Ultrasound physics

This presentation is about physics of ultrasonography.

1. PHYSICS OF ULTRASONOGRAPHY PRESENTED BY DR. ABHILASHA SINGH DNB RESIDENT

2. Applications Training for Service – Ravindran Padmanabhan 2 What is Sound ? Sound is a mechanical, longitudinal wave that travels in a straight line • Cannot travel through Vacuum Velocity of sound depends on the nature of medium.

3. Compression (Longitudinal) Waves Particle motion parallel to direction of wave travel 1 2 1 2 Wave Travel Motion of Individual Coil

4. 3 What is Ultrasound? • Ultrasound is a mechanical, longitudinal wave with a frequency exceeding the upper limit of human hearing, which is 20,000 Hz or 20 kHz. • Typically at 2 – 20 Mhz.

5. Ultrasound was first used for clinical purposes in Glasgow in 1956. Obstretician Ian Donald and engineer Tom Brown developed first prototype systems based on an instrument used to detect industrial flaws in ships.

6. 4 Basic Ultrasound Physics Velocity Frequency Amplitude Wavelength

7. 5 Velocity of sound - Speed at which a sound wave travels through a medium(cm/sec ) - Independent of frequency and depends primarily on the physical makeup of material through which sound is being transmitted - Determined by 1. Compressibility 2.D ensity - Velocity – Slowest in air/gas – Fastest in solids - Average speed of ultrasound in body is 1540m/sec

8. 6 Velocity Near Field Imaging Far Field Imaging Tissues closer appear on top and faster the waves return Tissues further appear at the bottom & slower the waves return

9. 7 Frequency Number of cycles per second Units are Hertz Ultrasound imaging frequency range 2-20Mhz In ultrasonic frequency range, the velocity of sound is constant in any particular medium, when frequency is increased the wavelength must decrease

10. 8 Frequency : Higher the freq Lower the penetration and Higher the resolution Low the freq higher the penetration and lower the resolution

11. 9 Wavelength • Distance over which one cycle occurs

12. 10 Velocity (v), Frequency ( ƒ), & Wavelength ( ) λ Given a constant velocity, as frequency increases wavelength decreases V = ƒ λ

13. Intensity OR Loudness Determined by the length of oscillation of the particles conducting the wave Greater the amplitude of oscillation, the more intense the sound Ultrasonic intensities are expressed in watts per square centimeter

14. Relative Sound Intensity Sound intensity is measured in decibels A bell is comparison of relative power of two sound beams expressed log- arithmically using the base 10 The number of decibels is obtained by multiplying the number of bels by 10

15. TRANSDUCER: It is a device that converts energy from one form to another . ULTRASOUND TRANSDUCER converts electric energy into sound energy and sound energy back into electric energy.

16. Ultrasound Transducer Acts as both speaker & microphone Emits very short sound pulse Listens a very long time for returning echoes Can only do one at a time Speaker transmits sound pulses Microphone receives echoes

17. How Ultrasound Image is constructed 1. Electrical Energy converted to Sound waves 2. The Sound waves are reflected by tissues 3. Reflected Sound waves are converted to electrical signals and later to Image  Acoustic Acoustic Impedance Impedance  Velocity  Frequency Velocity Frequency  Reflection  Amplification Reflection Amplification

18. 15 Pulse-Echo Method • Ultrasound transducer produces “pulses” of ultrasound waves • These waves travel within the body and I nteract with various tissues • The reflected waves return to the transducer and are processed by the ultrasound machine • An image which represents these reflections is formed on the monitor

19. TRANSDUCER DESIGN Matching layer Piezoelectric crystal Backing block Acoustic absorber Metal shield Signal cable

20. MATCHING LAYER It minimizes the acoustic impedence differences between transducer and the patient. Its impedence is intermediate to that of the soft tissue and the transducer. Its thickness is equal to one-forth of the wavelength, which is known as quarter wave matching Matching layer is made of perspex or plexiglass loaded with aluminium powder.

21. 2. PIEZOELECTRIC CRYSTALS Some naturally piezoelectrc occurring materials include Berlinite (structurally identical to quartz),cane sugar,quartz,Rochelle salt,topaz,tourmaline,and dry bone An example of man-made piezoelectric materials include barium titanate and lead zirconate titanate (PZT) They can be designed to vibrate in either the thickness or radial mode.

22. Piezoelectric Principle : Voltage generated when certain materials are deformed by pressure Reverse also true! Some materials change dimensions when voltage applied dimensional change causes pressure change when voltage polarity reversed, so is dimensional change V

23. Crystal layer : Molecules of piezoelectric crystal are polarized, one end is positive and other negative . When high frequency current is applied, it alternatively thickens and thins in its short axis, and generates ultrasound waves as a beam in air infront and back of the crystal face.

24. 3. DAMPING BLOCK Located on the backside of the crystal , made up of tungsten particles suspended in epoxy resin It absorbs backward US pulse and attenuates stray US signals . Transducer and damping block are separated from the casing by an insulator(rubber cork).

25. Function of damping block: In B- Mode operation, It must stop the vibration within a microsecond so that the transducer becomes ready to immediately receive the reflected echoes from the body

26. RESONANT FREQUENCY

27. TRANSDUCER Q FACTOR Refers to two characteristics of piezoelectric crystals: -purity of sound -length of time that the sound persists High Q transducer:- Nearly pure sound made up of narrow range of frequencies. Low Q transducer:- Whole spectrum of sound covering a much wider range of frequencies Ring down-time – Interval between initiation of the wave and complete cessation of vibrations.

28. Q = fo f 2 --- f 1 Where, Q = Q factor fo = resonance frequency f 2 = frequency above resonance at which intensity reduced by half f 1 = frequency below resonance at which intensity reduced by half Narrow range of sound frequencies and long ring down-time : Useful for Doppler ultrasound transducer Broad range of sound frequencies and short ring down-time : Useful for organ imaging because it can furnish short ultrasound pulses and will respond to a broad range of returning frequencies.

29. Spatial Pulse Length D epends on source & medium as wavelength increases, spatial pulse length increases Spat. Pulse Length = # cycles per pulse X wavelength (dist. / pulse) (cycles / pulse) (dist. / cycle) D istance in space traveled by ultrasound during one pulse

30. Calculate SPL for 5 MHz sound in soft tissue, 5 cycles per pulse (Wavelength=0.31 mm/cycle) SPL = 0.31 mm / cycle X 5 cycles / pulse = 1.55 mm / pulse Spat. Pulse Length = # cycles per pulse X wavelength

31. as # cycles per pulse increases, spatial pulse length increases as frequency increases, wavelength decreases & spatial pulse length decreases speed stays constant Spatial pulse length determines axial resolution Spat. Pulse Length = # cycles per pulse X wavelength Wavelength = Speed / Frequency

32. 40 Ultrasound - Internals

33. CHARACTERISTICS OF AN ULTRASONIC BEAM

34. Ultrasound beam characters: An unfocused ultrasound beam leaving a flat crystal has 2 parts: Initial cylindrical segment(near field or frensnal zone) Diverging conical portion ( far field or fraunhofer zone)

35. x ’ = r 2 λ Where, x ’ = length of Frensel zone (cm) r = radius of the transducer(cm) λ . = wavelength(cm) Zone longest with largest transducer and high frequency sound Zone shortest with small transducer and low frequency sound

36. The length of near field and divergence of the far field depend upon : A. FREQUENCY: higher the frequency longer the near fiel d s and less divergent the far field Depth resolution increases with higher frequencies Major drawback-Tissue absorption increases with increasing frequency B. CRYSTAL DIAMETER: increasing diameter increases the near field length but worsens the lateral and depth resolution.

37. FOCUSED TRANSDUCER

38. Ultrasound tissue interaction Reflection Refraction Absorption Attenuation Scattering

39. 1. REFLECTION A reflection of a beam is called ECHO. The production and detection of echoes forms the basis of ultrasound. Reflection occurs at the interface between two materials. It depends on the 1. tissue’s - “ACOUSTIC IMPEDENCE ” 2. beam’s angle of incidence If two materials have same impedence , no echo produced.

40. Acoustic Impedance Definition Acoustic Impedance = Density X Prop. Speed ( rayls ) (kg/m 3 ) (m/sec) increases with higher Density Stiffness propagation speed independent of frequency

41. Acoustic Impedance of Soft Tissue : Density: 1000 kg/m 3 Propagation speed: 1540 m/sec Acoustic Impedance = Density X Prop. Speed ( Rayls ) (kg/m 3 ) (m/sec) 1000 kg/m 3 X 1540 m/sec = 1,540,000 rayls

42. Differences in acoustic impedance determine fraction of intensity echoed at an interface If the difference in acoustic impedence is: Small –weak echo is produced and most of the sound waves will continue in second medium Large- strong echo is produced Very large- all sound waves will be totally reflected back. Example: tissue-air interface 99% of beam is reflected back.

43. Angle of incidence The amount of reflection is determined by the angle of incidence between the sound beam and reflecting surface The higher the angle of incidence(i.e., the closer it is to a right angle),the less the amount of reflected sound R = Z 2 – Z 1 2 X 100 Z 2 + Z 1 Where, R = percentage of beam reflected Z 1 =acoustic impedence of medium 1 Z 2 =acoustic impedence of medium 2

44. Lung-chest wall interface : 99.9% Kidney-fat interface : 0.64% Skull-brain interface : 44 % T = 4Z 1 Z 2 (Z 2 + Z 1 ) 2 Where, R = percentage of beam transmitted Z 1 =acoustic impedence of medium 1 Z 2 =acoustic impedence of medium 2 The sum of reflected and transmitted portions of sound beam must be 100%

45. 2.REFRACTION Incident reflective refraction Angle of incidence = angle of reflection Scattered echoes

46. REFRACTION

47. The angle of refraction is governed by Snell’s law, which is sin θ i = V 1 sin θ t V 2 where, θ i = incidence angle θ t = transmitted angle V 1 = velocity of sound for incident medium V 2 = velocity of sound for transmitting medium Refraction can cause artifacts, which cause spatial distortion (real structures are imaged in wrong location)

48. 3.ABSORPTION

49. Three factors determine the amount of absorption : 1) the frequency of the sound 2) the viscosity of conducting medium 3) the “relaxation time” of the medium Liquids – Low viscosity – Little absorption Soft tissues – H igh viscosity – Medium absorption Bones – Very high viscosity – High absorption Relaxation time is the time that it takes for a molecule to return to its original position after it has been displaced

50. The relaxation time is a constant for any particular material A molecule with a longer relaxation time may not be able to return completely before a second wave arrives Compression wave is moving in one direction and molecule in opposite direction and hence more energy required to reverse the direction of molecule and converted to heat. In soft tissues there is linear relationship between absorption of ultrasound and frequency The proper frequency is a compromise between the best resolution (higher frequency) and the ability to propagate the energy into the tissues(lower frequency)

51. QUARTER WAVE MATCHING Method of improving energy transfer is that of mechanical impedance matching A layer of material of suitable thickness and characteristic impedance is placed on the front surface of the transducer, the energy is transmitted into the patient more efficiently The thickness of matching layer must be equal to one fourth the wavelength of sound in the matching layer Z matching layer = Z transducer x Z soft tissue Use of quarter-wave matching will also improve the transmission of ultrasound pulses returning from tissues back into the transducer

52. 4.ATTENUATION

53. 5. SCATTERING Not all echoes are reflected back to probe. Some of them are scattered in all directions in a non uniform manner. More so with very small objects or rough surfaces. Part of scattering goes back to transducer and generate images is called BACKSCATTER.

54. A-MODE (Amplitude mode)

55. No memory is built into the display mechanism,so it discards previous pulses as it receives new ones. A permanent record is made by photographing the electronic display. Applications of A-MODE : Opthalmology-distance measurements Echoencephalography Echocardiography Detecting a cyst in breast Studying midline displacement in brain

56. TM MODE For the TM mode spikes are converted into dots,the dots move back and forth as indicated by arrows. To make a permanent record,the motion must be recorded over a period of time.This is accomplished by moving the line of dots to the top of scope and then gradually dropping them to bottom A record of sweep time is made with a camera using an exposure time longer than sweep time Disadvantage – Short time can be recorded A strip chart record can be as long as the operator desires and this method is increasing in popularity for echocardiography.

57. B-MODE (Brightness Mode)

58. Contact scaning – transducer is placed on patients skin with mineral oil on skin acting to exclude air and to ensure good acoustic coupling between transducer and skin. If the angle between the perpendicular from the tranducer surface surface and the interface to be imaged is greater than 5 o the amount of reflected ultrasound returning to the transducer will be too little to produce an image. Compound scanning motion is required to present the surface of the transducer to the wide variety of interface angles require lung imaging

59. Localization of one echo relative to another is accomplished with a small computer that is fed information by an arm containing three joints. Scanning arm serves: 1) it determines the spatial orientation of sound beam. 2) it constrains the motion of the transducer so that all components of a single image slice through the same plane in the patient

60. Gray scale imaging The purpose is to display the great variation of the amplitudes of echoes arising from tissues as varying shades of gray on a television monitor Possible by the development of the scan conversion memory tube (“scan converter”) It is similar to a cathode ray tube,the electron beam is used alternately to write the information on the target read the information to generate the signal sent to a television monitor and erase the target in preparation for receiving a new set of information Target – Silicon backplate about 25mm in diameter on which more than a million tiny squares of silicon “wafers” are placed.

61. Final picture is composed only of the strongest echo detected from each point of the scan ,rather than of a random addition of numerous signals(called “overwriting”) Two types of scan conversion memory tube: 1. Analog scan converter 2 . Digital scan converter In analog scan converter tube there is an objectionable flicker of the image viewed on the monitor, it is due to fact that tube must simultaneously store the image and transmit the image to the television monitor. Once stored image can be viewed for about 10 min before image deterioration begins.

62. Digital scan converter converts variation in amplitude of echo signal received by the transducer into binary numbers. They are free of gray scale drift Have much faster speed,eliminate flicker on monitor and can be viewed indefinitely Control of TV monitor can be used to adjust contrast and brightness

63. Controls They are designed to regulate the intensity of echos from various depths : 1.Time gain compensation 2.Delay 3.Intensity 4.Coarse gain 5.Reject 6. Near gain 7.Far gain 8.Enhancement

64. 1. TIME GAIN COMPENSATION TGC amplifies the signal proportional to the time delay between transmission and detection of US pulses. It amplifies and brings the signal in the range of 40- 50 dB. This process compensates for tissue attenuation and makes all equally reflective boundaries equal in amplitude irrespective of depth.

65. DELAY CONTROL : Regulates depth at which the TGC begins to augment weaker signals INTENSITY CONTROL : Determines the potential difference across the transducer. Increasing intensity produces more energetic ultrasonic beams and thus stronger echoes at all level. COARSE GAIN : Regulates the height of echoes from all depths REJECT : It discriminate echoes below a minimum amplitude. Cleans up the image by removing small useless signals. DELAY CONTROL : Regulates depth at which TGC begins to augment the weaker signal. NEAR GAIN CONTROL : Used primarily to diminish and not to enhance near echoes. FAR GAIN CONTROL : Used to enhance all distant echoes ENHANCEMENT CONTROL : Augment a localised portion of TGC curve.It gates a specific depth and enhances echoes within the gate to any desired level .

66. Ultrasound Beam Profile Beam comes out as a slice Beam Profile Approx . 1 mm thick Depth displayed – user controlled Image produced is “ 2D ” tomographic slice assumes no thickness You control the aim 1mm

67. Accomplishing this goal depends upon... Resolving capability of the system axial/lateral resolution spatial resolution contrast resolution temporal resolution Processing Power ability to capture, preserve and display the information

68. 29 Types of Resolution • Axial Resolution – specifies how close together two objects can be along the axis of the beam, yet still be detected as two separate objects – frequency (wavelength) affects axial resolution

69. • Lateral Resolution – the ability to resolve two adjacent objects that are perpendicular to the beam axis as separate objects – beamwidth affects lateral resolution

70. Factors affecting Width of the beam Distance from the transducer Frequency Side and grating lobe levels

71. • Spatial Resolution – also called Detail Detail Resolution – the combination of AXIAL and LATERAL resolution – some customers may use this term

72. Temporal Resolution the ability to accurately locate the position of moving structures at particular instants in time also known as frame rate

73. Contrast Resolution the ability to resolve two adjacent objects of similar intensity/reflective properties as separate objects - dependant on the dynamic range

74. Liver metastases

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76. COUPLING AGENTS

77. Artifacts are the errors in images produced by physical processes that affect ultrasound beam . They are potential pitfalls that might confuse the examiner . Some artifacts provide useful information for novel interpretation.

78.  REVERBERATION

79. RING DOWN ARTIFACTS

80. ACOUSTIC SHADOWING Tissues deeper to strongly attenuating objects like calcification, appear darker because the intensity of transmitted beam is lower. Example: Strong after shadowing due to gall stones. Rib shadow

81. ENHANCEMENT Seen as abnormally high brightness. Occurs when sound travels through a medium with attenuation rate lower than surrounding tissue. Example: Enhancement of tissues below cyst or ducts. Tissues deeper to gall and urinary bladder.

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