Ultrasound of nerves does have some special requirements as we are assessing small to very small structures. While “bigger” nerves like the median or ulnar nerve present with cross-sectional areas (CSA) from 8 to 15 mm2 and do therefore not impose a major problem in depiction, the lateral cutaneous femoral nerve or saphenous nerve will challenge your measurement tools with CSAs below the usual lower limit of 1 mm2.
Depending on the equipment you already have at hand, you should be able to start out with the bigger nerves to get an idea, but you will definitely reach a point where you might want to invest a little for also getting the details.
Ultrasound of nerves is usually carried out with linear transducers of different frequencies – depending on your needs:
Let´s start with the lower end of resolution that comes with a higher tissue penetration depth: a transducer working with 7-9 MHz. This is generally works well in the sciatic nerve – the biggest nerve in the human body; it is deep-lying and covered by thick layers of muscles. With every other nerve you will not be satisfied, because it will not give you the details.
The next category is somewhere around 12-15 MHz. For nerve ultrasound, this is the allrounder among transducers. It will give you sufficient resolution to check the major nerves of the limbs as well as the brachial plexus. If you want to invest in one probe – take one of those. It allows you to check median, ulnar, radial, peroneal and tibial nerves throughout their whole visible course and also judge their internal morphology to some extent.
If you want to extend your spectrum to the smaller nerves, the next step is somewhere around 18-24 MHz. This gives you details of nerves depicting themselves as only one hypoechogenic spot on your screen. You can find and follow e.g. the recurrent motor branch of the median nerve as well as the calcaneal branches of the tibial nerve.
To give you an idea how the different nerves look with the different transducers and why a different transducer may be the better choice, we put together Figures 1 & 2. Both show the maybe most classical view of all: the median nerve at the wrist. Depth is adjusted to 2 cm in all of the images, the focus set as superficially as possible, the preset is always the same.
Figure 1 demonstrates the general appearance of the median nerve, as an example of a larger nerve of the upper extremity.
The image on top is made with a transducer working with 9 MHz, the one in the middle with 15 MHz and the one at the bottom with 18 MHz.
Note the difference in a) depiction of details of internal structure of the nerve, but also in b) the field of view. With the lower frequency you get a lot more of the surroundings. Therefore you might sometimes use a lower frequency probe to localize the nerve and then switch to one with a higher frequency to get the details.
Figure 1. The median nerve at the wrist as seen with different transducers.
Figure 2 concentrates on a tiny branch of the median nerve as an example of a really small nerve that will only depict itself as a single, or two, fascicles: the palmar branch of the median nerve, which provides sensory innervation to the skin above the thenar.
Again you see 9 MHz at the top, 15 MHz in the middle and 18 MHz at the bottom.
In this case you will clearly have problems defining the nerve on a still image with 9 MHz, while you will see it in the other two images if you know where to look.
Figure 2. The palmar branch of the median nerve as seen with different transducers.
As finding nerves on stills is always difficult and definitely not recommended, we also added three videos and marked the palmar branch for you.
Have a look and see the difference!
So besides the difference in depiction, we want you to take one central message with you:
We hope that was useful for you.
Have a nice day and hope to hear from you soon.