Thread by @Thind888

1/
**Doppler knobology microskills: Wall filter**

I realized I haven't done an ultrasonography post for a while now so here goes.

Spectral doppler knobology is often overlooked but the basic settings include (i) Doppler Gain (ii) Scale (PRF) (iii) Sweep speed, and (iv) Baseline

2/ Just like B-mode ultrasound, doppler settings should be optimized to get the best tracing.

Here I'll talk about the less recongnized setting of "wall filter" and give a clinical example of how it may be useful to adjust.

But first, let's briefly review doppler physics.

3/ When ultrasound encounters a moving interface, the frequency of the reflected waves is "phase shifted" proportional to the velocity of the interface.

The direction of the phase shift depends on the direction of the object (positive if moving towards the ultrasound): image

4/ Technically speaking, wall filter is a 'high-pass filter'. It's a generic category of electronic filters that filters out lower frequencies (allows the higher frequencies to 'pass'). E.g. -EKG machine has both high- and low-pass filters.

So why is it called a wall filter?

5/ This is because it's original motive was to eliminate low-frequency high-amplitude doppler shifts from movement of the vessel wall in vessel doppler.

Side-note: Similarly, tissue doppler is nothing but spectral doppler with a low-pass filter (instead of a high-pass filter)!

6/ Wall filter setting can be adjusted in most machines by different levels of granularity. If this is set too high, it will filter out more velocities.

Hence, this setting should be set at a lower level when analyzing very low velocities such as venous flow (including, IRVD!).

7/ Clinical example: IRVD.

In this example, I had a great arterial signal but was unable to find a venous signal. The wall filter was originally set at 100 Hz. So all doppler shifts below this were being filtered.

This translates to all velocities <4.3 cm/sec being filtered -

8/ Note that when I reduced the wall filter, the venous velocities revealed themselves as a normal continuous flow. These were best seen at minimal wall filter (25 Hz)

However, completely turning the wall filter off adds a lot of noise from lower velocities due to tissue motion.

9/ Summary -

- Wall filter is a high-pass filter that is helpful in eliminating noise from vessel wall motion in vascular ultrasound
- This can be safely lowered when not expecting interference from fast moving tissue
- Especially consider lowering it when imaging low flows.

@ArgaizR @ThinkingCC @khaycock2 @katiewiskar @msiuba @siddharth_dugar @FoxStevenW @NephroP @iceman_ex

10/ Addendum:

Also sharing a few thoughts on the other doppler settings as per conversation with @cianmcdermott

(i) Scale - This is similar to adjusting depth in 2D. This is especially helpful while measuring velocities (e.g. portal vein pulsatility) - much easier on the right!

11/ (ii) Baseline - This can be adjusted to focus on the direction of flow being studied (positive: towards the probe, or negative: away from the probe). In PW doppler, adjusting baseline can avoid aliasing

In the attached image, doppler scale is maxed and baseline is shifted up

12/ (iii) Sweep speed - This setting can be quite helpful.

A classic example is analysis of tricuspid/mitral inflow velocity variation during evaluation of cardiac tamponade. Here, the sweep speed should be slowed to assess it better (25 mm/s is a good number).

13/ Contrarily, a high sweep speed is ideal when you have to trace the envelop.
E.g. when recording LVOT VTI, I increase the sweep speed to 150 mm/sec (especially in tachycardia with short ejection time). This is simply for convenience and more precise tracing.

14/ (iv) Doppler gain - This should be set similar to 2D gain - not too low, not too high. However, as opposed to 2D, setting doppler gain settings may affect measurements.

E.g. setting CW doppler gain too high may lead to overestimation of tricuspid regurgitant velocity.