Intraoperative Ultrasound in Spinal Surgery

Summary

Here, we present a protocol on the use of intraoperative ultrasound in spinal surgery, particularly in cases of intradural lesions and lesions in the ventral spinal canal when using a posterior approach.

Abstract

Since the 1980s, there have been several reports for the use of intraoperative ultrasound as a useful adjunct in spinal surgery. However, with the advent of newer cutting-edge imaging modalities, the use of intraoperative ultrasound in spine surgery has largely fallen out of favor. Despite this, intraoperative ultrasound continues to provide several advantages over other intraoperative techniques such as magnetic resonance imaging and computed tomography including being more cost-effective, efficient, and easy to operate and interpret. Additionally, it remains the only method for the real-time visualization of soft tissue and pathologies. This paper focuses on the advantages of using intraoperative ultrasound, especially in cases of intradural lesions and lesions ventral to the thecal sac when approaching posteriorly.

Introduction

Ultrasound is one of the most common diagnostic tools in medicine, particularly for visualizing pathology in the abdomen, limbs, and neck. However, its use to investigate cranial and spinal lesions is not currently widely utilized. In 1978, Reid was the first to report the use of ultrasound to visualize cervical cord cystic astrocytoma¹. Here, scans were performed with the patient's neck flexed to allow opening of the intralaminar window. Four years later, in 1982, Dohrmann and Rubin reported the use of ultrasound intraoperatively to visualize the intradural space in 10 patients². Pathologies identified with intraoperative ultrasound among the 10 patients included syringomyelia, spinal cord cysts, and intramedullary and extramedullary tumors. They further demonstrated the use of intraoperative ultrasound to guide catheters and probes for biopsy of tumors, drainage of cysts, and ventricular shunt catheter placement³. This allowed the real-time monitoring and precise positioning of probes/catheters, reducing inaccuracy and errors in placement. Following these initial reports, several others have published the use of intraoperative ultrasound for guiding spinal cord cyst drainage, intramedullary and extramedullary tumor resection, and syringo-subarachnoid shunt catheter placement^{4,5,6,7,8,9,10}. Additionally, it has been shown to also increase rate of complete resection of intra-axial solid brain tumors and spinal intradural tumors^11,12. Intraoperative ultrasound has also proven to be useful for the intraoperative surgical planning before manipulation of the tissue and subsequent visualization of adequate neural element decompression in patients with spine fractures^7,9,13,14,15.

With the advent of newer intraoperative technology allowing clearer visualization of soft tissues, such as magnetic resonance imaging (MRI) and computed tomography (CT), intraoperative ultrasound has become less common and a less favored intraoperative imaging modality among neurosurgeons today¹⁶. However, intraoperative ultrasound can have advantages over these newer technologies in certain operative cases (Table 1). Intraoperative ultrasound has shown to demonstrate better soft tissue visualization of intradural structures when compared with intraoperative CT (iCT) or cone-beam CT (cbCT)^9,17. While intraoperative MRI (iMRI) is useful where available because of the higher soft tissue resolution it provides, it is costly, time consuming and does not provide real-time images^{6, 16,18}. An example is in the circumstance of an intradural mass ventral to the thecal sac that the surgeon is unable to directly visualize. Additionally, despite being operator dependent, from our experience, intraoperative ultrasound is fairly simple to use and can be easily read without a radiologist.

Protocol

The protocol illustrated here follows the guidelines of the human research ethics committee at Brigham and Women's Hospital.

1. Preoperative Protocol

1. Assess patients with spinal pathology in clinic and determine eligibility for spinal surgery. Perform neurological assessment and obtain CT or MRI scan to identify spinal lesion.
2. Include patients who have an intradural pathology such as schwannoma, ependymoma, meningioma, astrocytoma, etc.; or patients who have a ventral compressive extradural pathology, such as a ventral thoracic herniated disc, fracture fragments ventrally, or a spinal bony tumor with ventral compression.
   NOTE: Pathology is determined by spinal imaging with CT or MRI. Exclusion criteria includes the patients who cannot tolerate surgery, or patients with an extremely poor prognosis.

2. Preparation for Surgery

1. Do not allow the patient to consume anything by mouth after the midnight before surgery.
   NOTE: The patient will be placed under general anesthesia and intubated by the anesthesiologist.
2. Position patient with their back exposed according to surgeon's preference for the spinal surgery.
3. Sterilize the surgical area with povidone-iodine by scrubbing the area.

3. Surgery

Note: This section of the protocol follows general spine surgery techniques that can be referenced from any reputable spine surgery technique textbook¹⁹.

1. Make an incision with a scalpel along the length of the spine over the appropriate vertebrae levels and continue to make a straight incision down until the bone is reached.
   NOTE: The size of the incision will depend on the size of the pathology. For example, if the tumor spans two vertebral levels, then at least two vertebral levels will need to be exposed. When the bone is exposed, an X-ray with a portable X-ray machine can be performed to verify the correct vertebrae.
2. Perform subperiosteal dissection by electrosurgical cautery and expose the spinous process which is visualized as a bulbous bony process. Turn the cutting edge ventrally and sweep across the laminar.
3. Use a combination of a Leksell bone plier and a highspeed drill to remove the bony lamina and spinous process to expose the ligamentum flavum underneath.
4. Use an angled Curette and Kerrison bone punch to remove the ligamentum flavum to reveal the dura mater underneath.
5. Use bipolar and hemostatic matrix to achieve hemostasis.
   NOTE: The success of a good ultrasound image relies upon a clean surgical field.

4. Intraoperative Ultrasound

1. Use a mobile ultrasound machine and a transducer probe with a 20 mm diameter.
   NOTE: The probe should have a 10 to 4.4 MHz frequency range. Any comparable device with similar probe diameter and frequency range should suffice.
2. After the bony removal and dura exposure, fill the surgical field with sufficient saline solution such that the ultrasound transducer probe can be submerged.
   NOTE: Generally, a range of 100-500 mL of saline solution is needed. The saline solution allows for acoustic coupling.
3. Turn on the ultrasound machine and place the ultrasound probe within the saline bath at the level of interest to begin acquiring images.
   NOTE: It is not necessary to place the probe directly touching the dura or spinal cord. Images are acquired on the ultrasound screen in real-time and can be interpreted immediately by the surgeon. Images on the screen can be captured at any time by pressing the Freeze button and can be saved by pressing the Save button.
4. Acquire real time images in the longitudinal plane by placing the ultrasound probe in line with the direction of the spinal canal to visualize the spinal cord and the lesion similar to the sagittal images from the MRI.
5. Acquire real time images in the transverse plane by placing the ultrasound probe perpendicular to the spinal canal to visualize the spinal cord and the lesion like the axial images from the MRI.
6. Acquire real-time images to verify the location of lesions that cannot be directly visualized, to correlate with the preoperative CT or MRI images, to guide surgical tool placement, and/or to confirm the resolution of pathology.
   NOTE: When needed, a small piece of sterile compressed sponge approximately 0.5 cm x 0.5 cm can be used as a hyperechoic surgical marker to be placed in the surgical field and help correlate the surgical location with the image location. This helps to locate the lesion during the surgery and also helps to identify the margin of the tumor.

5. Postoperative Follow-up

1. After discharge, have the patient return to the clinic within one month for follow-up.
2. Perform a neurological assessment and CT or MRI scans to confirm the resolution of the symptoms and pathology.

Results

On normal spine ultrasound imaging, the dura is an echogenic layer that surrounds the anechoic spinal fluid. The spinal cord is distinguished by its homogenous appearance and low echogenicity which is surrounded by an echogenic rim. This echogenic rim is due to the density shift from the spinal fluid to the spinal cord. The central canal appears as a bright central echo, while exiting nerve roots appear highly echogenic, particularly at the cauda equina¹⁶. Intraope...

Discussion

Intraoperative ultrasound in the spinal surgery has largely fallen out of favor with the advent of newer technology, however, it continues to provide several advantages over the other available imaging modalities such as MRI and CT^6,9,16,17,18. In addition to being inexpensive, in this protocol we also show that it is simple to use and can provide visualizatio...

Materials

Name	Company	Catalog Number	Comments
Aloka Prosound 5 mobile ultrasound machine	Hitachi	N/A	any comparable devices on the market should suffice
UST-9120 transducer probe.	Hitachi	UST-9120	Has a 20mm diameter with 10 to 4.4 MHz frequency range (any comparable compatible transducer should suffice).