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Imaging Planes
An important aspect of ultrasound and in particular of procedural ultrasound is your ability to correctly identify the structure and then placement of a needle in a very precise location. In this small chapter we will be reviewing some of the common terminology and planes used for optimal success. The principles described here span apply to all probe types.
Ultrasound Movements
Lets start by understanding how the ultrasound probe can be used to assess a structure. Sliding occurs when the probe follows the surface plane (the skin) and thus follows the contour of the skin. The probe-skin interface is a rectangular area. Tilting occurs when the probe-skin interface acts as a fulcrum and probe is moved along its major axis. Rocking is similar to tilting but the probe is moved along its minor axis. Rotation is changing the plane 90 degrees keeping the center of the probe-skin interface stationary. The following image explains the movements.
Ultrasound movements and manipulation. Adapted from Ramsingh 20018.
Imaging planes
Ultrasound imaging takes a very small slice or a plane of roughly 3mm thickness of a third dimensional structure. On the ultrasound display this 2d representation appears. To optimize the view of the structure of interest you need to move the prove as defined above when scanning the object of interest.
Lets define imaging planes. Transverse (or short axis) and long axis images are defined according to the spatial relationship between the ultrasound probe and the structure of interest. For example if the beam cuts the short axis of the vessel then it is a short axis view and it is what we see in the following example. In the following images we will be using the same 3D model of a vessel encased in a transparent structure. Within that solid structure is a small brown tissue that will later be used for reference.
Imaging planes. Image on the left displaying the position of the probe in relation to the major axis of the vessel. The image that will be displayed will be the short axis view. On the right, the plane cast by the probe will be cutting along the major axis of the vessel and thus a long axis view will be obtained.
Imaging planes2 . We see two probes A and B. The image is cutting along the short axis of the vessel and thus it is a short axis view. On the right, the ultrasound display. Probe A is tilted and the image is orthogonal the vessel while B is parallel the surface but cuts the vessel at an angle, making it appear larger than it actually is.
Needle position
Needle imaging by ultrasound on the other hand is either in plane or out of plane and in relation to the position of the axis of the needle (which is a line) and the plane generated by the ultrasound beam. So an out of plane needle would be coming perpendicular to the beam of the probe. In the images bellow we see an out of plan needle that has entered on the right hand side of the probe. The target on the following model is a space between the middle of the brown structure and the vessel. The needle is not moving in the following examples.
Short axis view with an out of plane needle. In this same 3D structure a single probe and the needle target being the space between the vessel and the brown structure. Notice that the center of the probe generates a plane which is what you see on the right side representing the display. Since the scanning plane is not pointed towards the needle tip (tilted opposite the needle tip) it seems as though the needle needs to be advanced more. This is a potentially dangerous situation as the needle perforates deeper structures.
Short axis view with an out of plane needle. Probe tilted towards the needle tip. Notice the scanning plane now sees the needle tip in the target space.
Live model and steps for success
Lets see a comparison of the above mentioned ideas to solidify the concepts. On the left hand side we see hand movements and on the right, the ultrasound display. Notice that the performer is making subtle movements of the probe and the hand manipulating the needle. Following the steps below when attempting to reach a structure will improve your success. Notice the left hand of the proceduralist as it remains anchored on the skin and the probe does not slide easily on the surface.
1. OOP Needle
Short axis view of the vessel while the needle is advanced from an out of plane (OOP) technique. The probe is stationary while we advance the needle into the plane generated by the probe. The needle appears as an hyperechoic structure that does not penetrate the vessel in this view.
2. In Plane Needle
Long axis of the vessel image while the needle is advanced from an In plane technique. The probe is stationary while we advance the needle into the plane generated by the probe. We can clearly see hyperechoic structure as it advances on the tissue. We confirm that the needle has penetrated the vessel.
3. Scan in short axis to confirm
The probe is placed back in the short axis view of the vessel and the needle remains stationary. In this case the probe is moved along the trajectory of the needle to visualize its tip.
The images noted above help with the sequence that is recommended for optimal success. Identify the vessel of interest and start with the short axis view of the vessel. The probe is now still while you advance the needle. Then move to a long axis view when you are close to the vessel and continue advancing the needle. Finish off by confirming placement of the needle in the short axis view while the needle remains stationary.
Although challenging, the in-plane technique which is associated with long axis visualization of the vessel leads to higher precision and fewer complications.
References.
1. Lamperti M, Bodenham AR, Pittiruti M, Blaivas M, Augoustides JG, Elbarbary M, Pirotte T, Karakitsos D, Ledonne J, Doniger S, Scoppettuolo G, Feller-Kopman D, Schummer W, Biffi R, Desruennes E, Melniker LA, Verghese ST. International evidence-based recommendations on ultrasound-guided vascular access. Intensive Care Med. 2012 Jul;38(7):1105-17. doi: 10.1007/s00134-012-2597-x. Epub 2012 May 22. PMID: 22614241.