IN THE NAME OF GOD

1. IN THE NAME OF GOD Doppler Physics and Instrumentation

2. Introduction Doppler ultrasound can detect the presence, direction, velocity and properties of blood flow in vessels. Several types of instrumentation are available, but the principles are the same

3. Doppler effect • The phenomenon is which an apparent change in the frequency of sound is observed ifthere is relative motion between the source of the sound and the receiver.

4. Doppler shift • Observedfrequency is influenced bymovementof the sourceor the receivereither toward or away from the other. • More or less pressure waves per unit of time strike the receiverand cause a higher or lower pitch to be heard.

5. Cont. • The difference between transmittedand observedfrequency orDoppler shift depends on how rapidly the sound source‚ receiver or both are . • By the velocity of sound in the medium . • Transmitted frequency.

6. Doppler shift equation • fD2=vfcosΦ/c. C=velocity of sound in tissue. • V=velocity of the interface • f=frequency • Φ=angle between incident beam and direction of target motion

7. Minimum shift at 90degreesis0 Cont.

8. Clinical consideration • Doppler shift is in the audible frequency range (typically between 200 to 15oooHz) • As the diameters of blood cells are much less than the wavelength, the Rayleigh scattering occurs • The intensity of the scattered is proportional to f4 • The speckles on the monitor image are the scattered sound wave • The intensity of scattered sound is proportional to the number of RBCs and thus indicates the quantity of blood in the sample volume • The optimum frequency (f0) in megahertz for Doppler examination is given by f0=90/R(mm) • R is the soft tissue distance to the region of interest in mm

9. Continuous-wave Doppler • Use 2 crystals in the transducer. • One to transmit the sound waves of constant frequencycontinuously. • The other to receive the reflected echoes continuously. • The sampling region is positioned by geometric arrangement of the crystals.

10. Beat frequency • The method used to measure a Dopplershift is based on the principle of wave interference. • A continuous transmitted wave offrequency . • B continuous reflected wave of constant frequency • C addition of the transmitted and received sound waves.

11. Instrumentation • An oscillator to emit a continuous single frequency. • The returning echo incident on the receiving crystal is converted to a radio frequency signal. • An RF amplifier increases the RF signal level. • This wave is demodulated to remove all but the beat frequency.

12. Pulsed-wave( PW) Doppler units use the echo- ranging principle . To provide quantitative depth information of the Doppler shift . Pulsed- wave Doppler

13. Instrument Design • In PW Doppler the basic CW circuit is modified to accommodate gating and to collect successive processed echoes in a sample – and – hold circuit . • The gate used for selecting the signal from desired depth • Multi gating is useful to acquire data from different depth at the same time (usually 6-32) • The clock functions as a master synchronizer for timing purposes ( PRF and gating ). • A single gate limits the interrogation to one depth along the line of sight .

14. Signal Processing • The received echo must be evaluated to determine whether the reflector is moving . • The echoes from different reflectors, one moving and the other stationary , are received after different time intervals • For stationary target the reference signal has the same frequency and phase as the transmitted pulse . • The dotted lines place the detected echoes on the same time scale as the reference signal.

15. Velocity Detection Limit • At a minimum, two pulses are required per beat cycle to define the beat frequency unambiguously. • This creates a very important limitation in PW Doppler scanning . • The maximum Doppler shift , fD(max), that can be detected is related to the PRF : • fD=PRF/2

16. A high PRF limits the depth that can be sampled , because a certain time is required to collect the echoes arising from that depth before the next pulse is sent out • Maximum reflector velocity (V max ) is described by a single equation : Vmax=c2/8fR cosΦ • To remove alising artifact: • Reduce PRF • Reject reverse flow • Increase detection angle • Change to CW

17. Pulse width bandwidth • PW include a spectrum of frequencies • It is necessary to have a burst with enough cycles • Increasing number of cycles results in lowering spatial resolution • Higher number of frequencies results in less SNR • It has higher attenuation in higher frequencies • Scattering depends of frequency

18. DIRECTIONAL METHODS • The received echo shifted in frequency above or below the reference signal depending on whether the motion is toward or away from the transducer . • Three processing methods : • Single–Sideband Detector • Heterodyne Detector • Quadrature Phase Detector

19. Single – Sideband Detector • The received signal consists of reflected echoes from both stationary and moving structures. • The signal from the radiofrequency amplifier is split into two components and then filtered. • One filter is designed to pass all frequencies above the reference signal and the other all frequencies below the reference signal. • The output of each filter is mixed with the reference signal and then filtered by a sideband filter in which all components except the shifted signals are removed. • This results in two separate signals corresponding to the forward and reverse signals corresponding to the forward and reverse motions.

20. Heterodyne Detector • Offset signal combines with the reference signal before being added the received to obtain the Doppler shift. • Displaces the Doppler shift to a new frequency range . • Forward and reverse differentiated by the magnitude of the Doppler shift compared with the offset frequency. • fD=fR-(f-5kHz) • fR(frequency of received signal) f(transmitted frequency)

21. Quadrature Phase Detector • Used directional technique. • Amplifier is split into two components. • Mixed with the reference signal. • After filtering the output from each channel contains a mixture of forward and reverse flow signals. • In time domain processing the presence of flow in one direction cause each channel to exhibit the same voltage variation as a function of time .

22. Duplex scanners • Combine real time imaging with CW or PW Doppler hence can: • Simultaneous display of B-mode and Doppler • Ability to define Doppler angle • Measurement of flow velocity • Real time flow information