Final answer:
Spectral broadening causes the Doppler tracing to not touch the baseline, indicating variation in blood flow velocities, which is common in vascular abnormalities.
Step-by-step explanation:
The reason why the Doppler tracing does not touch the baseline on either side is likely due to spectral broadening. Spectral broadening occurs when the range of velocities within the blood increases, often because of turbulent flow or other factors that cause a variation in blood flow velocities. This phenomenon leads to a filled-in Doppler waveform, where the clear window or spectral window, which normally appears between the baseline and the flow velocities, is obscured. This could indicate a disturbed flow pattern, which is common in areas of stenosis or other vascular abnormalities. A decrease in Doppler gain settings might reduce the amplitude of the waveform but wouldn't typically cause spectral broadening. Decreased rejection settings can lead to low-frequency noise, but it is also not the cause of spectral broadening. Finally, high filter settings remove low-velocity components from the signal, which could potentially reduce the appearance of spectral broadening but are not the cause of it.