General

Which axis is best to measure from?

Happy New Year!! Let’s kick 2015 off with a reader question.
Many measurements in Echo can be made from multiple locations (PLAX vs PSAX; A5C vs ALAX). Experienced sonographers will have a “favorite” way of imaging which is passed down to the next generation of trainee sonographers as the “correct” way, although we often do a poor job of verbalizing “Why” we measure one way over the other. Theories are thrown around, some hilarious, some reasonably valid…but at the end of the day all the techniques are probably just as good as each other; we just need to be aware of the pitfalls. The published guidelines suggest that for some measurements we can use either axis, however as an Echo.Guru reader pointed out recently, the measurements may be different between the two views… so which axis is best to measure from??

Today we will focus on the linear measurement (A-P) of the left atrium (LA) at end-systole.

Q:“Should we measure the LA from the PLAX or PSAX views?”

If we are measuring correctly, the measurement should be the same in both views. Discordance between the PLAX and PSAX measurements should draw your attention to the limitations of the technique and alert the sonographer to a possibly erroneous measurement.

As we rotate between the PLAX and PSAX views, the relationship between the transducer and heart remains constant. Therefore any argument that one gives a “better line-up” is flawed, it is just that we are aware of what is happening in the other view.

In the PLAX view below, we can appreciate that if we simply place a cursor through the aortic root and extend into the LA, then we are cutting a very poor angle through the atrium. This is not going to be a true minor axis dimension and will over-estimate the dimension. This suggests that the M-mode technique is not going to be possible in this case, as it is unlikely that we can swing the image across enough to obtain this correctly. This is the exact measurement that you are going to get by rotating around to a PSAX view at the level of the AoV – so for this patient the PSAX will also be overestimated. Note: I have deliberately used images which are fairly on-axis to illustrate that this is an issue for many of our patients (not just the oblique ventricles…)

PLAX clip prior to measuring LA dimension

PLAX clip prior to measuring LA dimension

Note the difference between the theoretical M-mode lineup (transecting through the aortic root) compared with the true minor axis.

Note the difference between the theoretical M-mode lineup (transecting through the aortic root) compared with the true minor axis (perpendicular to the major LA axis).

With 2D however, we can simply measure the true minor axis in the PLAX view by measuring from the aortic root across the LA. In this instance, the absolute difference between the two measurments is fairly small, but still illustrates the point that the two lines are cutting through different parts of the LA. The major issue is reproducibility and not knowing whether we are transecting through the middle of the LA (which we can appreciate by rotating around to the PSAX).

PLAXPSAX

The PSAX view has the advantage of showing whether we are transecting through the middle of the LA. The line transecting through the aortic root would be too far to the left of center (thereby underestimating the dimension). We can also appreciate whether we are including the origin of a pulmonary vein in the measurement (which would overestimate the dimension).

PSAX_cursor

 

PSAX_multiple-measurements

PSAX measurements of the LA. Note how much the numbers can change with only a little movement of the cursor. From the PLAX view you would not appreciate which is the right measurement. Similarly, in this image we can’t appreciate whether we are cutting obliquely through the long axis of the LA.

 

In other words, the PLAX shows us how far along the LA we are measuring, but the PSAX shows us whether we are in the center of the cylinder or not. If we are in the correct position, both measurements will be the same.PSAX_PLAX.fw

I should add that due to the complex (non-geometric) shape of the LA, the current ASE guidelines recommend using a biplane volume technique rather than a linear measurement. Personally, I believe there is still a role for the linear measurements as it acts as an error-check for our volumes/area and is still useful for serial comparison. Like everything in echo, it is all worthwhile if it is done properly! In practice, I measure from the PSAX view, but only if the line-up is reasonable from the PLAX view. In an ideal world, you would measure both the PLAX and PSAX views to ensure concordance between the measurements (and hopefully improving accuracy); however experience tells you when the measurement is going to be reliable, and when to not bother measuring at all. If you are pressed for time, I suggest trying to spend the time working on the biplane LA volumes. I look forward to reading the ASE’s revised “Chamber Quantification” paper coming out in the new year.

Thanks MS for asking another great question!! Keep the ideas coming… I will get to all the questions eventually!

Enjoy!
Echo.Guru

 

 


Image courtesy of Arztsamui at FreeDigitalPhotos.net

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2 Comments

  • Reply Ken Spencer January 2, 2015 at 3:38 pm

    Very nice to consider the limitations of each approach to LA measurement. Thank you

  • Reply ms January 2, 2015 at 11:55 pm

    Thanks so much. I check sometimes in PAX//PAS and LA volume when ever I am in doubt. .Thanks for sharing the nice demonstration!!

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