Hi Linh,
You are spot on, it is radial tunnel syndrome / PIN entrapment / radial tunnel / it is all the same thing.
I have a few ways of scanning this.
Option1: have the patient sitting opposite me with their arm outstretched, palm up. Place the transducer just proximal to the elbow crease on the anterograde/lateral aspect of the upper arm straddling the brachioradialis origin and the brachial muscle in short axis. Look between these muscles and you will see the radial nerve. Nice and big, echogenic and really easy to see. Follow it distally in short axis and you will see it turn into a flattened nerve as is divides into 2 branches. The branch on the medial side is the superficial branch and this is a sensory nerve (ignore this one for now). The more lateral branch is the deep branch (motor branch) and this is the PIN. Follow it distally and it passes a small vessel (recurrent radial artery) then is passes into the supinator muscle (between the heads) at the arcade of Frohse. This is the common site of compression from a fibrous band. You can follow it through the supinator for a distance but you need another trick to see the full length.
Option 2: The trick is to flex the elbow 90 degrees, point their thumb to the roof and scan the PIN in the same plane as before. In this position you can perform resisted supination provocation which often produces compression and displacement of the PIN and see if this recreates symptoms. Then you can follow the nerve for a short distance as before. To see the more distal part of the nerve all the patient to lay their palm flat on the bed and you can easily follow the PIN more distally until it passes out of the supinator (this is another site of potential compression, but not as common). Then it splits up into small branches to innervate the forearm extensor bellies.
Option 3: For dummies (so I like it) . Ask the patient to flex their elbow 30 degrees and lat their palm flat on the bed. Use your simple surface anatomy to scan the proximal radius in short axis. Easy right! Then the muscle wrapping around the radius is the supinator. Scan up and down the radius in short axis while watching the supinator muscle and you will see the PIN (often 2 or even 3 fascicle bundles) in short axis in the middle of the supinator muscle. Once you spot it rotate the transducer into the long axis of the nerve and you have a great PIN image. So easy !!
Steve.