E

Sound

• Sound is a vibration propagating as an acoustic wave.
• Acoustic waves are elastic waves.
• Sound propagates through mediums like solids, liquids, or gases.

Sound Categories

• Simple Sound
• Complex Sound
• Noise
• Clatter

Characteristics of Sound (Objective)

• Frequency ($f$)
• Period ($T$)
• Intensity ($I$)
• Wavelength ($\lambda$)
• Velocity ($v = f \lambda$)

Characteristics of Sound (Subjective)

• Sound Height (Pitch): Discrimination based on sharpness; high frequency = sharp sounds.
• Loudness (Audibility): Sensation degree; affected by intensity & frequency.
• Sound Timbre: Distinguishes sounds with same height & audibility; characterized by harmonic content.

Frequency of Acoustic Waves

• Infrasonic waves (infrasounds): f < 20 Hz
• Human acoustic region: 20 < f < 20000 Hz
• Ultrasonic waves (ultrasounds): f > 20000 Hz

Sound Propagation Velocity

Sound velocity ($v$) is determined by:

• Density of the medium ($d$)
• Elastic bulk modulus ($K$)
• Formula: $v = \sqrt{\frac{K}{d}}$

For gas, sound velocity is given by:

• $v = \sqrt{\frac{\gamma RT}{W}}$
  • $\gamma$: ratio of specific heats
  • $T$: temperature
  • $R$: universal gas constant
  • $W$: molecular weight

Sound Propagation Velocity (Examples)

• Air ($0^{\circ}C$): $331$ m/s
• Air ($20^{\circ}C$): $343$ m/s
• Oxygen: $317$ m/s
• Hydrogen: $1286$ m/s
• Water: $1450$ m/s
• Copper: $3560$ m/s
• Iron: $5130$ m/s

Wave Phenomena of Sound

Reflection

• Angle of incidence = angle of reflection: $\theta<em>i = \theta</em>r$
• Echo heard when reflected sound reaches ears after at least $0.1$ sec.
• Minimum distance (d) between observer & reflecting wall:
  • $v = \frac{2d}{t} \Rightarrow 2d = 340 \frac{m}{sec} \times 0.1 sec \Rightarrow d = 17 m$

Refraction

• Bending of sound waves when entering a medium where speed changes.
• Formula: $\frac{sin(\theta<em>i)}{sin(\theta</em>d)} = \frac{v<em>1}{v</em>2}$

Refraction (Non-linear Propagation)

• Occurs in the atmosphere due to temperature variations.

Diffraction

• Bending of waves around obstacles & spreading beyond openings.
• Occurs when opening/object size is small compared to wavelength.

Hearing

• Involves physics and physiology, leading to sound perception.

Outer Ear

• Includes pinna (auricle), ear canal, and tympanic membrane (eardrum).
• Ear canal length: ~$2.5$ cm.
• Sound waves vibrate the eardrum.

Middle Ear

• Air-filled cavity with three ossicles: malleus, incus, stapes.
• Connects to upper throat via Eustachian tube.
• Ossicles amplify sound pressure ($P = \frac{F}{S}$).
• Malleus receives vibrations, transmits to incus, then stapes, which rests on the oval window.

Inner Ear

• Bony labyrinth with liquid-filled channels.
• Cochlea: spiral-shaped organ for hearing.
• Cochlear duct contains Organ of Corti on the basilar membrane.
• Organ of Corti contains hair cells.
• Vibration from middle ear transmitted to cochlea.

Inner Ear cont.

• Initial stimulus magnitude determines basilar membrane motion amplitude.
• Basilar membrane moves at specific spot based on sound wave frequency.
• Higher frequency waves move the region near the cochlea base.

Inner Ear - Hair Cells

• Hair cells topped with stereocilia.
• Basilar membrane movement causes tectorial membrane movement & stereocilia bending.
• Electrochemical pulses are produced and transferred to the brain via the acoustic nerve.

Speech

• Involves nasal cavity, oral cavity, vocal folds, tongue, pharynx, epiglottis, larynx, trachea, lungs, etc.

Speech - Vocal Output

• Periodic sound from vocal folds is filtered by the vocal tract to produce output sound.

Sound Intensity-Pressure in Decibels

• $A = 10 \log<em>{10} (\frac{I}{I</em>o})$
  • $A$: intensity in decibels (dB)
  • $I$: intensity under examination in Watt/cm$\,^2$
  • $I_o$: reference intensity (minimum audible) = $10^{-16}$ Watt/cm$\,^2$

Sound Pressure in Decibels

• $A<em>p = 20 \log</em>{10} (\frac{P}{P_o})$
  • $A_p$: Sound pressure in decibels (dB).
  • $P$: Pressure of the sound under examination in Pa or N/m$\,^2$.
  • $P<em>o$: Reference sound pressure ($P</em>o = 2 \times 10^{-5}$ Pa).
• Relationship between intensity and pressure: $I = kP^2$
  • $k$: constant
  • $P$: sound pressure

Sound Intensity - Loudness

• $B = c \log I$
  • $B$: Loudness
  • $I$: Sound intensity
  • $c$: Constant (Weber-Fechner law)

Equal Loudness Curves

• Loudness quantified in phons.
• 1 phon = 1 dB at 1000 Hz.
• Equal loudness curve connects sounds producing same loudness despite different physical characteristics.

Auditory Effects

• Safe range, risk range, & danger range for sound levels.

Auditory Effects Cont.

• Auditory Fatigue: Temporary decrease in hearing; recovers after hearing rest.
• Occupational Hearing Loss: Long-term auditory fatigue leading to permanent deficits.

Auditory Effects - Non-Hearing

• Cardiovascular, nervous, reproductive, and psychological effects.

Sound Protection

• Peak of sound pressure ($P_{peak}$): max instantaneous pressure of noise.
• Daily noise exposure level ($L_{EX,8h}$): time-weighted average for 8-hour workday.
• Critical values: $L<em>{EX,8h} = 87$ dB and $P</em>{peak} = 200$ Pa.
• Lower action values: $L<em>{EX,8h} = 80$ dB and $P</em>{peak} = 112$ Pa.

Sound Protection - Measures

• Measures at the noise source.
• Measures in the noise path.
• Personal measures.

Sound-Noise Measurements

• Sound Level Meter: Provides sound level measurements at a specific location & time.
• Noise Dosimeter: Stores sound level measurements over time (e.g., 8-hour workday) to find the average reading.

Audiometry

• Examination with tuning fork.
• Pure tone audiometry.
  • Air conduction.
  • Bone conduction.

Audiometry - Audiograms

• Audiograms display hearing loss in decibels vs. frequency for air and bone conduction.

MEDICAL ULTRASOUND: Physics and Technology

A. PHYSICAL PRINCIPLES OF ULTRASOUND

1 Introduction

• Ultrasound waves: sound waves with f > 20 kHz.
• Medical imaging uses $1$ to $15$ MHz.
• Characterized by: frequency, wavelength, period, propagation speed.
• Average speed in soft tissues: $1540$ m/sec.
• Medical ultrasound generally uses pulses (not continuous beams).
• Pulse repetition period: time from the start of one pulse to the next.
• Pulse repetition rate: number of pulses per second.
• Pulse length ($L$): length over which the pulse exists.
  • Formula: $L = n \lambda$ ($n$ = cycles, $\lambda$ = wavelength).
  • Higher frequency = shorter pulse length.
  • Axial resolution limited by pulse length.
• Ultrasound imaging: pulse-echo technique.
  • Pulse sends echo back upon encountering organ surfaces.
  • Transducer emits & detects.
• Imaging based on time duration ($t$) of pulse path.
  • Structure imaged at depth $s = c(\frac{t}{2})$ ($c$ = speed of ultrasound).

2. Interactions of Ultrasound and Matter

• Intensity diminishes due to attenuation as it travels through tissues.
• Attenuation due to:
  • Reflection
  • Scattering
  • Absorption
• Attenuation coefficient: degree of attenuation in dB/cm.
• $I = I_o e^{-\alpha x}$
  • $I_o$: initial intensity
  • $I$: intensity after distance $x$
  • $\alpha$: attenuation coefficient
• Attenuation-frequency relationship: linear in soft tissues.
  • Attenuation coefficient: ~$0.5$ to $1$ dB/cm at $1$ MHz.
• Attenuation limits depth of propagation.
• Acoustic impedance ($Z$): difficulty of sound waves passing through tissues.
  • $Z = \rho c$ ($\rho$ = density, $c$ = speed).
  • Unit: rayl (1 rayl = 1 kg/m$\,^2$/s).

Acoustic Impedance Values

• Air/containing tissues: low acoustic impedance ($\approx 400$ Rayls).
• Soft tissues: ~$1.6 \times 10^6$ Rayls.
• Bones: high acoustic impedance ($\approx 7 \times 10^6$ Rayls).

Reflection

• Occurs at interface of two tissues of different acoustic impedance.
• Lateral incidence:
  • Part reflected, part refracted.
  • Angle of incidence = angle of reflection.
  • Snell's Law: $\frac{sin(\theta<em>i)}{sin(\theta</em>d)} = \frac{v<em>1}{v</em>2}$
• Perpendicular incidence:
  • Reflected wave returns to source.
  • $\frac{I<em>R}{I</em>o} = [(\frac{Z<em>2 - Z</em>1}{Z<em>2 + Z</em>1})^2]$
    • $I_R$: reflected wave intensity
    • $I_o$: incident wave intensity
    • $Z<em>1, Z</em>2$: acoustic impedances

Reflection percentages

• Greater impedance difference = stronger reflection.
• Air/soft tissue: ~$99\%$ reflected.
• Soft tissue/bone: ~$70\%$ reflected.

Scattering

• Occurs when wavelength is comparable to or shorter than the dimensions of the surfaces.
• Incident sound waves change orientation randomly.
• Small percentage backscattered to the transducer.
• Small inhomogeneities in parenchyma act as scattering centers.
  *

Rayleigh Scattering

• Scattering centers << wavelength.
• Intensity proportional to the fourth power of frequency: $I \sim f^4$
• Red cells = 'Rayleigh scatterers'.
• Doppler effect provides blood flow information.

Absorption

• Part of acoustic energy absorbed and converted into heat.
• Does not contribute to imaging.

B. ULTRASOUND PRODUCTION AND DETECTION

1. GENERAL ASPECTS

• Based on the piezoelectric effect.
• Force applied to crystal = electric potential develops.
• Potential is proportional to applied pressure.
• Detection: ultrasound wave -> electric potential.
• Conversely, electric potential ->crystal stretches/compresses.
• Alternating voltage -> vibration -> acoustic pressure fluctuations -> sounds.
• Piezoelectric crystal: converts electrical energy to mechanical (acoustic) energy and vice versa.
• Transducer: ultrasound emitting and detecting device (energy converter).
• Natural crystals (quartz) have piezoelectric properties.
• Modern systems use artificial crystals (lead-zirconium-titanium - LZT).

Piezoelectric Crystal Shape and Resonant Frequency

• Cylindrical (6-19 mm diameter, 0.2-2 mm thickness).
• Resonant frequency: greatest efficiency in energy conversion.
• Thickness determines resonant frequency ($f_o$).
• Thin crystals = high resonant frequencies; thick crystals = low resonant frequencies.

2. ENERGY TRANSFORMERS AND CHARACTERISTICS OF THE PRODUCED ULTRASONIC BEAM

• Piezoelectric crystals excited by short electrical pulses -> oscillation & ultrasound pulse emission.
• Absorbent material behind crystal reduces vibration time & pulse length (improves image).
• Layer of acoustic impedance material on the outer surface of the crystal to reduce reflections at the crystal-tissue interface and protect the crystal is facilitated.
• Near field (Fresnel zone): cylinder shape, diameter = crystal diameter.
  • Extends to distance $x = \frac{r^2}{\lambda}$ ($r$ = crystal radius).
  • Intensity varies significantly.
• Far field (Fraunhofer zone): conical shape.
  • Deviation angle $\theta$: $sin(\theta) = 0.61 \frac{\lambda}{r}$

Diameter Control of Beam

• Necessary for image quality.
• Focusing techniques:
  • Acoustic lenses: plastic with concave surface.
  • Refraction converges beam; reduces diameter in the focal zone.
  • Focal length: lens distance to minimum beam width.
  • Focal zone: region where beam width is < 2x beam width at focal length.
• Electronic focusing also used.
• Side lobes: small independent beams in lateral directions (undesirable: cause artifacts).

C. PULSE-ECHO ULTRASOUND SYSTEMS

1. General aspects

• Techniques based on pulse-echo principle.
• Main techniques: A-mode, B-mode, M-mode. B-mode with real-time imaging is the most widely used technique.
• A-mode (amplitude mode):
  • Information presented as peaks (height = echo amplitude).
  • Distance of two consecutive echoes corresponds to the distance of the surfaces that gave the reflection since the return time of each sound is proportional to the depth of the surface that gave the reflection.
  • Applications: accurate distance measurements (ophthalmology).
• B-mode (brightness mode):
  • Echo converted to a spot (intensity = echo amplitude).
• M-mode (motion mode):
  • Curves produced with distance (depth) vs. time.
  • Moving surfaces = curved lines; stationary structures = straight lines.
  • Used to study movement (heart walls & valves).

2. REAL-TIME ULTRASOUND IMAGING SYSTEMS

• Scan a body section with ultrasound beam.
• Scanning: mechanically or electronically.

Scanning Types

• Mechanical scanning:
  • Transducer movement is mechanical
  • Example: wheel with piezoelectric crystals attached.
    • Each crystal scans an angular sector.
    • Multiple crystals used at different frequencies.
• Electronic scanning:
  • Uses crystal arrays (no moving parts).

Electronic Scannning

• Piezoelectric crystal array: crystals next to each other, each with independent electrical connections.
• Allows individual or group activation.
• Achieved with linear phased arrays.
  • Stimulate crystals with time-delayed electrical pulses.
    • Beam direction determined by time-delayed sequence.
• Electronic focusing: applying a time difference to the excitation of the crystals.

D. DOPPLER ULTRASONOGRAPHY

1. Physical principles

• Doppler effect: frequency change due to relative motion between wave source and receiver.
• Used for circulatory system assessment (qualitative & quantitative).
• Ultrasound wave ($f<em>1$) directed at blood vessel, backscattered by red cells at a new frequency ($f</em>2$).
• $f_D = \frac{(2fvcos\alpha)}{c}$
  • $f<em>D$: Doppler frequency ($f</em>2 - f_1$)
  • $v$: relative speed of transducer - red cells
  • $\alpha$: angle between ultrasound beam & direction of movement of the red cells
  • $c$: speed of sound in tissues
• Doppler frequency depends on direction of blood flow.
  • \alpha > 90^{\circ}, cos \alpha < 0 \Rightarrow f2 < f1 (blood moving away).
• Doppler frequency lies in the acoustic range (< 20,000 Hz).
• Doppler techniques: determine $f_D$ to calculate flow velocity.
• Spectral analysis of Doppler signal: record information as distribution of frequencies or velocities, as a function of time.
• Doppler signal changes with time depending on the phase of the cardiac cycle.

2. Technological principles

• Three main techniques:
  • Continuous wave technique
  • Pulse wave technique
  • Color display technique
• Continuous wave systems:
  • Two piezoelectric crystals: one produces ultrasound, one detects scattering.
    • Crystals placed with a slight inclination so that the emitted beam intersects with the beam that returns to the detector.
    • Overlay area is the information-receiving area (detection field).
    • It is not possible to spatially locate the source of the scattering, which means that it is not possible to distinguish signals coming from different vessels located in the same detection field.
• Pulsed wave systems:
  • Have a transducer which functions as a transmitter and a receiver.
  • Gives the option of choosing the depth from which the Doppler signal is received.
    • Receives Doppler signal from a chosen depth.
• Duplex: combination of image (anatomical detail) and Doppler signal (blood flow data).
• Color Doppler technique: displays flow velocity information simultaneously in multiple areas, distributed over the entire or part of the real-time ultrasound image.
  * These regions are not selected by the operator but are distributed over both dimensions of the ultrasound to automatically provide color-coded Doppler signal information.

E. ULTRASONOGRAPHY IMAGE QUALITY

1. Axial resolution

• Resolution along the beam path.
• Depends on pulse length.
• $A$.
• High frequency ultrasound gives better axial resolution (shorter pulses), but are attenuated more rapidly.

2. Lateral resolution

• Resolution perpendicular to the beam axis.
• $B$.
• Depends on the width of the ultrasound beam.
• Beam focusing improves lateral resolution.

BIOLOGICAL EFFECTS

• No confirmed biological effects reported in patients or operators from diagnostic ultrasound.

Mechanisms

• Thermal mechanism: absorption of ultrasound energy and increase in tissue temperature.
• Cavitation: creation and collapse of microscopic bubbles in a fluid (can cause biological effects).
• Knowledge about the biological effects of ultrasound is relatively limited. Thus, is why every exposure must be justified and optimized.
• ALARA principle: acquiring diagnostic information with the least possible exposure of the patient.

PHYSICS OF LASER

Introduction

• 1917: Stimulated emission principle (A. Einstein)
• 1954: Maser - microwave amplification by stimulated emission of radiation (C. H. Townes, J. Gordon, H. Zeigler)
• 1957: Introduction of the acronym laser - light amplification by stimulated emission of radiation (G. Gould)
• 1960: Ruby laser (T. H. Maiman)
• 1960: He-Ne laser (Ali Javan)

Atomic Excitation and De-excitation

• $E_1$: low energy state (ground state)
• $E_2$: high energy state (excited state)

Spontaneous Emission The energy of the emitted photon:

• $h\nu = E<em>2 - E</em>1$
  • $h$: Planck’s constant
  • $\nu$: frequency of the photon
• $E<em>2 \rightarrow E</em>1 + h\nu$

Stimulated Emission

• $E<em>2 + h\nu \rightarrow E</em>1 + h\nu + h\nu$.
• Two photons have same energy, direction, & phase.

Absorption

• $E<em>1 + h\nu \rightarrow E</em>2$
• Absorption is not associated with photon emission.

Population Inversion

• $E_1$: low energy state (ground state)
• $E_2$: high energy state (excited state)
• In thermal equilibrium, Boltzmann's equation gives the ratio of atoms in each energy state:
  • $\frac{N<em>1}{N</em>2} = e^{\frac{(E<em>2 - E</em>1)}{KT}}$
    • $K$: Boltzmann’s constant
    • $T$: temperature in K
• E2 > E1 N1 > N2 \rightarrow \frac{N1}{N2} > 1.

Population Inversion continued

• A prerequisite for laser operation is:
• Higher energy state needs to be more populated than the lower energy state
• $N<em>2 > N</em>1$.

Laser Operation stages

Assume that the population inversion has been installed in the system ($N<em>2 > N</em>1$)

• Stage 1: Spontaneous emission
  • $E<em>2 \rightarrow E</em>1 + h\nu$
• Stage 2: Stimulated emission
  • $E<em>2 + h\nu \rightarrow E</em>1 + h\nu + h\nu$
    Stage 3: Stimulated Emission vs. Absorption
  • Stage 3: Stimulated emission is the desired effect for the amplification of electromagnetic radiation.
  • $E<em>2 + h\nu \rightarrow E</em>1 + h\nu + h\nu$
• Stage 3: Absorption is an undesired effect causing photon loss.
  • $E<em>1 + h\nu \rightarrow E</em>2$

Laser Operation

• Population inversion can not be achieved in a two-level laser.
• The process of excitation reaches to equilibrium with that of de-excitation leading to equal population in each of the energy states ($N<em>1 = N</em>2$).

Laser Operation - Types

• Three-level laser
• Four-level laser
  Fast transition
  LASER
  Fast transition
  LASER
  Fast transition
  Pumping

Laser Components

• Active medium (gain medium):
  • Atoms/molecules/ions undergoing stimulated emission to produce a laser beam.
  • Solid, liquid, or gas.
• Energy source (pump source):
  • Provides energy to medium for population inversion.
• Optical cavity (optical resonator):
  • Two parallel mirrors surrounding the active medium.
  • One mirror fully reflective; the other partially reflective.

Operation Mode

• Continuous wave operation
  • The output power of laser beam is maintained constant by the energy source
• Pulsed operation
  • The output power appears in repeated pulses of certain duration down to
• Q-switching
  • The population inversion builds up by inserting special elements in the optical cavity. The element removal releases high powered pulses with a duration of
• Mode locked
  • They generate pulses with a duration of

Laser Radiation Properties - Monochromaticity

• Laser radiation is highly monochromatic, more so than from any other source.

Laser Radiation Properties - Directionality

• Laser radiation is highly directional with very low divergence.

Laser Radiation Properties - Coherence

• Laser radiation is highly coherent (spatial & temporal).

Laser Radiation Properties - Brightness

• Brightness is defined as the emitted power per unit surface and per unit solid angle.
• Laser brightness is very high, even for low-powered systems.

Laser Categories

• Based on:
  • State of active medium
  • Spectral range of wavelength
  • Output power
  • Number of energy states participating
  • Excitation method of active medium

Laser Categories - Examples

• Solid state lasers
  • Active medium: glass or crystalline materials
  • Lasers: Ruby, Nd:Yag
• Gas lasers
  • Active medium: gas or mixture of gases
  • Lasers: He-Ne, Ar, CO2, N2
• Excimer lasers
  • Active medium: Inert gas
  • Lasers: ArF, KrF, XeF, Ar2, Kr2, Xe2
• Chemical lasers
  • Active medium: diatomic molecule
  • Lasers: HF, HCl, DF
    *Liquid lasers
  • Active medium: organic dyes
    • Dye Laser
• Semiconductor lasers
  • Active medium: semiconductor diode

Laser tissue interactions

• Laser-tissue interactions are absorption, transmission, reflection and scattering.
• The fraction of absorbed, scattered, transmitted and reflected radiation depends upon the optical properties of the tissue and the wavelength of the radiation.
• The absorbed radiation by the tissue can cause thermal and non-thermal effects.

Thermal effects

• $37^{\circ}C$: Normal tissue temperature
• $60-100^{\circ}C$: Photocoagulation (denaturation of biomolecules)
• $100^{\circ}C$: Vaporization (tissue dehydration)
• $300-400^{\circ}C$: Tissue carbonization
• $500^{\circ}C$: Tissue burn
• $37-60^{\circ}C$: Hyperthermia

Non-thermal effects

• Photomechanical action
  • Observed with high-powered lasers generating short pulses
    High amounts of energy are absorbed by a small area. The produced shock waves can cause the mechanical disruption of the target
• Photochemical action
  • The absorbed radiation can cause chemical changes (chemical bonds break): to the exposed tissues. The photodynamic therapy is based on the photochemical action.

Risks from laser

Risks related to radiation

Eye damage

Skin damage

Risks not-related to radiation

Chemical hazards

Electrical hazards

Eye exposure to laser

• The focus of the light rays to a small-sized spot (10-20 μm) significantly enhances the power density (Watt / cm2) on the retina compared to that at the cornea.
• A laser burn on the fovea centralis can cause loss of vision.
  Laser burns in the peripheral region of the retina have little or no impact of the vision The eye can focus both visible and near infrared light (400-1400 nm) onto the retina.

Self-defense mechanism

blink and aversion response (0.25 s)
The self-defense mechanism can not protect eyes when high-powered lasers are involved.

Skin exposure to laser

The penetration depth varies by the wavelength of the laser beam Lasers emitting in the visible and near infrared spectral regions penetrate more deeply than other lasers.

Radiation risk from lasers

Spectral range Eye Skin Ultraviolet C (100 nm - 280 nm) Photo-keratitis Erythema, Skin cancer Ultraviolet B (280 nm - 315 nm) Photo-keratitis Pigmentation Ultraviolet A (315 nm - 400 nm) Cataract Burn, Pigmentation Visible (400 nm 780 nm) Retinal damage Burn, Photosensitive reactions Infrared A (780 nm 1400 nm) Cataract, Retinal burn Burn Infrared B (1.4 μm 3.0 μm) Corneal burn, Cataract Burn Infrared C (3.0 μm 1000 μm) Corneal burn Burn

Classification of lasers

• Class 1 Safe under all conditions of normal use

• Class 1M Safe under all conditions except when it is viewed with magnifying optics such as telescopes and microscopes.

• Class 2 Safe due to the self-defense mechanism which restricts the exposure below 0.25 s.

Classification of lasers

*Class 2M
Safe due to self-defense mechanism provided that it is not viewed with optical instruments.

• Class 3R Potentially hazardous but the risk is low.
  Class 3B Hazardous for direct exposure No hazard from diffuse reflection Class 4 Hazardous for eye and skin after exposure to direct or scattered radiation. Hazards also apply after beam reflection.
• The maximum permissible exposure (MPE) is the energy density (/cm2) or power density (W/cm2) which is considered safe and it has negligible probability for causing damage to the eyes or skin.

The MPE is determined by the wavelength of the laser beam, the energy involved and the exposure time Time (s) MPE (W/m2) Nd:Yag laser Eye Skin 0. 25 127 31000 1. 0 70 11000 10. 0 50 2000

Maximum permissible exposure

Laser safety officer Classification of the laser syetem Appropriate signage Limited access in the controlled area for persons not involved in laser applications Training of the personnel in the laser system operation and use Training of the personnel in the laser radiation risks Administrative level

Laser safety

• Emergency power off button
• Beam interlocks
• Beam alignment
• Removal of reflective and flammable materials from the beam path
• Coverage of the windows if present in the lab
• Beam path below eye level Technical level
• Use of personal protective equipment (glasses)

Lasers in medicine

Ophthalmology - refractive surgery - retinal diseases - cataract

Dermatology - skin disorders - removal of tattoos Surgical applications LASIKGeneral surgery Otorinolaryngology Vascular surgery Neurosurgery Orthopedics Urology Gynecology Surgical applications Optical tomography Dopller velocitometry Patient set-up in diagnostic and therapeutic machines Diagnostic applications Photodynamic therapy Therapeutic applications

Kinematics

• Kinematics is the study of the geometry, the type, or the sequence of motion as a function of time is qualitative and quantitative analysis.
  Linear and angular

Kinetics

Angular and linear
A body is in equilibrium if A. The vector sum of all external forces is zero B. The vector sum of all external torques is zero Body dynamics Center of mass: the point at which all the mass is considered to be ‘concentrated’

Stress represents the force distribution inside a solid body when an external force acts.

Skeleton functions, loading of bones,

• Compression is a squeezing force. The opposite of compressive force is tensile force or tension. Whereas compressive and tensile forces act along the longitudinal axis of a bone, shear force acts parallel or tangent to a surface.
• During daily activities, compression is the most common form of loading on the spine. Spinal rotation creates shear stress within the discs. Tensile Force Force
  Change in Length or Angle
  Normal Strain Shear Strain
  Strain is the relative measure of the deformation of a bone as a result of loading. Loading of Bones A. Compression B. Tension C. Shear D. Torsion E. Bending
• Elasticity is the tendency of solid objects and materials to return to their original shape after the external forces (load) causing a deformation are removed. The stiffness is a measure of the resistance to deformation under the applied load

Osteoporo

• Osteoporosis is a disease characterized by reduction of bone mass and alteration of bone architecture resulting in increased bone fragility and increased fracture risk Osteopenia Osteopenia is a condition characterized by low bone density without fractures

Biomaterials

• Biomaterials may be natural or synthetic and are used in medical applications to support, enhance, or replace damaged tissue or a biological function. Basic properties A. Biocompatibility B. Fatigue strength C. Resistance to corrosion D. Cost effectiveness Physics of Magnetic Resonance General aspects Magnetic resonance is a phenomenon for the appeasement of which is necessary the application of a magnetic field and radio frequency, which is transmitted to the area to be measured or to the human body by pulses.

Physics of Magnetic Resonance

• Initially, the nuclei of the atoms are excited by absorbing electromagnetic energy emitted by a radio frequency emitting coil. Then the nuclei de-excite during their interaction with the molecular environment, emitting energy that is collected by a coil of the magnetic resonance system. That means that the signal of magnetic resonance is transmitted from the core of the atom. An important difference between magnetic resonance imaging and computer imaging or other systems is that, with the magnetic resonance process, it is possible to selectively image different characteristics of the human body tissues. The magnetic field of nucleons It is known that the atoms of all elements consist of the nucleus and the electrons that revolve around it. The nucleus consists of the nucleons, which are the protons and the neutrons.
• The protons are positively charged, and the neutrons have both a positive and negative charge, but their total charge is zero. The nucleons rotate around themselves, a property called spin. The rotation of the nucleons results in the movement of their electric charge. Moving charges are electric current. It is known that the electric current produces magnetic fields. The nucleons, therefore, produce their own magnetic field, so they are small magnets. The intensity and the direction of the magnetic field surrounding the core are determined by the magnetic field moment, which is a vector quantity with sense N
• B (image 1). The nucleus of the hydrogen atom is the simplest in nature, consisting of a single proton. The hydrogen atom is the most common element in the human body since the body of an adult consists of 65% water. That’s why in magnetic resonance (MR) the protons of the hydrogen atoms are used to produce the