Chapter 19 Flashcards
Doppler shift is also called…
Doppler frequency
The Doppler principle is used to measure?
blood cell velocities in circulation
The frequency of sound changes when
the sound source and the receiver move closer together or farther apart
The frequency does NOT change when
the distance between the sound source and the receiver remains constant
The process of extracting the low Doppler frequency from the transducer’s carrier frequency is
demodulation
Doppler shifts are created when
transmitted sound waves strike moving red blood cells
When blood cells move toward the transducer, the Doppler shift is
positive
the reflected frequency is higher than the transmitted frequency
When blood cells move away from the transducer, the Doppler shift is
negative
the reflected frequency is lower than the transmitted frequency
Doppler frequencies indicate what?
velocity NOT speed
Speed is
magnitude indicates the distance that a red blood cell moves in 1 second
units: cm/second or any distance/time
Velocity is
magnitude and direction
Example of speed and velocity
A car travels from New York to Florida at 50mph
speed: 50 mph (speed is magnitude)
velocity: 50 mph to the south (magnitude and direction)
The Doppler equation
Doppler shift = 2 X velocity of blood X transducer frequency X cos/ propagation speed
What is the relationship between the velocity of blood and Doppler shift?
Directly related
the faster the velocity, the greater the Doppler frequency
Why is there a 2 in the Doppler equation?
the 2 represents the fact that there are actually two Doppler shifts during a clinical ultrasound exam
1st: occurs when the sound wave from the transducer strikes moving blood cells
2nd: reflections return to the transducer
If the Doppler shift is measured with units of frequency, how do ultrasound systems report blood velocity?
modern ultrasound actually measure the frequency difference (in Hz) between the received and transmitted sound waves
The X-axis of a Doppler spectrum represents what?
time
The Y-axis of a Doppler spectrum represents what?
velocity
What is the relationship between the transmitted frequency and the Doppler shift?
directly related
if the transducer’s frequency is doubled, the measured Doppler shift will also be doubled
A Doppler study is done with a 2 MHz transducer, and the Doppler shift is 3 kHz. If the identical study is done with a 4 MHz transducer, the Doppler shift will be?
6 kHz
Doppler shift measured in Hz
directly related to velocity
directly related to transducer frequency
In order to measure the entire velocity the blood cells must be
parallel to the sound beam
If the sound beam and flow direction are not parallel, what determines how much of the velocity is measured?
depends on the cosine of the angle between the sound beam and the direction of motion
The most common clinical Doppler devices are
bidirectional
which distinguish the direction of flow toward or away from the transducer
Flow toward the transducer is
displayed above the baseline
Flow away from the transducer is
displayed below the baseline
Phase quadrature or quadrature detection
is commonly used signal processing technique for bidirectional Doppler
Continuous wave Doppler has how many crystals in the transducer?
2
one crystal transmits the sound wave and the other receives the signal
Advantage of continuous wave Doppler
ability to accurately measure very high velocities
Disadvantage of continuous wave Doppler
exact location of moving blood cells can’t be determined
signals from all blood cells in the region of overlap between the transmit and receive beams
lack of TGC
The limitation of the continuous wave Doppler is called
range ambiguity
Duplex imaging is
when simultaneous anatomic imaging and Doppler is used at the same time
Dedicated continuous wave transducer do not use what?
backing material
Not using backing material in a dedicated continuous wave transducer results in
undampened transmitted signal
narrow bandwidth
high quality factor (Q factor)
higher sensitivity
Dedicated continuous wave transducers have
increased sensitivity
the detect low amplitude reflections and small Doppler shifts
