1. EE 5340/7340, SMU Electrical Engineering Department, ©2004 1 Carlos E. Davila, Electrical Engineering Dept. Southern Methodist University slides can be viewed at: http:// www.seas.smu.edu/~cd/ee5340.html EE 5340/7340 Introduction to Biomedical Engineering Ultrasound Flowmetry II

2. EE 5340/7340, SMU Electrical Engineering Department, ©20042 A More Realistic Geometry u red blood cell (RBC) transmitted ultrasound reflected ultrasound tt rr ucos  r ucos  t affects first frequency shift affects second frequency shift

3. EE 5340/7340, SMU Electrical Engineering Department, ©20043 A More Realistic Geometry (cont.) (see HW problems) (1) (2) Doppler frequency shift: f d = f r - f t

4. EE 5340/7340, SMU Electrical Engineering Department, ©20044 Vector Notation transmitted ultrasound reflected ultrasound tt rr

5. EE 5340/7340, SMU Electrical Engineering Department, ©20045 Vector Notation (cont.) (1) can be written as: since: angle between these vectors > 90 o (3)

6. EE 5340/7340, SMU Electrical Engineering Department, ©20046 Vector Notation (cont.) (3) becomes: since:and we get: or:

7. EE 5340/7340, SMU Electrical Engineering Department, ©20047 Transmitter/Receiver Configurations the geometric configuration of the transmitter and receiver can greatly affect the sensitivity of the flowmeter:    d negative, sensitivity is maximized assume then Doppler shift depends primarily on angle between and

8. EE 5340/7340, SMU Electrical Engineering Department, ©20048 Transmitter/Receiver Configurations (cont.)   no sensitivity  

9. EE 5340/7340, SMU Electrical Engineering Department, ©20049 Transmitter/Receiver Configurations (cont.) tt rr  d negative, moderate sensitivity, also sensitive to transversal vessel wall motion

10. EE 5340/7340, SMU Electrical Engineering Department, ©200410 Angle of Transmitted Ultrasound hand-held flow probes are calibrated assuming  t = 45 o. if  t is too small, ultrasound undergoes too much attenuation since it has a longer path from transmitter to reflector and back to sensor. if  t is too large (close to 90 o ), sensor becomes too sensitive to transverse motion. n error due to mispositioning the sensor away from 45 o is not too severe (see HW).

11. EE 5340/7340, SMU Electrical Engineering Department, ©200411 Frequency Spectrum of Received Ultrasound f reflection from stationary objects transverse vessel wall motion forward blood flow reverse blood flow signal power f t f d > 0 f d < 0

12. EE 5340/7340, SMU Electrical Engineering Department, ©200412 Doppler Frequency Shift Varies due to: n Acoustic beam illuminates a relatively large volume of blood within the vessel n nonuniform velocity profile across section of vessel (e.g. laminar flow has lower velocity near vessel walls) acoustic beam is nonuniform, effective angles  t and  r vary over different locations n If acoustic beam is focused so as to illuminate a very small volume, then the reflector is seen for only a short period of time, this produces spreading in the frequency domain (e.g. in the limit the Fourier transform of an impulse function is a constant for all frequencies).

13. EE 5340/7340, SMU Electrical Engineering Department, ©200413 Continuous Wave (CW) Doppler Flowmeters -transmitter is excited continuously skin vessel oscillator transmitter RF amp mixer audio amp speaker zero-crossing detector LPF recorder LPF vrvr vtvt vdvd vava 2

14. EE 5340/7340, SMU Electrical Engineering Department, ©200414 CW Doppler Flowmeters (cont.) used trig identity:

15. EE 5340/7340, SMU Electrical Engineering Department, ©200415 CW Doppler Flowmeters (cont.) after LPF: note that flow direction is lost since n reverse blood flow cannot be distinguished from forward flow n for f t in the 2 - 10 MHz range, f d varies from 10 Hz to 10 kHz, corresponding to audio frequencies n audible output devices are small, hand-held devices which are easy to use. n useful for diagnosis of vascular occlusion

16. EE 5340/7340, SMU Electrical Engineering Department, ©200416 Zero-Crossing Detectors zero-crossing detector output: LPF output, crude flow signal: t t t hysteresis band v a must pass through entire hysteresis band to produce a pulse

17. EE 5340/7340, SMU Electrical Engineering Department, ©200417 Choice of Transmission Frequency n higher transmission frequencies tend to undergo better reflection (backscattering) when the reflectors are small relative the the wavelength of the ultrasound, as is the case with RBC’s, which produce Rayleigh scattering. n high frequencies are attenuated more than low frequencies. n for shallow vessels, typically use f t = 4 - 10 MHz n for deeper penetration, use f t = 2 MHz 2 conflicting criteria:

18. EE 5340/7340, SMU Electrical Engineering Department, ©200418 Bi-directional Flowmeters: Quadrature Phase Detection skin vessel oscillator transmitter RF amp mixer vrvr vtvt LPF vdvd vava 2 90 o phase shift 2 LPF v dq v aq tan -1

19. EE 5340/7340, SMU Electrical Engineering Department, ©200419 Quadrature Phase Detection (cont.) as seen before: after LPF: used trig identity:

20. EE 5340/7340, SMU Electrical Engineering Department, ©200420 Quadrature Phase Detection (cont.) tan -1 t sign of  d is retained

21. EE 5340/7340, SMU Electrical Engineering Department, ©200421 Hardware for  d Computation + _ vava 0 + _ v aq 0 MMV + _ LPF comparator difference amp arctan function not implemented in practice dd “one shot” a b c a’ d e

22. EE 5340/7340, SMU Electrical Engineering Department, ©200422 Timing Diagram: Flow Towards Transducer t t t a a’ b c d e vava v aq t t t t t

23. EE 5340/7340, SMU Electrical Engineering Department, ©200423 Timing Diagram: Flow Away From Transducer t t t a a’ b c d e vava v aq t t t t t

24. EE 5340/7340, SMU Electrical Engineering Department, ©200424 Pulsed Doppler Flowmeters n Rather than CW excitation, the transducer is excited for a short time interval, a pulse of ultrasound is transmitted. n The resulting echoes reflect the Doppler shifts occurring along different locations within the vessel. n Can use the pulse echo information to image the flow profile along a section of the vessel. Typically, transmit a 1  s pulse of 8 MHz ultrasound. n Can use a single transducer for both transmission and reception.

25. EE 5340/7340, SMU Electrical Engineering Department, ©200425 Pulsed Doppler Flowmeters (cont.) transmitted pulse: echo: near-wall reflection far-wall reflection from vessel center vtvt vrvr eight cycles of 8-MHz ultrasound 1  s t t vava v a changes much slower than v r t

26. EE 5340/7340, SMU Electrical Engineering Department, ©200426 Pulsed Doppler Flowmeters (cont.) skin vessel oscillator transmitter RF amp mixer audio amp LPF vrvr vtvt vdvd vava 2 1-2 n DEMUX n-bit counter quadrature phase detection can also be used “gating”

27. EE 5340/7340, SMU Electrical Engineering Department, ©200427 Pulsed Doppler Flowmeters (cont.) audio amp vava 1-8 DEMUX 3-bit counter v a scanned once for every transmitted pulse scan-1 scan-2scan-3 vessel center near wall far wall

28. EE 5340/7340, SMU Electrical Engineering Department, ©200428 Pulse Repetition Rate to avoid aliasing,  t must satisfy the Nyquist criterion: but 1/  t is the pulse repetition rate, this imposes constraints on the maximum range, R max :

29. EE 5340/7340, SMU Electrical Engineering Department, ©200429 Transit-Time Flowmeters vessel u  D downstream transit time: upstream transit time: invasive

30. EE 5340/7340, SMU Electrical Engineering Department, ©200430 Transit-Time Flowmeters (cont.) since  t is proportional to u fudge factors: laminar flow turbulent flow : average velocity along cross section of vessel

31. EE 5340/7340, SMU Electrical Engineering Department, ©200431 Stethoscope Pocket Doppler Flowprobe Built-in probe Approximate 8 MHz frequency Operates on a standard 9 volt battery Stethoscope fitted to a Telex earphone outlet courtesy of Park Medical

32. EE 5340/7340, SMU Electrical Engineering Department, ©200432 Pocket Doppler Flowprobe Pocket Doppler Flowprobe Built-in signal processing for displaying recordings Factory set amplitude control Plug-in probes Dimensions: height: 3.8 cam width: 7.9 cam depth: 14.6 cam Built-in speaker Auto-shutoff Approximate 8 MHz frequency Operates on a standard 9 volt battery Auxiliary earphone output courtesy of Park Medical

33. EE 5340/7340, SMU Electrical Engineering Department, ©200433 Features: Auto-shutoff Built-in speaker Auxiliary earphone output Rechargeable battery Low battery indicator Recorder output 8 MHz to 9.7MHz frequencies available Various welded-metal cases available Nondirectional courtesy of Park Medical

34. EE 5340/7340, SMU Electrical Engineering Department, ©200434 Bi-directional Flowmeter Chart recorder Two chart speeds 5 mm / sec 25 mm / sec 40 mm chart paper Large speaker Auxiliary earphone output Rechargeable battery AC coupled pneumoplethysmograph DC coupled pneumoplethysmograph AC coupled photoplethysmograph DC coupled photoplethysmograph Dual frequency Approximate 8 MHz frequency Approximate 4 MHz frequency courtesy of Park Medical

35. EE 5340/7340, SMU Electrical Engineering Department, ©200435 Some systems combine B-mode scanning and Doppler flowmetry © ATL