When using continuous-wave Doppler with spectral analysis, which operator-induced error would likely result in a falsely LOW frequency shift?

When using continuous-wave Doppler with spectral analysis, increasing the beam angle to 70 degrees can lead to a falsely low frequency shift due to the cosine relation involved in Doppler frequency calculations. The Doppler effect is influenced by the angle between the ultrasound beam and the direction of blood flow. As the angle increases, specifically approaching 90 degrees, the measured frequency shift decreases because the cosine of the angle approaches zero. At an angle of 70 degrees, the cosine value is around 0.34, which significantly reduces the observable frequency shift compared to a perpendicular angle (0 degrees). Therefore, the measured frequency shift may appear falsely low due to this increased angle.

In contrast, using a transducer with a lower frequency generally does not directly lead to a lower frequency shift but affects the depth of penetration and resolution. Adjusting for higher gain settings may enhance the signal, thereby not resulting in a falsely low frequency but possibly increasing the likelihood of noise. Leaving the wall filter off could potentially allow for all frequencies, including lower ones, to be processed, but it doesn’t inherently cause a low frequency shift as the angle does.