Chirp excitation for natural frequency optical coherence elastography

Area of Science:

• Biomedical Optics
• Tissue Biomechanics
• Optical Coherence Elastography

Background:

• Optical coherence elastography (OCE) measures tissue mechanical properties using sub-micron oscillations.
• Characterizing natural frequencies is crucial for understanding tissue biomechanics.
• Previous methods faced limitations in excitation bandwidth and sample stiffness.

Purpose of the Study:

• To compare broadband spectrum excitation methods for OCE: contact piezoelectric transducer (PZT) chirp and non-contact air-pulse.
• To evaluate their performance on agar phantoms and ex vivo porcine cornea under varying intraocular pressures (IOPs).
• To determine the suitability of each method for measuring natural frequencies across different sample types and conditions.

Main Methods:

• Investigated PZT chirp (0-5000 Hz) and air-pulse (0-840 Hz) excitations for OCE.
• Tested on 1.0-7.5% agar phantoms and ex vivo porcine cornea (IOPs 5-40 mmHg).
• Analyzed natural frequency quantification using modified Bland-Altman analysis.

Main Results:

• Air-pulse excitation quantified first-order natural frequencies in softer samples but struggled with stiffer samples and higher orders.
• Chirp excitation successfully measured both first-order (e.g., porcine cornea: 76-1240 Hz) and higher-order natural frequencies across all samples.
• A 20.4% bias was observed with chirp excitation, attributed to the PZT probe's contact.

Conclusions:

• Broadband chirp excitation offers superior performance for natural frequency OCE, especially for stiffer tissues and higher-order modes.
• Air-pulse excitation is limited to softer tissues and lower-order frequencies.
• Findings guide the development of OCE for advanced biomechanical characterization.