Fluorescence-mediated molecular tomography

Details Fluorescence-mediated molecular tomography Fluorescence-mediated molecular tomography

2000-11-27

2003-09-02

Paik, Sang (Department: 3742)

Surgery

Diagnostic testing

Measuring or detecting nonradioactive constituent of body...

C600S476000

Reexamination Certificate

active

06615063

ABSTRACT:

TECHNICAL FIELD
This invention relates to extracting quantitative, three-dimensional molecular information from living mammals and patients using fluorochromes and new optical tomographic imaging methods.
BACKGROUND
Molecular imaging can be broadly defined as the characterization and measurement of biological processes at the cellular and molecular level in mammals and human patients. In contradistinction to “classical” diagnostic imaging, for example, magnetic resonance (MR), computed tomography (CT), and ultrasound (US) imaging, molecular imaging analyses molecular abnormalities that are the basis of disease, rather than imaging the end effects of these molecular alterations. Specific imaging of molecular targets allows earlier detection and characterization of disease, as well as earlier and direct molecular assessment of treatment efficacy. Molecular imaging can theoretically be performed with different imaging technologies, up to now preferably with nuclear imaging technologies, e.g., PET and SPECT imaging) which have high sensitivity of probe detection. The IV administered imaging probes typically recognize a given target. Alternatively, some probes detectable by MR imaging have been developed (Moats et al., Angewandte Chemie Int. Ed., 36:726-731, 1997; Weissleder et al., Nat. Med., 6:351-5, 2000), although their detection threshold is generally in the micromolar instead of the pico/femptomolar range of isotope probes.
An alternative method is to use fluorescent probes for target recognition. For example, enzyme activatable fluorochrome probes are described in Weissleder et al., U.S. Pat. No. 6,083,486, and fluorescent molecular beacons that become fluorescent after DNA hybridization are described in Tyagi et al., Nat. Biotechnol., 16:49-53, 1998. Fluorescence activatable probes have been used in tissue culture and histologic sections and detected using fluorescence microscopy. When administered in vivo, fluorescence activatable probes have been detected by surface-weighted reflectance imaging (Weissleder et al., Nat. Biotechnol., 17:375-8, 1999); Mahmood et al., Radiology, 213:866-70, 1999. However, imaging in deep tissues (>5 mm from the surface), in absorbing and scattering media such as mammalian tissues, and quantitating fluorescence (and in particular fluorescence activation) has not been described.
To image light interactions in deeper tissues, light in the near infrared (near-IR or NIR) instead of the visible spectrum is preferred. Imaging with near infrared (near-IR or NIR) light has been in the frontier of research for resolving and quantifying tissue function. Light offers unique contrast mechanisms that can be based on absorption, e.g., probing of hemoglobin concentration or blood saturation, and/or fluorescence, e.g., probing for weak auto-fluorescence, or exogenously administered fluorescent probes (Neri et al., Nat. Biotech., 15:1271-1275, 1997; Ballou et al., Cancer Immunol. Immunother., 41:257-63,1995; and Weissleder, 1999). In either application, NIR photons undergo significant elastic scattering when traveling through tissue. This results in light “diffusion” in tissue that hinders resolution and impairs the ability to produce diagnostically interpretable images using simple “projection” approaches (transillumination), as in x-ray imaging.
During the last decade, mathematical modeling of light propagation in tissue, combined with technological advancements in photon sources and detection techniques has made possible the application of tomographic principles (Kak and Slaney, “Principles of Computerized Tomographic Imaging,” IEEE Press, New York, 1988, pp. 208-218); Arridge, Inverse Problems; 15:R41-R93, 1999) for imaging with diffuse light. Diffuse Optical Tomography (DOT) uses multiple projections and deconvolves the scattering effect of tissue. DOT imaging has been used for quantitative, three-dimensional imaging of intrinsic absorption and scattering (see, e.g., Ntziachristos et al., Proc. Natl. Acad. Sci., USA, 97:2767-72, 2000), and also Benaron et al., J. Cerebral Blood Flow Metabol., 20(3):469-77, 2000). These fundamental quantities can be used to derive tissue oxy- and deoxy-hemoglobin concentrations, blood oxygen saturation (Li et al., Appl. Opt., 35:3746-3758, 1996) or hematoma detection in diffuse media.
Although intrinsic-contrast for DOT imaging may be useful in certain situations, e.g., for functional brain activation studies or hematoma detection, these applications do not allow the extraction of highly specific molecular information from living tissues. Fluorochrome concentration has been measured by absorption measurements (Ntziachristos et al., 2000) or by fluroescence measurements in phantoms (Chang et al., IEEE Trans. Med. Imag., 16:68-77, 1997; Sevick-Muraca et al., Photochem. Photobiol., 66:55-64, 1997). However, previously described DOT systems and/or image algorithms have not been useful to obtain three-dimensional quantitation of fluorescence in deep tissues in living mammals.
SUMMARY
The invention is based on the discovery that in vivo fluorochrome signals from specific targeted molecular probes, e.g., probes targeted for specific enzyme activities or DNA sequences, can be localized in three dimensions in deep tissues and be quantitated with high sensitivity using a specially designed imaging system for this purpose and relying on self-calibrated image reconstruction, and new algorithms to extract molecular maps.
In general, the invention features a near-infrared, fluorescence-mediated molecular tomography (FMT) imaging system that includes a NIR light source to provide incident light; a multipoint incident illumination array to direct light into an object, e.g., an animal or human patient, from two or more separate excitation points; multiple optic fibers to transmit light from the light source to each point in the multipoint incident illumination array; a multipoint detection array to collect light, e.g., fluorescent light, emitted from the object from two or more separate collection points; a two-dimensional emitted light array to transmit light emitted from the object to a detector; multiple optic fibers to transmit light from each collection point to a corresponding point on the two-dimensional emitted light array; and a detector to detect and convert light emitted from each point of the two-dimensional emitted light array into a digital signal corresponding to the light emitted from the object.
In this system, the emitted light can be continuous wave (CW) light, time-resolved (TR) light, or both CW and TR light.
The system can further include a processor that processes the digital signal produced by the detector to provide an image on an output device. The output device can provide multiple images simultaneously. The processor can be programmed to process the digital signal by i) generating a corrected fluorescence measurement by subtracting a background signal and filter bleed-through signal from collected fluorescence measurements; ii) generating a corrected intrinsic signal measurement by subtracting a background ambient light signal from collected intrinsic signal measurements; iii) generating a self-calibrated fluorescence measurement by dividing the corrected fluorescence measurement by the corrected intrinsic measurement; iv) generating a corrected background-medium diffuse signal by subtracting the collected background ambient light signal from a collected diffuse signal; and v) generating a self-calibrated intrinsic measurement by dividing the corrected intrinsic signal measurement by the corrected background-medium diffuse signal.
In other embodiments, the processor can be programmed to process the digital signal by i) generating a self-calibrated measurement M=M
1
−M
3
/M
2
−M
4
, wherein M
1
is an emission wavelength fluorescence signal, M
2
is an intrinsic signal, M
3
is a background bleed-through signal, M
4
is a background ambient light signal; ii) generating a self-calibrated intrinsic measurement M′=log (M
2
−M
4
)/(M
5
−M
4
), wherein M
5
is a backgrou

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