Answer #5
The correct answer is E: 4 KHz. This question requires knowledge of the Doppler shift equation which is shown in the figure.
Here the operating frequency is 3 MHz, the speed of ultrasound in the medium is 1540 m/s in soft tissue, another number that you need to know. We are told to estimate this as 1500 m/s (to avoid need for a calculator) and angle the between the beam and blood flow is 0 degrees. The cosine of 0 degrees is 1 so we have Doppler shift = 2 x 3 MHz x 1m/s x 1/(1.5 x 10^3m/sec) = 6 MHz/(1.5 x10^3) = 4 KHz. Since the blood is moving away from the transducer the shift will be negative so it is technically -4KHz. However the question here only asks us about the magnitude of the shift. Note that 4 KHz is in the range of human hearing and hence we actually hear the Doppler shift from the machine when we pulse the blood flow across the LVOT. The other answers are not correct either because of order of magnitude(units) or math error.