Endomicroscopy in the Pancreaticobiliary Tree

The concept of confocal microscopy was first patented in 1957. Since then, this technology has been used with much success in the laboratory, and more recently this has translated into use in various clinical settings, first described in the field of gastroenterology in 2004 for diagnosing colorectal lesions.

Confocal laser endomicroscopy (CLE) combines the principles of confocal laser microscopy with the endoscope to allow high-resolution images of the gastrointestinal (GI) tract mucosa in real time. In essence, a low-power laser is shot through a pinhole and lens onto the tissue of interest at a certain depth. Only the light reflected back through the same lens and pinhole is detected by the collection system, which is on the same plane as the laser—thus the term “confocal.” Any scattered or unfocused reflected light is excluded from detection, leading to great spatial resolution at the plane of interest. Combined with a flexible endoscope, in vivo “optical biopsies” of the GI mucosa at the cellular level are made possible.

Confocal Laser Endomicroscopy

Although two confocal microscopy imaging systems exist, fluorescence and reflectance, the former is used primarily during GI endoscopy. With fluorescence, intravenous and topical contrast agents are used to highlight different structures. Intravenous fluorescein courses along the vasculature to highlight the extracellular matrix and lamina propria of the surface epithelium, allowing the differentiation of normal and abnormal structures. Topical agents such as acriflavine and cresyl violet achieve nuclear staining but are less popular in the West. In the reflectance type, no contrast agent is required, but it provides limited architectural analysis.

Intravenous fluorescein is the most commonly used contrast agent for CLE, with the most common dose being 2.5 to 5 mL of 10% sodium fluorescein. Becker et al. reported an optimal window ranging from 0 to 8 minutes after injection for viewing the time-dependent CLE imaging, with interpretable images lasting up to 30 minutes. Topical contrast involves removing the mucin layer with N -acetylcysteine or acetic acid, flushing with water, and using a spraying catheter to apply the agent. However, this nuclear staining has raised concerns for DNA damage and is not currently FDA-approved.

Two endoscopic modalities exist. Probe-based CLE (pCLE) uses a separate probe requiring an accessory channel, needle, or catheter, whereas endoscope-based CLE (eCLE) uses endoscopes with already-integrated CLE systems. Specifically, the pCLE system comprises a fiberoptic bundle with an integrated distal lens that is connected to a laser-scanning unit. Depending on the manufacturer, the required diameter of the accessory channel (0.9 to 2.8 mm), reusability of the probe (10 to 20 examinations), depth of imaging (40 to 70 µm), and resolution (1 to 3.5 µm) differ. The eCLE is too large for pancreaticobiliary imaging and is no longer commercially available, so with the focus of this chapter in mind, any mention of CLE going forward will refer to pCLE.

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