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

4. 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

5. Wave Speed c air = 331 m/s c salt 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

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? What is the wavelength of a 5MHz beam traveling into tissue?

7. 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.

8. 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)

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

10. Acoustic Impedance Z=  x c [kg/m 2 /sec] SI unit ([Rayl] =1 [kg/m 2 /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

11. Sound and pressure Sound waves cause a change in local pressure in the media Pressure (Pascal)=N/m 2 Atmospheric pressure 100KPa US will deliver 1 Mpa Intensity I (amount of energy per unit time and area) is proportional to P 2 This is the energy associated with the sound beam Temporal and Spatial intensity when dealing with time or space

12. 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%

13. 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

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

15. ReflectionSnell’s Law  i angle of incidence  t angle of transmittance

16. 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

17. 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

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

19. Axial beam profile Piston source: Oscillations of axial pressure in near-field (e.g. z 0 = (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 Near Field Fresnel Zone Far Field Fraunhofer zone US usually uses Fresnel Zone

20. 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):

21. 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

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

23. Array types a)Linear Sequential (switched) ~1 cm  10-15 cm, up to 512 elements b)Curvilinear similar to (a), wider field of view c)Linear Phased up to 128 elements, small footprint  cardiac imaging d)1.5D Array 3-9 elements in elevation allow for focusing e)2D Phased Focusing, steering in both dimensions

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

25. Ultrasound Imaging

26. 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)

27. 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)

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

29. 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

30. 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

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

32. 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

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

34. Linear switched

35. 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 velocity c: speed of sound  : angle between direction of blood flow and US beam Frequency Counter Spectrum Analyzer

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

37. CW Doppler example

38. Duplex Imaging Combines real-time B-scan with US Doppler flowmetry B-Scan: linear or sector Doppler: C.W. or pulsed ( f c = 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)

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

40. Duplex imaging (Pulsed Doppler)

41. US imaging example (4D)