CME article 6

 

Trouble Shooting Tips: Neck Vessel Doppler


Dr. Alpana Joshi

1.Why should the angle always be between 30 to 60 degrees?

The Doppler shift is given by
Fd = (2 Fo v/c) cos f

Fd is Doppler shift
Fo is the transmitted frequency
V is velocity of sound
C speed of sound in the medium
f Is Doppler angle

 

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For perpendicular beam of incidence, if f is 90, then cos f is 0, therefore there is no Doppler shift. If the angle is 0, then cos f is 1, and there is maximum Doppler shift. In practice however, this is mostly not possible. Therefore to get an optimum Doppler shift, the transducer beam is usually oriented to make a 30 to 60 degree angle with the arterial lumen. This gives an optimal color fill in. The angle is also important for measurements of velocities; it is known as angle corrected velocities. After obtaining the spectral tracing from a vessel the angle is corrected so that it is parallel to the vessel lumen. This gives us the true velocity. But in practice, when we try to attempt to do this (especially so in carotid tracing, which is parallel to the skin surface), the angle goes beyond 60. This will give a spurious increase in velocities.

For example: If angle is changed from 50 to 55, then the velocity difference moves from 45 cm/s to 50 cm/s. (5 cm/s)

If the angle is changed from 70 degrees to 75 degrees, the velocity difference moves from 84 cm/s to 112 cm/s (a difference of 28 cm/s). (See Fig 1)

Since the grading of stenosis in carotid arteries is done according to velocities, taking the angle corrected velocities beyond 60 will give a false positive high grade stenosis, since the velocities measured will be much higher than actual. Thus the angle should be kept between 30-60.

2. What happens if I cannot align the angle to absolutely parallel to the vessel without crossing 60 degrees Ė should I take a reading as it is?

As described earlier, it is vital to keep the angle between 30-60 degrees, max at 60 and not beyond. Another option is to use a wedge-shaped water bag kit, which can be attached to the transducer; this has an advantage as it gives same angle to the scanner image, so even if the vessel is parallel to the skin surface, it appears at the same angle (usually 30 degrees). See Fig 2, below.

 

3. I donít get a vertebral artery reading sometimes, what to do? Is it blocked?

Try the following:
1.     Reduce PRF
2.     Increase the color gain
3.     Sometimes hyperextension of neck with head turned to contra-lateral side and scanning proximally helps in getting the vertebral artery.

4.How do I confirm a subclavian steal? Is it commonly seen? 

In subclavian steal, there may be flow reversal or a to and fro pattern in vertebral artery (Fig 3)

These findings could be seen:

a)     vertebral flow is reversed throughout the cardiac cycle.

b)     Flow is bi-directional (forward in systole and reversed in diastole)

c)     Flow is normal with patient at rest.

In the latter 2, the steal may be intensified and optimally documented by inducing arm hyperemia. A BP cuff is placed around the arm and inflated above systolic pressure. This pressure is maintained for about 5 minutes & then abruptly released; the vertebral artery is then scanned: if steal is present, the waveform quickly inverts.

 

5.What is spectral broadening?

In the region immediately beyond an arterial stenosis (the post-stenotic zone), the high velocity, high pressure jet emanating from the stenotic lumen spreads out into a zone of relatively low velocity and pressure producing a non-laminar flow pattern. This is seen as spectral broadening.

6.What happens if a carotid bifurcation is high up in the neck, is they anyway to check out the ICA, besides Trans-cranial Doppler.

When there is a high bifurcation, it is difficult to evaluate the ICA and the ECA. In such patients, far postero-lateral approach may help. For this, it is necessary to turn the patientís head to the contra-lateral side and place the transducer well behind the sterno-cleido-mastoid muscle.

7. What is off-diameter scan? How does it affect plaque estimation?

In completely normal patients, a false positive diagnosis of plaque may occur if the vessel is scanned in an oblique (off-diameter) plane rather than true longitudinal plane; thus it is of great importance to scan along the diameter of the vessel when searching for plaques. In diseased vessels, the severity of plaque and degree of narrowing may be under or overestimated due to a similar malposition of the image plane.

 

High PSVs in distal CCA dut to ICA block & CCA thrombosis
 
Dr. Alpana Joshi
Associate Professor
Department Of Radiology
BYL Nair Hospital,
Mumbai, India.