The Golden Rule for Obtaining an On-Axis Image

Imaging the heart on-axis is possibly the most important (and difficult) task for a cardiac sonographer. Foreshortened or off-axis images provide a “false story” of the heart which can result in wall motion (and function) looking more vigorous than it really is. We may miss something completely, all just because we aren’t slicing through the heart correctly. Knowing how to correct an off-axis image shouldn’t be guesswork for the sonographer.

The #1 secret to on-axis imaging can be found in the following mantra….

Angle for the center, rotate for the sides (and always in that order).

It is such an important point, that I want you to say it out loud 3 times before we move on!!!!!

Angle for the center, rotate for the sides!!

Angle for the center, rotate for the sides!!

Angle for the center, rotate for the sides!!

If we look at a standard, on-axis view… in this case, the PLAX view. Note the aortic and mitral valves are opening, the LV infero-lateral wall and ventricular septum are contracting as two parallel lines, the LV apex is not visible from this window, and the aorta is opened out nicely and forms two parallel lines.

On-axis PLAX image

Now, lets look at the process for how I got this image.

1. Place transducer on chest

When I first put the probe on chest the image looked like the one below. The probe is just to the left of the sternum, placed perpendicular to the chest wall in the 3rd or 4th intercostal space, with the image marker is rotated to around 11 o’clock. Sounds like a textbook description, but that is only a guide. Don’t lose any sleep over this, everyone’s heart is in a slightly different position in the chest. It doesn’t matter if your transducer needs to be rotated differently for a particular patient…we are all different, but we display the images in a standard layout. Sometimes if we place the transducer in the “textbook” position it will look like this…

Off-axis image. Note the valves are closed down and it appears like the apex is closing into the LV.

This is a common scenario, where manipulation of the transducer is required to obtain the on-axis image. Experienced sonographers correct this without thinking, but often struggle to communicate to a trainee how to improve the image quickly. Trainees often just move around, adopting a “hope for the best” approach.

2. Mentally divide the image into thirds

This is where the Golden Rule comes into play. Divide the image into three rough zones – see image below. There will be a small section in the center of the image and a side-section on either side of the center.

Focus only on the center of the image initially.

 3. Tilt/Angle for the center

To then make adjustments to correct the image, angle for the center. Angle the transducer either way to open out the center region of the image. At this point, ignore the sides completely. Make sure that the AoV and MV are positioned within the center zone, and that the image is transecting through the center of both valves.

Here the center strip of the image is on-axis, but the apex still seems to be coming into the LV. The LV walls are not parallel with one another.

4. Rotate to open out the side thirds

 We are now happy that the center of the image is on-axis. It is important at this point that you hold the transducer very still and make sure you don’t slide, tilt or angle from this point. Very carefully rotate the transducer to open out the sides of the image. Often it will be a clockwise rotation, but if that doesn’t work, just rotate anti-clockwise instead. The important point is that you don’t tilt at this stage, and you limit your movements to a rotation. If you do slip…go back to the first step and tilt for the center – this MUST be the first movement, everytime.

Image is now tilted and rotated correctly.

It will seem like a slow process initially, but will become second nature very quickly… all of your imaging will become more efficient as you save time finding the correct image.

Additional thought exercise: 

Which structures are in the “center” and “side” zones for the PSAX view at the level of the AoV? What about the apical views??

What view do you routinely struggle with??

Post originally published November 24, 2014. Author – Tony Forshaw.