Abstract:
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.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/945,198, filed Jun. 20, 2007, the disclosure of which is incorporated herein by reference. 
    
    
     U.S. GOVERNMENT RIGHTS 
     This invention was made with Government support under grants NIH RO1 N539471, U54 CA105480 and K25CA106863 awarded by the National Institutes of Health. The U.S. Government has certain rights in this invention. 
    
    
     BACKGROUND 
     Diffuse Optical Tomography (DOT) is a technique wherein tissue is illuminated at multiple source points on a tissue surface with light having wavelengths ranging from visible light to near infrared (NIR). Light transmitted through the tissue from each source point is detected at multiple reception points on the tissue surface, and a measure of attenuation (absorption and scattering) along paths from each source point to each reception point is obtained to estimate the chromophore, fluorophore or scatterer concentrations. 
     Modeling of the absorption and scattering creates potentially high contrast images containing functional tissue information. For example, the heme group of myoglobin and/or hemoglobin absorbs visible and near infrared light, and the spectral characteristics of the absorption vary noticeably with the degree of oxygenation. Therefore, high contrast may be obtained between portions of tissue containing high concentrations of heme (such as blood and muscle) and portions of tissue containing low concentrations of heme (such as fat), and between highly oxygenated and poorly oxygenated or infarcted tissues. In particular, the high vascularity of tumors often provides them a significant hemoglobin content and a potentially high intrinsic optical contrast between the tumor and normal tissue. 
     Scattering and absorption can, however, be difficult to distinguish when DOT is performed with monochromatic, continuous-wave radiation because the transmitted or reflected light is diffuse and intensity is generally low. Distinctions between background noise, scattering, and absorption, may be improved by the use of modulated illumination and AC-coupled amplification at multiple wavelengths. To produce the modulated light, most existing DOT systems use either amplitude modulated laser diodes or light-emitting diodes (LEDs) that operate at different wavelengths. However, there are a limited number of available wavelengths of laser diodes or LEDs that generate enough power to provide a high signal-to noise ratio and lase at wavelengths suitable for probing biological chromophores. 
     Tunable titanium-sapphire lasers have been used for studies of optical absorption in mammalian tissues. For example, “Optical biopsy of bone tissue: a step toward the diagnosis of bone pathologies”, Pifferi et al., Journal of Biomedical Optics 9(3), 474-480 (May/June 2004) describes using a mode-locked, tunable from 700 to 1000 nanometers wavelength, titanium-saphire (Ti:Saphire) pulsed laser and a dye laser tunable from 650 to 695 nanometers with a single 1 mm optical fiber for delivering light to a single point on a heel and a single 5 mm optical fiber bundle for receiving light from the heel and delivering the light to a time-resolved photomultiplier-tube photodetector. 
     Time resolved transmittance spectroscopy has also been used in optical spectrometry of soft tissues. “Absorption of collagen: effects on the estimate of breast composition and related diagnostic implications”, Taroni et al., Journal of Biomedical Optics 12(1), 014021 1-4 January/February 2007, describes absorption spectrometry of the breast. The apparatus of Taroni is probably similar to that used by co-author Pifferi. The apparatus of Taroni provides spectra of, for example, absorption along a single path between two points through the tissue. The apparatus of Taorni does not provide images resolving inclusions within the tissue. Taroni acknowledges that spectroscopy may have a role in imaging but does not discuss how this might be done and does not discuss alternative light detection approaches. 
     SUMMARY 
     In one embodiment, a tomography system includes: a mode-locked, tunable laser for generating pulsed light of a predeterminable wavelength, the pulsed light characterized by a first pulse repetition frequency; an apparatus for applying the pulsed light to tissue at a source position; apparatus for collecting light from the tissue at a plurality of detector positions; an apparatus for transducing the collected light into a plurality of detection signals; an apparatus for supplying a reference signal characterized by the first pulse repetition frequency; and an image construction apparatus for receiving the plurality of detection signals and the reference signal and constructing a tomographic image of the tissue based at least in part on a comparison of the detection signals and the reference signal. 
     In one embodiment, a method for generating tomographic images of tissue includes: generating pulsed light from a pulsed, tunable laser, the pulsed light characterized by a first pulse repetition frequency; applying the pulsed light to a source position on the tissue; receiving transmitted light at a plurality of detector positions; transducing the transmitted light into electrical signals; generating a reference signal characterized by the first pulse repetition frequency; and constructing the tomographic image of the tissue based at least in part on a comparison of the electrical signals and the reference signal. 
     In one embodiment, a tomography system includes: a mode-locked, broadband tunable laser for generating pulsed light, the pulsed light characterized by a first pulse repetition frequency; an apparatus for applying the pulsed light to tissue at a source position; apparatus for collecting light from the tissue at a plurality of detector positions; a spectrally-sensitive apparatus for transducing the collected light into a plurality of detection signals embodying wavelength information; and an image construction apparatus for receiving the plurality of detection signals and the reference signal and constructing a tomographic image of the tissue. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram showing one exemplary optical imaging system embodiment using pulsed lasers for frequency domain diffuse optical tomography. 
         FIG. 2  is a flowchart illustrating one process for generating an optical image using the system of  FIG. 1 . 
         FIG. 3  is a block diagram showing an alternative embodiment having a photonic crystal fiber for transforming laser pulses into broadband light pulses and an array detector. 
         FIG. 4  is a block diagram of an alternative embodiment having a broadband illumination and using dichroic filters with multiple photodetectors. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     In the following description, the term light is used to represent electromagnetic radiation from the near infrared part of the spectrum into the visible range. The term light should therefore not be taken as a limitation to visible parts of the electromagnetic spectrum. 
     As discussed in more detail below, the disclosed systems and methods utilize a pulsed, tunable laser for near infrared diffuse optical tomography. The pulsed, tunable laser may for example be a mode-locked, pulsed-output, tunable laser driven by a continuous-wave pumping laser. One example of a pulsed-output, tunable laser is a Kerr-lens self-mode-locked (KLM) titanium sapphire (Ti:S) laser, which provides continuous wavelength tuning in a range from about 650-1100 nm and a power output of about 0.5 to 3 Watts in ultrafast pulses. Light pulses from a mode-locked Ti:S laser typically have pulse durations of between about five femtoseconds and a few picoseconds, with a pulse repetition frequency of between 70-90 MHz. The high power output of such lasers helps produce a high signal-to-noise ratio that improves spatial resolution relative to traditional laser diode systems. In addition, the wide tunable range allows data collection over a significant portion of the near-infrared wavelength range and reduces crosstalk between important chromophores, thereby improving spectral resolution. 
       FIG. 1  shows one exemplary optical imaging system  100 . System  100  includes a pumping laser  101  which may be a continuous-wave high-power laser diode, an argon ion laser or a frequency-doubled neodymium yttrium orthovanadate (Nd:YVO 4 ) laser, for pumping a mode-locked, tunable laser  102 . 
     Light, A 1 , emitted by laser  102  enters an optical transmit fiber  104  by way of a selector  106 . Selector  106  may be a rotating disk mechanically adjusted to sequentially select new source positions (i.e., transmit fibers  104 ). Tissue  108 , which may be disposed in a tissue distributor  110 , is exposed to light A 1  from one of transmit fibers  104 . Tissue  108  may contain inclusions, not shown, the inclusions may incorporate chromophores having absorption and scattering properties differing from other constituents of tissue  108 . 
     Exemplary tissue distributors  110  are described, for example, in T. O. McBride; B. W. Pogue; E. D. Gerety; S. B. Poplack; U. L. Österberg; K. D. Paulsen, “Spectroscopic diffuse optical tomography for the quantitative assessment of hemoglobin concentration and oxygen saturation in breast tissue”, Applied Optics, 38(25), 5480-5490, 1 Sep. 1999 and B. W. Pogue; M. Testorf; T. McBride; U. Österberg; K. Paulsen, “Instrumentation and design of a frequency-domain diffuse optical tomography imager for breast cancer detection”, Opt. Exp. 1, 391-403 (1997). A tissue model, or phantom, may be used in place of tissue  108  during development and calibration. 
     An optode is a device that serves as the optical equivalent of an electrode—it serves to couple light from a transmit fiber  104  into tissue  108 , or from tissue  108  to receive fiber  112 . Distributor  110  incorporates multiple receive optodes at different positions around tissue  108 , as well as multiple transmit optodes at multiple positions around tissue  108 . 
     Light exiting tissue  108  is collected by receive optodes of distributor  110 , each receive optode couples this light into one of several receive fibers  112 . Each receive fiber  112  couples light to a detector  114 . Receive fibers  112  may be multiplexed into a single detector  114 , or multiple detectors may be provided. Detector(s)  114  may include one or more of photomultiplier tubes (PMTs), avalanche photodiode modules, charge coupled devices (CCDs) or other known detectors. Each detector  114  transduces received light from receive fiber  112  into a modulated electrical signal, B, that is sent to a data acquisition device  118  of a computer  119 , via a communication path  117 , when only intensity is being measured. Where multiple detectors  114  are used, communication path  117  may represent multiple signal pathways. 
     However, in order to allow for amplification and precise measurement of a delay in the detected light resulting from migration of the light through tissue  108 , detected signals B, which have the pulse repetition frequency of laser  102  (typically between 70-90 MHz), are heterodyned to a frequency that is sampleable by a clock of high resolution data acquisition device  118 , typically in the range from 5 Hz to 200 KHz. Data acquisition device  118  feeds information to computer  119 . 
     Signals B may be measured either in sequence or in parallel by data acquisition device  118 . Sampling in sequence may be facilitated by a multiplexer within data acquisition device  118 . 
     Heterodyning of electrical signals B is accomplished by a mixer(s)  116 , driven by a local signal generator  122 . Mixer(s)  116  combine signal B, received via communication path  115 , with a secondary signal, C, received from a splitter  124 . The difference between signal B and signal C produces detected mixed signal D characterized by a beat frequency that may be sampled by data acquisition device  118 . For example, when signal B has a pulse repetition frequency of 80 MHz and signal C has a frequency of 79.998 MHz, the difference between signal B and signal C yields signal D having a beat frequency of 2 KHz, which is readily amplifiable with AC-coupled amplifiers and measurable by data acquisition device  118 . 
     Reference signal C is generated by a signal generator  122 , and, in order to stabilize the beat frequency, is phase locked (synchronized) to a predetermined ratio of the pulse repetition frequency of laser  102  by a phase locked loop  120 . Phase locked loop  120  may be an integrated circuit. A portion of reference signal C is also sent, via splitter  124 , to a second mixer  126  where it is mixed with signal A 2 , which is a clock signal having the same pulse repetition frequency as light A 1 ; this creates a reference signal E that may be sampled by data acquisition device  118 . In the absence of reference signal C, signal A 2  may be transmitted as a non-heterodyned signal to data acquisition device  118 . 
     Detected mixed signal D and reference signal E are acquired by data acquisition device  118  and read by computer  119 , which measures the amplitude and phase of these signals to determine an attenuation of light A 1  due to transmission through tissue  108 . 
     Computer  119  may contain a processor, executing image construction software known in the art, and a display. Images may also be recorded to a memory of computer  119  for later study. 
     In one example, tunable laser  102  may be scanned across a wavelength range, e.g., from 690 nm to 1000 nm, in set increments, e.g., 5 nm, thereby providing for data acquisition at sixty-three wavelengths. In another example of operation, laser  102  is set to lase at a particular wavelength (e.g., under control of software instructions executed by computer  119 ), and light is distributed through each one of transmit fibers  104  in sequence, e.g. fiber  104 ( 1 ) . . . fiber  104 ( n ). In this way, the present systems and methods utilize the pulse repetition frequency of laser  102  to mimic the intensity modulation of laser diodes in a traditional DOT system. Upon completion of a first cycle (i.e., after transmit fiber  104 ( n ) has been illuminated), laser  102  is tuned to a new wavelength (e.g., λ 1 +5 nm), and the process of illuminating each transmit fiber  104  is repeated. 
     In an embodiment wherein laser  102  is set to lase at particular wavelengths, at least three wavelengths in the range from 650 to 1100 nanometers are used to permit distinguishing oxygenated from non-oxygenated heme groups. 
     Changes may be made to system  100  without departing from the scope hereof. For example, an optical fiber that acts as a transmit fiber  104  in one instance may become a receive fiber  112  when a new transmit fiber  104  is selected by selector  106 . In another example, mixer  116  may form part of a photomultiplier tube. 
     The heterodyne technique for amplifying and detecting transmitted light, using phase lock loop  120  and mixer  116 , permits high sensitivity, noise immunity, and rapid data acquisition. This high sensitivity and rapid data acquisition in turn permits use of the system to observe dynamic changes in chromophores, such as may occur during neural or muscle activity. Since noise photons, including photons originating from other sources of illumination such as room lighting, those originating from fluorescence in tissue  108 , and infrared photons radiated by the tissue  108 , arrive with random timing with respect to the laser pulses, these photons tend to cancel each other and are ignored by the system. The system therefore has improved noise rejection compared to single-photon-counting systems such as that of Pifferi and Taroni. 
       FIG. 2  shows a flowchart illustrating one process  200  for generating an optical image using system  100  of  FIG. 1 . In step  202 , a specific wavelength of light (e.g., 690 nm) is generated by tunable laser  102 . In an example of step  202 , a processor of computer  119  may execute software containing instructions for an illumination wavelength, pulse repetition frequency, etc. of tunable laser  102 . In order to implement these instructions, computer  119  may control some or all of the components of system  100 . In step  204 , light is applied to tissue  108  through a transmit fiber  104 . After the light migrates through tissue  108 , it is received by receive fibers  112  at multiple reception points on tissue  108  in step  206 . In step  208 , the received light is transduced into a detection signal. In an example of step  208 , detector  114  converts light received through receive fibers  112  into electrical signals B. Step  210  is optional. In step  210 , the detection signal is heterodyned to a lower frequency by mixing with a secondary signal to provide a resultant signal (e.g., signal D). In step  212 , the detection signal, having either a heterodyned or non-heterodyned pulse repetition frequency, is compared to a reference signal. If the detection signal is heterodyned, the reference signal is also heterodyned to the same pulse repetition frequency. If the detection signal is non-heterodyned, the reference signal is also non-heterodyned. In step  214 , a tomographic image is constructed by a processor of computer  119  based at least in part on the comparative data. 
     In another embodiment, as illustrated in  FIG. 3 , a photonic crystal fiber  302  is used to generate a pulsed broadband or “white” light source. Photonic crystal fibers are microstructured light guides that, in some cases, when stimulated by laser pulses, can act as broadband lasers. For example, when pumped by mode-locked pulsed laser  304  crystal fiber  302  generates a pulse of light having energy essentially evenly distributed from the infrared to green wavelengths. The light from crystal fiber  302  is distributed by transmit selector  306  into transmit optical fibers  308  one at a time in sequence. Light from each transmit fiber  310  is provided to tissue  312  through a optode for each transmit fiber in tissue distributor  314 . 
     Light is collected by receive optodes of tissue distributor  314  and transmitted by receive optical fibers  318  to a slit  320 . At slit  320  the receive optical fibers are placed adjacent to each other such that light transmitted by all fibers  318  illuminates a short line at the slit  320 , and that light received from each fiber  318  illuminates a particular nonoverlapping portion of the short line; this line is imaged by a spectrographic imager  321 . 
     Within spectrographic imager  321 , light from slit  320  is focused by lenses  322  into a dispersive device  324  oriented such that light of the line is spread by wavelength into a rectangle on photodetector array  326 . Dispersive device  324  may be a prism or may be a diffraction grating as known in the art of spectroscopy. Light from each fiber  318  is dispersed by wavelength across a row of pixels on photodetector  326  that does not overlap the rows of pixels illuminated by other fibers  318 . Light at each pixel of detector  326  is light of a specific wavelength received from a particular fiber. 
     Additional lenses  328  may be present in the optical path as necessary to properly focus spectra from receive fibers  318  onto photodetector array  326 . 
     For convenience in illustration, dispersive device  324  and photodetector array  326  are rotated by ninety degrees with respect to slit  320  as viewed in  FIG. 3 . 
     Signals representative of light received by each pixel of photodetector  326  are passed to digital signal processor  330 . Digital signal processor also receives a synchronization signal  332  from transmit selector  306  and determines a spectra of light transmission along each path through tissue  312  from each transmit optode of distributor  314  to each receive optode of distributor  314 . The spectra of light transmission along each path are used by image reconstruction  334  to produce a tomographic image of inclusions within tissue  312 . 
     In an embodiment, photodetector  326  is electronically gated by a reference signal  336 . Detected photons arriving during and immediately after light pulses emitted by photonic crystal fiber laser  302  are accumulated and averaged to produce a “lighted” signal. Detected photons, such as noise photons, arriving at other times are accumulated and averaged to produce a “dark” signal. The spectra of light transmission along each path is determined in part by subtracting the dark signal from the light signal. 
     Combined broadband spectroscopy and frequency domain spectroscopy may be useful in disease or physiological imaging where the spectral content of the signal can be acquired rapidly. 
     In an alternative embodiment particularly suited to rapid data acquisition, several pumping lasers  304  and photonic crystal fiber lasers  302  are used. In this embodiment, transmit selector  306  may act as a multipole switch, permitting several transmit fibers  310  to be coupled to different crystal fiber lasers  302  simultaneously. In this embodiment, crystal fiber lasers  302  are sequenced electronically and at a high rate such that paths through tissue  312  from different transmit fibers to receive fibers  318  can be distinguished. Similarly, if a crystal fiber laser  302  is provided for each transmit fiber  310 , transmit selector  306  may be deleted. 
     In an alternative embodiment, illustrated in  FIG. 4  with reference to  FIG. 3 , a pumping laser  304 , photonic crystal fiber laser  302 , transmit fibers  310 , optodes and tissue distributor  314 , receive fibers  318 , and slit  320  are provided similar to the embodiment of  FIG. 3 . Light from slit  320  is transmitted through a lens system  402  and through a series of dichroic filters  404 ,  406  to separate the light into separate beams corresponding to differing wavelengths of light received from receive fibers  318 . Each of these separate beams is received by a photodetector  408 ,  410 ,  412  of a type suitable for receiving light of wavelengths of that beam. Photodetectors  408 ,  410 ,  412  may be linear array photodetectors such that each pixel elements of the photodetectors  408 ,  410 ,  412  receive light corresponding to particular fibers of receive fibers  318 . Signals from photodetectors  408 ,  410 ,  412  are processed by digital signal processor  414  and passed to an image reconstruction computer  416 . 
     Specific applications in which the present system may be used include: assessment of disease or response to therapy; monitoring uptake or retention of drugs or dyes, which are optically absorbing or scattering; detection of epidural and subdural hematomas; and imaging of tumors. For example, the disclosed systems and methods may be used to generate images useful in the diagnosis and/or treatment of breast cancer, brain cancer, prostate cancer, aneurisms, hematomas, tumors, cysts, heart disease, renal artery stenosis, peripheral vascular disease and vulnerable plaques in arteries. 
     The changes described above, and others, may be made in the systems and methods described herein without departing from the scope hereof. It should thus be noted that the matter contained in the above description or shown in the accompanying drawings should be interpreted as illustrative and not in a limiting sense. The following claims are intended to cover all generic and specific features described herein, as well as all statements of the scope of the present systems and methods, which, as a matter of language, might be said to fall there between.