28.1 Introduction

Intradural spinal tumors represent a minority of all tumors involving the spinal canal, though they present unique challenges for the surgeon. One of these is visibility: Due to their location deep to the dura mater, they are not readily apparent following laminectomy, as opposed to epidural tumors, which often are (Fig. 28‑1). Intramedullary lesions can be even more challenging to visualize, requiring significant myelotomy for exposure. Risks of operating on intradural lesions include incomplete resection and spinal cord injury. Careful planning with preoperative imaging is essential (Fig. 28‑2), though intraoperative image guidance can help mitigate risk by providing real-time information to the surgeon to guide dural opening, refine goals of surgery, and estimate extent of resection. Ultrasound represents a rapid, readily accessible, reliable, and cost-effective imaging modality to serve this intraoperative role.

28.2 Background

Ultrasound technology relies on sound waves that, by definition, have a frequency exceeding what is audible to the human ear. These waves are emitted from a transducer, travel through space, and generate a signal when they are reflected back toward the transducer by an object. The majority of medical applications of ultrasound utilize frequencies in the range of 3 to 18 MHz. ¹

The earliest patents for ultrasonic devices were filed in 1912; however, the use of ultrasound for medical diagnosis was not documented until 1942 by Karl Dussik, an Australian neurologist who published a case report of ultrasound use for detecting an intracranial lesion. The routine use of medical ultrasound did not occur until the 1960s, when ultrasound devices became increasingly commercially available. ² Multiple case reports and case series from this decade describe ultrasound use for intracranial lesions. ^{3 ,​ 4 ,​ 5} The first documented use of ultrasound for visualizing a spinal intradural lesion was in 1978 by Reid. ⁶ Within the literature, there is evidence of increasing interest in the use of ultrasound for cases of spinal intradural tumors as the utility of ultrasound is better understood and as ultrasound technology becomes more ubiquitous within the operating room.

There are multiple commercially available medical ultrasound machines which have documented use for neurosurgical cases. At our institution, we utilize the Hitachi ProSound Alpha 7 machine with the Hitachi “hockey-stick” transducer probe for spinal intradural lesions. Other ultrasound manufacturers include GE, Philips, Siemens, and Toshiba (Fig. 28‑3).

The most commonly known and utilized mode of ultrasound imaging is B-mode imaging, which uses a linear array of transducers to generate a two-dimensional image of a plane through the body. The brightness of different areas on the image depends on the ability of various tissues to reflect sound waves, referred to as echogenicity. Brighter areas represent tissue that reflects more of the incident ultrasound wave and are therefore termed hyperechoic. Darker areas on the image represent weak ultrasonic reflection and are termed hypoechoic. High-density lesions include many solid tumors and are generally hyperechoic (bright) on ultrasound imaging, while fluid-filled areas such as intracranial ventricles and tumor cysts reflect ultrasound waves poorly and are hypoechoic. Doppler or duplex ultrasound is another commonly used mode to visualize blood flow magnitude and direction.

28.3 Uses

Various uses of intraoperative ultrasound during the resection of intradural tumors have been described, including real-time operative planning, diagnosis, and evaluation of resection.

28.3.1 Evaluation of Exposure

Intraoperative ultrasound can be used initially at the time of dural exposure to verify correct localization over the tumor. Evaluation of the extent of bony exposure bilaterally as well as superiorly and inferiorly is prudent to avoid a narrow working corridor during tumor resection. Ultrasound can be used to verify exposure of dura both above and below the level of the lesion, as well as laterally to the margins of the spinal canal. If inadequate exposure is found, appropriate widening of the exposure prior to dural opening should be performed to minimize the potential for bone fragments and blood to enter the thecal sac (Fig. 28‑4).

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29 Stereotactic Radiosurgery for Treatment of Primary and Metastatic Spinal Tumors 3 Intraoperative Ultrasound in Spine Surgery: A Versatile and Useful Adjunct 10 Navigated Subaxial Cervical Spine Pedicle Screw Instrumentation 16 The Role of Navigation in the Resection of Primary Spinal Tumor 23 Navigated and Robotic Anterior Odontoid Peg Fracture Fixation 31 Mounting the Learning Curve with New Technologies

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