The Invisible Made Visible: How Glowing Arteries Guide the Surgeon’s Hand

November 03 • Surgical Innovation & Applied Technology

How do neurosurgeons manage to see something as invisible as blood flow inside the brain—while the brain is still alive and moving?

Waiting for the elevator to arrive, I pulled out my phone to check LinkedIn, only to find a surgical video of a green artery pulsing through the circle of Willis. Instantly, I clicked and was taken to a clipping aneurysm case. What caught my attention was how clearly I could see the vessel—no guessing, no searching, no struggle to identify where it was. The title of this post: How AR Fluorescence Imaging Supports Neurovascular Surgery.

I had no idea that, until recently, surgeons actually had to pause a procedure just to figure out where the blood was flowing. And in brain surgery, seeing a single vessel clearly can mean the difference between safety and catastrophe.

This article is written for anyone interested in the field of medicine, more specifically surgery and biomedical engineering. If you are curious about the use of imaging and its advancements in modern surgery, then this article would be of value. The language is clear and concise for various levels of understanding.

The Evolution of Visualization

During complex vascular surgeries, surgeons once had to pause the procedure just to visualize blood flow. Real-time visualization was missing; instead, they relied on pre-operative imaging, plus visual and tactile feedback, to judge whether their actions were correctly directed. Naturally, this sometimes led to errors, especially when the anatomy around the operative field was misunderstood.

With the introduction of fluorescence, vessels of interest could finally "shine" under white light. This was a major step in visualization. Surgeons could stop the surgery, take a snapshot of the area, and study the anatomy with better clarity. But—they still had to stop the surgery. This interruption was inefficient, and it left room for new errors: the surgeon saw the vessel only at one moment in time, and once the operation continued, it became easy to lose orientation if the vessel wasn't perfectly tagged or exposed.

The next step was to make fluorescence live. And that is exactly what Leica developed: a system that allows surgeons to observe fluorescent vessels in real time, directly through the microscope, while they operate. This reduces confusion, maintains the flow of microsurgery, and ultimately improves patient outcomes.

This is how Prof. Dr. med. Nils Ole Schmidt describes integrating augmented reality into his neurovascular workflow:

"With the inclusion of the aneurysm using GLOW800 — we regularly use ICG fluorescence angiography — but now, with GLOW800, it can be incorporated into the neurosurgical procedure much more fluently because you can view it inside the microscope. It has false colors, and we see the anatomical relationship between the aneurysm and the fluorescence, which we were not able to see when we used the conventional ICG. So it's much more integrated into the normal flow of microsurgery."

Operating on an Aneurysm

Now, we will step into the surgeon's shoes. We will study a neurovascular condition and operate on it using fluorescence imaging.

A patient is diagnosed with an aneurysm of the internal carotid artery—a major artery in the neck that supplies oxygenated blood to the brain, eyes, and surrounding structures. In this case, the aneurysm is located in the circle of Willis, at the center of the brain. The aneurysm is detected on MRI, which shows a bulging area of the artery caused by damage to the vessel wall that leads to blood accumulating in a weakened section of the vessel. As pressure increases, the aneurysm can enlarge and may eventually rupture, causing a subarachnoid hemorrhage—a life-threatening bleeding into the brain and the surrounding spaces.

To prevent this from happening, surgeons can use different techniques. In this case, we choose clipping, where micro-metallic clips are applied across the neck of the aneurysm to prevent the pressure from increasing in that area and to restrict blood flow into the weakened sac. (Another option is to use a catheter to fill the aneurysm with a coil, stopping blood flow into it, but clipping is the technique used here.)

Into the OR

We begin by performing a craniotomy (cranio = bone around the brain, tomy = cutting) at the appropriate angle and side of the head. We position the microscope, then make an incision through the dura to access the vessels.

ICG is injected intravenously, and the dye circulates through the blood vessels within seconds, reaching our area of interest. At this moment, the microscope uses its imaging system to shine near-infrared (NIR) excitation light onto the dye. The microscope also provides normal white-light illumination for the anatomy. When the NIR light excites the ICG dye, it emits a slightly different wavelength back, which the microscope captures and displays in a pseudo-color of choice, such as green.

As we dissect around the aneurysm, we see the anatomy under white light, while the blood vessels appear in a bright pseudo-colored fluorescence signal. This allows us to visualize live blood flow in the vessel while we operate.

We then identify the aneurysm, its branches, and the anatomical relationship between the vessel and the aneurysm so that critical vessels are not mistaken for the aneurysm itself. This is where GLOW800 adds major value—it highlights the aneurysm and clearly differentiates it from nearby vessels that may look similar under white light alone.

Clipping and Verification

We place the clip across the aneurysm neck, which stops the blood flow into it. Because the fluorescence travels with the circulating blood, our imaging should now show no NIR emission from the aneurysm if the clipping was successful. This confirms that the aneurysm has been excluded. At the same time, the rest of the vessel should continue to show smooth fluorescence, verifying that normal blood flow is preserved.

How It Works

The way this system functions is by separating and recombining different wavelengths of light to reveal information the human eye cannot see on its own. Fluorescence from ICG becomes excited at around 790 nm, which lies in the near-infrared spectrum. During surgery, the microscope projects both normal white light and NIR excitation light onto the operative field. The ICG dye circulating in the vessels absorbs the NIR light and then emits a slightly longer wavelength back—this is the fluorescence signal. The multispectral sensor inside the microscope detects this emission independently from the white-light image and transforms it into a pseudo-color that the surgeon selects (green, blue, magenta, etc.), depending on what creates the best contrast. The surgeon can also adjust the opacity of the fluorescence, essentially choosing how dominant or subtle the signal should appear.

To improve clarity, the system uses a digital homogenization filter that corrects brightness. Earlier fluorescence techniques often produced a bright vessel surrounded by a darker field of view, which made interpreting the anatomy challenging. With homogenization, the surgeon can adjust luminance from 0% to 100%, creating a uniform brightness across the entire image while the vessel remains distinctly highlighted. A similar control exists for intensity: at high magnifications, light naturally decreases, so the system scales the fluorescence signal to keep it visible without overexposing the tissue.

After processing, the microscope merges the white-light anatomy and the fluorescence data into a single real-time high-definition 3D view. This allows the surgeon to move through different planes of the anatomy, maintain spatial awareness, and visualize vascular flow dynamically as decisions are being made. The result is a cleaner, more intuitive image that keeps the surgical field natural while enhancing the structures that matter most.

The Future of Surgery

Innovations like these inspire the future of patient care. Procedures become safer while remaining ethical. The skills of the surgeon and the operating team are still at the center, and this device simply facilitates their work instead of replacing it. Now we can visualize the direction of blood flow in the most complex and dangerous organ to operate on—the brain.

Its impact does not only benefit neurovascular surgeons; it supports many other specialties and even medical students, offering them a powerful way to see anatomy come alive. This technology transforms structures that were once invisible into something clear and observable.