Successfully reported this slideshow.
We use your LinkedIn profile and activity data to personalize ads and to show you more relevant ads. You can change your ad preferences anytime.

Physic Of Ultrasound


Published on

Lecture on physic of u/sound ( for post-basic dip. student session 2011/2012 )

Published in: Education, Business, Technology

Physic Of Ultrasound

  1. 1. Physics AndInstrumentation of Ultrasound
  3. 3. Bats navigate using ultrasound
  4. 4. Bats make high-pitched chirps which are too high forhumans to hear. This is called ultrasoundLike normal sound, ultrasound echoes off objectsThe bat hears the echoes and works out what causedthem•Dolphins also navigate with ultrasound•Submarines use a similar method called sonar•We can also use ultrasound to look inside the body…
  5. 5. • Ultrasound – Cyclic sound pressure with a frequency greater than the upper limit of human hearing. • Human Ear  Audible Range Frequency?
  6. 6. The human ear can onlyThe human ear canonly respond to theaudible frequency range~ 20Hz - 20kHz to theaudible frequency range ~20Hz - 20kHz
  7. 7. Medical sonography(ultrasonography) Ultrasound-based diagnostic imaging technique used to visualize muscles and internal organs, their size, structures and possible pathologies or lesions. APPLICATIONS? ADVANTAGES & DISADVANTAGES?
  8. 8. Diagnostic applications• Cardiology• Gynaecology & Obstetrics• Ophthalmology• Abdomen• Urology- to determine, for example, the amount of fluid retained in a patients bladder.• Musculoskeletal - tendons, muscles, and nerves• Vascular - arteries and veins• Interventional biopsy - emptying fluids, intrauterine transfusion
  9. 9. Therapeutic applications• Therapeutic applications use ultrasound to bring heat or agitation into the body.• Therefore much higher energies are used than in diagnostic ultrasound.
  10. 10. ULTRASOUND PHYSICSFormat What is sound/ultrasound? How is ultrasound produced Transducers - properties Effect of Frequency Image Formation Interaction of ultrasound with tissue Acoustic impedance Image appearance
  11. 11. Sound? Sound is a mechanical, longitudinal wave that travels in a straight line Sound requires a medium through which to travel
  12. 12. CATEGORIES OF SOUND Infrasound (subsonic) below 20Hz Audible sound 20-20,000Hz Ultrasound above 20,000Hz Nondiagnostic medical applications <1MHz Medical diagnostic ultrasound >1MHz
  13. 13. In 1826 DanielColladon, a Swissphysicist, and CharlesSturm, a Frenchmathematician,accurately measured itsspeed in water. Using along tube to listenunderwater (as Leonardoda Vinci suggested in1490), they recorded howfast the sound of asubmerged bell traveledacross Lake Geneva.Their result--1,435meters persecond in water of 1.8degrees Celsius (35degrees Fahrenheit)--wasonly 3 meters per secondoff from the speedaccepted today.
  14. 14. Compression wave
  15. 15. Acoustic Variables• Period• Wavelength• Amplitude• Frequency• Velocity
  16. 16. Acoustic Variables
  17. 17. Acoustic Variables
  18. 18. Amplitude, A (m) The maximum displacement that occurs in an acoustic variable.
  19. 19. Why we use different frequency?
  20. 20. Basic Ultrasound Physics Amplitudeoscillations/sec = frequency - expressed inHertz (Hz)
  21. 21. 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. Medical Ultrasound 2MHz to 16MHz
  22. 22. ULTRASOUND – How is it produced?Produced by passing an electrical current through a piezoelectrical (material that expands and contracts with current) crystal
  23. 23. Human Hair Single CrystalMicroscopic view of scanhead
  24. 24. In ultrasound, the following events happen:1. The ultrasound machine transmits high- frequency (1 to 12 megahertz) sound pulses into the body using a probe.2. The sound waves travel into the body and hit a boundary between tissues (e.g. between fluid and soft tissue, soft tissue and bone).3. Some of the sound waves reflect back to the probe, while some travel on further until they reach another boundary and then reflect back to the probe .4. The reflected waves are detected by the probe and relayed to the machine.
  25. 25. 1. The machine calculates the distance from the probe to the tissue or organ (boundaries) using the speed of sound in tissue (1540 m/s) and the time of the each echos return (usually on the order of millionths of a second).6. The machine displays the distances and intensities of the echoes on the screen, forming a two dimensional image.
  26. 26. Piezoelectric material ACapplied to a piezoelectric crystal causes it to expand and contract – generating ultrasound, and vice versa Naturally occurring - quartz Synthetic - Lead zirconate titanate (PZT)
  27. 27. Ultrasound Production Transducer produces ultrasound pulses (transmit 1% of the time) These elements convert electrical energy into a mechanical ultrasound wave Reflectedechoes return to the scanhead which converts the ultrasound wave into an electrical signal
  28. 28. Piezoelectric Crystals  Thethickness of the crystal determines the frequency of the scanhead Low Frequency High Frequency 3 MHz 10 MHz
  29. 29. Frequency also affects the QUALITY of the The frequency vs. Resolution ultrasound image  The HIGHER the frequency, the BETTER the resolution The LOWER the frequency, the LESS the resolution A 12 MHz transducer has very good resolution, but cannot penetrate very deep into the body A 3 MHz transducer can penetrate deep into the body, but the resolution is not as good as the 12 MHz Low Frequency High Frequency 3 MHz 12 MHz
  30. 30. Broadband vs. Narrowband Amplitude Frequency
  31. 31. Broadband vs. NarrowbandNerve Visualisation: 5-10 MHz 6-13 MHz By altering the transmit frequencies one transducer replaces several transducers View a range of superficial to deep structures without changing transducers
  32. 32. Transducer DesignSize, design andfrequencydepend upon theexamination
  33. 33. Image FormationElectrical signal produces ‘dots’ on the screen Brightness of the dots is proportional to the strength of the returning echoes Location of the dots is determined by travel time. The velocity in tissue is assumed constant at 1540m/sec Distance = Velocity Time
  34. 34. Image Formation‘B’ mode
  35. 35. Interactions of Ultrasound with Tissue  Reflection  Refraction  Transmission  Attenuation
  36. 36. Interactions of Ultrasound with Tissue  Reflection  The ultrasound reflects off tissue and returns to the transducer, the amount of reflection depends on differences in acoustic impedance  The ultrasound image is formed from reflected echoes transducer
  37. 37. Refraction reflective refractionScatteredechoes Incident Angle of incidence = angle of reflection
  38. 38. Interactions of Ultrasound with Tissue Transmission  Some of the ultrasound waves continue deeper into the body  These waves will reflect from deeper tissue structures transducer
  39. 39. Interactions of Ultrasound with Tissue Attenuation  Defined - the deeper the wave travels in the body, the weaker it becomes -3 processes: reflection, absorption, refraction  Air (lung)> bone > muscle > soft tissue >blood > water
  40. 40. Interactions of Ultrasound with Tissue• Acoustic impedance (AI) is dependent on the density of the material in which sound is propagated - the greater the impedance the denser the material.• Reflections comes from the interface of different AI’s • greater ∆ of the AI = more signal reflected • works both ways (send and receive directions) Transducer Medium 1 Medium 2 Medium 3
  41. 41. Interaction of Ultrasound with Tissue• Greater the AI, greater the returned signal • largest difference is solid-gas interface • we don’t like gas or air • we don’t like bone for the same reason GEL!!• Sound is attenuated as it goes deeper into the body
  42. 42. • Z (Rayls) = Density (kg/m³) x Speed (m/s)
  43. 43. • Incident beam has normal incidence 90 degree (perpendicular incidence) on the tissue interface, the magnitude of reflection can be calculated (IRC)• α  Z values
  44. 44. Attenuation & Gain Sound is attenuated by tissue More tissue to penetrate = more attenuation of signal Compensate by adjusting gain based on depth  near field / far field  AKA: TGC
  45. 45. Ultrasound Gain Gain controls  receiver gain only  does NOT change power output  think: stereo volume Increasegain = brighter Decrease gain = darker
  46. 46. Balanced Gain Gain settings are important to obtaining adequate images. bad far fieldbad near field balanced
  47. 47. Reflected Echo’s Strong Reflections = White dots Diaphragm, tendons, bone ‘Hyperechoic’
  48. 48. Reflected Echo’sWeaker Reflections =Grey dots  Most solid organs,  thick fluid – ‘isoechoic’
  49. 49. Reflected Echo’s No Reflections = Black dots  Fluid within a cyst, urine, blood ‘Hypoechoic’ or echofree
  50. 50. What determines how far ultrasound waves can travel? The FREQUENCY of the transducer  The HIGHER the frequency, the LESS it can penetrate  The LOWER the frequency, the DEEPER it can penetrate  Attenuation is directly related to frequency
  51. 51. Ultrasound Beam Depth• Need to image at proper depth• Can’t control depth of beam • keeps going until attenuated• You can control the depth of displayed data
  52. 52. 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
  53. 53. Goal of an Ultrasound System  The ultimate goal of any ultrasound system is to make like tissues look the same and unlike tissues look different
  54. 54. Accomplishing this goal dependsupon...  Resolving capability of the system  axial/lateral resolution  spatial resolution  contrast resolution  temporal resolution  Processing Power  ability to capture, preserve and display the information
  55. 55. 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 – frequency resolution
  56. 56. Types of Resolution Lateral Resolution  the ability to resolve two adjacent objects that are perpendicular to the beam axis as separate objects  beamwidth affects lateral resolution
  57. 57. Types of Resolution Spatial Resolution  also called Detail Resolution  the combination of AXIAL and LATERAL resolution - how closely two reflectors can be to one another while they can be identified as different reflectors
  58. 58. Types of Resolution Temporal Resolution  the ability to accurately locate the position of moving structures at particular instants in time  also known as frame rate
  59. 59. Types of Resolution Contrast Resolution  the ability to resolve two adjacent objects of similar intensity/reflective properties as separate objects - dependant on the dynamic range
  60. 60. Liver metastases
  61. 61. Ultrasound ApplicationsVisualisation Tool:Nerves, soft tissue massesVessels - assessment of position, size,patencyUltrasound Guided Procedures in realtime – dynamic imaging; central venousaccess, nerve blocks
  62. 62. ImagingKnow your anatomy – Skin, muscle,tendons, nerves and vesselsRecognise normal appearances –compare sides!
  63. 63. Skin, subcutaneous tissue Epidermis Loose connective tissue and subcutaneous fat is hypoechoic Muscle interface Muscle fibres interface Bone
  64. 64. Transverse scan – Internal Jugular Vein and Common Carotid Artery
  65. 65. Summary•Imaging tool – Must have the knowledge tounderstand how the image is formed•Dynamic technique•Acquisition and interpretation dependantupon the skills of the operator.