Thank you for your response and the well written explanation of the history of angle correction. I am going to present this information to the university vascular instructor along with the ICAVL standards recommending 60 degrees or less. When I explain this to my students who are the better physics minded, they get it.
And thanks to everyone who posted this blog.
Kristy Peeler LPN, RVT
The Heart Group, PC
From: UVM Flownet on behalf of Debbie Anderson
Sent: Tue 1/27/2009 9:42 AM
To: [log in to unmask]
Subject: Re: angles
Kristy, here is a note from Frank Miele... The 60 degree angle is a ubiquitous dilemma. A few pieces of information might help clarify.
It is important to realize that 60 degrees is a very suboptimal angle for Doppler in terms of errors and artifacts. According to Doppler theory, it should not matter what angle is used since angle correction can be employed. This theoretical assumption is false since it ignores the fact that there is always error in specifying the Doppler angle (no one can place the flow indicator exactly in the true direction of flow all of the time) and that there are many artifacts that are angle dependent. Since the Doppler Effect is affected by the cosine of the Doppler angle, and since the cosine is very non-linear with angle, the error associated with the Doppler angle is also very non-linear. The worst Doppler angle is clearly 90 degrees at which there is infinite error and no amount of angle correction can "fix" the problem. The best angle is 0 or 180 degrees (the cosine changes very slowly around 0 and 180 degrees - if you know calculus it is determined by the first derivative (the slope)). One thing is for sure, angles greater than 60 degrees yield horrendous error and should be avoided as much as possible. However, 60 degrees is not very good either. 50 degrees is certainly better than 60 degrees, and of course 40 degrees is better still. Additionally, artifacts such as spectral spread are exacerbated with larger angles, again leading to the conclusion that angles smaller than 60 degrees are better. (Note that for carotid studies, at 60 degrees, spectral broadening can sometimes result in an overestimation of the peak velocity by as much as 50%.)
So how then did 60 degrees become a standard? The answer is related to what was practical. In the early days before electronic steering, "steering" was manual. Since vessels tend to be parallel to the skin surface, the worst possible situation for Doppler existed unless the transducer was manually angled (without manually angling the probe, an angle of 90 degrees would result, yielding no Doppler information). As it turns out, the transducer can be manually angled about 30 degrees while maintaining good skin contact and still be before the point of actually having to push so hard as to affect flow in the vessel being assessed. If the steering angle is 30 degrees, the resulting Doppler insonification angle (Doppler angle) is 60 degrees. Furthermore, in the early days, spectral analysis was not performed (this was well before the days of applying Fast Fourier Transforms with slick DSP chips) and velocity information was not presented. Instead, the detected frequency shift was presented. As it turns out, the cosine of 60 degrees is 0.5, which when used in the denominator of the calculation to correct for the angular effect, is simply a factor of 2, a factor that anyone can mathematically apply in their head.
The standard then became 5 MHz transducers at 60 degree Doppler angles. The correlations were performed using this standard. Now move ahead to the point at which electronic steering was created. The question was what angle should now be used. Initially, many systems had very limited steering, so maintaining a 60 degree angle within a patient, let alone from patient to patient, was challenging, but usually "doable." Since all the correlations were at 60 degrees, the 60 degree criterion was preserved.
Now move ahead to modern times. Most systems now can achieve angles better than 60 degrees pretty routinely. So why continue to use 60 degrees? Quite often the answer is historic. Since that was the standard, many people now mistakenly believe that 60 degrees gives the best Doppler. In contrast, many labs now recommend a range of 45 to 60 degrees (a very acceptable practice). There is a reason that many give for specifying a range and not just specifying 60 or less. That argument is associated with repeatability. The goal is to maintain repeatability in how studies are performed both within a lab and from lab to lab. From a mathematical standpoint, this argument is not completely true since the error bands associated with 0 to 60 degrees are encompassed within the error bands associated with 45 to 60 degrees. Furthermore, with vessel tortuosity, often times maintaining a 60 degree angle is almost impossible and often leads to the use of suboptimal views and steering angles. In these cases, the correct answer is to use the lower angle and make a note. The interpreter should already be acutely aware of potential velocity variations (acceleration effects) associated with the tortuosity. The best position nowadays is probably to try to maintain angles between 45 and 50 degrees, realizing that there will be times when larger or smaller angles will just make more sense. Of course, you should always perform correlation studies to show that your Doppler measurements are consistent with the current gold standard.
Debbie K. Anderson
Pegasus Lectures, Inc.
972.668.1842 (direct line)
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From: UVM Flownet [mailto:[log in to unmask]] On Behalf Of Kristy Peeler
Sent: Monday, January 26, 2009 9:59 AM
To: [log in to unmask]
I work in a small city with a new ultrasound program at the university. There is also a new vascular ultrasound program with students rotating through my lab. I am finding these students are being taught that the angle of insonation should be 60 degrees and nothing else no matter how the vessel is coursing. My students are telling me that he other surgeons' lab and the general US depts. in the 2 hospitals also incorporate this same school of thought. For me, it has always been 60 degrees or less. Is this common practice in other communities? I would appreciate your feedback.
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