Fluorescence Imaging Offers Advanced Surgical Navigation

Fluorescence imaging is a rapidly evolving form of surgical navigation. Surgical guidance aims to answer fundamental questions for the surgeon: Where is the surgeon located in the operative field? Where is the target structure? What critical structures are close and may be at risk? Answers to these questions orient surgeons to anatomical landmarks and may improve outcomes.

A brief history of surgical navigation
Neurosurgery introduced the earliest forms of surgical navigation, including the first clinical stereotaxic instruments in the 1950s. Modern computing in the 1960s and 1970s, followed by revolutionary advances in medical imaging through the turn of the century (e.g., CT, MRI, ultrasound, positron emission tomography, single photon emission computed tomography), led to anatomical and functional imaging modalities that underpin today’s state-of-the-art navigation technologies. In the past 30 years, imaging-based surgical navigation has become common in many surgical disciplines, including craniomaxillofacial surgery, otorhinolaryngology, and orthopaedics.

Key limitations continue to drive innovation of navigation technologies. Stereotaxic frames mechanically link preoperative imaging and patient anatomy to a unified coordinate system, but stereotaxic instrumentation is cumbersome and reduces access to the surgical field. Anatomical models do not account for unique or altered anatomy from pathological states (e.g., tumors, infection, trauma). Preoperative imaging, although invaluable for surgical planning, is of limited intraoperative value due to the deformation of tissues from altered positioning and dissection. These limitations highlight the need for real-time imaging to dynamically track the surgeon’s operative position relative to the patient’s anatomy, to identify critical structures, and to assess tissue function.

Radiological imaging-based navigation—primarily with CT and MRI—improves upon the inherent limitations of preoperative imaging alone. However, the former exposes the patient to additional ionizing radiation, and the latter can be cost- and time-prohibitive. Intraoperative ultrasound is non-ionizing and can be cost-effective, handheld, and real-time. Yet, ultrasound is prone to artifacts, requires specialized expertise for image interpretation, and cannot resolve structures containing bone or air (e.g., lungs, bowel).

Optical imaging encompasses a wide range of technologies capable of providing high resolution, a wide field of view, and real-time information in the surgical field, and it is non-ionizing. For example, frameless stereotaxic solutions, first conceptualized by Roberts et al in the 1980s, reduce the surgeon’s reliance on conventional instrumentation and instead rely on wireless signals, such as optical stereovision or structured light, to determine the precise spatial location of the surgeon’s tool relative to the 3D topography of the surgical field.

Role of fluorescence in surgical navigation
Fluorescence-guided surgery (FGS) is an emerging form of optical surgical navigation. The term “fluorescence” was introduced in the 1850s by G. G. Stokes, who observed that certain materials, when exposed to specific wavelengths of light, reemitted light of a longer wavelength (i.e., light of a different color). The phenomenon involves the absorption of a photon by a fluorescent molecule—fluorophore—that enters an electronically excited state. The excited molecule then spontaneously undergoes non-radiative decay, emitting a photon of a longer wavelength. This wavelength (spectral) shift between the excitation light and the reemitted light (i.e., fluorescence) is known as Stokes shift. Fluorescence guidance involves illuminating the surgical field with a specific wavelength of light, then capturing the spectrally distinct, reemitted light with a specialized imaging system.

Fluorescent agents used in FGS can be broadly categorized by their mechanism of tissue labeling: non-targeted intravascular perfusion agents (i.e., vascular angiography), targeted agents that molecularly bind to specific biomarkers (i.e., tissue-specific labeling), and enzymatically or metabolically activated agents (i.e., physiological environment-specific labeling). The primary applications of FGS are tissue perfusion assessment, lymph node mapping, critical structure mapping (e.g., bile ducts, ureters, nerves), and tumor imaging.

The first example of FGS came in the 1950s, when intravascular fluorescein was used to enhance contrast to intracranial neoplasms. Perhaps the most well-known application of FGS is 5-aminolevulinic acid (5-ALA)-induced protoporphyrin IX fluorescence to guide glioma resection, first reported by Stummer et al in the early 2000s. Today, multiple FGS imaging systems and fluorescent agents are FDA-approved for clinical use. FDA-approved fluorescent agents include: indocyanine green (ICG) for cardiovascular and lymphatic angiography, bile duct imaging, and gastrointestinal tract imaging (2000s); fluorescein for angiography of the retina and iris vasculature (2000s); 5-ALA for high-grade glioma resection (2010s); and most recently, OTL-38 for identification of ovarian and lung cancer lesions and LUM015 for identification of breast cancer (2020s).

The future of fluorescence-guided surgery
The rapidly advancing field of biomedical optics promises performance augmentation of FGS technologies. Multichannel imagers enable spectrally resolved measurements of fluorescence through the simultaneous detection of multiple fluorophores. Fluorescence measurements can be temporally resolved (i.e., fluorescence lifetime imaging), offering additional contrast to target tissues, such as tumors (see recent work by Pal et al). Other forms of optical imaging can be integrated into FGS systems as well, such as spectroscopy or spatial frequency domain imaging, to quantify optical properties of tissues, oxy- and deoxy-hemoglobin concentrations, and tissue oxygen saturation over a wide field of view. These quantitative datapoints add to the suite of anatomical and functional information available to the surgeon for enhanced surgical navigation.

The rise of robotic surgery, starting in the mid-1990s and expanding through the 2000s with the FDA approval of the first da Vinci system, represents an opportunity for these optical imaging technologies. Synergy exists between advanced optics and robotic surgery, given that surgical robots frequently rely on optical imaging for interfacing the surgeon with the surgical field. The integration of fluorescence guidance into existing optical subsystems of surgical robots—already a growing trend—is a natural evolution of cutting-edge surgical guidance technology.

The authors’ research group explores the application of fluorescence guidance to cancer, nerve, and necrotizing infections (NSTI, a.k.a., necrotizing fasciitis). Their work in soft-tissue sarcoma has demonstrated useful fluorescence contrast in a first-in-human clinical trial deploying a near-infrared fluorophore that targets the epidermal growth factor receptor, overexpressed in many sarcomas. The authors have subsequently combined this technology with the use of nerve-specific fluorescence, working with the Gibbs Lab (Oregon Health and Science University, Portland, Oregon) (Fig. 1). In NSTIs, results have demonstrated significant ICG signal voids in patients with true NSTIs compared to other major soft-tissue infections with similar presentations, and a consequence of pro-thrombotic tissue factor production by NSTI-causing bacteria, producing microthrombi in affected tissues (Fig. 2).

The potential applications of FGS are far-reaching, and an unprecedented number of FGS technologies (imaging systems and fluorescent agents) have been recently approved or are currently in clinical trials for approval. For example, in the past 5 years alone, the number of imaging systems dedicated for ICG-based FGS that have secured 510(k) FDA approval has increased dramatically, including macroscopic, microscopic, endoscopic, robotic, and handheld imagers. Although only two molecularly targeted fluorescent agents are currently FDA-approved for clinical use (i.e., OTL-38, LUM015), several are under evaluation in clinical trials, including protease-activated AVB-620 for labeling breast cancer tissue, ALM-488 for labeling nerves, EMI-137 for labeling colon cancer and lymph node metastases, and BLZ-100 for labeling pediatric central nervous system cancers, to name a few.

Surgical navigation has evolved greatly over the past 70 years, encompassing imaging modalities beyond those routinely familiar to orthopaedic surgeons. The next decade is likely to see an unprecedented increase in FGS imaging systems, approved fluorescent agents, and their synergistic use.

Eric R. Henderson, MD, directs the Sarcoma and Connective Tissue Oncology Program at Dartmouth College in Hanover, New Hampshire, and is an associate professor of orthopaedics and adjunct faculty at the Thayer School of Engineering.

Samuel S. Streeter, PhD, is a scientist in the Department of Orthopaedics at Dartmouth Hitchcock Medical Center in Lebanon, New Hampshire, and an assistant professor at the Geisel School of Medicine at Dartmouth College.

References

1. Roberts DW, Strohbehn JW, Hatch JF, et al: A frameless stereotaxic integration of computerized tomographic imaging and the operating microscope. J Neurosurg 1986;65(4):545-9.
2. Stummer W, Novotny A, Stepp H, et al: Fluorescence-guided resection of glioblastoma multiforme by using 5-aminolevulinic acid-induced porphyrins: a prospective study in 52 consecutive patients. J Neurosurg 2000;93(6):1003-13.
3. Pal R, Lwin TM, Krishnamoorthy M, et al: Fluorescence lifetime of injected indocyanine green as a universal marker of solid tumours in patients. Nat Biomed Eng 2023;7(12):1649-66.