On the feasibility of imaging peripheral nerves using acoustic radiation force impulse imaging

Abstract

Regional anesthesia is preferred over general anesthesia for many surgical procedures; however, challenges associated with poor image guidance limit its widespread acceptance as a viable alternative. In B-mode ultrasound images, the current standard for guidance, nerves can be difficult to visualize due to their similar acoustic impedance with surrounding tissues and needles must be aligned within the imaging plane at limited angles of approach that can impede successful peripheral nerve anesthesia. These challenges lead to inadequate regional anesthesia, necessitating intraoperative interventions, and can cause complications, including hemorrhage, intraneural injections and even nerve paralysis. ARFI imaging utilizes acoustic radiation force to generate images that portray relative tissue stiffness differences. Peripheral nerves are typically surrounded by many different tissue types (e.g., muscle, fat and fascia) that provide a mechanical basis for improved image contrast using ARFI imaging over conventional B-mode images. ARFI images of peripheral nerves and needles have been generated in cadaveric specimens and in humans in vivo. Contrast improvements of >600% have been achieved for distal sciatic nerve structures. The brachial plexus has been visualized with improved contrast over B-mode images in vivo during saline injection and ARFI images can delineate nerve bundle substructures to aid injection guidance. Physiologic motion during ARFI imaging of nerves near arterial structures has been successfully suppressed using ECG-triggered image acquisition and motion filters. This work demonstrates the feasibility of using ARFI imaging to improve the visualization of peripheral nerves during regional anesthesia procedures.

Figures

FIG. 1

In vivo B-mode (left) and ARFI (right) images of the tibial and common peroneal nerves in a 29-year old subject, just distal to their bifurcation from the sciatic nerve in the popliteal fossa. The ARFI image was generated using two excitation focal zones at 15 and 20 mm with the VF10-5 linear array. The peak displacement in the ARFI image is 4 μm. ECG data acquisition gating was not used in generating the ARFI image. Notice that the two nerves are clearly delineated as stiffer (darker) circular structures in cross-section; their location is not readily apparent in the B-mode image (they have been outlined in yellow (tibial) and green (popliteal) based on the ARFI image boundaries). The improvement in nerve contrast is over 600% in the ARFI image compared with the B-mode image.

FIG. 2

In vivo B-mode (left) and ARFI (right) images of the same sciatic nerve shown in figure 1, but in a more proximal location in the upper third of the thigh, imaged longitudinally from a posterior approach using a VF7-3 linear array. Green arrows have been used to highlight the nerve location in both the B-mode and ARFI images; the arrows are in identical positions in both images.

FIG. 3

In situ distal cadaveric sciatic nerve with an 18 gauge needle intentionally piercing the upper left quadrant of the nerve sheath, imaged using the VF10-5 linear array. There is significant decorrelation deep to the needle due to poor SNR but the tip of the needle and the edge of that decorrelation can be seen inside the left border of the sciatic nerve, which appears dark (stiff) in the ARFI image. The nerve contrast improvement is~300% in the ARFI image compared with the B-mode image.

FIG. 4

In situ cadaveric sciatic nerve after an intentional intraneural injection with 3 cc of saline in the left half of the nerve imaged using the VF10-5 linear array. The location of the nerve has been circled in the B-mode image using some circumferential landmarks from the corresponding ARFI image. The needle was removed before the image was acquired. Notice the ARFI image contrast reversal in the left half of the nerve that has been infused with saline compared with the right half of the nerve that was not injected.

FIG. 5

B-mode and ARFI images of the brachial plexus in vivo from an interscalene approach pre-injection (top row), after a 2 cc saline injection (middle row) and after an additional 4 cc saline injection, for a total of 6 cc saline injected into that site (bottom row). Imaging was performed using the VF10-5 linear array. Pre-injection, the needle (yellow arrows) was placed adjacent to the lower left aspect of the nerve bundle (green arrows). Notice that the substructures of the nerve bundle spread apart and are displaced to the right after the saline injections. Some of the nerve substructures translated over 5 mm laterally in response to the injection. This translation is more apparent in the ARFI images than the corresponding B-mode images.

FIG. 6

In vivo B-mode and ARFI images of the rectus abdominus muscles and rectus sheath in a 29-year old subject. The posterior sheath is located at a depth of 14–16 mm in these images; the anterior surface is the target for local anesthetic injection. The ARFI image highlights the greater mechanical contrast that exists in this targeted layer compared with the more superficial fascial planes (i.e., it is darker, representing greater relative stiffness compared to the adjacent muscle and superficial fascia).