Pulsed lasers in frequency domain diffuse optical tomography and spectroscopy
A diffuse optical tomography system incorporating a mode-locked, tunable laser produces pulsed light that may be used to interrogate tissue with high spatial and spectral resolution. The detection signal may be heterodyne shifted to lower frequencies to allow easy and accurate measurement of phase and amplitude. Embodiments incorporating wavelength-swept, tunable, lasers and embodiments using broadband photonic fiber lasers with spectrally-sensitive detectors are described.
1. A tomography system, comprising:
a pulsed, tunable laser for generating pulsed light of a predeterminable wavelength and a first pulse repetition frequency,
wherein the pulsed, tunable laser generates a reference signal characterized by the first pulse repetition frequency;
a transmit fiber coupled to apply the pulsed light to tissue at a source position;
a plurality of receive fibers each coupled to receive light from the tissue at a receive optode position, wherein there are a plurality of receive optode positions;
detectors for transducing the collected light into a plurality of detection signals,
a computer for receiving the plurality of detection signals and the reference signal and constructing a tomographic image of the tissue;
a signal generator for generating a secondary signal characterized by a second pulse repetition frequency;
a first mixer for mixing the secondary signal with the detection signals to form first heterodyned signals characterized by a third frequency; and
a second mixer for mixing the secondary signal with the reference signal to form a second heterodyned signal characterized by the third frequency;
wherein the computer is configured to receive the first and second heterodyned signals and construct the tomographic image of the tissue based at least in part on a comparison of the first and second heterodyned signals, and configured to use the reference signal to control reception of the detection signals.
2. The tomography system of claim 1 , wherein the first and second heterodyned signals have frequencies in a range of 5 Hz to 200 KHz.
3. A method for generating tomographic images of tissue, comprising:
(a) generating pulsed light from a pulsed, tunable laser, the pulsed light characterized by a wavelength and a first pulse repetition frequency;
(b) applying the pulsed light to a source position on the tissue;
(c) receiving light at a plurality of detector positions on the tissue;
(d) transducing the received light into electrical signals;
(e) generating a reference signal characterized by the first pulse repetition frequency; and
(f) constructing the tomographic image of the tissue based at least in part on a comparison of the electrical signals and the reference signal;
(g) generating a secondary signal characterized by a second pulse repetition frequency;
(h) mixing the electrical signals with the secondary signal to produce first heterodyned signals characterized by a third frequency;
(i) mixing the reference signal with the secondary signal to produce a second heterodyned signal characterized by the third frequency; and
(j) constructing the tomographic image of the tissue based at least in part on a comparison of the first and second heterodyned signals.
4. The method of claim 3 , further comprising altering the wavelength of the pulsed light and repeating steps (a)-(j).
5. The method of claim 4 , wherein the wavelength of pulsed light is set to at least three wavelengths at least five nanometers apart in the range of 650 and 1100 nanometers.