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Ultrasound Physics

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Ultrasound Physics

  1. 1. Ultrasound Physics
  2. 2. Characteristics of Sound • Requires medium for its transmission. • Compression and rarefaction. • Propagation.
  3. 3. Ultrasound waves • Range • Longitudinal waves • Length of wave • Wave once generated continue in original direction until it is either reflected, refracted or absorbed
  4. 4. Velocity of Sound • Independent of frequency • medium How velocity is related to: • Compressibility • Density • Intensity
  5. 5. Transducers • Converts an electrical signals into ultrasonic energy that can be transmitted to tissues & vice versa • Constituents :- 1. Piezoelectrical crystals 2. Two electrodes 3. Backing block 4. Acoustic insulator(rubber ) 5. Plastic housing
  6. 6. • Two basic modes of transducers 1. Continuous mode – doppler studies 2. Pulse mode
  7. 7. Curie temperature Resonant frequency Transducer Q factor •Two characteristics- purity of sound & length of sound •High Q transducer •Low Q transducer
  8. 8. Advantage of High frequency over low frequency Depth resolution is superior & near zone is long Dis-advantage Tissue absorption increases with increased frequency
  9. 9. Interaction between ultrasound & matter 1. Reflection 2. Refraction 3. Absorption
  10. 10. Reflection • Reflected wave sound produces image • It depends on 1. Acoustic impedance 2 Angle of incidence
  11. 11. Acoustic impedance • Impedance is product of tissue density with velocity of sound in the material. Angle of incidence Higher the amount of angle less is the amount of sound reflected Specular interface Examples diaphragm,walls of the vessels boundaries of many organs
  12. 12. Refraction • Bending of waves as they pass from one medium to another is called refraction • Interfaces that are either smaller than the wavelength or not smooth are nonspecular interface • Example Rbc,liver parenchyma
  13. 13. Absorption • It means conversion of ultrasonic to thermal energy • It depend on 1. Frequency of sound 2. Viscosity of medium 3. Relaxation time
  14. 14. Ultrasonic display • A Mode– displayed as spikes projecting from baseline. Spike height is proportional to echo intensity. It is static one- dimensional mode. • TM Mode– echoes produced by moving structures as dots. • B Mode– produces a picture of a slice of tissue. Echoes displayed as dots. • Gray scale Imaging– It’s a B mode type displaying large amplitude of echoes arising from tissues as varying shades of gray
  15. 15. • Real time imaging– produces multiple images in a very short period • Two types 1. Mechanical– Oscillating & Rotating wheel 2. Electronic array– Linear array & phased or steered array
  16. 16. PRINCIPLES OF DOPPLER ULTRASOUND
  17. 17. BASIC PRINCIPLE • Doppler ultrasound is a technique for making non-invasive velocity measurements of blood flow. • Christian Doppler was the first to describe the frequency shift that occurs when sound or light is emitted from a moving source. • Ultrasound is transmitted into a vessel and the sound that is reflected from the blood is detected. Because the blood is moving, the sound undergoes a frequency (Doppler) shift.
  18. 18. Uses of doppler • Determination of anatomy. • Determination of small vessel flow distribution. • Detection of vascular dilatation, obstruction. • Evaluation of intravascular disease and evaluation. • Detection of extra vascular flow.
  19. 19. 1) Continuous wave doppler • Without B MODE imaging 2) Pulsed wave doppler • Pulsed Doppler ultrasound is a technique for measuring the velocity of blood in a small sample volume • Used in general & obstetric ultrasound & also used to provide data for doppler sonograms & colour flow images. Disadvantage – “Aliasing” 3) Color Doppler
  20. 20. DOPPLER INDICES

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