FTIR Microscopic Imaging of Large Samples with 4x and 15x Infrared Objectives: A Case Study of a Carcinoma Tissue Section

Significance of the Topic

Fourier transform infrared (FTIR) microscopic imaging provides label-free chemical maps of biological tissues, enabling molecular diagnostics, disease staging and research into biochemical alterations. Expanding the field of view (FOV) while maintaining spectral fidelity and spatial resolution is critical for analyzing large tissue sections efficiently. This study illustrates how low-magnification infrared objectives can accelerate survey scans and how high-magnification optics resolve cellular features, thereby enhancing throughput and diagnostic value.

Objectives and Overview of the Study

The main goals were to compare 4× and 15× infrared objectives in FTIR imaging of a laryngeal carcinoma tissue section, to assess differences in:

• Field of view and acquisition speed
• Data volume and processing demands
• Spectral quality and classification accuracy
• Spatial resolution for histopathologically relevant details

The study employed mosaic scanning to cover an 8 × 6 mm tissue region, followed by chemometric classification into five histological classes.

Methodology and Instrumentation Used

An Agilent Cary 620 FTIR microscope coupled to a Cary 670 spectrometer collected transmission spectra at 4 cm⁻¹ resolution with a 64×64 Focal Plane Array (FPA). Two objectives were compared:

• 4× reflective objective (1230×1230 µm FOV, 19.2 µm pixel)
• 15× Cassegrain objective (350×350 µm FOV, 5.5 µm pixel)

Mosaics required 7×6 tiles for 4× (4.3 GB, 1 h) versus 27×19 tiles for 15× (42.4 GB, 10 h) at 16 co-added scans per spectrum and UDR = 4 (up to 3975 cm⁻¹). Data were processed in R using the hyperSpec toolbox; spectra with artifacts were removed. Linear discriminant analysis (LDA) classified five tissue types based on spectra from the 4× data set and applied to the 15× images.

Main Results and Discussion

Key findings included:

• Spectral consistency between objectives across 1200–1800 cm⁻¹, confirming minimal impact of magnification on biochemical signatures.
• Significant reduction in acquisition time and file size when using the 4× objective for survey scans.
• High-resolution imaging with the 15× objective resolved microstructures, such as blood vessel lumina and cellular borders.
• LDA classification successfully mapped normal epithelium, connective tissue, inflammation, dysplasia and carcinoma, demonstrating robust tissue discrimination at both scales.
• Data processing demands were mitigated by under-sampling, optional binning, and use of an external cluster for large mosaics.

Benefits and Practical Applications

The complementary use of 4× and 15× objectives offers:

• Rapid screening of large tissue regions to locate areas of interest.
• High-resolution follow-up to characterize cellular pathology.
• Reduced instrument time and storage demands for routine histopathology workflows.
• Reliable chemometric classification for QA/QC in biomedical research and clinical studies.

Future Trends and Potential Applications

Advances that will further enhance FTIR microscopic imaging include:

• Larger FPA detectors (e.g., 128×128) to combine wide FOV with fine resolution.
• Faster scanning modes and improved under-sampling strategies to halve acquisition time.
• Integration with machine learning for automated tissue classification and biomarker discovery.
• Multimodal approaches linking FTIR maps with Raman, mass spectrometry or optical microscopy for comprehensive molecular histology.

Conclusion

This case study demonstrates that a 4× IR objective greatly expands survey imaging capabilities while preserving spectral integrity, and a 15× objective remains essential for resolving histological detail. The combined approach optimizes throughput, data volume and diagnostic information, offering a flexible workflow for large-area chemical imaging in biomedical and industrial applications.

References

• Beleites C., Popp J., Krafft C., Kansiz M. FTIR Microscopic Imaging of Large Samples with 4× and 15× Infrared Objectives: Application Note 5991-1363EN, Agilent Technologies, 2014.
• European Regional Development Fund (EFRE), Project FKZ B714-07037 and B715-07038.