Fluorescence imaging

Mechanism of fluorescence generation

The method is based on the ability of specific molecules—fluorophores (fluorophore)—to respond to external radiation. The signal generation process can be divided into several sequential steps:

• First, the fluorophore absorbs the incident light (absorbed light).
• The acquired energy causes an electron to transition from the ground state (ground state) to a higher energy level—the excited state (excited state). This transition occurs through a succession of virtual excited states.
• The electron stays at the new level for a very short time. Following a brief transient excited lifetime (transient excited lifetime), it tends to return to a lower energy level.
• The return is accompanied by the release of accumulated energy, which is liberated in the form of emitted light (emitted light). This exact process is called emission (emission), and this luminescence is what diagnostic devices detect.

Optical changes in pathological tissues

Tumor and precancerous cells alter tissue architecture, which directly affects how it interacts with light. There are three key optical changes during malignant progression:

1. Increased optical absorption (optical absorption). This occurs due to nuclear enlargement, increased total DNA content, and irregular chromatin clumping (chromatin clumping).
2. Increased optical scattering (optical scattering). Tumors require active nourishment and therefore trigger angiogenesis. The formation of new vessels increases vascular density and local hemoglobin concentration, which scatters light more strongly.
3. Changes in fluorescence. The luminescence of different tissue layers changes in opposite directions. Epithelial fluorescence increases due to accelerated metabolism in precancerous areas. At the same time, stromal fluorescence may decrease due to structural remodeling within the extracellular matrix (extracellular matrix).

Clinical application: surgery and endoscopy

Although optical imaging possesses superb spatial resolution (spatial resolution) and sensitivity (sensitivity), its major drawback is poor tissue penetration (poor tissue penetration). Therefore, the method is used where direct visual access is available: in operating rooms and during endoscopy.

Intraoperative imaging (Fluorescent guidance) Surgeons use luminescence for precise tumour margin delineation (tumour margin delineation) and to locate malignant or sentinel nodes (malignant/sentinel nodes). This is critically important for performing a complete resection. Practical examples:

• In neurosurgery, luminescence induced by 5-aminolevulinic acid (5-ALA) is used. It increases the rate of complete resection of malignant gliomas.
• In ovarian cancer, experimental probes targeting folate receptors are being tested to help surgeons easily find and remove disseminated lesions.

Endoscopic screening During routine examination of the gastrointestinal tract, bronchi, and urinary tract, the method helps detect occult dysplastic lesions (occult dysplastic lesions) that are invisible under normal white-light imaging (white-light imaging). A classic example is Barrett's esophagus: Using autofluorescence imaging (autofluorescence imaging), a physician can visualize low-grade dysplasia: it appears as a purple area against a normal green mucosal background. This effect is achieved by detecting the luminescence of the tissue's own (endogenous) molecules without administering dyes.

The role of molecular imaging in oncology

Functional and molecular diagnostic imaging methods differ fundamentally from traditional X-rays.

• They study subtle processes that require signal amplification (amplification) or enhancement for detection.
• There is always a trade-off between the spatial resolution of the image and the sensitivity of the method.
• Instead of the familiar high-resolution anatomical images, these methods generate spectra or so-called parameter maps (parameter maps) with lower resolution.

For these technologies to work successfully in routine clinical practice, functional data must be combined with traditional anatomical imaging. This approach is analogous to creating hybrid images, such as PET/CT (PET/CT).

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Fluorophore physics: lifetime in the excited state
• Application of 5-aminolevulinic acid (5-ALA) in neuro-oncology
• Screening of Barrett's esophagus and occult dysplastic lesions
• The role of folate receptors in targeted ovarian cancer surgery
• Creation of parameter maps in functional diagnostics

Other topics in the "Diagnostic Radiology in Oncology" section

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