USRA - Scanning Technique

Nerve

Take advantage of a real time anatomical examination every time a transducer is placed on the skin surface of a patient.
Perform a complete and systematic anatomical survey in the region. This is similar to reading a chest x-ray.
Nerves may not be readily visible at the first glance but vessels, muscles and bones are easily identifiable under ultrasound. These structures will define the target nerve location.
It is important to visualize structures that should not be entered or trespassed (e.g., the first rib and pleura in the supraclavicular region).

Finding the Sciatic Nerve in the Popliteal Region

First identify 1) the femur (Figure A), 2) the popliteal artery (PA, Figure B), and 3) the muscles (semitendinosus and semimembranosus ST/SM, medially and biceps femoris laterally, BF). This defines the region where the sciatic nerve will be found.
The sciatic nerve is consistently superficial and posterior to the femur, superficial and lateral to the popliteal vessels and between the muscle layers (Figure C).
Slightly angle the transducer towards the foot (caudad) to capture a clearer nerve image (Figure D).
Note the hyperechoic area medial to the PA is NOT a nerve structure (arrow, Figure B).

	Arrow = hyperechoic area PA = popliteal artery ST and SM = semitendinosus and semimembranosus muscles
Arrowheads = sciatic nerve PA = popliteal artery ST and SM = semitendinosus and semimembranosus muscles	Arrowheads = sciatic nerve PA = popliteal artery

Needle Tip

Finding the needle tip during ultrasound guided nerve block can be technically challenging. This is particularly true with the out of plane needle insertion technique.

1. Transducer Movement to Locate the Needle Tip During Out of Plane Needle Insertion

A. The needle is inserted out of plane with the transducer.

The needle image is not seen because:

The transducer is still far away from the needle thus the beam is not crossing the needle or
The beam hitting the needle is deflected away from the transducer and not returning to the transducer because of the angle of incidence (less than 90 degrees)

B. Maneuver # 1:

Tilt the needle tip to a more superficial position by decreasing the angle of insertion (blue arrow, i.e., dropping the hand); this will bring the needle and the beam closer to a 90 degree angle of incidence.
It is also useful to wiggle the needle tip from side to side or slightly in and out (a small jabbing movement) until the needle is seen (white arrowhead showing the needle shaft).

C. Maneuver # 2:

To find the needle tip, it is important to move the transducer towards the needle tip and then away from the needle tip. This scanning movement will determine whether the observed bright dot is the shaft or the tip of the needle. The needle tip is indicated by a white dot that is deepest in the tissue (white arrowhead). The dot will disappear once the transducer is no longer over the needle tip.

2. Hydro Location Technique

It can be technically challenging to locate the needle tip when the needle is inserted at a steep angle (> 45 degrees) and when the target is > 4-5 cm deep.

Injection of a small amount of fluid (0.5-1 mL) through the needle will create an image of tissue expansion on ultrasound. This will indicate the location of the needle tip. Dextrose 5% solution (a non conducting medium) is injected if electrical stimulation is desired for nerve confirmation. Alternatively, saline or local anesthetic (conducting medium) can be injected if nerve stimulation is not required.

Demonstration of Hydro Location During Infraclavicular Block

Figure A = pre-injection Arrowhead = nerve AA and AV = axillary artery and vein PMM and PMiM = pectoralis major and minor muscles	Figure B = needle tip is within PMiM as indicated by local tissue expansion (asterisk ) Arrowhead* = nerve AA and AV = axillary artery and vein PMM and PMiM = pectoralis major and minor muscles
Figure C = needle tip is now deep to PMiM and anterior to AA as indicated by local fluid expansion (asterisk) although the tip is not visualized Arrowhead* = nerve AA and AV = axillary artery and vein PMM and PMiM = pectoralis major and minor muscles

3. Echogenic Needle

It is easier to detect a needle with an echogenic tip. The figure shows an example of the echogenic tip needle (Hakko™ Medical Co. LTD Japan) with 3 hyperechoic dots at the needle tip (arrows).

Scanning Technique - Localization