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Ultrasound

Ultrasound. Sound waves. Sounds are mechanical disturbances that propagate through the medium Frequencies <15Hz Infrasound 15Hz<Frequencies <20KHz Audible sound Frequencies>20Khz Ultrasound Medical Ultrasound frequency 2 -20MHz Some experimental devices at 50MHz. Velocity and frequency.

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Ultrasound

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  1. Ultrasound

  2. Sound waves • Sounds are mechanical disturbances that propagate through the medium • Frequencies <15Hz Infrasound • 15Hz<Frequencies <20KHz Audible sound • Frequencies>20Khz Ultrasound • Medical Ultrasound frequency 2 -20MHz • Some experimental devices at 50MHz

  3. Velocity and frequency • For sound waves the relationship between frequency/velocity and wavelength is c = f x  • Speed of sound depends on the material sound travels • Velocity is inversely proportional to compressibility the less compressible a material is the greater the velocity • Average velocity in tissue 1540 m/sec (air 331m/sec, fat 1450 m/sec) The difference in speed of sound at the boundaries determines the contrast in US

  4. Wave Speed • cair= 331 m/s csalt water= 1500 m/s B = Bulk Modulus  = density Bulk modulus measures stiffness of a medium and its resistance to being compressed Speed of sound increases with stiffness of material k = adiabatic bulk modulus  = density

  5. Wave speed cnt • Changes in speed DO NOT affect the frequency so only the wavelength is dependent on the material. What is the wavelength of a 2MHz beam traveling into tissue? What is the wavelength of a 5MHz beam traveling into tissue?

  6. Wave speed cnt • Changes in speed DO NOT affect the frequency so only the wavelength is dependent on the material. What is the wavelength of a 2MHz beam traveling into tissue? 0.77mm What is the wavelength of a 10MHz beam traveling into tissue? 0.15mm The wavelength determines the image resolution Higher frequency -> higher resolution Penetration is higher at smaller frequencies.

  7. Penetration and resolution • Thick body parts (abdomen) • Low frequency ultrasound (3.5 - 5 Mhz) • Small body parts (thyroid, breat) • High frequency (7.5 - 10 Mhz)

  8. Interference • Waves can constructively and destructively interfere • Constructive interference -> Increase in amplitude (waves in phase) • Destructive interference -> Null amplitude (waves out of phase)

  9. Acoustic Impedance • Z= x c [kg/m2/sec] SI unit ([Rayl] =1 [kg/m2/sec]) • Independent of frequency • Air -> Low Z • Bone -> High Z • Large difference in acoustic impedence in the body generate large reflections that translate in large US signals • Example going from soft tissue to air filled lunghs ->BIG REFLECTION

  10. Sound and pressure • Sound waves cause a change in local pressure in the media • Pressure (Pascal)=N/m2 • Atmospheric pressure 100KPa • US will deliver 1 Mpa • Intensity I (amount of energy per unit time and area) is proportional to P2 • This is the energy associated with the sound beam • Temporal and Spatial intensity when dealing with time or space

  11. Sound and pressure • Relative sound intensity (dB) (Bels => B, 1B=10dB) • Relative intensity dB= 10 log(I/Io) Io original intensity, and I measured intensity • Negative dB -> signal attenuation -3dB -> signal attenuated of 50%

  12. Attenuation • Loss by scatter or absorption • High frequency are attenuated more than low frequencies • Attenuation in homegeneous tissue is exponential • A 1Mhz attenuation in soft tissue is 1 dB/cm, 5 MHz -> 5dB/cm • Bone media attenuation increases as frequency squared. • Absorbed sound ->heat

  13. Reflection • Echo -> reflection of the sound beam • The percentage of US reflected depends on angle of incidence and Z • Similar to light

  14. Reflection Snell’s Law • i angle of incidence • t angle of transmittance

  15. Transducer • Made of piezoelectric material • Crystals or ceramics • Stretching and compressing it generate V • Lead-zirconate-titanate (PZT) • A high frequency voltage applied to PZT generate high freq pressure waves Are generators and detectors

  16. Q factor • Q factor is the frequency response of the piezoelectric crystal • Determines purity of sound and for how long it will persist • High Q transducers generate pure frequency spectrum (1 frequency) • Q=operating frequency/BW • BW bandwidth • High Q -> narrow BW • Low Q->broad BW

  17. Transducer backing • Backing of transducer with impedance-matched, absorbing material reduces reflections from back  damping of resonance • Reduces efficiency • Increases Bandwidth (lowers Q)

  18. Axial beam profile • Piston source: Oscillations of axial pressure in near-field (e.g. z0= (1 mm)2/0.3mm = 3 mm) • NF Variation in pressure and amplitude • Caused by superposition of point wave sources across transducer (Huygens’ principle) • Side lobes = small beams of reduced intensity at an angle to the main beam Far Field Fraunhofer zone Near Field Fresnel Zone US usually uses Fresnel Zone

  19. Lateral beam profile • Determined by Fraunhofer diffraction in the far field. • Given by Fourier Transform of the aperture function • Lateral resolution is defined by width of first lobe (angle of fist zero) in diffraction pattern • For slit (width a): • For disc (radius r, piston source):

  20. Focused transducers • Reduce beam width • Concentrate beam intensity, increasing penetration and image quality • All diagnostic transducers are focused • Focal zone – Region where beam is focused • Focal length – distance from the transducer and center focal zone

  21. Focusing of ultrasound • Increased spatial resolution at specific depth • Self-focusing radiator or acoustic lens

  22. Array types • Linear Sequential (switched) ~1 cm  10-15 cm, up to 512 elements • Curvilinearsimilar to (a), wider field of view • Linear Phasedup to 128 elements, small footprint  cardiac imaging • 1.5D Array3-9 elements in elevation allow for focusing • 2D PhasedFocusing, steering in both dimensions

  23. Array resolution • Lateral resolutiondetermined by width of main (w) lobe according to Larger array dimension  increased resolution • Side lobes (“grating lobes”) reduce resolution and appear at g a w

  24. Ultrasound Imaging

  25. Imaging • Most ultrasound beam are brief pulses of 1 microsecond • Wait time for returning echo • Object must be large compared to wavelength • Signal is amplified when returned (echo is small signal)

  26. A-mode (amplitude mode) I • Oldest, simplest type • Display of the envelope of pulse-echoes vs. time, depth d = ct/2 • Pulse repetition rate ~ kHz (limited by penetration depth, c  1.5 mm/s  20 cm  270 s, plus additional wait time for reverberation and echoes)

  27. A-mode (amplitude mode) • Or space! Also M mode! depth

  28. A-mode II • Frequencies: 2-5 MHz for abdominal, cardiac, brain; 5-15 MHz for ophthalmology, pediatrics, peripheral blood vessels • Applications: ophthalmology (eye length, tumors), localization of brain midline, liver cirrhosis, myocardium infarction • Logarithmic compression of echo amplitude (dynamic range of 70-80 dB) Logarithmic compression of signals

  29. M mode or T-M mode • Time on horizontal axis and depth on vertical axis • Time dependent motion • Used to study rapid movement – cardiac valve motion

  30. B-mode clinical example Static image of section of tissue Brighter means intensity of echo

  31. B-mode (“brightness mode”) • Lateral scan across tissue surface • Grayscale representation of echo amplitude Add sense of direction to information-> where did echo come from

  32. Real-time B scanners • Frame rate Rf ~30 Hz: • Mechanical scan: Rocking or rotating transducer + no side lobes - mechanical action, motion artifacts • Linear switched array d: depth N: no. of lines

  33. Linear switched

  34. Frequency Counter SpectrumAnalyzer CW Doppler • Doppler shift in detected frequency • Separate transmitter and receiver • Bandpass- filtering of Doppler signal: • Clutter (Doppler signal from slow-moving tissue, mainly vessel walls) @ f<1 kHz • LF (1/f) noise • Blood flow signal @f < 15 kHz • CW Doppler bears no depth information v: blood flow velocityc: speed of sound: angle between direction of blood flow and US beam

  35. v [10cm/s] t [0.2 s] CW Doppler clinical images • CW ultrasonic flowmeter measurement (radial artery) • Spectrasonogram: Time-variation of Doppler Spectrum f t

  36. CW Doppler example

  37. Duplex Imaging • Combines real-time B-scan with US Doppler flowmetry • B-Scan: linear or sector • Doppler: C.W. or pulsed (fc = 2-5 MHz) • Duplex Mode: • Interlaced B-scan and color encoded Doppler images limits acquisition rate to 2 kHz (freezing of B-scan image possible) • Variation of depth window (delay) allows 2D mapping (4-18 pulses per volume)

  38. Duplex imaging example (c.w.) www.medical.philips.com

  39. Duplex imaging (Pulsed Doppler)

  40. US imaging example (4D)

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