ABSTRACT
Doppler flow measurements As was discussed earlier in Chapter 2, the Doppler effect provides a unique capability for ultrasound to measure blood flow (Evans and McDicken, 2000; Jensen, 1996). Upon insonification by an ultrasound beam, the echoes scattered by blood carry information about the velocity of blood flow. Blood flow measurements are frequently performed in a clinical environment to assess the state of blood vessels and functions of an organ. Ultrasonic Doppler instruments allow a measurement of instantaneous blood flow velocity. Combined with pulse-echo instruments, instantaneous flow rate in a blood vessel as a function of time and cardiac output can be measured noninvasively with ultrasound. At present, very few clinical options are available to do so. Figure 5.1 shows an ultrasound beam of frequency f insonifying a blood vessel, making an angle of θ relative to the velocity v. Here it is assumed that blood flows in a vessel with a uniform velocity v. The returned echoes are Doppler shifted. The Doppler shift frequency fd is related to the ultrasound frequency f by Equation (2.43):
f
v c
fd 2 cos
=
θ
where c is the sound velocity in blood and may be assumed to be 1540 m/s. The Doppler-shifted frequencies happen to be in the audio range for blood flow velocities in the human body for an ultrasound frequency between 1 to 15 MHz.
