ULTRASOUND PHYSICS

PHYSICS BEHIND USG TRANSDUCERS, TYPES OF TRANSDUCERS

1. ULTRASOUND PHYSICS TRANSDUCERS TRANSDUCER JELLY SEMINAR BY DR NAVNI GARG

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

5. COMPRESSION WAVE

6. PROPERTIES OF SOUND WAVE Ultrasound obeys the wave equation : u = v λ 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 D ependant 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.

16. COMPOSITION OF TRANSDUCER

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 T his occurs when an ultrasound beam passes, at an angle other than 90 degrees, from one tissue into another with change in velocity. I t increase with the increasing angle of incidence . I t 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

35. TYPES OF ELECTRONIC SCANNING LINEAR ARRAY PHASED ARRAY

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

43. Thank you