Numerous techniques and devices are known and available for imaging structures within opaque or turbid objects, such as biological tissue.
For example, X-ray imaging has been widely used to provide shadow images of obstructed structures. Such imaging relies on opacity of dense materials, such as bone structure, relative to surrounding tissue, to attenuate the intensity of X- radiation passing directly through the object to a recording medium, such as photographic film or fluorescent media. The recording medium is exposed to a spatial pattern of radiation corresponding to the intensity of the incident attenuated radiation, thus providing data or shadow image representation of a spatial distribution of internal structural components of differing size and density at different locations in the object being imaged.
In order to obtain a three dimensional view of the imaged object it is necessary to irradiate the object sequentially from all angles, using complex mechanical scanning devices for changing the relative positioning of the source and detector with respect to the imaged object, as with computerized tomography, in order to enable imaging of a single "slice". Other imaging techniques use still other forms of ionizing radiation, such as gamma radiation, to obtain similar results. In addition to these disadvantages, it is necessary to introduce contrast agents into the sample in order to image soft tissue. Such a process adds to the complexity of imaging living subjects.
Similarly, magnetic resonance imaging techniques and ultrasound imaging techniques are known, providing information in the form of images descriptive of the position, size and shape of objects. In NMR (nuclear magnetic resonance) imaging a spatially varying magnetic field is applied to the object while in ultrasound imaging a sonic wave is applied to the object. Mechanical scanning may be necessary for ultrasound imaging in order to apply and to receive the waves to the selected segment to be imaged. Moreover, two dimensional N.times.N imaging using NMR exploits the frequency domain and the phase domain, and N sequential data acquisitions are required in the phase direction for such imaging. The acquired data detected by an RF pickup coil are then processed to provide an image of the internal structure of the object.
Moreover, there are known techniques for use of light beams for producing an image, wherein one or more light rays travel through an object and the exiting rays are passed through a zone plate to a detector array to provide information relative to the internal structure of the object. Such techniques, however, are subject to significant errors in attempting to image a preselected site. Specifically, resolution is a function of the wavelength. Typical wavelengths for laser generated light are in the range of 0.5 to 1.0 micron. Thus, in order to use a beam of coherent light to image a selected element of an object it is necessary to position the object with extreme accuracy. Positioning errors of less than a micron result in significant errors.
In order to obtain resolution of the order of 1 mm it would be necessary to use electromagnetic radiation having wavelengths in the range of 1 to 10 cm. However, use of this band of frequencies would expose the object to radiation in the microwave region. Such radiation is particularly susceptible to absorption by water in the tissue being imaged. As is known, electromagnetic radiation in the microwave region is thus typically used for heating and cooking. Accordingly, it could be quite harmful to attempt direct optical imaging utilizing electromagnetic radiation having wavelengths appropriate for standard resolution.
Additionally, similarly to X-ray and gamma imaging, known attempts at imaging using rays of partially coherent light rely on shadow-imaging, utilizing only the relatively small number of "prompt" photons which pass directly through the object, in a relatively straight line and with minimal deflection and diffusion. However, a much larger number of photons which pass through the tissue, and which thus potentially carry much more information, are ignored. These are photons which pass through the tissue by the wave diffusion process, hence passing more slowly, at attenuated intensity and undergoing scattering and interference with one another. For various reasons such diffusive rays have been overlooked by the prior art as a source of imaging information.
The theory of propagation of intensity modulated laser beams in turbid media by photon diffusion has been studied and equations have been developed describing wave propagation characteristics. Specifically, attenuation and phase delay of the modulated wavefront have been described in a homogeneous medium, for diffusive waves having a coherent front. The equations were tested for description of photon migration in turbid media, and were used in conjunction with a frequency domain analysis to determine linear scattering and absorption coefficients of a homogeneous, infinite, turbid medium. Fishkin et al., "Diffusion of Intensity Modulated Near-Infrared Light in Turbid Media", Tissues, SPIE, Los Angeles, January, 1991.
In a more recent development, it has been proposed to image tissues using intensity-modulated near-infrared light provided by a pulsed laser, using the increased distance of maintained coherence for diffusional waves and the greater depth of propagation for the lower frequency intensity modulation wavefront than for the higher frequency optical field wavefront. Nonetheless, the reported technique has not contemplated a manner in which a particular volume of interest (voxel) at a desired depth may be selected for imaging.
In summary, the known imaging methods and devices suffer from various drawbacks. For example, known techniques do not permit simultaneous imaging of all points of a desired volume of interest within an object. Nor are the known techniques capable of providing information beyond the presence and shape of various structures within an imaged volume. Moreover, where reflectance imaging is used, signals from voxels close to the object surface tend to overwhelm the photodetector, so that selection of deeper voxels is made impractical. Further, various techniques of the prior art require mechanical scanning of an object in order to image a desired voxel, rely on sequential data acquisition, and are incapable of imaging a voxel in real time. Others of the known methods require exposure to ionizing radiation, administration of radioactive contrast or tracing agents, may be invasive in nature, expensive to administer, and suffer from various other deficiencies.
There is accordingly a need for method and apparatus for obtaining data descriptive of characteristics of a turbid medium, and more specifically of tissue, and of its internal structure, which are not subject to the deficiencies of the prior art.
There is a more particular need for a method and apparatus for imaging an object utilizing electromagnetic radiation and permitting resolution in the range of 1 mm, while avoiding use of radiation having wavelengths in the range of 1-10 cm and the harmful effects thereof.
There is a more specific need for a non-invasive method and apparatus for imaging an object, free of requirements for exposure to ionizing radiation and administration of contrast or tracing agents, free of a need for mechanical scanning, and capable of simultaneously providing imaging data representative of a selected volume of interest.
Moreover, there is a need to be able to reflectively image voxels which are deep within the object, without overwhelming the photodetector by signals from voxels close to the surface.
There is still a further need for a method and apparatus for obtaining images or data descriptive of characteristics of a selected voxel of an object by exposure to intensity modulated light beams or other non-ionizing radiation, wherein the acquired images or data are descriptive of characteristics or physical parameters in addition to physical size or presence of structures within the object.