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Physical Principles Of Ultrasound

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Physical Principles Of Ultrasound

  1. 1. Physical Principles of Ultrasound Grateful appreciation to Richard A. Lopchinsky, MD, FACS and Nancy H. Van Name, RDMS, RTR, and Marlene Kattaron, RDMS ©2000 UIC All Rights Reserved.
  2. 2. Course Objectives • Identify history & define ultrasound • Define piezoelectric effect • Define frequency & wavelength; identify their relationship • Define bandwidth • Define attenuation; identify relationship to frequency • Define resolution & its components; identify relationship to frequency • Identify basic transducer types • Define electronic array • Differentiate between sector & linear array • Identify types of image display • Identify artifacts useful to diagnosis • Discuss safety of medical ultrasound
  3. 3. History of Ultrasound • Piezoelectricity discovered by the Curies in 1880 using natural quartz • SONAR was first used in 1940’s war-time • Diagnostic Medical applications in use since late 1950’s
  4. 4. Ultrasound: Physical Definition • Sound waves greater than 20,000 Hertz or cycles per second Infrasound Ultrasound <20 Hz >20,000 Hz
  5. 5. Ultrasound: Medical Definition • Diagnostic Medical Ultrasound is the use of high frequency sound to aid in the diagnosis and treatment of patients. • Frequency ranges used in medical ultrasound imaging are 2 - 15 MHz
  6. 6. Piezoelectric Effect • Definition: The principle of converting energy by applying pressure to a crystal. • The reverse of the piezoelectric effect converts the energy back to its original form.
  7. 7. Piezoelectric Effect and Ultrasound Transducers • A transducer converts one type of energy into another. • Based upon the pulse-echo principle occurring with ultrasound piezoelectric crystals, ultrasound transducers convert: – Electricity into sound = pulse – Sound into electricity = echo
  8. 8. Pulse • Pulse of sound is sent to soft tissues • Sound interaction with soft tissue = bioeffects • Pulsing is determined by the transducer or probe crystal(s) and is not operator controlled
  9. 9. Echo • Echo produced by soft tissues • Tissue interaction with sound = acoustic propagation properties • Echoes are received by the transducer crystals • Echoes are interpreted and processed by the ultrasound machine
  10. 10. Frequency • Number of complete cycles per unit of time • Man-made transducer frequency is predetermined by design • Ultrasound transducers are referred to by the operating, resonant or main frequency
  11. 11. Frequency Units • One cycle per second = one Hertz (Hz) • One thousand Hertz = One kilohertz (KHz) • One million Hertz = One megahertz (MHz) Example: a 7.5 MHz transducer operates at 7,500,000 cycles per second
  12. 12. Wavelength • Definition: The distance between consecutive cycles of sound. Transducer frequency Transducer wavelength
  13. 13. Transducer Frequencies • 2.5 MHz • Deep abdomen, OB/Gyn • 3.5MHz • General abdomen, OB/Gyn • 5.0 MHz • Vascular, Breast, Gyn • 7.5 MHz • Breast, Thyroid • 10.0 MHz • Breast, Thyroid, Superficial veins, Superficial masses
  14. 14. Bandwidth • All ultrasound transducers contain a range of frequencies, termed bandwidth • Broad bandwidth technology produces medical transducers that contain more than one operating frequency, for example: – 2.5 - 3.5 MHz for general abdominal imaging – 5.0 - 7.5 MHz for superficial imaging
  15. 15. Attenuation • Definition: The reduction in power and intensity as sound travels through a medium. Transducer frequency Depth of penetration • Higher frequencies attenuate, or are absorbed, faster than lower frequencies
  16. 16. Attenuation
  17. 17. Time Gain Compensation • Operator controlled adjustment to compensate for the attenuation of sound as it travels into the tissue • Must be adjusted manually for each tissue type examined and may be manipulated throughout an exam to optimize the image
  18. 18. RESOLUTION • The ability to differentiate between structures that are closely related, both in terms of space and echo amplitude • Wavelength (frequency) dependent – Gray Scale Resolution – Axial Resolution – Lateral Resolution
  19. 19. Frequency vs. Resolution Transducer frequency Resolution and image detail • Higher frequency transducers provide better image resolution – better gray scale resolution – improved ability to distinguish fine detail
  20. 20. Frequency and Resolution 3.5 MHz 7.5 MHz
  21. 21. Gray Scale Resolution • Adequate gray scale resolution allows for the differentiation of subtle changes in the tissues • Dynamic Range determines how many shades of gray are demonstrated on an image
  22. 22. Dynamic Range Decreased DR Increased DR
  23. 23. Axial & Lateral Resolution • Spatial Resolution describes how physically close two objects can be and displayed separately. – Axial: along the beam path – Lateral: perpendicular to beam path • All current equipment has an overall spatial resolution of 1.0 mm or less.
  24. 24. Frequency Summary • High frequency • Low frequency – improved – poorer resolution resolution – depth of penetration – full depth of loss penetration – higher frequency – lower frequency transducers for transducers for general superficial uses abdominopelvic uses
  25. 25. Machine Components Transducer Beam Former Receiver Memory Display
  26. 26. Transducer Types • Mechanical • Electronic – Oscillating – Linear Arrays – Rotating – Curved Arrays – Phased Arrays
  27. 27. Electronic Arrays • Groups of piezoelectric material working singly or in groups Transducer 1 2 3 4 5 6 7 8 126
  28. 28. Electronic Transducers • Sector Array • Linear Array – crystals are placed – crystals are placed parallel or in parallel concentric rings – transducer face is – transducer face is curved flat – produces sector or – produces pie-shaped image rectangular image
  29. 29. Display Field of View • Field Of View -- the display of the echo amplitudes • shape dependent on transducer type and function
  30. 30. Field of View Shapes • SECTOR FOV • LINEAR FOV • produced by • produced by oscillating linear arrays rotating curved arrays phased arrays • typically used in • typically used in abdominal application superficial application
  31. 31. Sector Linear
  32. 32. Display Modes • B Mode • B Color • M Mode • D Mode or Doppler – spectral – audio – color • Color/Doppler/PowerAngio -- slow flow
  33. 33. B-Mode M-Mode
  34. 34. Color Power Doppler Doppler
  35. 35. Duplex and Triplex Imaging
  36. 36. Artifacts • Portions of the display which are not a “true” representation of the tissue imaged • Medical Diagnostic Ultrasound imaging utilizes certain artifacts to characterize tissue
  37. 37. Artifacts • The ability to differentiate solid vs. cystic tissue is the hallmark of ultrasound imaging • Acoustic Shadowing and Acoustic Enhancement are the two artifacts that provide the most useful diagnostic information
  38. 38. Shadowing • Diminished sound or loss of sound posterior to a strongly reflecting or strongly attenuating structure – Strong reflectors • large calcifications, bone – Strong attenuators • solid tissue, significantly dense or malignant masses
  39. 39. Shadowing
  40. 40. Enhancement • Increased through transmission of the sound wave posterior to a weakly attenuating structure • Gain curve expected a certain loss or attenuation with depth of travel – Occurs posterior to • simple cysts or weakly attenuating masses
  41. 41. Enhancement
  42. 42. Bioeffects • Prudent use assures patient safety • Effects at intensities higher than those used in diagnostic medical ultrasound include: cavitation sister chromatid exchange
  43. 43. AIUM Statement • “No confirmed biological effects on patients or operators caused by exposure at intensities typical of diagnostic ultrasound… • ...current data indicate that the benefits… outweigh the risks.”
  44. 44. Summary • Ultrasound > 20,000 Hz • Piezoelectric Effect = pulse-echo principle • Frequency & wavelength are inversely proportional • Broad bandwidth enables multihertz probes • Attenuation & frequency are inversely related • Resolution determines image clarity • Electronic Arrays may be sector or linear • Display mode chosen determines how image is registered • Shadowing & Enhancement are the artifacts most used in ultrasound diagnosis • Diagnostic Medical Ultrasound is safe!