121-144 Flashcards
Describe the relationship between the frequency of an emitted ultrasound wave and the Doppler shift that is produced when the sound beam reflects off of a moving red blood cell?
The higher frequency transducer will produce a larger Doppler shift than the lower frequency transducer.
Ex. Assume that a Doppler study is performed with a 2-MHz transducer, resulting in a 1,500Hz Doppler shift. If the study is then performed with a 4 MHz transducer, the resulting Doppler shift will be 3,000 Hz.
Two types of Doppler systems used in the clinical setting are:
Pulsed and continuous-wave
What is the primary advantage of pulsed Doppler?
Range resolution (aka range specificity)
What is the primary advantage of continuous-wave Doppler?
No maximum velocity that can be measured, thus, very high velocities can be accurately measured.
What is the primary DISADVANTAGE of pulsed Doppler?
It’s inability to accurately measure high velocities.
This results in aliasing because the high velocities show as blood flowing in the opposite direction.
What is the primary DISADVANTAGE of continuous-wave Doppler?
Range ambiguity.
It is not possible to obtain information about red blood cell velocity at a specific depth because continuous wave Doppler processes Doppler shift at all depths. So there is no range resolution with continuous-wave Doppler
Assume that a pulsed Doppler system produces 6,000 pulses per second. (The system has a PRF of 6kHz)
What is the Nyquist limit and why is the Nyquist limit important?
Nyquist limit is the highest Doppler frequency that can be accurately processed by a pulsed Doppler system without the occurrence of aliasing. It is equal to one-half of the systems PRF.
For this problem the PRF is 6,000Hz, so any Doppler shift greater than 3,000Hz will be subject to aliasing.
What is duplex ultrasound scanning?
AKA duplex imaging, refers to an ultrasound study that utilizes simultaneous real-time, two-dimensional imaging and Doppler assessment.
Allows the sonographer to accurately position the Doppler sample volume at a location of physiologic interest by means of 2D anatomical images.
Which component of an ultrasound system processes Doppler signals from a reflected sound wave?
By a device called a demodulator.
The demodulator extracts the Doppler shift data from the reflected echoes.
What is directional Doppler?
Used to identify both the speed and the direction of red blood cells.
If Doppler shift is positive the reflected frequency is greater than the emitted frequency, and red blood cells are moving toward the transducer.
If Doppler shift is negative the reflected frequency is less than emitted frequency and the red blood cells are moving away from the transducer.
Directional Doppler uses phase quadrature processing.
What is nondirectional Doppler?
Used is clinical settings in which presence of blood is important but direction of flow is not.
Nondirectional determines whether there is adequate blood flow in the arteries of the foot after a catheter has been inserted into the femoral artery.
Uses non-coherent processing
What is spectral analysis?
The study or interpretation of a collection.
What is the role of spectral analysis in Doppler ultrasound?
The process of spectral analysis makes it possible to decode the complex Doppler signals into their constituent frequencies and obtain more detailed information about the velocities of the moving red blood cells.
What is the Fast Fourier Transform (FTT) ?
A computerized, mathematical technique used to identify the various frequencies that combine to form the complex Doppler waveform.
A technique of digital spectral analysis
What is the role of the Fast Fourier Transform in current diagnostic Doppler systems?
The standard technique of spectral analysis in modern pulsed and continuous-wave ultrasound systems.
Are exceedingly accurate and can be performed numerous times per second in modern ultrasound systems
