Session 10 Doppler Instrumentation

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Session 10 Doppler Instrumentation Powered By Docstoc
					Doppler
        Doppler Effect or Shift

  is the change in frequency (and wavelength) due to
  motion of a sound source, receiver, or reflector.
 is the difference between the received & transmitted
  frequencies measured in Hertz (Hz).
 is used to determine the velocity of the moving reflectors
  (Recall: velocity is speed & direction.)
            If the source is moving
            toward the receiver, or if the
            receiver is moving toward
            the source, or if the
            reflector is moving toward
            the source and receiver, the
            received wave has a higher
            frequency than would be
            experienced without the
    O       motion.
X
        X
A moving source
(red blood cells)
approaching a
stationary receiver
(transducer), the cycles
are compressed in front
of the source as it
moves into its own
wave.
The source motion shortens the
    wavelength ahead of it

as observed by a stationary receiver
  in front of the approaching source

    wavelength =  frequency
Conversely,
if the source motion
is away,
the received wave has
a lower frequency
&  wavelength
    Only the moving reflector is of interest for
         diagnostic Doppler ultrasound.

The amount of  or  in the frequency
depends on the:
   speed of motion
   angle between the wave propagation
    direction
    & the motion direction
   frequency of the source’s wave
Doppler equation

- relates the detected Doppler shift
 (∆F) to those factors that affect it

          ∆F = 2 x F0 x V x COS 
                    C
           ∆F = 2 x F0 x V x COS 
                     C

F0 is the central sound frequency
(aka - transmitted, emitted, incident, or
ongoing frequency) transmitted by the
transducer.

       ∆F and F0 are directly related;
   if F0 is doubled, then ∆F is doubled
  ∆F = 2 x F0 x V x COS 
                 C

V is the velocity of the moving
  reflector (red blood cells)

       ∆F and V are directly related;
   if V is doubled, then ∆F is doubled
      ∆F = 2 x F0 x V x COS 
                     C

COS  is the cosine of the angle
between the direction of blood flow & the
axis of the beam (the Doppler angle).

        If the cosine is doubled,
       the Doppler shift doubles.
             sin & cos (in brief)

 A is the starting point (in our
  case, the blood cell), the
  side opposite of angle A is
  called ‘a’
 C is the right angle; the
                                    c
  hypotenuse (the side of the           a c
                                                a
  triangle opposite of angle C)
  is called ‘c’                             c
                                                    a
 B is the remaining angle; the
  side of the triangle opposite A       C       C   C
  angle B is called ‘b’
                                    sin A = a/c
As  A , side ‘a’ also ;
eventually ‘a’ will equal ‘c’
                                         a
As  A approaches 90°,
ratio a/c becomes closer to 1                 a
                                     c   c
When  A equals 90°,
a/c = 1                                           a
                                         c

     sin A = 1                 A
    when  A = 90°
    or the sin 90° = 1
                                            cos A = b/c
  As  A , side ‘b’ ;
  eventually side ‘a’ = side ‘c’.

  As  A approaches 90°, the
  ratio b/c becomes closer to
  0.                                    c   a c
  When  A equals 90°,                              a
  b/c = 0,                                      c
                                                          a
 cos A = 0 when  A = 90°          A       b       b     b
    (or the cos 90° = 0)
∆F = 2 x F0 x V x COS 
     C

 Cosine values range from 0 to 1.
 It is an inverse relationship.

      As the angle , the cosine .

Example
             cosine of 0° = 1;
             cosine of 90° = 0
       ∆F = 2 x F0 x V x COS 
                      C

C is the speed of sound in the medium.
The speed of sound is approximately
1540 m/s or 1.54 mm/s.

If the speed of sound in the medium ;
the value of the detected Doppler shift .
       C is inversely related to ∆F
     Detection of Doppler Shift

Doppler ultrasound systems do not
use the Doppler equation to calculate
the Doppler shift.

The ultrasound system compares the
frequency of the received echo (fr) to the
frequency of the transmitted pulse (ft).
Doppler Frequency (fd) =
Reflected Frequency (fr) - Transmitted Frequency (ft)


  When:
   received frequency = transmitted frequency
     there is no Doppler shift
   received frequency > transmitted frequency
    Doppler shift is positive
   received frequency  transmitted frequency
    Doppler shift is negative
         Factors influencing the magnitude of
         the Doppler shift frequency
1.   The Doppler shift frequency occurs in the
     audible range.
Example: A probe emits 5.0 MHz ultrasound beam
   striking the red blood cells traveling toward the
   transducer. The unit detects the reflected
   frequency to be 5.007 MHz. The transmitted &
   received frequencies are in the ultrasonic
   range, but the Doppler shift frequency is .007
   MHz (7,000 Hz) (audible range).
Typical Doppler shifts range: –10 kHz to +10 kHz
     Factors influencing Doppler shift
     frequency

2.   As the angle between the transducer & flow
      (COS ), the Doppler shift 
3.   If the RBCs are moving toward the
     transducer, the received frequency is higher
     than the transmitted frequency (shown as a
     positive Doppler shift).
4.   If the RBCs are moving away from the
     transducer, the received frequency is lower
     than the transmitted frequency (shown as a
     negative Doppler shift).
         Factors influencing Doppler shift
         frequency
5.   If there is no motion of the RBCs, the reflected
     frequency = transmitted frequency; the
     Doppler shift is zero.
6.   The faster the flow velocity, the higher the
     Doppler shift. Flow speed & Doppler shift,
      with vessel diameter2.
7.   If there is a  in concentration of RBCs or if you
     are performing the Doppler exam of the vessel
     off to one side, there may be a  in intensity
     (hard to hear & see on spectral display).
     Factors influencing Doppler shift
     frequency

8.  Since the RBCs are much smaller than the
    wavelength of the sound beam, Rayleigh
    scattering occurs.
9.  the transducer frequency will  scattering
    &  the Doppler shift
10. Although higher frequencies produce more
    scatter, they are attenuated more rapidly.
       Factors influencing Doppler shift
       frequency

11. Since these reflectors are very weak in
    intensity, lower frequency transducers may be
    needed to obtain Doppler information at
    deeper depths.
12. Modern transducer technology takes
    advantage of the wide bandwidth emitted by
    the transducer by allowing imaging at high
    frequencies and then downshifting into lower
    frequencies to acquire the Doppler information.
Spectral Doppler Instrumentation
         Spectral Doppler Instrumentation

                 Two Different Types
1.   Continuous wave (CW) Doppler - 2 crystals
     located at slight angles to each other in the
     probe, one is for transmitting & the other for
     receiving
2.   Pulsed wave (PW) Doppler - a single crystal
     element or an array transmits & receives the
     Doppler information.
    Continuous wave (CW) Doppler

  2 crystals located at slight angles to
  each other in the probe, one for
  transmitting & the other for receiving
 The CW transducer is not damped
 The transmitting crystal is transmitting
  sound 100% of the time
   Continuous wave (CW) Doppler


Transmitted beam zone & receiving zone
overlap because the beam is directional
A moving structure in this region
of overlap (sample volume) will
create a Doppler signal.

A CW instrument detects flow that
occurs anywhere within the
intersection of the beams.
With this large sample volume, CW Doppler
systems can give complicated and
confusing presentations if two or more
different motions or flows are included in
the sample volume, like when 2 vessels are
being scanned at the same time.
            No anatomical image is displayed,
            just a waveform on a monitor




Only bi-directional instruments can distinguish between
positive or negative Doppler shifts (forward & reverse flow)
Components of a CW Doppler system
    1. Oscillator
    2. Doppler detector
    3. Phase quadrature
       detector
    4. Spectrum analyzer
    5. Spectral display device
Oscillator

 The oscillator produces the continuously
 alternating voltage with a 2-10-MHz
 frequency applied to the source
 transducer element.

 The oscillator that is set to equal the
 operating frequency of the transducer
 determines the ultrasound frequency.
The transducer assembly has a separate
receiving transducer element that produces
voltages with frequencies equal to the
frequencies of the returning echoes.

The reflected scatterer motion ultrasound
and the source transducer will have
different frequencies.
Doppler detector
 The Doppler detector detects the
 difference (Doppler shift) and relays
 it to the audio speaker at this
 frequency.

  Doppler shifts are typically 1/1000
 of the operating frequency putting
 them in the audible range.
      Doppler detector
1. Amplifies the echo voltages it receives,
   detects the Doppler shift information, &
   determines motion direction from the
   sign of the Doppler shift.
2. The difference is zero for echoes
   returning from stationary structures
Phase quadrature detector
 The phase quadrature detector
 determines the direction and
 divides the Doppler shift voltages
 into separates forward and reverse
 channels.
 The outputs are sent to separate
 loudspeakers so that forward &
 reverse Doppler shifts can be heard
 separately
Spectrum analyzer
 Doppler shifts are also sent through
 a spectrum analyzer to a spectral
 display to show positive & negative
 Doppler shifts above and below the
 display baseline, which represents
 zero Doppler shift for observation
 and evaluation.
Spectral display device
 The spectral display device is a cathode-
 ray tube where the Doppler shifts (that
 continually change over the cardiac
 cycle) are displayed as a function of time
 with appropriate real-time frequency-
 spectrum processing.
 These displays provide quantitative data
 for evaluating Doppler shifted echoes.
The displayed Doppler information is stored
in digital memory before display so that it can
be frozen and backed up over the last few
seconds of information prior to freezing.

To convert a display correctly from Doppler
shift versus time to flow speed versus time,
the Doppler angle must be accurately
incorporated into the calculation process.
      Wall filter (wall-thump filter)

  - is an electronic filter that allows the
  sonographer to adjust what level of
  frequencies are to be processed, thus
  eliminating high-intensity, low frequency
  Doppler shift echoes (clutter).
 Frequently used in cardiac sonography to
  eliminate (reject) frequencies caused by heart
  or vessel wall and cardiac valve motion with
  pulsatile flow to an acceptable value.
     Wall filter (wall-thump filter)
-rejects the strong tissue structure echoes that
would overwhelm the weaker echoes from the
blood. These echoes have low Doppler shift
frequencies because the structures do not
move as fast as the blood does.
The upper limit of the filter is adjustable over a
range of about 25 to 3200 Hz.
If not properly used, the filter can erroneously
alter assumptions about the diastolic flow and
distal flow resistance.
    Other Types of CW

1. Hand-held, nondirectional devices are
   simpler, yielding only an audible output.
2. Analog zero-crossing detector devices
  Analog zero-crossing
     detector devices
- provide an instantaneous average
Doppler shift that varies over cardiac
cycle. This device counts how often the
Doppler shift voltage changes from
negative to positive per second.

          The higher the count,
        the higher the frequency.
The count is represented on the vertical
axis & the horizontal axis represents time
on a 2-d graph, on a strip-chart recorder
that produces a hard copy.

 Some systems have spectral displays.
   CW Advantages
1. ability to measure very high velocities
2. small probe sizes
3. ability to use high frequencies
4. large sample volume is helpful when
   searching for a Doppler maximum
   associated with a vascular or valvular
   stenosis
CW Disadvantages
1. lack of imaging
2. all Doppler shifts will be displayed
   so that specific vessels cannot be
   interrogated by themselves; this is
   a range ambiguity
Pulsed wave (PW) Doppler

 - uses a single crystal element or
 an array to transmit & receive
 Doppler information

     The required Doppler frequency shift
      detection requires longer ultrasound
      pulses than that used for imaging
Pulsed wave (PW) Doppler

1. Sample volume is placed in the vessel
   where the Doppler information is
   wanted
2. The operator adjusts the location &
   length of the range gate (sample gate,
   sample volume, range gate) to isolate
   the signal from the desired depth.
 The width of the sample volume is
   equal to the beam width
Pulsed wave (PW) Doppler
Sample Volume Placement

 Increasing the gate size  the # of
  Doppler signals picked up; longer gate
  lengths are used when searching for the
  desired vessel
  the gate length results in a narrow
  range of velocities and flow location &
  are used for spectral analysis and
  evaluation.
Shorter gate length improves
the quality of the spectral display
 Pulsed wave (PW) Doppler

 Motion information obtained from a
  specific depth is called range gating
 Allows 2D gray scale imaging & Doppler
  information of the vessel, known as Duplex
  (anatomic imaging & flow measurement)
 Flow direction for an individual anatomical
  region is determined by a positive or
  negative Doppler shift
Components of a PW Doppler system

1. Pulser
2. Doppler detector
3. Phase quadrature detector
4. Spectrum analyzer
5. Spectral display device
     Pulser
- pulses 5-30 cycles of voltage that drive
  the transducer.
- This is necessary to determine the
  Doppler shifts of returning echoes
  accurately.

Recall: imaging pulses are 2-3 cycles long
     Doppler detector

- processes the echo voltages from the
  transducer coming from reflectors at a given
  depth based on their arrival time (13 s/cm
  rule), amplifies and demodulates them, &
  compares their frequency to the pulser
  frequency (Doppler shifts are determined).
  Echoes arrive from the sample volume depth
  in pulsed form at a rate = PRF
PW vs. CW Doppler

 PW Doppler instrument obtains sample
 of Doppler shifts as opposed to the CW
 instrument that detects the complete
 Doppler shift.
 The PW instrument is a sampling system
 with each pulse yielding a sample of the
 Doppler shift signal.
PW Doppler Advantages

 Range Resolution - only information
  detecting motion or flow from this
  selected depth gate will be processed
  allowing the sonographer to know
  exactly from which vessel the Doppler
  information is derived
 Permits duplex imaging
       PW Doppler Disadvantages

 cannot accurately measure high velocities
 due to an artifact called aliasing
   Aliasing appears as the top of the spectrum
    wrapping around into the opposite half of
    the baseline
   Aliasing is caused by high velocities of the
    reflectors being too fast for the sampling
    rate (PRF)
  Nyquist Limit

 Aliasing will occur when the Doppler
 shift exceeds the Nyquist limit, which is
 1/2 the PRF

Recall: the depth of the sample volume
 controls the PRF
Adjusting for Aliasing

 Aliasing may be corrected by
  properly adjusting the spectral
  baseline and/or the spectral scale
 If aliasing still occurs using a lower
  frequency and/or shallower depth
  may correct the problem
 If aliasing still persists a CW
  transducer should be used
      Duplex Instruments

Duplex Instruments are Pulsed-Doppler
instruments combined with gray-scale
sonography. They image anatomic structures
as well as analyze motion & flow at a known
point in the anatomic field.

Imaging allows intelligent positioning of the gate
& angle correction in a pulsed-Doppler system
        Points to understand
 Understand what creates a Doppler Shift
 Recognize what factors influence the Doppler
  shift
 Know the cos of 0° & 90°
 Explain the differences & similarities between
  CW Doppler & Pulsed Doppler
 Define the Nyquist limit
 Define Aliasing & know how to compensate for it

				
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posted:11/17/2012
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