1. BMI 1 FS05 – Class 8, “US Instrumentation” Slide 1 Biomedical Imaging I Class 8 – Ultrasound Imaging II: Instrumentation and Applications 11/02/05

2. BMI 1 FS05 – Class 8, “US Instrumentation” Slide 2 Generation and Detection of Ultrasound

3. BMI 1 FS05 – Class 8 “US Instrumentation” Slide 3 Piezoelectric effect I Conversion of electric energy into mechanical energy and vice versa in materials with intrinsic el. dipole moments (structural anisotropy). Electric field (~100 V) causes re-orientation of dipoles  deformation Deformation causes shift of dipoles  induces Voltage Examples of piezoelectric Materials: Crystalline (quartz), Polycrystalline ceramic (PZT, lead zirconium titanate), Polymers (PVDF) Crystalline: Quartz (SiO 2 )

4. BMI 1 FS05 – Class 8 “US Instrumentation” Slide 4 Piezoelectric effect II Polycrystalline (e.g., ferroelectric, PZT) Polymers (PVDF) “  phase” (not p.e. active) “  phase” (p.e. active)

5. BMI 1 FS05 – Class 8 “US Instrumentation” Slide 5 Transducer Q-factor Disc of piezoelectric material (usually PZT) shows mechanical resonance frequencies f res  Resonance curve (Q-factor  High Q: strong resonance (narrow curve) Low Q: strongly damped, weak resonance (broad curve) Tradeoff of high Q: +Efficient at f res (high signal-to-noise ratio) –Pulse distortion  f lo-Q A (f res ) = 0 dB - 3 dB  f hi-Q Amplitude Frequency

6. BMI 1 FS05 – Class 8 “US Instrumentation” Slide 6 Transducer resonances Transducer (disc) has mechanical resonances at frequencies Lowest (fundamental) resonance frequency (standing wave): Crystal Length of crystal, L c (c: speed of sound, : wavelength) time Transducer ends have 180  phase difference (=  = /2)

7. BMI 1 FS05 – Class 8 “US Instrumentation” Slide 7 Transducer backing Backing of transducer with impedance-matched, absorbing material reduces reflections from back  damping of resonance Reduces efficiency Increases Bandwidth (lowers Q)

8. BMI 1 FS05 – Class 8 “US Instrumentation” Slide 8 Transducer–tissue mismatch Impedance mismatch causes reflection, inefficient coupling of acoustical energy from transducer into tissue: Z T  30 MRayl Z L  1.5 MRayl  I t /I i = 0.18 Solution: Matching layer(s) increases coupling efficiency damps crystal oscillations, increases bandwidth (reduces efficiency) Transducer Load (tissue) ItIt IiIi IrIr ZTZT ZLZL

9. BMI 1 FS05 – Class 8 “US Instrumentation” Slide 9 Matching layers A layer between transducer and tissue with Z T > Z l > Z L creates stepwise transition Ideally, 100 % coupling efficiency across a matching layer is possible because of destructive interference of back reflections if layer thickness = /4 Z l chosen so that I r,1 = I r,2 : Problems: Finding material with exact Z l value (~6.7 MRayl) Dual-layer: Matching Layer Load (Target) Transducer ZTZT ZlZl ZLZL ItIt IiIi I t,L I r,1 I r,l I r,2  = /4  = /2

10. BMI 1 FS05 – Class 8 “US Instrumentation” Slide 10 Pulsed vs. C.W. mode Low bandwidth: No backing, matching possible High efficiency (SNR) High-Q Strong “Pulse ringing”  c.w. applications Large Bandwidth: Pulsed applications Backing, matching Low-Q Lowered efficiency

11. BMI 1 FS05 – Class 8 “US Instrumentation” Slide 11 Axial beam profile Piston source: Oscillations of axial pressure in near-field (e.g. z 0 = (1 mm) 2 /0.3mm = 3 mm) Caused by superposition of point wave sources across transducer (Huygens’ principle) Function, see Webb Eq. (3.30)

12. BMI 1 FS05 – Class 8 “US Instrumentation” Slide 12 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):

13. BMI 1 FS05 – Class 8 “US Instrumentation” Slide 13 Axial and lateral resolution Axial resolution = 0.5  c, determined by spatial pulse length (  = pulse duration). Pulse length determined by location of -3 dB point. Lateral resolution determined by beam width (-3 dB beam width or - 6 dB width)

14. BMI 1 FS05 – Class 8 “US Instrumentation” Slide 14 Focusing of ultrasound Increased spatial resolution at specific depth Self-focusing radiator or acoustic lens

15. BMI 1 FS05 – Class 8 “US Instrumentation” Slide 15 Transducer arrays Linear sequential array  lateral scan Linear phased array for beam steering, focusing

16. BMI 1 FS05 – Class 8 “US Instrumentation” Slide 16 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

17. BMI 1 FS05 – Class 8 “US Instrumentation” Slide 17 Array resolution Lateral resolution determined by width of main lobe according to Larger array dimension  increased resolution Side lobes (“grating lobes”) reduce resolution and appear at w a g

18. BMI 1 FS05 – Class 8 “US Instrumentation” Slide 18 Radiation pattern Contributions of different terms to pattern: Example for: a = g = 2 w = 32 a g w

19. BMI 1 FS05 – Class 8, “US Instrumentation” Slide 19 Ultrasound Imaging

20. BMI 1 FS05 – Class 8 “US Instrumentation” Slide 20 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)

21. BMI 1 FS05 – Class 8 “US Instrumentation” Slide 21 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 Time-Gain Compensation

22. BMI 1 FS05 – Class 8 “US Instrumentation” Slide 22 M-mode (“motion mode”) Recording of variation in A scan over time Cardiac imaging: wall thickness, valve function see Fig. 3.17

23. BMI 1 FS05 – Class 8 “US Instrumentation” Slide 23 M-mode clinical example B-Mode / M-Mode image of mitral valve

24. BMI 1 FS05 – Class 8 “US Instrumentation” Slide 24 B-mode (“brightness mode”) Lateral scan across tissue surface Grayscale representation of echo amplitude

25. BMI 1 FS05 – Class 8 “US Instrumentation” Slide 25 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

26. BMI 1 FS05 – Class 8 “US Instrumentation” Slide 26 Linear switched

27. BMI 1 FS05 – Class 8 “US Instrumentation” Slide 27 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

28. BMI 1 FS05 – Class 8 “US Instrumentation” Slide 28 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]

29. BMI 1 FS05 – Class 8 “US Instrumentation” Slide 29 CW Doppler example

30. BMI 1 FS05 – Class 8 “US Instrumentation” Slide 30 Pulsed Doppler – single volume Time gate (range gate) is used to define depth location Sample volume ~mm 2 Center or carrier frequency 2-10 Mhz Pulse repetition rate 1/T~ kHz Red Box? Demodulation of signal, see Webb, pp.138

31. BMI 1 FS05 – Class 8 “US Instrumentation” Slide 31 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)

32. BMI 1 FS05 – Class 8 “US Instrumentation” Slide 32 Modern US instrument

33. BMI 1 FS05 – Class 8 “US Instrumentation” Slide 33 Duplex imaging example (c.w.) www.medical.philips.com

34. BMI 1 FS05 – Class 8 “US Instrumentation” Slide 34 Duplex imaging (Pulsed Doppler)

35. BMI 1 FS05 – Class 8 “US Instrumentation” Slide 35 US imaging example (4D)