Sound in Medicine PDF

Sound in Medicine Sound is a mechanical disturbance/energy that propagates through a medium at specific velocity by the compression and rarefaction of the medium particles. Propagation mechanism of sound: as the sound waves travel through air, it will cause loca...

Sound in Medicine Sound is a mechanical disturbance/energy that propagates through a medium at specific velocity by the compression and rarefaction of the medium particles. Propagation mechanism of sound: as the sound waves travel through air, it will cause local increase and decrease in the atmospheric pressure. The pressure increase is called compression, and the decrease is called rarefaction. Sound waves spread outward as longitudinal wave which means that the motion of the medium particles is in the same direction of sound propagation. Sound propagates in a medium (compressions and rarefactions). Sound waves When an object vibrates, the particles around the medium vibrate. The disturbance produced by the vibrating body travels through the medium but the particles do not move forward themselves. A wave is a disturbance which moves through a medium by the vibration of the particles of the medium. Sound waves are called mechanical waves and it require medium for transmission. 1  When a vibrating object moves forward, it pushes and compresses the air in front of it forming a region of high pressure called compression (C). When the vibrating object moves backward, it forms a region of low pressure called rarefaction (R). In these waves the particles move back and forth parallel to the direction of propagation of the disturbance. Such waves are called longitudinal waves. Sound wave diagram Compressions are the regions of high pressure and density where the particles are crowded and are represented by the upper portion of the curve called peak (crest). 2  Rarefactions are the regions of low pressure and density where the particles are spread out and are represented by the lower portion of the curve called bottom (trough). Characteristics of a sound wave There are five main characteristics of sound waves: wavelength, amplitude, frequency, time period, and speed. Wavelength (λ): the distance between adjacent crests, measured in meters. (The wavelength of a sound wave indicates the distance that wave travels before it repeats itself). Amplitude: is the maximum displacement from the rest or central position, measured in meters. Frequency (𝒇): is defined as the number of complete waves produced per unit time, measured in inverse seconds, or Hertz (Hz).1 Hz =1 cycle per second. - Audible sound: refers to frequencies range from 20 Hz to 20 kHz. - Infrasound: refers to sound frequencies below the audible range (20 Hz). - Ultrasound: refers to frequency range above the audible range (20 kHz). Period: the time it takes for one complete wave to pass a given point, measured in seconds. 𝟏 𝑻= (1) 𝒇 Speed: the horizontal speed of a point on a wave as it propagates, measured in meter /second. Multiplying the frequency ƒ of the sound wave by its wavelength λ results in the velocity ν of the sound propagation: 𝐯=𝒇𝝀 (2) 3  - The speed of sound is more in solids, less in liquids and least in gases. - The speed of sound also depends on the temperature of the medium. If the temperature of the medium is increase, the speed of sound is more. Sharpness (intensity) and loudness of sound The Sharpness of sound depends on the frequency of vibration. If the frequency is high, the sound has high intensity and if the frequency is low, the sound has low intensity. Reflection of Sound Sound gets reflected at the surface of a solid or liquid and follows the laws of reflection. 1. The angle of incidence is equal to the angle of reflection. 2. The incident ray, the reflected ray and normal at the point of incidence all lie in the same plane Echo If we shout or clap near a reflecting surface like tall building or a mountain, we hear the same sound again. This sound which we hear is called echo. It is caused due to the reflection of sound. To hear an echo clearly, the time interval between the original sound and the echo must be at least 0.1 s. Since the speed of sound in air is 344 m/s, the distance travelled by sound in 0.1 s = 344 m/s x 0.1 s = 34.4 m So to hear an echo clearly, the minimum distance of the reflecting surface should be half this distance that is 17.2 m. 4  Reverberation Echoes may be heard more than once due to repeated or multiple reflections of sound from several reflecting surfaces. This causes persistence of sound called reverberation. In big halls or auditoriums to reduce reverberation, the roofs and walls are covered by sound absorbing materials like compressed fiber boards, rough plaster or draperies. Sound Applications in Medicine Percussion in Medicine (the body as a drum) Percussion is a diagnostic procedure in which the physician or nurse knock on parts of the body and listen to the obtained sound (sound of empty barrel or drum). Percussion could be used to diagnose solid masses, abnormal cavities within an organ and lung diseases involving fluids in the chest. Stethoscope It is a simple hearing aid permits physician or nurse to listen to sound made inside the body primarily in the heart and lungs. The sound of heartbeat reaches the doctor’s ears by multiple reflection. Such process called auscultation. 5 Modern stethoscopes have two chest pieces: a “bell” chest piece applied lightly to the skin to pick up low frequency sounds and a “diaphragm” chest piece pressed firmly to the skin to minimize low frequency sound and therefore enable hearing high-frequency sounds. Other parts of stethoscope are the tubing for sound transmission and the earpieces. Parts of a Stethoscope. Ultrasound Imaging Ultrasounds are produced by piezoelectric effect (the ability of certain materials to generate an electric charge in response to applied mechanical stress). A device that converts electrical energy to mechanical energy or vice versa is called a transducer. Each transducer has a natural resonant frequency of vibration. The basic for the use of ultrasound in medical imaging is the partial reflection of sound at the surface between two media that have different 6 acoustical properties. Typical frequencies for medical imaging are ranged between 1 and 10 MHz Pulses of ultrasound are transmitted into the body by placing the vibrating crystal in close contact with the skin, using water or jelly paste to eliminate the air. This gives good coupling at the skin and greatly increases the transmission of the ultrasound into the body and of the reflected echoes (rebound) back to the detector. Diagram of ultrasound waves emitting from ultrasound probe. 7  Usually the same transducer that produces the pulse serves as the detector. The return signals are then amplified and displayed on an oscilloscope (a devise used to display and analyze the waveform of electronic signals). Doppler Effect The Doppler Effect refers to the property of waves that results in a change in frequency of the wave. The Doppler Effect is used in medical imaging to produce a Doppler ultrasound image showing whether or not movement, such as blood flow or heartbeat is normal. Doppler ultrasound images are normally color coded, with different colors representing different velocities relative to the ultrasound head. Doppler ultrasound image uses color coding to show different rates of movement of the tissues being imaged. Color Doppler image showing leakage of blood through a hole in the septum separating the left and right ventricles. 8 Doppler ultrasound can be used to detect of  Leakage of blood through heart walls - holes  Backflow of blood through faulty valves  Poor blood flow due to fat deposits in arteries  Irregular flow due to heart malfunction Physiological effects of Ultrasound in Therapy The primary physical effect produced by ultrasound is temperature increase of tissue. This temperature rise is due to the absorption of acoustic energy in the tissue. The magnitude of physiological effects of ultrasound depends on the frequency and amplitude of the sound wave. At the very low intensity levels, used for diagnostic work, no harmful effects have been observed. As the power is increased, ultrasound becomes useful in therapy. Ultrasound is used as a deep heating agent at continuous intensity levels of about 1 W/cm2 and as a tissue-destroying agent at intensity levels of 103 W/cm2. 9