What operator-induced error can result in a falsely LOW frequency shift while assessing the internal carotid artery?

When assessing the internal carotid artery with Doppler ultrasound, increasing the beam angle to a high degree, such as 70 degrees, can lead to a falsely low frequency shift measurement. This is due to the cosine effect in Doppler ultrasound, where the frequency shift is influenced by the angle of incidence between the ultrasound beam and the direction of blood flow. As the angle increases, especially beyond 60 degrees, the cosine of the angle decreases, which in turn causes a dramatic reduction in the observed frequency shift. This makes it appear as if the velocity of blood flow is lower than it actually is, leading to inaccurate assessments of hemodynamics.

In contrast, the other given choices do not create the same effect on frequency shift measurements. Overdriving the Doppler signal gain may amplify noise and clutter but does not directly influence the angle of measurement or result in a falsely low frequency. Changing to a higher-frequency transducer would not inherently affect the frequency shift, but can influence penetration and resolution. Leaving the wall filter on could help eliminate low-frequency artifacts and noise but would not lead to a falsely low reading regarding the Doppler frequency shift. Thus, increasing the beam angle to 70 degrees is the factor that directly results in the incorrect assessment of blood