A Direct Comparison of Near-Infrared Imaging Camera Systems for Detecting Intracranial Tumors

 

Objective Distinguishing neoplasm from normal brain parenchyma in real time, in the operating room, is critical for the neurosurgeon. Systemic injection of 5-aminolevulinic acid (5-ALA), a visible light fluorophore prodrug, has been used in meningioma surgeries with variable benefit. Near-infrared (NIR) fluorophores, such as indocyanine green (ICG), have advantages over visible light fluorophores, mainly due to greater tissue penetration by the fluorescence and reduced background fluorescence. We have recently described a technique for NIR fluorophore accumulation within brain tumors, including meningioma, craniopharyngioma, and schwannomas—“Second Window ICG.” We chose to compare a state-of-the-art neurosurgical microscope (System 1) to a commercially available NIR visualization platform (System 2) to better assess our capabilities to measure NIR fluorescence in the operating room.

Methods The two imaging systems (Leica OH6 and VisionSense Iridium) were compared both in vitro and in vivo. Serial dilutions of ICG from 0.47 to 1,007 µg/L were imaged with both systems under the same conditions and NIR signal for each concentration was recorded. Each system's sensitivity and dynamic range for NIR fluorescence were documented and analyzed. In addition, the two systems were used in the operating room to image intracranial tumors in 12 patients who were a part of our larger SWIG (second window ICG) study. Each patient received 5 mg/kg of intravenous ICG infusion ∼24 hours prior to surgery. In the operating room, tumor resections were conducted using conventional technique, and NIR imaging was performed with both systems prior to dura opening, immediately after dura opening but with the cortex intact, and after exposure of the tumor below the cortex, or upon extra-axial exposure in skull base surgery.

Results In vitro, System 2 demonstrated greater ICG sensitivity and detection range (System 1: 1.5–251 µg/L with maximum SBR of 3.25 vs. System 2: 0.99–503 µg/L with maximum SBR of 12). Similarly, System 2 demonstrated greater NIR sensitivity in vivo. In the cohort of 12 patients, a total of 26 intraoperative NIR images were taken with System 1 and 24 with System 2. Prior to dura opening, System 1 was not able to reliably detect the ICG fluorescence, with an SBR of 1.2 ± 0.15, while System 2 was able to detect NIR fluorescence from the tumor with an SBR of 2.0 ± 0.69 (p = 0.0168). Similarly, after dural opening, System 1 and System 2 detected NIR fluorescence with SBRs of 1.4 ± 0.23 and 4.6 ± 1.8, respectively (p <0.0001). Finally, with the tumor in direct line of sight, System 1 and System 2 detected ICG fluorescence with SBRs of 1.6 ± 0.30 and 6.8 ± 1.9, respectively (p <0.0001).

Conclusion Dedicated NIR imaging platforms can outperform conventional microscopes in intraoperative NIR detection with significantly greater sensitivity and detection range. Future microscopes that take advantage of this improved NIR sensitivity and incorporate it into the neurosurgery operative workflow could enhance the use of intraoperative NIR fluorescence to detect neoplasm and improve patient outcome.