Vascular Access - Basics - Imaging Technique
Imaging Technique
Here, we shall discuss on the techniques to acquire images and, also, to perform needle guidance:
1. Real Time Guidance
2. Short Axis Approach
3. Long Axis Approach
Here, we shall discuss on the techniques to acquire images and, also, to perform needle guidance:
1. Real Time Guidance
2. Short Axis Approach
3. Long Axis Approach
1. Real Time Guidance
What does "Real Time" mean? In terms of vascular access, it means the needle is guided under ultrasound whilst maintaining the needle tip in view during the entire process. As such way, you know where the needle is advancing towards.
It is a complex technique requiring ambidextrous skills (one hand maneuvering the probe with the other guiding the needle) and hand-eye coordination (eyes on the monitor, not at the field, during the real time guidance process). It takes some practice to develop these skills.
What does "Real Time" mean? In terms of vascular access, it means the needle is guided under ultrasound whilst maintaining the needle tip in view during the entire process. As such way, you know where the needle is advancing towards.
It is a complex technique requiring ambidextrous skills (one hand maneuvering the probe with the other guiding the needle) and hand-eye coordination (eyes on the monitor, not at the field, during the real time guidance process). It takes some practice to develop these skills.
2. Short Axis Approach
The vessel of interest is imaged in its short axis, and the needle is guided into the vascular lumen with this view. The needle, also, is imaged in its short axis as well with this approach. On the monitor, the vessel will be displayed as a circular/ovoid anechoic structure, and the needle tip will be an hyperechoic dot.
The vessel of interest is imaged in its short axis, and the needle is guided into the vascular lumen with this view. The needle, also, is imaged in its short axis as well with this approach. On the monitor, the vessel will be displayed as a circular/ovoid anechoic structure, and the needle tip will be an hyperechoic dot.
Staggered Sliding:
As the animation illustrates, place the probe to image the vessel in short axis. Insert the needle until you see a hyperechoic dot. Once you see it, stop advancing the needle. The question you have to ask yourself is, whether that dot is the needle tip or body. Keeping the needle in place, slowly slide the probe forward until the dot disappears. Just when it disappears, it indicates the probe is, now, distal to the tip (the last image of the hyperechoic dot is the tip). Then, keeping the probe in place, advance the needle until the hyperechoic dot appears on the screen again. Repeat this (staggered sliding) process until the tip is inside the lumen.
As the animation illustrates, place the probe to image the vessel in short axis. Insert the needle until you see a hyperechoic dot. Once you see it, stop advancing the needle. The question you have to ask yourself is, whether that dot is the needle tip or body. Keeping the needle in place, slowly slide the probe forward until the dot disappears. Just when it disappears, it indicates the probe is, now, distal to the tip (the last image of the hyperechoic dot is the tip). Then, keeping the probe in place, advance the needle until the hyperechoic dot appears on the screen again. Repeat this (staggered sliding) process until the tip is inside the lumen.
The key emphasis with the short axis technique, and, also, as a major challenge for the novice, is ensuring that the needle body is not mistaken for the needle tip.
As you may recall BOTH are hyperechoic dots on display. If the body is mistaken for the tip, the needle would have penetrated much deeper than you think (see animation).
If you are not sure if you are looking at the tip or the body. FIRST stop advancing or withdrawing the needle - keep it stationary. THEN slide the probe forward to find the tip (you know you have found the tip when the hyperechoic dot just disappears).
If you are not sure if you are looking at the tip or the body. FIRST stop advancing or withdrawing the needle - keep it stationary. THEN slide the probe forward to find the tip (you know you have found the tip when the hyperechoic dot just disappears).
This is a clip of real time needle guidance in the short axis in a phantom model.
Note that the needle tip is maintained throughout the entire process.
Note that the needle tip is maintained throughout the entire process.
3. Long Axis Approach
The vessel of interest is imaged in its long axis, and the needle is guided into the vascular lumen with this view. The needle, also, is imaged in its long axis as well with this approach. On the monitor, the vessel will be displayed as a tubular anechoic structure, and the entire needle will be imaged as a hyperechoic linear structure.
The vessel of interest is imaged in its long axis, and the needle is guided into the vascular lumen with this view. The needle, also, is imaged in its long axis as well with this approach. On the monitor, the vessel will be displayed as a tubular anechoic structure, and the entire needle will be imaged as a hyperechoic linear structure.
Fixed Imaging:
As the animation illustrates, place the probe to image the vessel in long axis. Place the needle right at the edge of the probe as shown. Advance the needle IN PLANE with the probe, and you will see its length on the display. As you may appreciate with this technique once the view is acquired, the hand holding the probe stays stationary. The only hand that moves is the one that advances the needle. Also, given the length of the needle is imaged simultaneously, there is no ambiguity to distinguish between the tip and body as per integral to the short axis approach.
The key emphasis with the long axis technique, and, also, as a major challenge for the novice, is ensuring that the tranducer's US beam is IN PLANE with the needle.
You can image that the US beam's width and the needle's diameter are both very narrow. A slight misalignment will render the needle disappear from the monitor (see animation).
The key is to keep the probe-hand very steady (anchor it by placing your wrist on the surface).
As the animation illustrates, place the probe to image the vessel in long axis. Place the needle right at the edge of the probe as shown. Advance the needle IN PLANE with the probe, and you will see its length on the display. As you may appreciate with this technique once the view is acquired, the hand holding the probe stays stationary. The only hand that moves is the one that advances the needle. Also, given the length of the needle is imaged simultaneously, there is no ambiguity to distinguish between the tip and body as per integral to the short axis approach.
The key emphasis with the long axis technique, and, also, as a major challenge for the novice, is ensuring that the tranducer's US beam is IN PLANE with the needle.
You can image that the US beam's width and the needle's diameter are both very narrow. A slight misalignment will render the needle disappear from the monitor (see animation).
The key is to keep the probe-hand very steady (anchor it by placing your wrist on the surface).
This is a clip of real time needle guidance in the long axis in a phantom model.
Note that the in order to image the length of the needle, the US beam must be IN PLANE with it.
Note that the in order to image the length of the needle, the US beam must be IN PLANE with it.
Most novice prefers the short axis technique, because the long axis demands VERY steady hands to ensure the needle and probe are in plane. It is good to master both techniques as they have their advantages and drawbacks. If one technique fails, you may deploy the other. For example:
- Long straight vessels (eg. central vasculature) : Long axis approach is easier to perform
- Winding vessels (eg. peripheral vasculature): Short axis approach is easier to perform
Key Messages:
- Real time US demands visualizing the needle TIP during the ENTIRE guidance process
- Short axis: the hyperechoic dot seen on the monitor is either the needle tip, or the needle body
Practical Tips:
- Long axis: keep the US beam and needle in plane by anchoring your hands