Ultrasound orientation is essential to understand what is being seen in point-of-care ultrasound. There are two key aspects: how the indicator is oriented relative to the screen and how the probe and indicator are placed and oriented relative to the patient. As described in Chap. 2, the conventional ultrasound image is a two-dimensional plane composed of frames made up of scan lines that are updated many times a second to create a moving image.
Ultrasound orientation can be challenging because it involves understanding how a two-dimensional plane may cut through a three-dimensional object in any of the three standard planes (sagittal, transverse, or coronal), as well as any oblique orientation between those planes. However, one of the great advantages of ultrasound is the ability to obtain images from a variety of different approaches, and a thorough understanding of orientation will allow you to take advantage of this.
There are two rules for indicator-to-screen orientation in standard emergency ultrasound imaging, which uses the same convention as general radiology imaging:
The top of the screen is closer to the probe. Bottom of the screen shows structures farther away from the probe.
The left side of the screen, as it is viewed, corresponds to the side of the probe marked with an indicator.
Imaging performed by cardiology specialists uses a different and opposite convention for rule 2, which will be discussed in more depth later.
The “indicator” may differ widely between manufacturers, and is typically a bump or a groove. It is important to verify your orientation prior to beginning any exam. This should be done by placing a small amount of gel on the side of the probe where you believe the indicator to be, and confirming that the side with the gel corresponds to the left of the screen as it is viewed (Figs. 3-1a and b).
Once the indicator-to-screen orientation is understood and verified, the probe is placed on the patient and images are viewed on the screen.
The top of the screen will show structures closer to where the probe is placed, with the bottom of the screen showing structures farther away from the probe face. The left of the screen will show structures toward where the indicator is directed.
Another way to think about this is to think of holding the probe in front of you, with the indicator on your left. In this way you are looking through the plane of the ultrasound, with the left side of the screen being on your left.
When you are located on the right side of the patient, a sagittal plane will be obtained by directing the indicator to your left (toward the patient’s head), and this will be the left of the screen. For a transverse plane, think of yourself as looking up from the patient’s feet, with right-sided structures on the left of the screen.
Most of emergency ultrasound can be performed in this orientation, with the indicator (screen left) being toward the patient’s right, patient’s head, or in the 90° arc between these directions.
There are three standard anatomic planes (Fig. 3-2):
- Transverse (or axial)
- Sagittal (or longitudinal)
- Coronal (or frontal)