Fadah-doppler Shift Uss

7/31/2019 Fadah-doppler Shift Uss

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QUESTIONS

Explain what you understand by Doppler shift.

Of what interest are the Doppler methods inclinical ultrasonic.

Sketch a diagram showing the basic element of the continuous ultrasonic Doppler shift detector .


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Write short notes on:

Pulsed wave system Continuous wave system. Directionally sensitive system. Range gated system. Two dimensional scanning .

ANSWERS The Doppler shift is the difference between the

incident frequency and reflected frequency. Ultrasound wave is generated in a continuous

manner from the transmitting transducer at aparticular frequency known as the incidentfrequency.


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Sound reflected to the receiver transducer from

stationary sources is of equal frequency to theincident beam but reflected sound from movingstructures is altered or shifted according to thevelocity and direction of the moving structure.

Returning echo originate from the cellularelements of the blood, particularly the redblood cells.

The change of frequency is proportional to thevelocity of the interface. The higher thetransducer frequency or the faster the interfacemoves, the greater the doppler frequency shift


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It is greatest when the beam strikes a vessel at

small angle. The smaller the angle between thesonic beam and the flow direction, the greaterthe vector of motion towards transducer andthe greater the Doppler shift.

Motion at right angle to the transducer showsno Doppler effect.

The difference, the Doppler shift usually falls

within the frequency range detectable byhuman ears and after amplification, thisDoppler shift is the audio signal.

The Doppler shift equation is as follows


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DOPPLER EQUATION 1 (Dopplershift)

cv f

f D cos2 0

f d is Doppler frequency (shift) 0 = frequency of source v = speed of reflector

c = speed of sound in medium


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A great deal can be learnt about the status of the circulatory system with a Doppler device.

It can be used to detect blood flow or itsabsence in both arteries and veins.

It can be used to identify vascular constrictionwith their associated edgy currents and venturi jets.

Useful in detecting fetal heart motion earlierthan any other method.`


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PULSED WAVE SYSTEM

Pulsed Doppler is similar to a conventional pulse echoinstrument in that bursts of uss are emitted repetitively at aprecisely controlled rate into the tissues. A new pulse is nottransmitted until echoes from the previous pulse has beendetected.

The depth at which a returning signal originated can bedetermined by knowing the time of transmission of thesignal and the time of its return i.e. the time of flight to aninterface and its subsequent return. Short burst of ussabout 0.5 to 1.0 micro secs can yield good axial resolutionwith separation of interfaces to within about 1mm.

Pulsed Doppler instrument provide both velocity andposition information simultaneously.


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Determining the Doppler shift of an echo at aspecific depth requires that the ultrasonic

transducer produce very short duration burst at aprecise rate of repetition. After transmission of thisshort burst of uss, the receiving transducer may beturned on(gating on) for a short period at a specifictime therefore, only signals arriving from a specifictissue depth are available for detection.

The same transducer is usually used for transmitting and receiving . The depth from whichthe signal originates is controlled by the length of time after pulse transmission before thetransducer is allowed to receive returning signals(the time at which the gate is turned on)


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The axial length over which the signal originates

is determined by the length of time the gate isturned on. A high Pulse Repetition Frequency limits the depth at which a pulsed Dopplertransducer can receive echoes. PRF alsodetermines the maximum Doppler shiftfrequency that can be detected.

With short pulses used in imaging it is notpossible to get accurate Doppler flowinformation.


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DIRECTIONALLY SENSITIVE SYSTEM

COLOUR FLOW IMAGING This is a directionally sensitive systemwhich provides a 2D visual display of moving blood in thevasculature superimposed upon the conventional grey scale image.Velocity and direction are determined for multiple position within asubarea of the image and then color encoded. Shades of red for

blood moving towards the transducer and shades of blue for blood moving away from the transducer . Turbulence i.e. variationin flow direction show green or yellow

Two dimensional color flow system does not use the full Dopplershift information because of lack of time and or lack of parallelchannels necessary for real time imaging.

The depth of each color depends on the velocity of flow stationarytissue appear grey. Compared with normal B scan imaging, theDoppler pulse is longer, being a compromise between accuratedepth information requiring a short pulse and accurate velocityinformation requiring a longer pulse.


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CONTINUOUS WAVE SYSTEM

This is the simplest and the least expensivedevice for measuring blood velocity. Twotransducer are required , with onetransmitting the incident uss and the otherdetecting the resultant continuous echoesboth are contained in a single head and are

slightly angled. They are chosen as regardsoperating frequency in the range 2 to 10MHzaccording to the depth of the vessel.


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The transmitter is energized continuously with aradiofrequency alternating voltage of frequency f. Thereceiver R listens continuously to the back scattered wavesof frequency f coming from the cross over area or sensitivevolume. A Doppler signal having the difference frequency(f- f) is extracted electronically.

Frequency analyzer is used to produce a spectrum of Doppler frequencies that can be displayed on the screen.

The Doppler signal can be heard through a loud speaker orhead phone as a rushing sound. The higher the pitch thegreater the velocity, the harsher the sound the greater theturbulence.

With a continuous wave it is not possible to locate themoving reflector or to distinguish between the flow in twooverlapping vessels at different depth in the beam.


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RANGE GATED SYSTEM This is also known as Duplex scanning . Doppler measurement is

combined with a real time B scan image. Most of the time is spentin the Doppler mode, the B scan image is updated once a second.The imaging frequency is chosen to optimize resolution.

A real time B mode image is produced , and with its help a line of

sight is chosen for the Doppler beam. Along its cursors are set toidentify the sampling volume, which is positioned over the vessel inwhich blood flow is to be measured.

If the vessel is clearly defined, the angle theta can be read off toallow measured frequency shift to be converted into blood flowvelocities. The diameter of the lumen of the vessel can also beestimated to allow volume flow rate to be calculated.


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The interval between pulses must be long

enough for the successive Doppler signals notto overlap. A high PRF is chosen for superficial vessels and a lower one for deeper vessels .AS the range setting is increased, the PRF isreduced and the TGC automatically increased.

The Doppler signals can be analyzed anddisplayed as a time velocity spectrum orsonogram.


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TWO DIMENSIONAL SCANING

This is achieved by sweeping a pulsed ultrasound beamover the volume of interest and displaying echo signalsusing the brightness modulated dot on a displayedscreen. In general, the brightness of the dot is

proportional to the echo signal amplitude .Echo position Is based on the delay time between the

pulse initiation and the reception of the echo, usingthe speed of sound in soft tissue(1540m/sec). The 2Dimage is progressively built up or continuously updatedas the beam is swept through the object.

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Fadah-doppler Shift Uss