Welcome to part 3 of our series on how to set up your own neuromuscular ultrasound lab! If you missed part 1 dealing with local settings, or part 2 about the equipment you can find it here:
This time we think about
1. How to define our presets and why we should do so and
2. How we document our findings optimally.
If you have comments or ideas – we are happy to hear them!
Now let´s get started. Fasten your seatbelts and get ready for our third chapter:
3. THE PROTOCOL
a. The presets
Every time you start scanning a nerve, you want an optimal image at the push of a button. Therefore it is definitely a good investment of time to define the settings of this button once and enjoy profits for the next years. We suggest you start with the bigger nerves of the upper extremity, defining the settings at the point of pick-up – which will be quite superficial in most of the cases. Choose a depth that allows you to get a good overview, but also reasonable insight into the nerve you want to scan. Adjusting the depth down to the bone is a good rule of the thumb in the extremities.
Pic 1. The median nerve in the forearm as an example of adjustment of depth
a. A little bit to much depth, you are literally scanning through the whole forearm. The nerve is there, but detail gets lost
b. optimal adjusment of depth – you see the nerve as close as possible, but also have an overview on the innervated muscles – the superficial and the deep finger flexors. See the bone in the lower right corner.
c. A little too close – you see of the nerve as much as you can, but miss the surrounding and will lose the nerve in the next centimeters of scanning proximal, because it will dive deep to the pronator teres.
Adjust the gain to a medium level, not turning the image too bright, choosing the color spectrum of your machine that suits your eyes best. Frequency is not the major point when looking at the main trunk, as it is quite big and won´t cause any problems with identification. Something around 10-12 MHz will be enough for a good pic at the wrist and also higher up the forearm, as the nerve lies deep to the superficial flexor and pronator teres. Depending on your machine you will have different options of image optimization that will usually better define edges and also fascicular structure of the nerve – switch them on and off for seeing if they assist you well.
Pic 2. Different levels of brightness achieved by adjustment of gain
a. Image too dark. Details will get lost.
b. Optimal adjustment of brightness. You have all levels of brightness in your image.
c. Image too bright. Details will get lost.
The next buttons of interest are those of time gain compensation (TGC). These can be understood as the “volume of the depth”. The more tissue the ultrasound beam has to penetrate, the more it gets attenuated. This leads to a fading of brightness in the deeper layers. With TGC you enhance the gain in the depth, thereby achieving an evenly distributed brightness on your image.
Pic 3. Different levels of brightness in the depth achieved by adjustment of TGC
a. The more superficial parts of the image are well defined, but the deeper layers of muscle and the bone get dark or lost.
b. Optimal adjustment of TGC. A good and evenly distributed brightness throughout the pic.
c. Here we have a little overcompensation of the attenuation, with the deeper layers even brighter than the more superficial ones.
For scanning of the lower limb nerves like tibial or sciatic nerve, you will have to choose different settings. Both you would pick up in the popliteal fossa, not as superficial as the upper limb nerves. Both would then dive even deeper, under the flexors of the knee or the ankle/foot. The depth in which you would expect the nerve is around 3 cm, therefore your settings should be around 5 cm to get a first overview and – in case of the tibial – also picture the guiding structure – the popliteal artery – adequately. The focus is set at the expected 3 cm. You might have to turn down frequency a little for the tibial and a little more for the sciatic (9-10 MHz are good for most patients). Adjust your TGC (time gain compensation) for the lower levels, to get enough brightness in the deeper layers.
In the deeper lying nerves, sometimes image optimization makes it even harder, so try if turning it off helps you with tracing the sciatic nerve over its whole course.
If you got your optimal image at the point of pick-up, save the settings to ease your future daily work.
PS: And don´t forget that you have to adjust your settings while scanning! One hand on the transducer, the second constantly optimizing the image!
b. Documentation of findings
1. The stills
THE basis of your diagnosis. As in most of the cases you are chasing a pathological alteration, you want to get the message across in your key images. If you are dealing with a segmental swelling of a nerve – which is the case in about 95% of your scans – you will have to prove the concept: proximally the nerve is normal, then the nerve suddenly changes and distally the nerve is normal again.
This results in at least 3 stills in transverse view you will have to save. Usually you will also use the still showing the point of maximum swelling for measurement of the cross-sectional area. Additionally, you should document the problem on a second plane – the corresponding longitudinal view. This can of course be a problem in lesions, e.g. trauma, extending over a longer segment. If you have the option of panoramic view on your machine, this is the perfect moment for it – to capture the whole extent of the lesion and also be able to measure distances. If you haven´t got panoramic view, try to catch multiple stills along the change, which at least allow a rough estimate of its expansion.
Pic 1. Documentation of stills in a classical ulnar neuropathy at the elbow (UNE):
a. normal-sized ulnar nerve proximal on the medial head of the triceps muscle.
b. site of maximum enlargement in transverse view – documentation including cross-sectional area.
c. normal-sized ulnar nerve a little distal to entrapment between the heads of the flexor carpi ulnaris muscle.
d. longitudinal view with panoramic view mode.
Note that sometimes catching a proper longitudinal view can be quite tricky, especially when it comes to lesions close to bony spurs like the fibular head.
Our advice is to put the nerve in the middle of the image, maybe stabilize the probe with two hands and slowly turn by 90°. If you lose the nerve – don´t waste time trying to find it again or trying to distinguish it from surrounding tendons. Just turn the probe back and start again. It´s completely normal that this takes some time to optimize!
In some cases, a hypervascularization within the nerve might be of interest. This especially applies to vasculitis of the peripheral nervous system or if you are searching for a tenosynovitis as a cause of entrapment neuropathy.
Hypervascularization can be found in other disease entities, but has not turn out to be an important marker for e.g. disease severity (like in idiopathic entrapment neuropathy). In other disorders, you would expect it to be important, but you rarely ever see it. This especially applies to inflammatory neuropathies like GBS (Guillain-Barre-Syndrome) or CIDP (chronic inflammatory demyelinating polyneuropathy) and also to neuralgic amyotrophy.
Anyway, if you search for hypervascularization, just make sure that your pulse repetition frequency is low, maybe around 3-5 cm/s, as you are looking for a slow flow of small intraneural vessels.
Pic 2. Hypervascularization in a tenosynovitis de Quervain. Note the low pulse repetition frequency of 2 cm/s for the slow flow!
2. The videos
Videos are your friend! You should write that down or maybe print it and pin it on the wall of your ultrasound lab next to your screen. In our experience this is one of the most frequent mistakes of beginners with neuromuscular ultrasound. No matter how sure you are after finishing the examination – you will forget where exactly the lesion was and you will take that one more pic of the nerve that you can´t classify later.
A short sequence across the lesion is so helpful if you ever have to open the patient file again. We consider a video you can start and stop manually the easiest to handle. Not one second more or less than you need. In case your findings are unremarkable, we usually only document stills.
Video 1. Video documentation of the same patient as shown above with UNE.
Sidenote: Label your images
Don’t spend hours and hours thinking about where now exactly that pathology was or maybe you are not even sure which nerve you documented. This is not easy from just seeing a nerve and a muscle below – which is a very frequent situation. Also arteries are often close to nerves and therefore won´t help you. It doesn’t matter if you do shortcuts like: “left median upper arm” or if you use that sketches of the human body on which you just have to change the position of the probe, just promise us: you label everything.
We hope that was helpful for you. If you have anything to add or comment – we are happy to hear!
See you soon.
Your Sonocampus Team