1 / 35

Biomedical Imaging I

Biomedical Imaging I. Class 8 – Ultrasound Imaging II: Instrumentation and Applications 11/02/05. Generation and Detection of Ultrasound. Piezoelectric effect I.

gwyn
Download Presentation

Biomedical Imaging I

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Biomedical Imaging I Class 8 – Ultrasound Imaging II: Instrumentation and Applications 11/02/05

  2. Generation and Detection of Ultrasound

  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 (SiO2)

  4. Piezoelectric effect II “a phase” (not p.e. active) “b phase” (p.e. active) Polycrystalline (e.g., ferroelectric, PZT) Polymers (PVDF)

  5. Dflo-Q Amplitude A (fres) = 0 dB - 3 dB Dfhi-Q Frequency Transducer Q-factor • Disc of piezoelectric material (usually PZT) shows mechanical resonance frequencies fres Resonance curve (Q-factor) • High Q: strong resonance(narrow curve) • Low Q: strongly damped, weak resonance (broad curve) • Tradeoff of high Q: • Efficient at fres (high signal-to-noise ratio) • Pulse distortion

  6. Transducer resonances • Transducer (disc) has mechanical resonances at frequencies • Lowest (fundamental) resonance frequency (standing wave): (c: speed of sound, : wavelength) Transducer ends have 180 phase difference (=  = /2) time Crystal Length of crystal, Lc

  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. Transducer–tissue mismatch • Impedance mismatch causes reflection, inefficient coupling of acoustical energy from transducer into tissue: ZT 30 MRaylZL 1.5 MRayl  It/Ii = 0.18 • Solution: Matching layer(s) • increases coupling efficiency • damps crystal oscillations, increases bandwidth (reduces efficiency) ZL ZT Load (tissue) Transducer Ii It Ir

  9. Matching layers ZL • A layer between transducer and tissue with ZT>Zl>ZLcreates stepwise transition • Ideally, 100 % coupling efficiency across a matching layer is possible because of destructive interference of back reflections if • layer thickness =/4 • Zl chosen so that Ir,1=Ir,2 : • Problems: Finding material with exact Zl value (~6.7 MRayl) • Dual-layer: Zl ZT MatchingLayer Load (Target) Transducer It Ii Ir,1 It,L Ir,l Ir,2  = /4  = /2

  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. Axial beam profile • Piston source: Oscillations of axial pressure in near-field (e.g. z0= (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. 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. Axial and lateral resolution • Axial resolution = 0.5t c, determined by spatial pulse length (t = 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. Focusing of ultrasound • Increased spatial resolution at specific depth • Self-focusing radiator or acoustic lens

  15. Transducer arrays • Linear sequential array  lateral scan • Linear phased array for beam steering, focusing

  16. 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

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

  18. Radiation pattern • Contributions of different terms to pattern: Example for: a = l g = 2 l w = 32 l w a g

  19. Ultrasound Imaging

  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. 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) Time-Gain Compensation Logarithmic compression of signals

  22. M-mode (“motion mode”) • Recording of variation in A scan over time • Cardiac imaging: wall thickness, valve function see Fig. 3.17

  23. M-mode clinical example • B-Mode / M-Mode image of mitral valve

  24. B-mode (“brightness mode”) • Lateral scan across tissue surface • Grayscale representation of echo amplitude

  25. 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

  26. Linear switched

  27. 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

  28. 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

  29. CW Doppler example

  30. Pulsed Doppler – single volume • Time gate (range gate) is used to define depth location • Sample volume ~mm2 • Center or carrier frequency 2-10 Mhz • Pulse repetition rate 1/T~ kHz • Red Box? • Demodulation of signal, see Webb, pp.138

  31. 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)

  32. Modern US instrument

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

  34. Duplex imaging (Pulsed Doppler)

  35. US imaging example (4D)

More Related