Medical Physics Notes Part 1_2 PDF

# Acoustics Acoustics is a branch of physics that studies mechanical waves, including vibrations, sound, infrasound, and ultrasound. Acoustics observes the generation, propagation of mechanical waves, their interaction with the environment, and the process of hearing. ## What is a mechanical wave? - Mechanical wave (MW) is a spreading disturbance (forced displacement of particles ) in an elastic medium. MW transfers energy but not mass. - The MW may take the form of an elastic deformation in solids or a variation of pressure in gases. ## What is sound? - Sound is called any MW/mechanical vibrations producing hearing perception. ## Totality of mechanical waves - Totality of mechanical waves can be illustrated in a diagram spectrum of mechanical waves consisting of: - Low frequency, mechanical waves - INFRASOUND - Acoustic range (sound) - mechanical waves with frequency from 20 Hz to 20 KHz - High frequency, mechanical waves - ULTRASOUND ! Sound propagation can only occur in elastic media. The vibrations in elastic matter are transmitted consecutively from particle to particle, causing recurrent alteration of density. ## Variations of pressure - If the source of the sound is located in an air environment, its vibrations produce additional movement of the particles, which results in variations of pressure: $P(t) = \Delta P sin(\phi)$. - **Where:** - $\Delta P$ (picture below) is the maximal pressure increase/decrease (+ or -). - $\phi$ is the phase of the given mechanical wave showing the rate of alterations. **Diagram of a mechanical wave** A diagram showing a mechanical wave is displayed, containing the following information: - **X-axis:** distance - **Y-axis:** pressure - **Labels:** - Compressions (C), rarefactions (R) - Wavelength - High Pressure - Low Pressure ## Sources of sound - Sound is a mechanical wave. - Sound can only be transmitted through elastic media. - Sound transfers energy, but not mass. ## Mechanical waves Mechanical waves propagate as two forms of matter vibrations: - Longitudinal - Transversal ### Longitudinal waves - Longitudinal sound waves are typical of gases and liquids and particle movements are illustrated below: **Diagram of a Longitudinal wave** The diagram depicts a longitudinal wave with the following: - **X-axis:** direction of propagation - **Y-axis:** Pressure - **Labels:** - Rarefaction - Compression - Wave's motion - Displacement ### Transversal waves - Transversal waves are typical for solids, where particles displacement occurs perpendicularly to wave propagation: **Diagram of a Transversal wave** The diagram shows a transversal wave with the following: - **X-axis:** direction of propagation - **Y-axis:** Displacement - **Labels:** - Amplitude - Wavelength - Direction of travel ## Physical characteristics of sound - **Sound intensity I:** $I = E/t.S$ - **DEF:** the sound energy transmitted per second through unit area, placed perpendicularly to the sound propagation. Intensity is measured in watt per square meter. [W/m2] - **Sound pressure P:** $P = \Delta P sin(\phi)$ added to atmospheric $P_{atm}$ **Diagram of a mechanical wave** A diagram showing a mechanical wave is displayed, containing the following information: - **X-axis:** distance - **Y-axis:** pressure - **Labels:** - Compressions (C), rarefactions (R) - Wavelength - High Pressure - Low Pressure - **The relation between intensity and pressure:** $I = p²/2Za$ - **Sound frequency - f** - **DEF:** Frequency is the number of vibrations (full alternations of the sound pressure or repetitions of motion of the particles) per second. It is measured in Hz - 1 Hz equals to 1 vibration per second. - **A quantity that is directly related with f is the period: the time duration of 1 vibration** - **Sound velocity (SV) (or speed of sound)** is a quantity that depends on the properties of the medium through which sound propagates. - **For example:** At 20° C SV in the air is about 340 m/s, but in water SV is about 1500 m/s. The exact relationship between sound speed and matter properties treats elasticity as the major factor. - **Temperature dependence of the SV:** **Diagram of Temperature Dependence on Speed of Sound** - **X-axis:** Temperature (°C) - **Y-axis:** Velocity (m/s) - Shows data points and a linear trend line representing the dependence of speed of sound on temperature - **Sound wavelength:** The distance between two consecutive sound fronts $ \lambda = v.T = v/f$ **Diagram of a wavelength** - Illustrates a wave with labeled points A, B, C - Labels: - $\lambda$ - Wavelength - **Acoustic impedance Z:** $Z = p.v$, p-density of the given medium, v – sound velocity - **Sound propagation through a certain medium** is determined from acoustic impedance Z of the latter. When the sound wave encounters a border between two different media, a part of the wave reflects, and the other part passes due to differences in acoustic impedances Z1 and Z2. The degree of reflection/transmission depends on the difference between Z1 and Z2: $a_r^2 = (Z_2 - Z_1)²/ (Z_2 + Z_1)²$ **Diagram of reflection/transmission** - Diagrams illustrate reflected, incident, and tranmitted pulses - **The sounds are of two types:** simple tones and complex tones - **DEF:** sound is called a simple tone if the vibration has a sinus shape and possesses a certain frequency. (on the figure below - two simple sounds - 100 and 500 Hz) - **DEF:** A sound is called complex when it consists of several simple tones as superposition of mechanical waves. (on the figure below - third axis wave) **Diagrams of simple and complex sounds** - The diagram contains graphical representations of two simple sounds and one complex sound. - The x-axis is time (milliseconds) and the y-axis is amplitude. - Each simple sound is represented by a sinusoidal wave. The complex sound is represented by a complex waveform formed by the superposition of the two simple sounds. - **Each complex sound can be illustrated by a graph representing its acoustic spectrum:** combination of certain amplitudes and frequencies proportional to the participant simple waves frequencies (figure below). **Diagrams of acoustic spectrums** - Two spectra of sound are illustrated. For each spectrum, the x-axis represents frequency (Hz) and the y-axis represents amplitude (units are not specified). The spectrums display the amplitudes of different frequencies present in the sound, showing how these frequencies contribute to the overall sound. - **The minimal frequency is called the basic frequency fo (on the pictures above - 100 and 200 Hz) for the two given sounds. The basic sound has always a maximal amplitude. The other components of the vibration are 2f0 (two fold higher), 3f0 (three fold higher) ... and their amplitudes are smaller.** ## Psychophysical characteristics of sound - Each physical (objective) characteristic of sound corresponds to a respective psychophysical (subjective) analog. - Human perception of sound is based on fundamental physiological Weber - Fechner law: - Perception ~ log(stimulus) - (Perceptions increase logarithmically with the stimulus) - Intensity of the sound has a subjective analog: - **Sound Intensity level E (E is defined at 1000 Hz sound frequencyonly) ** - $E = k. Ig(I/I_o)$, where: - I is the intensity of the sound - $I_o$ is its threshold (at 1000 Hz the threshold is approximately $10^{-12}~W/m²$ - the lowest in power vibration audible for human ears) - Intensity level is measured in bel B/ decibel dB (at k=1 and k=10 respectively) - **Loudness of the sound** - L is a quantity representing subjective human perception of the sound magnitude. Loudness is defined by equation: - $L = k. Ig(I/I_o)$ - **Where:** - k is a coefficient depending on the frequency - I is the intensity of the given sound - Io is the threshold of audibility of this sound. - **L is measured in Phon. On the figure below red lines represent equal-loudness contours. Each point of these curves corresponds to certain frequency and sound level in dB, making the relation loudness-intensity level accessible.** **Diagram of Loudness contours** - **X-axis:** Frequency (Hz) - **Y-axis:** Level (dB SPL) - Shows data points and a linear trend line representing the dependence of speed of sound on temperature - **Weber-Fechner low explanation:** **Diagram of Weber-Fechner law** - The diagram illustrates the relationship between sound intensity and nerve impulse frequency. It shows that doubling the sound doesn't double the nerve impulse rate. - **Why is it that doubling the sound intensity to the ear does not produce a dramatic increase in loudness?** We cannot be absolutely sure, but it appears that there are saturation effects. Nerve cells have maximum rates at which they can fire, and it appears that doubling the sound energy to the sensitive inner ear does not double the strength of the nerve signal to the brain. This is just a model, but it seems to correlate with the general observations which suggest that something like ten times the intensity is requried to double the signal from the inner ear. - One difficulty with this "rule of thumb" for loudness is that it is applicable only to adding loudness for identical sounds. If a second sound is widely enough separated in frequency to be outside the critica band of the first, then this rule does not apply at all. - While not a precise rule even for the increase of the same sound, this rule has considerable utility along with the just noticeable difference in sound intensity when judging the significance of changes in sound level. ## Audibility area - On the diagram below, the closed area between the threshold of audibility and the threshold of pain/feeling represents the audibility area. All of the sounds we can perceive are inside this area. Our ears are most sensitive to sounds from 1000 - 4000 Hz range. Human speech is spread over 400-1000 Hz frequency range. **Diagram of Audibility Area** - The diagram displays the threshold of audibility in decibels as a function of frequency in cycles per second. The graph shows that the human ear is most sensitive to sounds between 1000 Hz and 4000 Hz. The region between the threshold of audibility and the threshold of feeling represent the audibility area, which includes all the sounds that humans can perceive. ## Pitch of the sound - Pitch of the sound is a subjective characteristic, corresponding to sound frequency. The higher the frequency - the higher the pitch. - The pitch of the human voice is a physiological feature. It is determined by the vibrating characteristics of the vocal cords (their basic resonant frequency). - **For example**: male basic frequency is in the range of 80-100 Hz and men's voice sounds with lower pitch, whereas females basic frequency of vocal cords is about 400-500 Hz, and this is the reason for the higher pitch of the female voice. ## Timbre - Timbre characterizes complex sounds. Timbre allows different sound sources to be distinguished by their acoustic spectra. ## Sound as a diagnostic instrument ### Auscultation - The basic tool for auscultation is a stethoscope. Mechanical stethoscopes amplify sound due to a standing wave phenomenon. - There are optimal recording sites (sites to place a chest piece of a stethoscope) for the various heart sounds (figure below). - Sound conductivity of a stethoscope is characterized by some features: - Firm application of the chest piece makes the diaphragm taut with pressure, thereby causing an attenuation of low frequencies; - Loose-fitting earpiece causes leakage, which reduces the coupling between the chest wall and the ear. - **On the picture - the frequency sensitivity of the stethoscope is shown (the sound range of the stethoscope application).** **Diagram of Heart Valves and Heart Sounds** - The diagram depicts the human heart with labels indicating the location of heart valves: Aortic, pulmonary, left AV, and right AV. It also shows the location of where heart sounds can be heard. **Diagram of the Stethoscope Frequency Respone** - The diagram presents a frequency spectrum of a stethoscope. - **X-axis:** Frequency (Hz) - **Y-axis:** Gain - Shows the response of a stethoscope to different frequencies. - The stethoscope is most sensitive to frequencies between 100 Hz and 500 Hz, capturing the important heart sounds and murmurs. ### Phonocardiography - Heart sounds and murmurs have extremely small amplitudes with frequencies from 0.1 to 2000 Hz. Therefore, the recording device must be carefully selected for wide band frequency response characteristics. - Specially designed acoustically quiet environments are needed for noise-free recording of heart sounds. The device that is used for recording of these tones and their transformation into audible sounds is called a phonocardiograph. **Diagram of Phonocardiogram** - The diagram shows a recording of phonocardiograph, which represents recordings of heart sounds and murmurs in different conditions. - **X-axis:** Time - **Y-axis:** Sound Amplitude - Each sound wave is labeled (1st, 2nd, 3rd, Normal, Atrial, Aortic stenosis, Mitral regurgitation, Aortic regurgitation, and Mitral stenosis). - **Labels:** Systole, Diastole - **Phonocardiogram (picture above)** is a recording of the heart sounds and murmurs. The method allows for eliminating subjective interpretation of the heart sounds. It enables the evaluation of the heart sounds and murmurs regarding electric and mechanical events in the cardiac cycle. Evaluation of the result is based on the basis of changes in the wave shape and various timing parameters. ### Audiometry - Audiometry is a method for experimental examination/determination of the personal threshold of audibility. It is carried out by means of a generator of simple tones with certain frequencies (125, 250, 500, 1000, 2000, 4000, 8000 Hz). Audiometry is used to determine the relation between objective sound Intensity and subjective Loudness. **Diagram of Audiometry Curve** - The diagram shows the subjective loudness of sound in dB as a function of frequency in Hz. - **X-axis:** Frequency (Hz) - **Y-axis:** Loudness (dB) - The figure emphasizes that the relation between objective sound intensity and subjective loudness is not linear, demonstrating human ear's sensitivity to different frequencies. ## Sound therapy ### Extracorporeal Lithotripsy: - EL is based on the concept of remote destruction of kidney and bladder stones by sound waves. When a mechanical wave (of type shock wave) passes through the stone, the latter starts to vibrate. A resonance occurs and the amplitude of vibration reaches the critical value at which the structure stability is impaired. The stone is destroyed into small pieces. The latter is due to the rapid increase of pressure inside the stone and impairing of its structure stability. **Diagram of Extracorporeal Lithotripsy** - The diagram illustrates the setup and operation principle of Extracorporeal Lithotripsy. - Two diagrams are displayed: - **fig. A:** Shows the setup of lithotripsy device, representing a shock wave generator and reflectors - **fig. B:** Depicts the operating principle with a shock wave generator focusing a beam on kidney stones. ## Ultrasound - Ultrasound (US) waves are spread over a wide range of frequencies: 20 kHz – 20 MHz. - US waves carry higher energy compared to the same amplitude sound (due to their high frequency). - US waves can interact with the biological tissues (due to the similarity in size of different bio-structures and US wavelength), which determines its significance in medicine. - US vibrations are well absorbed by tissues. Being reflected or scattered by small objects US serve as information carriers in diagnostic imaging. ### Production of US - US is produced by means of a piezoelectric effect. PEE occurs in some crystals due to their specific crystal lattice: normally electric charges are placed symmetrically, but being deformed, the crystal, the symmetry of the lattice is impaired and polarization occurs - this phenomenon is called direct piezoelectric effect. - Conversely, if the electric charge is loaded on the opposite walls of the crystal, it experiences electric forces deforming the lattice (reverse piezoelectric effect). If this deformation appears at more than 20000 times per second, the crystal will vibrate with the US frequency, and it becomes the US source. The crystals of this type can simultaneously generate and receive US wave. The device built on piezo-crystal is called a transducer and serves as both - a generator and a receiver of US. **Diagram of Piezoelectric Effect** - Two diagrams are displayed: - The first diagram shows an unloaded piezoelectric crystal with symmetrical charge distribution. - The second diagram shows a deformed piezoelectric crystal, where the charges are shifted due to strain, resulting in a net charge displacement. The charges are no longer symmetrical, creating a dipole moment. ## Physical properties of US, determining its application as a diagnostic instrument: - The wavelength of US is shorter than that of the audible sound (example: if two sound waves with frequencies of 2 KHz and 2 MHz are spreading in air their wavelengths will be 75 cm and 0.75 mmrespectively). - Short wavelength enables US to interact with small structures (dimensions of mm and less) in human tissues. - As a consequence, US can produce resonant vibration within tissues, and being reemitted or reflected by them, it carries useful information about their location, intactness, constitution. ## US imaging - US sonography is based on the analysis of US reflected by tissues. - Reflections occur when US encounters boundary between different types of tissues (due to the difference in acoustic impedances). **Diagram of US imaging** - The diagram depicts the US propagation, reflection, and transmission through tissues. Two tissues - "emit" and "wait" - are separated by a boundary. - The US wave is emitted from "emit", and then part of the wave is reflected from the boundary, while the other part transmits through the boundary. - **The distribution of reflected/penetrated US beam is determined by:** - Reflection coefficient : $a_r^2 = (Z_2-Z₁)²/ (Z_2 + Z₁)²$ - **When US falls from the air to the patient's skin ar equals approximately 1 (Z2 skin >> Z₁ air). Therefore, almost 100% of the wave reflects back to the air. Thus the US wave does not penetrate into the body and no diagnostic information is received. This problem is abolished by the use of contact gel. The gel has the same acoustic impedance as the skin (tissues). The transducer is immersed in gel and US penetrates into the body without losses when it crosses the border gel-skin:** **Diagram of US penetration through the skin** - The diagram illustrates the US penetration through the skin. It depicts a US transducer placed on the skin surface, with the US beam traveling through a layer of conductive gel and penetrating into the underlying tissue. ## US sonography types - **A** - **amplitude scan**. The image represents a straight line with spikes. Each spike corresponds to a border between tissues. The main diagnostic parameter is the amplitude of spikes - the larger the difference between acoustic impedances of tissues, the greater the amplitude. This method is applicable to simple structures. - **B** - **brightness scan images represent each small detail of the tissue observed as a bright/dark spot, depending on the value of acoustic impedance. This method is suitable for compound structures examination**. - This is the default mode that is produced by any ultrasound / echo machine. It is a 2 dimensional cross-sectional view of the underlying structures. It is made up of ... This is the most intuitive of all modes to understand. The field of view is the portion of the organs or tissues that are intersected by the scanning plane. Depending on the probe used, the shape of this field could be a sector - commonly seen with Echo and abdominal ultrasound probes or rectangular or trapezoid - seen with superficial or vascular probes. - **Multiple images of the field or frames are generated every second on the screen, giving an illusion of movement. A frame rate of at least 20 frames per second is needed to give a realistic illusion of motion.** - **On a grey scale, high reflectivity (bone) is white; low reflectivity (muscle) is grey and no reflection (water) is black. Deeper structures are displayed on the lower part of the screen and superficial structures on the upper part.** - **The main uses for 2-D mode are to measure cardiac chamber dimensions, assess valvular structure & function, estimate global &segmental ventricular systolic function, and improves accuracy of interpretation of Doppler modalities.** - **While this mode is useful to accurately represent the 2-dimensional structure of the underlying tissues, it does not resolve rapid movements well, and may misrepresent the 3-dimensional nature of structures.** **Diagram of US imaging modes** - The diagram shows two US images: - The left image is an A-mode scan, showing a linear display with spikes representing different tissue layers. - The right image is a B-mode scan, displaying a two-dimensional grayscale image of the tissue with different brightnesses representing different tissues. - **On the figure above - comparison between A- and B- mode images, used in echography of eye.** ### M-mode - This mode allows investigation of movable structures. Initially, a 2-D image of the object is produced, and a single scan line is placed along the area of interest. The M-mode will then show how the structures intersected by that line move toward or away from the probe over time. - The M-mode has good temporal resolution, so it is useful in detecting and recording rapid movements. We can also correlate and time events with ECG or respiratory pressure waveforms traced alongside the M-mode tracings. - The M-mode is commonly used for measuring chamber dimensions and calculating fractional shortening and ejection fraction. **Diagram of M-mode** - The diagram shows a typical M-mode recording. The x-axis represents time (milliseconds), and the y-axis represents the distance, measured in millimeters. - The M-mode tracing shows the displacement of heart structures (like valve leaflets) as a function of time, providing visual information about the moving heart. ## Doppler's effect - The Doppler effect is named after its inventor Christian Doppler. - Doppler effect is called the change in frequency of a wave as perceived by an observer moving relative to the source of the wave. - Doppler's effect can occur in any process involving waves, including ultrasound. - **Let consider a person (receiver) who is moving toward a sound source with velocity v. The receiver will perceive sound with higher frequency than the source frequency (respectively - the wavelength will be shorter than at the source) because moving against the sound propagation, he will encounter more than one wavefront per second.** **Diagram of Doppler Effect** - The diagram depicts the scattering of an ultrasonic wave from a moving object. - the wave travels from the emitter towards the object that is moving toward the receiver - The diagram illustrates that the frequency of the received signal (f) is higher than the transmitted frequency (fo) due to the Doppler effect caused by the object's movement. - **If the person (receiver) is going away from the source – the effect is reverse: perceived sound has a lower frequency and a longer wavelength, compared with the source.** - These phenomena are called blue shift and red shift respectively and are present in other wave phenomena such as light. - **Measurement of Doppler shift allows calculation of bloodstream velocity. The frequency of reflected US changes with:** - $ \Delta f = 2.v Cos(\theta).f_o /c $ - $\Delta f $ = $f_r - f_o$ - $f_r$ is the reflected wave frequency - v is the velocity of the blood stream - c is the US velocity in soft tissues (~1540 m/s) - $f_o$ is the US frequency - $\theta$ is the angle between blood flow and the US axis of propagation (Doppler's angle) **Diagram of Doppler Velocity Measurement** - The diagram shows a blood vessel and a US beam. - The US beam is emitted toward the blood vessel, and the Doppler shift is measured to determine the velocity of blood flow. - The labels indicate the US frequency (fo), the reflected frequency (fr), the velocity of blood flow (v), and the Doppler angle (θ). - The angle represents the angle between the direction of blood flow and the direction of the US propagation. ## Ultrasound applications in therapy - US can influence tissues, producing favorable therapeutic effects upon them. The character of the effects derived is related to US physical properties and the type of tissues. Different influences can be achieved selecting the US mode of generation - continuous or pulsed. - **Pulsed US (duty cycle of 20%) cannot produce thermal effects because of dissipation of energy absorbed during the pause of the cycle. Ultrasound energy creates also mechanical forces independent of thermal effects, thereby causing biologic effects that are not related to temperature rise alone, such as cavitation, torque forces, oscillatory shear, radiation, pressure, and microstreaming.** - **MEDICINE only: The interaction of ultrasound with gas bubbles or contrast agents causes rapid and potentially large changes in bubble size. This process, termed cavitation, may increase temperature and pressure within the bubble, and thereby cause mechanical stress on surrounding tissues, precipitate fluid microjet formation, and generate free radicals. Gas-containing structures (e.g., lungs, intestines) are most susceptible to the effects of acoustic cavitation. Ultrasound wavelength has an important role in bubble formation and growth: short wavelength ultrasound (observed at higher frequencies) does not provide sufficient time for significant bubble growth; therefore, cavitation is less likely under these circumstances compared with long wavelengths. The short half-life of cavitation nuclei prevents most cavitation-related biological effects, unless ultrasound contrast agents are also present. Contrast agents markedly reduce the threshold intensity for cavitation. However, because of the relatively high viscosity of blood and soft tissue, significant cavitation is unlikely and cavitation has not been shown to occur with the ultrasound exposure commonly used during a diagnostic examination.** - **In general:** pulsed US is used to produce: - Increase of skin and cell membranes permeability resulting in calcium influx enhancement. - Also increase of: - Mast cell degranulation - Macrophage activity - Rate of protein synthesis - Oppose inflammatory processes. ## Thermal effects - **The thermal action of US is caused due to oscillations of tissues (cells, molecules) when US is passing through them. The amount of heat produced depends on the intensity of the ultrasound, the time of exposure, and the specific absorption characteristics of the tissue. As much as 70% of the total temperature increase associated with ultrasound occurs within the first minute of exposure, but temperature continues to rise as exposure time is prolonged.** - **Better thermal effect is achieved in highly absorbing structures (rich in collagen as the main absorber)** - joints, bones, connecting tissue. - **The relative protein content of each tissue, since absorption coefficients of tissues are directly related to protein content; absorption coefficients vary between 1 (skin, tendon, spinal cord) and 10 (bone) dB/cm MHz.** - **Such effect is derived by high intensity, high frequency, continuous US. Since muscles are well vascularized, they do not undergo sizeable heating up because blood flow continuously carries away a large part of heat produced.** - **Main effects of heating are:** - Acceleration of metabolic processes. - Alteration of nerve conductivity. - Enhanced blood circulation. - Improved extensibility of soft tissues, incl. muscle elasticity. - **The mentioned phenomena are beneficial in US diathermy (heating therapy) to:** - Heat up bones and joints. - Treat of arthritis. - Strengthen bones. ## US sonophoresis - Ultra-sonophoresis is a method for drug delivery into localized areas assisted by a collimated ultrasound beam directed toward these areas. **Diagram of Sonophoresis** - This diagram depicts the process of sonophoresis, showing a skin layer with a localized application of ultrasound waves. - The ultrasound application is used to facilitate the penetration of drug molecules (represented as "molecules in gel") deeper into the skin. ## HIFU - Ultrasound Therapy (HIFU) stands for High Intensity Focused Ultrasound, a system to treat cancer pathologies and/or to improve the quality of life of patients. - It is compatible with other therapies and may be a viable alternative to traditional surgery. - Despite the great advances made in the field of preventive and therapeutic, cancer remains the second leading cause of death in developed countries and is among the top three leading causes of death in developing countries. **Diagram of HIFU** - The diagram shows a focused transducer emitting ultrasonic waves. - The diagram illustrates that a high-intensity focused beam of ultrasound can be used to create localized heating and can be used to destroy cancerous tissue. ## Ablation and therapeutic procedures - In medicine, the term ablation indicates the creation of a necrosis of a portion of biological tissue. The techniques of thermal ablation are therapeutic procedures that aim to destroy diseased tissue (typically cancers) by a thermal heating without damaging adjacent structures vital. - **The cells that make up the tissue, in fact, cannot withstand high temperatures, and suffer damage in different amounts according to the temperature range of which they are subjected.** To understand how heat interacts with biological tissue, you can define some variables, values, temperature-associated cellular damage: - **40 °C** - Slight increase in temperature causes the cellular homeostasis - **40 °C-45°C** - Value of hyperthermia moderate - **46 °C** - Cells begin to suffer irreversible damage but with slow speed - **50 °C - 52 °C** - Cells undergo irreversible damage with a reduced speed - **60 °C - 100°C** - Coagulation necrosis, irreversible cell damage involving the main cytosolic enzymes, mitochondrial complexes and histone-nucleic acids, and thermal damage also occurs in the course of a few days. - **105°C and over** - Vaporization of the cells and subsequent carbonization. ## Infrasound - **IS** - vibrations in elastic media with a frequency from 0 to 20 Hz. - Humans do not hear these vibrations. - Natural sources of IS are: Earthquakes, volcanoes, sea waves (tsunami), typhoons, waterfalls. - Some animals use IS to communicate - elephants, hippopotamus. - **Artificial sources:** some machines, transport systems, factories, large air conditioners. - IS is absorbed weakly due to its long wavelength. For example, if two waves with frequencies of 1000 Hz and 10 Hz, respectively, propagate through the same medium - the latter will be absorbed ten times less! Therefore, IS penetrates deeper than audible sound. - Because of its low frequency, IS causes vibrations of large objects (machines, buildings, ships) and also can produce resonance vibrations in the human body - internal organs and body cavities have characteristic resonant frequencies. - Heart - 1 to 1,5 Hz - Circulation of the blood flow across the body - 1,2 Hz - Vestibular apparatus - 0,5 to 13 Hz - Most of Internal organs - 2 to 8 Hz - Head - up to 20 Hz - **Exposure to infrasound has been demonstrated to affect recipients with symptoms including fear, sorrow, depression, anxiety, nausea, chest pressure, and hallucination.** It can cause objects to move through vibration, and some believe the body's internal organs can be affected. - It is suggested that levels above 80 decibels at frequencies between 0.5 to 10Hz may start to affect the vestibular of the inner ear, thus causing disorientation. - Any high volume sound can trigger the body to react by increasing respiration, heart rate, and blood pressure, but when they cannot actually hear the sound, recipients are left with no explanation for the sudden onset of these symptoms. This may then lead to further effects caused by the minds' possible reaction to the unknown, as outlined below. - Ocean waves are known to sometimes generate infrasound, and it has been suggested to have been a possible "trigger" causing ships crews to abandon their craft in fear, only to have the ship later found mysteriously drifting about unmanned. - The range of infrasound is generally accepted to be between 0-20 Hz, with a specific area of interest between 17 and 19 Hz. Tests by NASA have revealed that the human eyeball resonates at around 18Hz, to which infrasound exposure may cause a reaction and lead to hallucinations. - Infrasound occurs quite naturally at some locations and possible causes include storms, earthquakes, waterfalls, volcanoes, ocean waves, and wind reacting with structures such as chimneys. - Some animals are sensitive to these low frequency vibrations and may appear to "foresee" approaching storms and earthquakes. Elephants are known to use infrasound as a form of communication over long distances. - Subsonic sound can travel long distances, pass through walls and may be amplified in tunnel-like structures. Standard hearing protection is of little use for subsonic sound as it often can pass straight through and may even be amplified. - **Table 18.1 Effects of Mechanical Vibration on Man** - **Specific Vibrational** - **Sound Frequency** - **Estimated** - Major body resonances - 3 Hz-300 Hz - 140-150 dB - Effects on postural control - 0.1 Hz-10 KHz - 140-155 dB - Motion sickness - 0.1 Hz-1 Hz - 140-145 dB - Bluring of vision - 3 Hz-1 KHz - 140-155 dB - Disturbance of breathing, speech - 1 Hz-100 Hz - 140-150 dB - Interference with task performance - 3 Hz-1 KHz - 140-155 dB ## Processes of transfer ### Diffusion - DEF: **Diffusion is the net movement of molecules or atoms from a region of high concentration (or high chemical potential) to a region of low concentration (or low chemical potential).** **Diagram of Diffusion** - The diagram displays a simple schematic representing diffusion. - It shows a box divided into two compartments: - One compartment contains a higher concentration of circles, representing molecules or atoms. - The other compartment contains a lower concentration of molecules. - Arrows indicate the direction of diffusion, where molecules are moving from the higher concentration to the lower concentration. - **Quantitatively, the mass of transferred substance can be evaluated using Fick's laws of diffusion:** - $J = -D.dc/dx$, where: - J is a mass flux - the mass flowing per second through unit area (M/S.t). - D is the diffusion coefficient. - dc/dx is the concentration gradient - **or:** - $M = -D.dc/dx.S.t$ ### Gradient - Gradient is a vector quantity representing the rate of change of a given scalar quantity over the distance. Its value equals to the difference in quantity's values in two points/regions, and

Medical Physics Notes Part 1_2 PDF

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