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Ultrasound Physics

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  1. Ultrasound Physics Have no fear Presentation by Alexis Palley MD Department of Emergency Medicine Cooper University Hospital

  2. “How does it do that?” Lecture Objectives: • Review basic physics vocabulary • Explain the principles of sound waves • Use ultrasound physics to explain how images are produced • Teach how to use these principles to help your diagnostic abilities

  3. Basics • Sound is energy traveling though matter as a wave • The wave travels by compressing and rarefacting matter • Depending on the matter- the wave will travel at different velocities or directions • U/S probes emit and receive the energy as waves to form pictures

  4. Physical Principles

  5. Cycle Cycle • 1 Cycle = 1 repetitive periodic oscillation VOCAB

  6. Frequency • # of cycles per second • Measured in Hertz (Hz) -Human Hearing 20 - 20,000 Hz -Ultrasound > 20,000 Hz -Diagnostic Ultrasound 2.5 to 10 MHz (this is what we use!) VOCAB

  7. 1 second = 1 Hertz frequency 1 cycle in 1 second = 1Hz VOCAB

  8. High Frequency • High frequency (5-10 MHz) greater resolution less penetration • Shallow structures vascular, abscess, t/v gyn, testicular

  9. Low Frequency • Low frequency (2-3.5 MHz) greater penetration less resolution • Deep structures Aorta, t/a gyn, card, gb, renal

  10. Wavelength • The length of one complete cycle • A measurable distance VOCAB

  11. Wavelength Wavelength VOCAB

  12. Amplitude Amplitude • The degree of variance from the norm VOCAB

  13. Producing an image • Probe emits a sound wave pulse-measures the time from emission to return of the echo • Wave travels by displacing matter, expanding and compressing adjacent tissues • It generates an ultrasonic wave that is propagated, impeded, reflected, refracted, or attenuated by the tissues it encounters

  14. Producing an image • Important concepts in production of an U/S image: • Propagation velocity • Acoustic impedance • Reflection • Refraction • Attenuation

  15. Propagation Velocity • Sound is energy transmitted through a medium- • Each medium has a constant velocity of sound(c) • Tissue’s resistance to compression • density or stiffness • Product of frequency (f) and wavelength (λ) c=fλ • Frequency and Wavelength therefore are directly proportional- if the frequency increases the wavelength must decrease.

  16. Propagation Velocity • Propagation velocity Increased by increasing stiffness Reduced by increasing density • Bone: 4,080 m/sec • Air: 330 m/sec • Soft Tissue Average: 1,540 m/sec

  17. Impedance • Acoustic impedance (z) of a material is the product of its density and propagation velocity Z= pc • Differences in acoustic impedance create reflective interfaces that echo the u/s waves back at the probe • Impedance mismatch = ΔZ

  18. Acoustic Impedance • Homogeneous mediums reflect no sound • acoustic interfaces create visual boundaries between different tissues. • Bone/tissue or air/tissue interfaces with large Δz values reflect almost all the sound • Muscle/fat interfaces with smaller Δz values reflect only part of the energy

  19. Refraction • A change in direction of the sound wave as it passes from one tissue to a tissue of higher or lower sound velocity • U/S scanners assume that an echo returns along a straight path • Distorts depth reading by the probe • Minimize refraction by scanning perpendicular to the interface that is causing the refraction

  20. Reflection • The production of echoes at reflecting interfaces between tissues of differing physical properties. • Specular - large smooth surfaces • Diffuse – small interfaces or nooks and crannies

  21. Specular Reflection • Large smooth interfaces (e.g. diaphragm, bladder wall) reflect sound like a mirror • Only the echoes returning to the machine are displayed • Specular reflectors will return echoes to the machine only if the sound beam is perpendicular to the interface

  22. Diffuse Reflector • Most echoes that are imaged arise from small interfaces within solid organs • These interfaces may be smaller than the wavelength of the sound • The echoes produced scatter in all directions • These echoes form the characteristic pattern of solid organs and other tissues

  23. Reflectors Specular Diffuse

  24. Attenuation • The intensity of sound waves diminish as they travel through a medium • In ideal systems sound pressure (amplitude) is only reduced by the spreading of waves • In real systems some waves are scattered and others are absorbed, or reflected • This decrease in intensity (loss of amplitude) is called attenuation.

  25. The Machine

  26. Ultrasound scanners • Anatomy of a scanner: • Transmitter • Transducer • Receiver • Processor • Display • Storage

  27. Transmitter • a crystal makes energy into sound waves and then receives sound waves and converts to energy • This is the Piezoelectric effect • u/s machines use time elapsed with a presumed velocity (1,540 m/s) to calculate depth of tissue interface • Image accuracy is therefore dependent on accuracy of the presumed velocity.

  28. Transducers • Continuous mode • continuous alternating current • doppler or theraputic u/s • 2 crystals –1 talks, 1 listens • Pulsed mode • Diagnostic u/s • Crystal talks and then listens

  29. Receiver • Sound waves hit and make voltage across the crystal- • The receiver detects and amplifies these voltages • Compensates for attenuation

  30. TGC (time gain compensation) Manual control Selective enhancement or suppression of sectors of the image enhance deep and suppress superficial *blinders Gain Manual control Affects all parts of the image equally Seen as a change in “brightness” of the images on the entire screen *glasses Signal Amplification

  31. Changing the TGC

  32. Changingthe Gain

  33. Displays • B-mode • Real time gray scale, 2D • Flip book- 15-60 images per second • M-mode • Echo amplitude and position of moving targets • Valves, vessels, chambers

  34. “B” Mode

  35. “M” Mode

  36. Image properties • Echogenicity- amount of energy reflected back from tissue interface • Hyperechoic - greatest intensity - white • Anechoic - no signal - black • Hypoechoic – Intermediate - shades of gray

  37. Anechoic Hyperechoic Hypoechoic

  38. Image Resolution • Image quality is dependent on • Axial Resolution • Lateral Resolution • Focal Zone • Probe Selection • Frequency Selection • Recognition of Artifacts

  39. Axial Resolution • Ability to differentiate two objects along the long axis of the ultrasound beam • Determined by the pulse length • Product of wavelength λ and # of cycles in pulse • Decreases as frequency f increases • Higher frequencies produce better resolution

  40. 5 MHz transducer Wavelength 0.308mm Pulse of 3 cycles Pulse length approximately 1mm Maximum resolution distance of two objects = 1 mm 10 MHz transducer Wavelength 0.15mm Pulse of 3 cycles Pulse length approximately 0.5mm Maximum resolution distance of two objects = 0.5mm Axial Resolution

  41. Axial Resolution screen body

  42. Lateral Resolution • The ultrasound beam is made up of multiple individual beams • The individual beams are fused to appear as one beam • The distances between the single beams determines the lateral resolution

  43. Lateral resolution • Ability to differentiate objects along an axis perpendicular to the ultrasound beam • Dependent on the width of the ultrasound beam, which can be controlled by focusing the beam • Dependent on the distance between the objects

  44. Lateral Resolution screen body

  45. Objects within the focal zone Objects outside of focal zone Focal Zone Focal zone Focal zone

  46. Linear Array Curved Array Probe options

  47. Ultrasound Artifacts • Can be falsely interpreted as real pathology • May obscure pathology • Important to understand and appreciate

  48. Ultrasound Artifacts • Acoustic enhancement • Acoustic shadowing • Lateral cystic shadowing (edge artifact) • Wide beam artifact • Side lobe artifact • Reverberation artifact • Gain artifact • Contact artifact

  49. Acoustic Enhancement • Opposite of acoustic shadowing • Better ultrasound transmission allows enhancement of the ultrasound signal distal to that region

  50. Acoustic Enhancement