2D Transducer Position and Orientation
Three-dimensional insights can be gained by considering two-dimensional images obtained with a transducer at different positions. The figure immediately below depicts long axis image schematics with the transducer positioned relatively far dorsally. You are invited to consider how the short axis image would appear from this transducer position; short axis scan planes are suggested by heavy lines.
I maintain that this is the "correct" transducer position for performing M-mode recordings or 2D measurements in the short axis plane. Here are some of the short axis view features we could expect:
- The right ventricular space is large and the interventricular septum is readily visible as a result of the contrast provided by the right ventricular cavity.
- The scan plane misses the trabecular septum of the right ventricle. This is appreciated by the fact that the right ventricular side of the septum appears smooth. You will typically see the main septal papillary muscle "floating free" as a circular echogenic structure within the right ventricular lumen. As an exercise, fan towards the apex so that you can see the appearance of this papillary muscle where it attaches to the septum. The rough appearance of the septal trabeculation will be more apparent there also.
- Both mitral leaflets are clearly visible; the scan plane engages both leaflet tips at the same fan angle.
- It only takes a few degrees of fanning angulation to transition from the left ventricular view to the heart base (aorta/left atrial view).
- As you complete this fanning angulation, the far field border of the heart remains about the same distance from the transducer.
- At the level of the aortic root, the aorta appears as an echodense ring and all three aortic cusps are engaged; the interventricular septum is not engaged.
The subsequent schematic depicts a transducer positioned that is too far ventral (for M-mode or 2D short axis quantification). When trying to obtain short axis images from this position there are a number of visual cues to suggest suboptimal transducer positioning.
Again, imagine the short axis views suggested by slices at the solid heavy lines.
- The right ventricular space is small and there may be very little right ventricular lumen to act as contrast for septal visualization. In some cases, the right ventricular side of the septum may not be visualized at all. You should be watching the 2D image for a clearly demarcated septum since this is one of the measurements that you will be making, whether your technique is 2D or M-mode.
- The scan plane passes through the trabecular septum. On the right ventricular side of the septum, this appears as a roughness or irregularity of the septum. Inclusion of the trabecular septum (2D or M-mode) significantly increases the measured thickness as does inclusion of the right ventricular septal papillary muscle. This is not trivial; it will make the difference between normal versus abnormal measurements.
- Only one mitral leaflet is engaged at a time. The most typical recording depicts the anterior leaflet only and the far field wall of the heart is the left atrium, not the left ventricular endocardium as intended.
- It takes a lot of fanning (large fanning angulation) to get from the "left ventricular" slice up to the aorta.
- The wall of the left ventricle and left atrium literally get farther and farther away from the transducer while fanning towards the heart base. This can occur due to left atrial enlargement, of course, but you'd better consider your transducer positioning if you are reaching for the depth knob (to increase depth) after fanning towards the heart base.
- The aorta does not appear as a discrete ring structure since you're not slicing it (anywhere near) squarely at the root. Typically the near wall of the aorta will appear with varying degrees of "meat" on it as a result of including part of the interventricular septum in the slice.
We now consider the appearance of the long axis image, dependent on how far cranial or caudal ( i.e. which rib space ) the transducer is placed in. In the schematics immediately below, the left-hand side depicts cranial transducer positioning and the right-hand schematics depict caudal transducer positioning. Which transducer position your employ depends on what you're trying to accomplish which will be shown further below. The schematics also happen to depict a potentially marked angular dependency of the standard M-mode left atrial measurement, i.e. the one where the M-mode beam passes directly through the aorta. The cranial transducer position typically makes this LA dimension come out larger as shown. There are of course other limitations to this measurement, but I feel that this is one that contributes substantially to quantitative differences.
Just as the short axis image suggests the dorsal-ventral positioning of the transducer, there will be visual cues in the long axis image that will suggest whether the transducer is in the cranial or caudal position.
- A long axis image from the more cranial rib space that divides in the left ventricle evenly between the two major papillary muscles (interpapillary groove) tends to include portions of both the left atrium and left ventricular outflow tract along with the mitral valve. It is not necessarily a classic 4 chamber view.
The schematic on the right is an attempt to depict how the long axis scan plane (dark vertical line) is oriented relative to the structures as imaged in the short axis view. As suggested by the actual long axis image, a scan plane that passes directly between the papillary muscles may engage the aorta (not a classic 4 chamber view). Note that on color flow Doppler, pulmonary venous return entering the left atrium from the vein near the far field wall may be prominent due to proximity of the vein to the scan plane.
- From the cranial transducter position, the cranial (anterior) or caudal (posterior) papillary muscles are approximately the same distance from the transducer when fanning to engage either muscle in the scan plane. The left-hand image below depicts the caudal (posterior) papillary muscle and the right-hand image depicts the cranial (anterior) one as evidenced by inclusion of the left ventricular outflow tract. The subsequent short-axis schematic shows how both papillary muscles remain roughly equidistant from the transducer.
- In contrast, placement of the transducer at a more caudal rib space tends to allow production of the classic 4 chamber view if the scan plane is such that it passes between the papillary muscles. This is the appropriate view (and transducer position) to obtain the left atrial long axis dimension. However it is typical that the scan plane on this view does not slice the major chord of the left ventricle; the dimension of the LV may not be as great as for the cranial transducer position. In other words, it may not be possible from this rib space to slice the left ventricle at the major chord without hitting the cranial (anterior) papillary muscle. My viewpoint is that the caudal transducer position is not appropriate for LV measurements. However it IS the correct position for long axis LA measurements. The schematic shows how this scan plane includes the body of the LA at its largest dimension.
- Compared to the more cranial transducer position, this caudal position tends to result in the caudal (posterior) papillary muscle being closer to the transducer than the cranial (anterior) muscle. Fanning towards the posterior papillary muscle makes it appear as if the muscle originates at the center of the left ventricle, i.e. close to the transducer, as it does indeed. The cranial (anterior) papillary muscle continues to appear as though attached at the far field endocardial wall.