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Wavelength Tuneable Frequency Domain Photon Migration Spectrometer for Tissue-like Media

Frequency domain spectrometers use intensity modulated light to quantitatively interrogate turbid media. The modulation frequencies employed are in the radiofrequency range. Intensity modulated light launched into a turbid medium generates photon density fluctuations with wave like character that oscillates at the modulation frequency. These density fluctuations are named diffuse photon density waves, and it has been shown that the amplitude and phase of the photon density wave inside the medium depends on its optical properties. Hence by measuring the amplitude and phase of the photon density wave the optical properties of the medium can be estimated. This is the basic working principle of a frequency domain photon migration spectrometer. Frequency domain spectrometers fabricated with laser diodes are limited to discrete wavelengths thereby making compromises on the information about the media under test. In this research a wavelength tuneable frequency domain spectrometer was constructed by modulating the output intensity of a titanium: sapphire laser using an acousto-optic modulator. A low noise avalanche photodiode module in conjunction with a lock-in amplifier was used to measure the amplitude attenuation and phase lag inside a turbid sample. The frequency domain spectrometer was tested for accuracy and precision by estimating the optical properties of an important tissue simulation phantom, Intralipid , at a representative wavelength 790 nm. The results indicated that the spectrometer estimates absorption with an accuracy of 10%. The instrument estimates the absorption and reduced scattering coefficients with a precision of 3% and 6%, respectively. Optical properties of Intralipid were measured from 710-850 nm in the therapeutic window. The results were compared with published data measured by other methods and similar frequency domain techniques. The absorption coefficient agrees within 10% with results from a time domain measurement. The reduced scattering coefficient was within the error limits of other reported measurements. At 750 nm the reduced scattering agrees within 5% with the results from a continuous wave, time domain and within 1% from another frequency domain measurement, and at 811 and 849 nm this agreement is within 9%. A Mie theory prediction of the reduced scattering coefficient based on a measurement of the particle size distribution by a Mastersizer 2000 is larger than the frequency domain results by 6%. The spectrometer was used to determine the optical temperature coefficient of Intralipid , exploring its potential as a non invasive temperature monitoring device. The measured minute change in the absorption coefficient suggests a minimum observable temperature change of 4'C, which for most practical applications means that the precision needs to improve. The effect of glucose on the optical properties of Intralipid indicates that the absorption coefficient decreases steadily at 730 nm up to 1000mg/dL. The reduced scattering coefficient decreases with increasing glucose concentration at most of the wavelengths. This work quantified the absorption and reduced scattering of Intralipid over a larger wavelength range (in the therapeutic window) than before. This is the first time the effects of temperature on the optical properties of a turbid medium monitored with a frequency domain spectrometer. Specific information about the precision and accuracy which can be achieved with the current technology is documented. Current precision is not sufficient for many applications that would benefit from separation of absorption and scattering.
Type of thesis
Cletus, B. (2010). Wavelength Tuneable Frequency Domain Photon Migration Spectrometer for Tissue-like Media (Thesis, Doctor of Philosophy (PhD)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/3964
The University of Waikato
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