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Doppler

Chapter 19 Edelman

QuestionAnswer
The change in frequency as an object approaches or departs is called what? Doppler shift or Doppler frequency
The process of extracting the low Doppler frequency from the transducer's carrier frequency is called what? Demodulation
When the Doppler shift is positive, blood is moving toward or away from the transducer? Toward
To find the Doppler shift, subtract what frequency from what other frequency? Reflected frequency - transmitted frequency
When the reflected frequency is higher than the transmitted frequency, the Doppler shift is positive or negative? Positive
Doppler frequencies indicate speed or velocity? Velocity
Speed is purely a what? Magnitude
Velocity is defined by what and what? Magnitude and a direction
Doppler shift is directly related to what three things? Velocity of blood, transducer frequency and the cosine of the angle.
An ultrasound machine measures the frequency difference between sound waves, which is measured in Hz, but the computer puts the Hz into the Doppler equation to calculate and display what? The velocity of blood
A 100% accurate measurement of the velocity of blood is measured when blood is moving parallel or perpendicular to the sound beam? Parallel
When an angle exists between the direction of flow and the sound beam, the measured velocity is what? Less than the true velocity
Can Doppler shifts and velocities be measured when the sound wave is perpendicular to the blood flow? No, the measured velocity will be 0
What is the cosine of 60 degrees? 0.5
How many crystals does continuous wave Doppler require? Two
What is the greatest advantage of CW Doppler? It's ability to accurately measure very high velocities.
What is the biggest disadvantage of CW Doppler? Range ambiguity, or it's inability to determine the exact location of the moving blood cells.
Simultaneous anatomic imaging and Doppler is called what? Duplex imaging
True or false? Dedicated CW transducers do not contain backing material. True, which makes them have a narrow band width, a high quality factor and a higher sensitivity.
How many crystals are needed in a pulsed wave Doppler? Only one
What is the greatest advantage of PW Doppler? Range resolution, or the ability to select the exact location where velocities are measured.
The small marker used to select the area for the pulsed wave to measure is called what? The sample volume or gate
What does a pulsed wave measure? Time of flight for a sound pulse to travel to and from the gate.
What is the biggest disadvantage of PW Doppler? Aliasing, or inaccurate measurement of high velocity signals. They also have low quality factor, lower sensitivity and wide bandwidth pulses.
True or false? Both imaging and pulsed Doppler are created from pulsed ultrasound and can be performed with a single crystal transducer. True
Studies that simultaneously perform imaging and pulsed Doppler are called what? Duplex examinations
When a very high velocity display reaches the top of the spectral display and then wraps around the bottom of the display, the flow at the very top of the display is called _________ and the spectrum at the bottome is called ____________. Nyquist limit and aliasing
Aliasing occurs only in what type of Doppler? Pulsed wave
Aliasing occurs when what is too low compared to the measured blood velocities? Doppler sampling rate
The Nyquist limit equals what? One half of the PRF or pulse repetition frequency
The definition of the Nyquist limit The highest Doppler frequency or velocity that can be measured without the appearance of aliasing.
Two ways to avoid aliasing are: Raise the Nyquist limit or reduce the Doppler shift
True or false? With pulsed wave, velocities are sampled many times per second. True, the sampling rate of pulsed Doppler is the system's pulse repetition frequency.
When the sample volume is deep, the PRF is _______ and the Nyquist limit is _______, This creates aliasing. Low and low
When the sample volume is shallow, the PRF is ______ and the Nyquist limit is ______. No aliasing occurs. High and high
True or false: Higher frequency transducers create more aliasing? True
Created by: gillybruursema
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