Principle of usg imaging, construction of transducers

20 de May de 2017

Más contenido relacionado


Principle of usg imaging, construction of transducers

  1. Principle of USG imaging, construction of transducers and USG controls DR. DEV LAKHERA
  2. Topics • Properties of sound wave • Propagation of sound wave • Transducer components • Workings of a transducer • Interaction between sound and matter • Ultrasonic image display • USG controls
  3. Sound as a wave • Mechanical • Require a medium for transport • Normal auditory frequency – 20Hz-20 KHz • Ultrasonic - > 20 KHz • Diagnostic imaging – 1 MHz – 20 MHz
  4. • Longitudinal waves travel with alternate compression and rarefaction.
  5. • Wavelength (λ) – bet two compression bands • Time (T) to complete a single cycle is called the period. • frequency (f ) -number of complete cycles in a unit of time.
  6. Propagation of sound (velocity) Depends on:  Density  Resistance to compression 1540 m/sec
  7. • Sound travels slowest in gases, intermediate in liquids and fastest through solids • Body tissues behave as liquids (avg 1540 m/sec) • V = Freq x Wavelength (velocity constant in a medium) • Freq – inc  wavelength - dec
  8. Propagation of sound (intensity) Depends on:  Amplitude of oscillation Db Defines the Brightness of the image Irrespective of the Freq the Amp remains constant The Higher the Amp the brighter the image and the lower the more darker the images Returning Waves
  9. Frequency Higher the freq Lower the penetration and Higher the resolution Low the freq higher the penetration and lower the resolution
  10. Formation of USG image 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
  11. Transducer • Device that converts one form of energy into another
  12. Components • Piezoelectric crystal • Electrodes with conducting material • Backing block • Coaxial cable • Casing
  13. • Backing block
  14. Piezoelectric crystals • Piezoelectric effect- certain materials on application of electric energy change their physical dimensions • Naturally occurring: Quartz • PZT- Lead zirconate titanate
  15. • Dipoles –geometric pattern • Plating electrodes • Voltage applied in a pulse causes this crystal to vibrate
  16. Receive • Echoes reflect back and from each tissue interface and cause physical compression of crystal element • Dipole change their orientation • Causes generation of voltage  received and displayed
  17. Characteristics of a USG Beam • Fresnel zone- Determined by radius of transducer • Fraunhofer zone (divergent part) • Fresnel zone- Increases with frequency and diameter
  18. Advantage of high frequency beams • Superior superficial resolution , longer frensel zone • Tissue absorption increases with increasing frequency so low frequency beam required to penetrate thick parts • Larger transducers however reduce side to side resolution.(now reduced due to focused transducers)
  19. Phase array transducer Linear transducer Convex transducer- C60
  20. Interactions between sound and matter Reflection Refraction Attenuation Scattering
  21. REFLECTION Images are produced by the reflected portion of beam Percentage of reflected beam depends upon 1.Tissue’s acoustic impedance 2.Beam’s angle of incidence
  22. Acoustic Impedance • How much resistance an ultrasound beam encounters as it passes through a tissue. • Acoustic impedance depends on: the density of the tissue (d, in kg/m3) the speed of the sound wave (c, in m/s) • Amplitude of returning echo is proportional to the difference in acoustic impedance between the two tissues
  23. Acoustic Impedance • Two regions of very different acoustic impedances, the beam is reflected or absorbed • Acoustic impedance of tissue is constant (speed of transmission is constant) Examples of impedance for bodily tissues (in kg/(m2s)): •air 0.0004 × 106 •lung 0.18 × 106 •fat 1.34 × 106 •water 1.48 × 106 •kidney 1.63 × 106 •blood 1.65 × 106 •liver 1.65 × 106 •muscle 1.71 × 106 •bone 7.8 × 106
  24. • Tissue - air interface – 99.9 % beam is reflected • Coupling agent is needed • Ultrasound gel
  25. Angle of incidence • Higher the angle of incidence lesser is the reflection
  26. Specular reflector Diaphragm Wall of urine-filled bladder Endometrial stripe
  27. Echogenicity (caused by Reflection) Anechoic Hypo-Echoic Hyper-Echoic
  28. Scattering • Redirection of sound in several directions • Caused by interaction with small reflector or rough surface • Only portion of sound wave returns to transducer
  29. Refraction • Sound passes from one medium to other at an angle change in velocity but frequency is constant so there is a change in wavelength. • Causes a change in direction • spatial distortion
  30. Absorption • Due to frictional forces opposing the movement of particles in a medium • Utrasonic energy  Thermal energy • Depends on 1) frequency 2) viscosity of the medium 3) relaxation time of the medium
  31. • The deeper the wave travels in the body, the weaker it becomes • The amplitude of the wave decreases with increasing depth Attenuation
  32. Ultrasonic display • Electronic representation of data • A – mode • M – mode • 2D B mode •
  33. Amplitude Modulation (A- mode) • line through the body with the echoes plotted on screen as a function of depth • Stronger echoes produce larger spinkes
  34. Motion mode (M- Mode) • pulses are emitted in quick succession • organ boundaries that produce reflections move relative to the probe • commonly in cardiac and fetal cardiac imaging
  35. B-mode or 2D mode: (Brightness mode) • Most used imaging mode • Produces a picture of a slice of tissue • Brightness depends upon the amplitude or intensity of the echo
  36. USG Imaging Controls • TGC- Time gain compensator • Near gain • Far gain • Intensity • Coarse gain • Reject • Delay • Enhancement
  37. Time gain compensator TGC adjusts the degree of amplification of echoes
  38. Amplitude • Intensity control- Increases the potential difference between transducer • Coarse gain – Increases the height of all echoes proportionately • Reject control- It helps remove echoes below a minimum amplitude
  39. • Delay – Regulates the depth at which the TGC begins to augment the weaker signal • Q-scan - automatically optimises key imaging parameters
  40. • General mode • Resolution mode (high frequency setting) • Penetration mode • (low frequency setting)
  41. • 2D – sets to default B-mode • Depth • Zoom
  42. • Power doppler Uses amplitude of Doppler signal to detect moving matter • Pulse wave doppler Emits USG in pulses Lower velocity • Continuous wave doppler Transducer emits and receives continuously High velocity Color flow Type of power doppler emits pulses
  43. Tissue Harmonic Imaging- Selectively removes phase aberrations generated by variation of velocity between interfaces
  44. Speckle reduction filter • ultrasound speckle image degradation and loss of contrast. • Interaction of generated acoustic fields grainy appearance
  46. 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
  47. 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
  48. Types of Resolution • Spatial Resolution • also called Detail Resolution • the combination of AXIAL and LATERAL resolution • some customers may use this term
  49. Types of Resolution • Contrast Resolution • the ability to resolve two adjacent objects of similar intensity/reflective properties as separate objects
  50. 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 • VERY IMPORTANT IN CARDIOLOGY