Ultrasound is a valuable tool for visualizing peripheral nerves, offering insights that aid in various medical procedures and diagnostics. This non-invasive imaging modality provides real-time views of nerve structures, assisting healthcare providers in understanding nerve health. It is particularly useful for identifying structural changes, compressions, or pathologies affecting nerves, complementing other diagnostic methods. The ability to visualize nerves directly makes ultrasound an effective technique for both diagnostic purposes and guiding interventions.
Visual Characteristics of Nerves
Nerves display distinct appearances on ultrasound depending on the viewing angle. In a transverse (short-axis) view, a normal nerve often appears as a “honeycomb” structure. This results from hypoechoic fascicles separated by hyperechoic septa, corresponding to the epineurium. The overall shape is typically oval to round.
In a longitudinal (long-axis) view, nerves present a “railroad track” appearance, characterized by parallel hypoechoic fascicles interspersed with hyperechoic connective tissue. Echogenicity can vary, with some normal nerves appearing generally hypoechoic depending on their type and anatomical location. For instance, nerve roots, trunks, and cords, as well as the ulnar nerve at the elbow, may naturally appear darker.
Nerves exhibit specific physical properties under ultrasound. They are generally compressible, changing shape under probe pressure and regaining it when released. Nerves also demonstrate anisotropy, meaning their appearance changes significantly with slight alterations in the ultrasound beam angle. This property requires precise probe positioning for optimal visualization.
Essential Ultrasound Techniques for Nerve Localization
Effective nerve visualization relies on specific techniques and machine adjustments. A high-frequency linear array probe (8-15 MHz) is preferred for superficial nerves, providing high-resolution images. Proper probe orientation, holding the transducer perpendicular to the nerve, optimizes image quality and ensures accurate measurements.
Optimizing machine settings like gain (brightness) and depth (focal zone) significantly improves nerve visibility. Ample ultrasound gel is also important to maintain good probe-skin contact, preventing image artifacts and facilitating smooth probe movement.
Dynamic scanning is a practical technique for following nerve pathways and confirming their identity, involving subtle probe movements along the expected course. Clinicians often use “traceback” or “trace forward” methods from known anatomical landmarks. Patient positioning also optimizes nerve access and visibility, often requiring specific limb adjustments.
Distinguishing Nerves from Surrounding Structures
Differentiating nerves from structures like tendons, blood vessels, and muscle fascia requires careful observation. Tendons can appear fibrillar in longitudinal view but typically show a “sliding sign” with muscle contraction, unlike nerves. They also have a more uniform hyperechoic appearance compared to the mixed echogenicity of nerves.
Blood vessels can be mistaken for nerves due to their tubular shape, but they collapse under probe pressure, unlike nerves. Color Doppler ultrasound definitively distinguishes vessels by showing blood flow, which is absent in nerves. Pulsations may also be visible in arteries.
Muscle fascia, the connective tissue surrounding muscles, appears as bright lines but lacks the distinct fascicular pattern or tubular/honeycomb architecture of nerves. Confirming identity often involves dynamic scanning, tracing its course, and observing its behavior under compression or movement.
Common Nerve Scan Targets
The median nerve at the wrist is a common target, located superficially within the carpal tunnel, just deep to the flexor retinaculum. It appears as hypoechoic fascicles surrounded by hyperechoic epineurium. Enlargement or flattening can indicate conditions like carpal tunnel syndrome.
The sciatic nerve, a large nerve in the thigh, is a frequently scanned structure. It appears as a prominent, multi-fascicular structure in the posterior thigh, usually between the hamstring muscles. Tracing it proximally from the popliteal fossa helps confirm its identity.
The brachial plexus, a collection of nerves in the neck and axilla, can be identified by its honeycomb appearance. Its components course through anatomical landmarks like the scalene muscles or adjacent to the axillary artery. Identifying these structures often involves tracing them from their origins or well-known adjacent vascular landmarks.