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  2. CHARACTERISTICS OF SOUND • A sound beam is similar to x-ray beam in that both are waves transmitting energy but important difference is that x-rays pass through a vacuum where as sound require a material medium ( solid , liquid , gas ) for transmission, they will not pass through the vacuum. • Sound must be generated mechanically by vibrating body matter
  3. HISTORY OF ULTRASOUND • Piezoelectricity discovered by Pierre and Jacques Curie in 1880 using natural quartz • SONAR was first used in 1940s war time • Diagnostic medical applications in use since late 1950’s
  4. • A : uniform distribution of molecules in a medium • B: movement of the piston to the right produces a zone of compression • C: withdrawl of the piston to left produces a zone of rarefraction • D: alternate movement of the piston to right and left establishes a longitudinal wave in the medium
  6. PROPERTIES OF SOUND WAVE • Ultrasound obeys the wave equation : u = vλ where v = frequency ( Hz , number of cycles / sec. ) u = velocity of sound ( meter / sec.) λ = the wavelength ( which is distance between two successive compression. meter.)
  7. FREQUENCY • Frequency refers to the number of cycles of compressions and rarefactions in a sound wave per second, with one cycle per second being 1 hertz. • Medically used ultrasound involves 1-10 MHz frequencies .(1 -10 million Hz /sec.).
  8. WAVELENGTH • The wavelength is the distance traveled by sound in one cycle, or the distance between two identical points in the wave cycle i.e. the distance from a point of peak compression to the next point of peak compression. • It is inversely proportional to the frequency.
  9. Wavelength is one of the main factors affecting axial resolution of an ultrasound image • Smaller wavelength • Higher frequency • Higher resolution • Lesser penetration • Therefore, higher frequency probes (5 to 10 MHz) provide better resolution but can be applied only for superficial structures and in children. • Higher wavelength • Lesser frequency • Less resolution • Deeper penetration • Lower frequency probes (2 to 5MHz) provide better penetration albeit lower resolution and can be used to image deeper structures.
  10. PROPAGATION VELOCITY • The propagation velocity is the velocity at which sound travels through a particular medium • Dependant on the compressibility and density of the medium. • The average velocity of sound in soft tissues such as the chest wall and heart is 1540 metres/second.
  11. COMPRESSIBILITY • The velocity of sound is inversely related to the compressibility of the conducting material. That means less compressibility of material , the more rapidly transmits the sound. • Sound waves move slowly in the gas because the molecules are far apart and are easily compressed. • Solids > liquids > gases
  12. DENSITY • Dense materials have large molecules with large inertia : difficult to move or stop once in motion • Propagation of sound requires rhythmic starting and stopping of particles • Density is inversely related to velocity
  13. AMPLITUDE/INTENSITY • It is a measure of the degree of change within a medium, caused by the passage of a sound wave and relates to the severity of the disturbance • Determined by the length of oscillation of particle • Greater amplitude = more intense sound
  14. • Sound intensity is measured in decibel (dB). • Ultrasonic intensities are expressed in power / unit area (watts/cm2)
  15. TRANSDUCER • Transducer is the device which generates ultrasound wave . • Transducers are used to convert an electric signal into ultrasonic energy that can be transmitted into tissue , and to convert ultrasonic energy reflected back from the tissue into an electric signal.
  17. • The most important component is a thin (0.5 mm) piezoelectric crystal element located near the face of the transducer . • The piezoelectric crystal consist of lead zirconate titanate or PZT. • The front and back faces of the crystal are coated with a thin conducting film to ensure good contact with the two electrodes that will supply the electric field used to strain the crystal.
  18. • Crystal is made up of numerous dipoles arranged in a geometric pattern. • Dipole is a polarized molecule, one end positive and other end negative . • The positive and negative ends arranged so that an electric field will cause them to realign thus changing the dimensions of the crystal.
  19. • No current flows through the crystal • Plating electrodes behave as capacitors and it is the voltage between them that produces the electric field which causes change in crystal shape
  20. • When the high frequency voltage pulse is applied across the crystal , the crystal vibrates like a cymbal that has been struck a sharp blow and generates sound waves. • The backing block must stop the crystal vibration within a microsecond because the transducer must be ready immediately to receive reflected waves (echoes) from tissue interface.
  21. • As the sound pulse passes through the body ,echoes reflect back towards the transducer from each tissue interface. These echoes carry energy and they transmit their energy to the transducer , causing a physical compression of the crystal element . This compression forces the tiny dipoles to change their orientation , which induces a voltage between the electrodes
  22. • The voltage is amplified and serves as the ultrasonic signal for display on television monitor. • Compression force and associated voltage are responsible for the name piezoelectricity which means “ pressure “ electricity.
  23. • Naturally occurring materials possess piezoelectric properties : Quartz • Man made material ( ferroelectrics ) : Barium titanate lead zirconate titanate
  24. • Curie Temperature : is the temperature at which polarization is lost. • Heating the piezoelectric crystal above the Curie temperature reduces it to a useless piece of ceramic so transducer should never be autoclaved.
  25. Resonant frequency : • The thickness of piezoelectric crystal determines its natural frequency called its resonant frequency. • The crystal is designed so that its thickness is equal to exactly half the wavelength of the ultrasound to be produced by the transducers. • Thickness = wavelength/2
  26. Transducer Q Factor : • Two characteristics :- purity of sound & the length of time that the sound persists. • A high Q transducer produces a nearly pure sound made up of narrow range of frequencies. • A low Q transducer produces whole spectrum of sound covering wider range of frequencies. • The interval between initiation of the wave and complete cessation of vibration is called the “ ring down time “.
  27. • High Q : useful for doppler USG transducers because it furnishes narrow range of sound frequencies • Low Q : useful for organ imaging because it can furnish short ultrasound pulses and will respond to a broad range of returning frequencies
  28. • The Q factor can be controlled by altering the characteristic of the damping block. • Damping block consist of powered rubber and tungsten blended with an epoxy resin. • Ratio of tungsten to resin is chosen to satisfy the impedance requirements • Rubber is added to increase the attenuation of sound in the backing block.
  29. RECEPTION OF ULTRASOUND • 1. Reflection : • Both ultrasound and light obey the law of reflection , the angle of incidence and the angle of reflection are equal. • The factor that determines the percent of the incident beam undergoing reflection is a property , peculiar to various tissues , called acoustic impendence
  30. • Acoustic impendence Z = p u rayls • where p is density , u is velocity of sound in cm/sec. • The velocity of sound in all soft tissue is virtually same 1540 m/sec. • So , Z α p. example air and bone.
  31. • As sound waves pass from one tissue to another , the amount of reflection is determined by the difference in the impedances of the two tissues .
  32. • At a particular angle of incidence known as the critical angle , total reflection occurs at the skin
  33. REFRACTION • This occurs when an ultrasound beam passes, at an angle other than 90 degrees, from one tissue into another with change in velocity. • It increase with the increasing angle of incidence . • It passes deeper into the body where it gives rise to artifacts. • If angle of incidence is less than 3 degrees, very little refraction seen.
  34. ABSORPTION • Due to friction among molecules in their back – forth movement , reduction in intensity of the ultrasound beam occurs as it traverse matter. Friction results in degradation of part of molecules kinetic energy to heat. • The greater the frequency , the greater the attenuation coefficient. This means high frequency beam shows less penetration than a low frequency beam. • Attenuation in soft tissue is 1 dB/cm/MHz
  36. TYPES OF TRANSDUCERS The ultrasound transducers differ in construction according to • Piezoelectric crystal arrangement • Aperture ( footprint ) • Operating frequency ( which is directly related to the penetration depth )
  37. SECTOR TRANSDUCER • Crystal arrangement : phased array • Footprint size : small • Operating frequency : 1-5 MHz • Ultrasound beam shape : sector, almost triangular • Use : small acoustic windows ,mainly ECHO, gynecological ultrasound, upper body ultrasound
  38. LINEAR TRANSDUCER • Crystal arrangement : linear • Footprint size: usually big ( small for hockey transducers ) • Operating frequency : 3-12 MHz • Ultrasound beam shape : rectangular • Use : USG of superficial structures e.g. obstetrics ultrasound , breast,thyroid,vascular ultrasound
  39. CONVEX TRANSDUCER • Crystal arrangement : curvilinear • Footprint size : big ( small for the micro convex transducers ) • Operating frequency : 1-5 MHz • Ultrasound beam shape • Use : useful in all USG types except ECHO, typically abdominal ,pelvic and lung ( micro convex transducer )
  40. TRANSDUCER JELLY/COUPLING AGENT • Air and other gases impede sound waves • At tissue-air interface, more than 99.9% of the beam is reflected so none is available for further imaging • Jelly acts as a special aqueous conductive medium for the sound waves • Prevents the formation of bubbles between the transducer and the patient’s skin • Acts as a lubricant
  41. PROPERTIES • Non allergenic • Odourless • Non staining • Harmless • Neutral ph • Easily removable with tissue or towel
  42. USG GEL INGREDIENTS • Water • Carbomer : synthetic high molecular weight polymer of acrylic acid cross linked with allyl sucrose and containing 50-68% of carboxylic acid groups. Neutralized with alkali hydroxide to make it water soluble. • EDTA • Propylene glycol : organic oil compound that doesnot irritate the skin and helps retain moisture • Glycerine and trolamine : neutral colorless gel that absorbs moisture from air • Colorant : occasionally used, usually blue color
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