Chapitre 4: The basics of Ultrasound PDF

UNIVERSITY OF BLIDA1 Faculty of Natural and Life Sciences 2nd year LMD Academic year :2024/2025 Prepared by: Dr. DJELLOUT and Dr. KABACHE Chapter 4: “The basics of Ultrasound” The objectives of the Course 1. Determine the characteristics of ultrasounds waves 2. Under...

UNIVERSITY OF BLIDA1 Faculty of Natural and Life Sciences 2nd year LMD Academic year :2024/2025 Prepared by: Dr. DJELLOUT and Dr. KABACHE Chapter 4: “The basics of Ultrasound” The objectives of the Course 1. Determine the characteristics of ultrasounds waves 2. Understand the principle of transmitting and receiving ultrasounds wave 3. Understand the principle of ultrasound image formation 4. Determine the types of ultrasound image Introduction Ultrasound science combines modern electronics and acoustics, with its effects known since 1812 but widely applied only after 1945 for submarine detection. Since then, ultrasound has expanded into fields such as biology, medicine, and industry, particularly with advances in electronics during the 1970s. The growing number of ultrasonic devices highlights its importance in industrial and medical contexts. This chapter introduces the applications and principles of ultrasound. It covers the physical characteristics of acoustic waves, explores its biological uses and propagation in biological environments, and explains the basic principles of ultrasound. Understanding these fundamentals is key to comprehending how ultrasound imaging systems, such as echographs, work and create images. Ultrasound applications Ultrasound is used in a wide of applications in the metal industry. Their use is more recent in the plastic and composite materials sectors. Ultrasonics are implemented by : - Medicine: including ultrasound and focused ultrasound thermotherapy; - In agriculture: by vibration of water which turns into aerosol and supplies the root system with oxygen; - Remote sensing: for sonar; - In telemetry: to measure distances; - In the automotive industry, to avoid barriers; - Non-destructive control 1  1-Physical acoustics Acoustics is the study of the properties of sound waves, their production, propagation and effects. 2- Definition of sound An wave is energy that moves and causes a disturbance to move in a medium. We cite two categories of waves: - Mechanical waves require a physical medium to propagate. - Electromagnetic waves can propagate in a vacuum. Sound is a mechanical wave produced by any solid, liquid or gaseous body that enters into vibration and spreads in an elastic medium (which deforms under the passage of a mechanical wave, and regains its initial form when it disappears), before being transmitted to our ear (figure 1). Three elements allow the existence of sound:  A source, producing mechanical vibration.  A carrier medium, transmitting this vibration.  A receiver, the ear, which receives this vibration. Figure 1: Three elements allow the existence of sound. 3- Mode of propagation Sound wave or acoustic wave is a longitudinal wave that propagates by compression and decompression of molecules in a material medium (figure 2). The longitudinal waves have the same direction of vibration as their direction of displacement. 2  Figure 2: Mode of propagation. This diagram effectively demonstrates how a longitudinal wave propagates through a medium, showing the alternating regions of compression and rarefaction as the wave travels. Figure 3: Mode of propagation. The first line in the diagram represents a medium at rest, where there is no disturbance or vibration occurring. This means: 1. No compression or rarefaction: The particles are evenly spaced, indicating that no wave has started propagating yet. 2. No oscillation: Since the particles are stationary, there are no mechanical vibrations, and therefore no sound is being generated or transmitted. Sound is a mechanical wave that relies on particle movement in a medium to propagate. 3  4- Classification of sound waves The frequencies audible to humans range from 20 Hz to 20,000 Hz (20 kHz). These values may vary depending on age and the individual. Below 20 Hz are infrasounds, and above 20 kHz are ultrasounds. These frequencies do not produce a sensation of sound. Figure 4: Acoustics waves and ultrasound (Sound waves can be classified based frequency range). 5- Physical parameters of the sound a. Frequency Frequency is the number of periodic oscillations per second. It is expressed in Hertz (Hz). The frequency is the opposite of the period. f = 1/T With : f: frequency in Hz ; T: period in Second (s). A frequency of 9 Hz will correspond to 9 oscillations per second. 4  b. Wavelength The wavelength is the distance travelled by an acoustic wave during a period. λ = C/f = V/f With: λ: (Lambda) in meters. V or C: (celerity or speed) in m/s (340 m/s in air). f: frequency in Hertz. Figure 5: Wavelength. It can be seen that the higher the frequency, the shorter the wavelength (figure 6). 5  Figure 6: Relationship between Wavelength and Frequency. The higher the frequency, the shorter the wavelength. The lower the frequency, the longer the wavelength. c. Velocity (speed) of sound The speed of sound depends on the nature of the medium (solid, liquid or gas), the temperature and pressure of the medium. The speed of propagation of sound is:  340 m/s in the air at 20° C;  331 m/s in the air at 0° C;  1480 m/s in water at 20° C ;  6000 m/s in steel. The denser the medium, the higher the speed of sound propagation. 6  Table 1. Speed of sound in different medium Temperature will also play a role: the warmer the medium, the more the agitation of the molecules that compose it increases and promotes transmission. In contrast, the speed of sound decreases in a cold environment. At 0°C, the speed of sound waves is 331 m/s in the air. Table 2. Speed of sound in the air as a function of temperature. Temperature Speed of sound -10°C 325 m/s 0 °C 331 m/s 10 °C 337 m/s 20 °C 343 m/s 30 C 349 m/s 5-4- The sound intensity When you are near a sound source, the sound is always louder than when you move away from it. This phenomenon is due to the distribution of sound energy in space To translate this phenomenon, a quantity called “Sound intensity” is defined. The sound intensity or volume distinguishes a loud sound from a low sound, and is related to the amplitude (sound pressure) of sound vibrations A. When the amplitude of the wave is large, the intensity is great and therefore the sound is louder. 7  Figure 7: Sound intensity. 6- Pure sound and Complex sound a. Pure sound Sound is pure (or Simple) when it is composed of a single frequency. This is the simplest sound that can exist. In nature, such sound does not exist. Only an electronic device can generate this type of sound. Figure 8: Pure Sound. b. Complex sound A complex sound is the addition of several pure sounds. Figure 9: Complex Sound. 8  7- Production of ultrasonic waves The ultrasonic transducer is a crystal capable of converting electrical energy into mechanical energy (ultrasound) and vice versa. The mechanism of operation of an ultrasonic probe is based on the physical effect called piezoelectric. Piezoelectricity (Piezo, that is to say pressure) Figure 10 : Production of ultrasond (indirect effect). Figure 11: Reception of ultrasound (direct effect). 8- The principle of ultrasound (the echo) Ultrasound is a term made up of two words: echo and graph which means to draw the echo (reflected wave), which is an imaging technique for visualizing on screen structures of the human and animal body using ultrasonic waves. 9  An ultrasonic transmitter emits short, high-frequency sound pulses at regular intervals. When they encounter an object, they reflect back to the receiver as an echo. The distance from the target is calculated by the time elapsed between the signal being sent and the echo being received. Figure 12: The principle of ultrasound (the echo). Figure 13: Production of ultrasound (ultrasound probe). On the left is the direct effect, the image on the right shows the indirect effect. 9- Ultrasound frequency, image quality and organ depth The ultrasound technique uses ultrasonic waves of frequency ranging from 1MHz to 20 MHz (up to 50MHz for the eye), it depends on the organs or biological tissues to be visualized. The ultrasound system uses a probe, computer system and visualization system. 10  The element that will emit the ultrasound will be the probe. This last one will send waves in a delimited perimeter. Ultrasound uses different frequencies of waves depending on the use to be made. - 1.5 - 4.5 MHz: One can study deep areas such as the abdomen and this to the order of a few millimeters. - 5 MHz: Target intermediate structures such as the heart at a scale below the millimeter. - 7 MHz: Small structures near the skin such as veins or arteries. - 10 - 18 MHz: It will be used to study small animals or for surface imaging. In ultrasound, the low frequency of ultrasound has a high energy at the high frequencies, so the distance traveled by the frequencies bases is greater than that of the high frequencies and the quality of the ultrasound image is low. This frequency is used for deep organs. Table 3. Ultrasound frequency, image quality and organ depth Why is there an echo? The echo is due to the change of the propagation medium, because each medium has its own resistance to ultrasound, this resistance is called acoustic impedance of the medium. 11  Figure 14: Impedance. The speed is generally higher in gravel, followed by compacted sand, then soft sand, and finally seawater (which offers the greatest resistance). Acoustic impedance (Z) is a measure of how much resistance a medium offers to the passage of sound waves. 10- Acoustic Impedance The acoustic impedance Z is a very important quantity to characterize a medium. It has resistance to the propagation of sound wave and is given by: 𝒁= 𝝆. 𝒗 with: Z: Acoustic impedance in Kg/𝑚2. s 𝝆: Density of the medium in Kg/𝑚3 𝒗 : Speed of propagation in m/s Figure 15 : Acoustic Impedance. 12 Table 4. Acoustic impedance for different medium. 11- UltraSound interaction with matter When an ultrasonic wave encounters obstacles (organs), several phenomena can be observed:  Reflection  Refraction These phenomena are responsible for the formation of the ultrasound image. Figure 16: Different phenomena of US interaction with matter. 13  12- Echography image formation Before an echography, a gel will be applied to the part to be studied to improve contact between the skin and the probe and to minimize interference in the transition of the waves from the probe to the area under study. Through a probe in contact with the skin, the doctor can view on a screen the images obtained, which allows him to diagnose pathologies without risk and pain for the patient. The echography image is obtained by ultrasound after passing through the different tissues, are reflected to the probe. The latter alternately acts as transmitter and receiver in extremely short time intervals, of fractions of seconds. In the time that the probe receives the ultrasound, the echograph analyzes two parameters that affect the image; Two important physical parameters or graders are required to form an ultrasound image: 1. The distance between the ultrasound probe and the object (organ) 2. The intensity of the echo received by the probe. a. The distance The receiver calculates the distance from the organ based on the time elapsed between the signal being emitted and the echo being received. Figure 17: Principle of echography Path traveled by the sound wave is twice the distance between the transmitter and the object. Knowing that v and t are known: 𝟐𝒅 𝒕.𝒗 𝒕= => 𝒅 = 𝒗 𝟐 The distance allows to trace (delimit) the contour of the object. 14  Figure 18: Echography image formation. b. Intensity of the echo The amplitude or intensity of the reflected echo gives consistency to the obstacles encountered by representing them in shades of grey. Figure 19 : Intensity of the echo. Knowing the echo return times, their amplitudes and their frequencies, informations can be determined on the nature, depth and thickness of the tissues traversed. Figure 20: Example of an echography image. 15  13- Types of Echography images The different structures of the human (animal) body give different ultrasound images according to their nature: a. Anechogenic image Water and liquids (urine, blood, cysts,...) transmit perfectly the ultrasound without reflecting them, therefore without giving echoes; they are said to be anechogenic, they appear black on the images. Figure 21: Anechogenic image. b. Echogenic image Homogeneous organs with high fuid content (foie, rate, reins, thyroïde, prostate...) cause moderate and regular ultrasound reflection, they are echogenic and appear in shades of grey on the images. Figure 22: Echogenic image. 16  c. Hyperechogenic image Heterogeneous tissues such as fat, bone,... reflect much more ultrasound, they are hyperechogenic and appear in very light gray (white color) on the images. Figure 23: Hyperechogenic image. Conclusion The use of ultrasound plays a crucial role in various fields and has become an indispensable tool. By introducing the basic concepts of ultrasound, we gain a better understanding of the phenomena involved during its propagation through different media. Furthermore, its application in non-destructive testing highlights its simplicity, precision, and, most importantly, its effectiveness. 17