another function of the piezoelectric element is the reception of echoes that return from the object being studied. The piezoelectric element, upon receiving a returning echo, converts the acoustic pressure into an electrical signal.

16 PIEZOELECTRIC EFFECT Electrical energy Mechanical energy Although sound may be transmitted and received using a single piezoelectric element, both functions cannot be performed simultaneously. Pulsed transducers utilize the same piezoelectric element(s) during separate transmit and receive intervals

17 Huygens Principle Sound waves produced by imaging transducers originate as numerous points on the surface of a piezoelectric element and serve as sources of small individual sound wavelets. Acoustic interference occurs when two or more wavelets are produced at the same time. Their amplitudes at any point in space may be added together to determine their combined effect. If the amplitude of the resultant wave is larger than either of the original waves, the waves are said to interfere constructively. If the resultant amplitude is smaller than one of the individual waves, the waves are interfering destructively. With constructive interference, the individual waves become tangent to each other and have the same phase relationship. This normally results in a sound beam with most of the useful energy present along the primary path of propagation. Constructive interference may be present off-axis of the primary beam path, resulting in side lobes.

18 Huygens Principle Side lobes often contribute to image artifacts. The multi-element structure of transducer arrays also contribute to side lobes (often called grating lobes). Apodization is a technique that applies variable-amplitude voltages across individual array elements. This changes the relative power from the individual elements to reduce the intensity of grating lobes. Subdicing is an additional technique used to reduce or eliminate grating lobes. This is accomplished by dividing each element of an array into many smaller subelements with each subelement group wired together to operate electrically as a single element.

19 LONGITUDINAL WAVE PROPAGATION Sound waves in a material produce particle motion that is back and forth along the direction of travel. This back and forth motion, termed longitudinal wave propagation, may be represented graphically. The positive peaks on the graph represent the condensations (where there is a high molecular concentration). The negative peaks on the graph represent the rarefactions (where there is a low molecular concentration).

20 WAVE PARAMETERS The period (P) of a sound wave is the time of one cycle. The wavelength (l) is the distance between two successive points in a pulse that are in the same state of compression. It is the actual distance traveled during a complete cycle. The amplitude (A) is an indication of the relative intensity (loudness) of the sound. The higher the frequency, the shorter the period and wavelength. The pulse duration is the period multiplied by the number of cycles in the pulse. The spatial pulse length is the wavelength multiplied by the number of cycles in the pulse.

21 WAVE PARAMETERS The number of cycles in a pulse is determined by the amount of damping that is present in a transducer. Damping is due to the presence of the transducer s backing material. Increased damping causes a reduction in the number of cycles. Damping also causes an increase in the transducer s bandwidth (BW), which is the range of different frequencies contained in an ultrasound pulse.

22 WAVE PARAMETERS AND EXAMPLES Period = 1 Frequency Wavelength = Velocity Frequency Pulse Duration = Period x Number of Cycles Spatial Pulse Length = Wavelength x Number of Cycles DAMPING FREQUENCY PERIOD WAVELENGTH NUMBER OF CYCLES PULSE DURATION SPATIAL PULSE LENGTH Increase Decrease Decrease Decrease Decrease Decrease Increase Increase Increase Increase Increase Decrease Decrease Decrease Decrease Increase Increase Increase The number of cycles in a pulse is not the same as the frequency of the sound, which is the number of cycles per unit time that a transducer, which is operating continuously, is designed to produce.

23 SAME DAMPING & AMPLITUDE DIFFERENT FREQUENCY & PHASE Frequency = 5.0 MHz Number of Cycles = 3 Period = 0.2 µs Pulse Duration = 0.6 µs Wavelength = mm Spatial Pulse Length = mm Frequency = 2.5 MHz Number of Cycles = 3 Period = 0.4 µs Pulse Duration = 1.2 µs Wavelength = mm Spatial Pulse Length = mm 3-cycle pulse shorter periods shorter wavelengths shorter pulse duration shorter spatial pulse length 3-cycle pulse longer periods longer wavelengths longer pulse duration longer spatial pulse length

24 SAME FREQUENCY, AMPLITUDE, & PHASE DIFFERENT DAMPING Frequency = 5.0 MHz Number of Cycles = 3 Period = 0.2 µs Pulse Duration = 0.6 µs Wavelength = mm Spatial Pulse Length = mm Frequency = 5.0 MHz Number of Cycles = 4 Period = 0.2 µs Pulse Duration = 0.8 µs Wavelength = mm, Spatial Pulse Length = mm 3-cycle pulse same periods same wavelengths shorter pulse duration shorter spatial pulse length 4-cycle pulse same periods same wavelengths longer pulse duration longer spatial pulse length

25 SAME FREQUENCY & DAMPING DIFFERENT AMPLITUDE & PHASE Frequency = 5.0 MHz Number of Cycles = 4 Period = 0.2 µs Pulse Duration = 0.8 µs Wavelength = mm Spatial Pulse Length = mm Frequency = 5.0 MHz Number of Cycles = 4 Period = 0.2 µs Pulse Duration = 0.8 µs Wavelength = mm Spatial Pulse Length = mm 4-cycle pulse same periods same wavelengths same pulse duration same spatial pulse length 4-cycle pulse same periods same wavelengths same pulse duration same spatial pulse length

26 DAMPING vs. BANDWIDTH DAMPING Increase Decrease BANDWIDTH Increase Decrease Damping also causes an increase in the transducer s bandwidth (BW), which is the range of different frequencies contained in an ultrasound pulse.

27 HIGH DAMPING vs. NO DAMPING SAME FREQUENCY. DIFFERENT DAMPING Center frequency = 5.0 MHz Range = 3.75 MHz to 6.25 MHz Number of Cycles = 2 Bandwidth = 2.5 MHz Center frequency = 5.0 MHz Range = 4.9 MHz to 5.1 MHz Continuous Wave Bandwidth = 0.2 MHz Pulse-echo Damped Wide Bandwidth CW Not damped Narrow Bandwidth The continuous wave results in a non-pulsed wave with cycles that repeat indefinitely at the transducer s resonant frequency.

28 Answers to the following ELEVEN practice questions were derived from material in the textbook:

29 Question 1 Ultrasound waves that are traveling through a medium consist of electromagnetic and radio frequencies compressions and refractions condensations and refractions electromagnetic and ionizing frequencies compressions and rarefactions Pages 2 and 5

30 Question 2 What is the difference between audible sound and ultrasound? Ultrasound waves are always longitudinal Audible sound waves are always longitudinal Audible sound has a higher frequency Ultrasound has a higher frequency Ultrasound waves are ionizing Page 2

31 Question 3 A piezoelectric element produces a voltage when postprocessing is used preprocessing is used an acoustic pressure is present on its surface the receiver gain is increased the attenuation increases Page 3

32 Question 4 A decrease in the thickness of a piezoelectric element will result in a greater pulse duration an increase in the propagation speed a decrease in the quality factor if the bandwidth decreases an increase in the frequency of the transducer a higher duty factor Page 3

33 Question 5 The resonant frequency of an ultrasound transducer is dependent on damping the backing material the thickness of the piezoelectric element the amplitude of the voltage applied to the transducer the Curie temperature Page 3

34 Question 6 What does A, B, and D represent on the graph? amplitude, period, velocity pulse duration, duty factor, amplitude wavelength, duty factor, pulse duration period, wavelength, velocity wavelength, spatial pulse length, pulse repetition period Pages 5 and 6

35 Question 7 If the frequency is doubled, the duty factor will increase the period will double the lateral resolution will be poorer the wavelength will double the wavelength will be one-half Pages 5 and 6

36 Question 8 The speed of ultrasound in soft tissue is closest to 1.54 mm/sec 1540 km/sec 1.54 km/msec 1540 mm/sec 1540 m/s Page 2

37 Question 9 Ultrasound energy is traveling through the same medium. If the frequency is doubled, the propagation speed is quartered quadrupled doubled halved unchanged Pages 2 and 3

38 Question 10 The propagation speed is highest in bone tissue fat water muscle Pages 2 and 3

39 Question 11 A single pulse of ultrasound from a transducer contains a range of frequencies contains sound at the nominal frequency of the transducer only contains sound at the resonant frequency of the transducer only has a narrow bandwidth contains continuous waves Page 7

40 END OF LESSON 02

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