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Frequency Modulated Spectroscopy
What is Frequency Modulated Near Infrared Spectroscopy?
Frequency Modulated Near Infrared Spectroscopy (FMNIRS) is a non-invasive technique that uses, sinusoidally, intensity-modulated near infrared light to quantitatively interrogate both the absorption and scattering properties of tissue.
A number of physiological processes alter the relative amounts of hemoglobin, myoglobin, water, and fat in tissue, affecting the absorption and scattering properties of the tissue.
By measuring the tissue absorption changes distinctly from the effects of tissue scattering at various wavelengths, the amount of hemoglobin, myoglobin, and water in tissue can be quantitatively determined.
Mapping the absorption within the tissue in a quantitative manner allows visualization of the spatial contribution from each of the components within the tissue. These spatial maps of absorption and scattering provide insight into changes in tissue metabolism and physiology.
How does FMNIRS work?
Near infrared frequency domain instruments use a source whereby the intensity is modulated at radio frequencies (MHz). A diode laser modulated via a laser driver forms the source.
FMNIRS functional schematic
The modulated light propagates through the tissue and is detected by an avalanche photodiode. A second reference channel modulated at a frequency slightly displaced from the laser modulation frequency is used in a heterodyne detection scheme where the reference and the transmitted laser signal are mixed to produce a difference frequency.
This frequency down-conversion brings the detected radio frequency signal down to a lower intermediate frequency where conventional phase detection methods can be applied.
By comparing a reference signal to the detected signal using phase sensitive detection, the amplitude and phase of the signal can be interrogated. The degree of attenuation of the optical light as a function of phase-lag from the reference channel enables one to extract the absorption and scattering properties of the tissue.
Schematic of FMNIRS signal decomposition into scattering and absorption components
By modeling the amplitude and phase of the light transmission using a light transport model, a quantitative measure of the light absorption and scattering in the tissue can be obtained.
How can FMNIRS be used?
Frequency domain devices can be used in determining the optical characteristics of human tissues, particularly breast tissue, brain tissue and skeletal muscle for quantitative oxygenation and perfusion measurements.
Related Information
Read more about Frequency Modulated Spectroscopy:
- Diffuse Reflectance Spectroscopy
- Non-linear Optical Microscopic Imaging
- Fluorescence Imaging
- MRI Studies
- Bioluminescence Imaging
- Molecular Imaging - Spectroscopy
- Optical Coherence Tomography
- Raman Spectroscopy
- Speckle Imaging
- Thermal Imaging
- High Throughput Diagnostics
- Diagnostic Technologies
