Abstract:
A scanning module for imaging through scattering media is provided. The scanning module image through scattering media while alleviating adverse effects of the weak transmission through highly scattering media. The injection of photons is optimized so that the overall transmission is increased compared to the conventional art. Cross-talk effects in a multi-port geometry are eliminated thereby increasing parallelism.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a scanning module for imaging through scattering media. More particularly, the present invention is directed to a scanning module for imaging through scattering media while alleviating adverse effects of the weak transmission through highly scattering media. According to the present invention the injection of photons is optimized so that the overall transmission is increased compared to the conventional art. In addition, the present invention eliminates cross-talk effects in a multi-port geometry thereby increasing parallelism. 
     2. Background of the Invention 
     Infrared imaging through scattering media is an area of research that has created enormous interest. A main target application is breast cancer detection, in which the use of a near infrared short-pulse laser is particularly appropriate. Obtaining images of the interior of the breast is complicated by the extensive scattering of light in such a medium. The scattering property of the breast causes a weak transmission through a thick slab and blurring of the image obtained from the transmitted light. The use of ultra-short laser pulses combined with an ultra-fast and ultra-sensitive detection system reduces the adverse effects of scattering. 
     The trajectory of a photon propagating inside a scattering medium can be predicted only on a statistical basis. In addition to the probability of being absorbed, the photons are subject to numerous scattering events. In a slab medium that is highly scattering and weakly absorbing, such as the human breast, most photons are reflected toward the entrance surface after traveling only a few millimeters into the tissue. Other photons are absorbed by the medium or transmitted to the output surface where they can be detected. In the case of typical breast thickness and optical parameters, 0.001 to 1% of the injected photons are transmitted to the output surface. 
     The light transmitted to the output surface carry the most significant information about the interior of the medium, since the reflected light carry information predominantly from a region close to the input surface. In previous systems as shown in FIG. 1, images of the interior of a scattering medium  24 , such as the human breast, have been obtained by scanning a laser beam  20  of small diameter along the input surface  22  and detecting light emerging from a small area along the output surface  26 . The grey region  25  indicated in the dotted line in FIG. 1 illustrates the region through which the photons pass when traveling from the injected point to detector  29 . As discussed previously, such an imaging technique is complicated by the strong scattering property of the medium: the number of transmitted photons is very weak compared to the number of injected photons and the scattering causes a strong blurring of the image. There is a need for developing techniques that alleviate the blurring and the weak transmission associated with the scattering. 
     Referring to FIG. 1, when a single photon is injected at the input surface  22  of a scattering slab  24 , it has a certain probability to be transmitted to the opposite surface  26  at some point and after some traveling time. Similarly, when injecting an infinitely short light pulse at the input surface  22 , a certain proportion of light (typically weak) is transmitted. The transmitted light can then be collected over a certain spatial region of the output surface and during a certain time interval. In other words, when arriving at the output surface  26 , the input pulse is spread spatially and temporally forming a spatio-temporal distribution of the transmitted light. 
     Theoretical expressions (the diffusion model) allow to correctly predict the spatio-temporal distribution of the emerging light from the output surface corresponding to the injection of an ultra-short light pulse at the input surface of a homogeneous scattering slab. FIGS. 2 and 3 illustrate examples of the theoretical temporal and spatial distributions of the transmitted light in the case of a 40 mm thick scattering slab having a scattering coefficient μ s ′=1 mm −1 , an absorption coefficient μ a =0.002 mm −1  and a refractive index n=1.34. The scattering coefficient μ s ′ is the probability of the photon being isotropically scattered per infinitesimal pathlength, and the absorption coefficient μ a  is the probability of the photon being absorbed per infinitesimal pathlength. FIG. 2 shows the temporal distribution of the spatially-integrated transmission, and FIG. 3 shows the spatial distribution of the time-integrated transmission. In FIG. 2, the time origin corresponds to the entry of the light pulse in the medium. In FIG. 3, the radial position 0 corresponds to the point of the output surface facing the injection point of the input surface. As can be seen in this particular case, the part of the input pulse that reaches the output surface emerges into a 40 mm Gaussian spot temporally spread during a time interval of about 5 ns. 
     Similar to the transmission mechanism, when a single photon is injected at the input surface of a scattering slab, it has a certain probability of being reflected back to the input surface at some point and after some travel time. When injecting an infinitely short pulse at the input surface, a certain proportion of light (typically very high) is reflected. The reflected light can then be collected over a certain spatial region of the input surface and during a certain time interval. In other words, the reflected pulse is spread spatially and temporally forming a spatio-temporal distribution of the reflected light. 
     Again, the diffusion model provides theoretical expressions for the spatio-temporal distribution of the emerging light from the input surface corresponding to the injection of an ultra-short light pulse at the input surface of a homogeneous scattering slab. FIGS. 4 and 5 show examples of the theoretical temporal and spatial distributions of the reflected light in the case of a 40 mm thick scattering slab having a scattering coefficient μ s ′=1 mm −1 , an absorption coefficient μ a =0.002 mm −1  and a refractive index n=1.34. FIG. 4 shows the temporal distribution of the spatially-integrated reflection and FIG. 5 shows the spatial distribution of the time-integrated reflection. In FIG. 4, the time origin corresponds to the entry of the light pulse in the medium, and in FIG. 5, the radial position 0 corresponds to the injection point. As can be seen for this particular case, the part of the input pulse that is reflected back emerges into a 2 mm Gaussian spot temporally spread during a time interval of about 50 ps. 
     The scanning process illustrated in FIG. 1 is typically slow due to the weak number of transmitted photons and the typically required averaging. One way to achieve a faster scan is to use parallelism, that is a duplication of the scanning process in many ports  60 , as illustrated in FIG.  6 . Each port  60  performs a scan over a reduced region allowing for a faster process. As far as possible, the different ports  60  must be independent and thus the spacing between two adjacent ports must be sufficiently large to avoid cross-talk effects. The cross-talk (interaction between ports) is superimposed on the useful signal (the light traveling from the input to the output of a single port). The larger the port spacing, the weaker the cross-talk will be, but the larger the scanned region for each port  60 . As a result, there is a trade-off between the cross-talk and the scanning speed. For the particular case of FIG. 3, to avoid significant cross-talk, the port spacing must be about 40 mm. In general cases, the port spacing must be approximately equal to the scattering slab thickness. 
     The use of optical fibers to carry light is appropriate for the present application, especially when parallelism is used. All-fiber dividers (couplers) are available allowing for an easy separation of a main laser beam into many ports. Furthermore, the light is brought close to the medium and collecting fibers can be used allowing for a safe an efficient scanner. 
     SUMMARY OF THE INVENTION 
     These and other deficiencies of the prior art are addressed by the present invention which is directed to a scanning module for imaging through scattering media while alleviating adverse effects of the weak transmission through highly scattering media. 
     An object of the present invention is to provide an apparatus for optimizing the injection of light such that the overall transmission is increased compared to the conventional art. 
     Another object of the invention is to provide an apparatus for eliminating cross-talk effects in a multi-port geometry for an increase in parallelism. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other attributes of the present invention will be described with respect to the following drawings in which: 
     FIG. 1 is a schematic view of a scanning system through a scattering medium as known in the prior art; 
     FIG. 2 is a graph representing theoretical temporal distribution of a spatially-integrated transmission; 
     FIG. 3 is a graph representing theoretical spatial distribution of a time-integrated transmission; 
     FIG. 4 is a graph representing theoretical temporal distribution of a spatially-integrated reflection; 
     FIG. 5 is a graph representing theoretical spatial distribution of a time-integrated reflection; 
     FIG. 6 is a depiction of a parallel scanning process utilizing four ports; 
     FIG. 7 is a diagram illustrating the trajectories of transmitted and reflected photons in a scattering medium resulting from the injection of photons at an input surface; 
     FIG. 8 is a diagram illustrating the trajectories of transmitted and reflected photons in a scattering medium resulting from the injection of photons at an input surface using a re-injection reflector according to the present invention; 
     FIG. 9 is a graph showing the gain of input re-injection as a function of the slab thickness, according to the present invention; 
     FIG. 10 is a graph showing the gain of input re-injection as a function of the absorption coefficient according to the present invention; 
     FIG. 11 is a diagram showing optimized injection using an optical fiber and re-injection reflector according to the present invention; 
     FIG. 12 is a diagram showing an optimized light collection arrangement using a re-injection reflector according to the present invention; 
     FIG. 13 is a diagram showing a usual two-port geometry using optical fibers; 
     FIG. 14 is a graph showing cross-talk effects on a detected signal in a two port geometry; 
     FIG. 15 is a diagram showing a two-port geometry using optical fibers of adjusted length according to the present invention; and 
     FIGS. 16 a  and  16   b  are graphs of detected signals at ports  1  and  2  in a two-port geometry using optical fibers of adjusted length, as shown in FIG. 15, according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As discussed previously, most of the photons injected into a scattering medium  24  are reflected back to the input surface  22  after traveling only a few millimeters in the medium  24 . Referring to FIG. 7, a reflected photon exits the medium  24  and has no chance to contribute to the detected signal at the output surface  26 . Nevertheless, referring to FIG. 8, if the photons hit a reflector  28 , they will be reflected back into the scattering medium  30 , having a certain probability of reaching the output surface  26  and contributing to the detected signal. The employment of such reflectors  28  can be used to significantly improve the overall transmission. Furthermore, referring to FIGS. 2-5, it is apparent that the reflection process is fast and covers a small area compared to the transmission process. Therefore, re-injected photons will contribute to the transmission without significantly changing the spatio-temporal distribution of the transmission. 
     It should be noted that such reflection naturally occurs partially due to the Fresnel reflection between the scattering medium and the external medium, since most of the time, an index mismatch exists. 
     Assuming that the laser beam  20  is introduced into the scattering medium  24  by traversing an infinitely large reflector  28  through a hole  32  of negligible dimension, the gain in the total transmission can be calculated using Monte Carlo simulations. FIG. 9 shows the total gain of input re-injection as a function of slab thickness for a scattering medium having a scattering coefficient μ s ′=1 mm −1 , an absorption coefficient μ a =0.002 mm −1  and a refractive index n=1.34. The points in FIG. 9 represent the calculated values. 
     FIG. 10 shows the gain of input re-injection as a function of the absorption coefficient for a 40 mm thick slab having a scattering coefficient μ s ′=1 mm −1  and a refractive index n=1.34. Again, the calculations are represented by points. From the forgoing it is found that the gain of input re-injection depends significantly only on the absorption coefficient since the dependency upon the slab thickness is very weak. It can be shown that the gain dependency upon the scattering coefficient is very similar to the dependency upon the slab thickness. For weakly absorbing media, such as the human breast, input re-injection could result in an increase of the transmission by approximately a factor of five. 
     The most important factor, in order to take advantage of re-injection, is to introduce the laser beam  20  through a hole  32  in the reflector  28  that is as small as possible. Such an injection can be easily achieved using an optical fiber  34  as illustrated in FIG.  11 . Most of the time, a transparent medium  36  must be used between the scattering medium  24  and the external medium (typically air) mainly in order to maintain the slab shape when the scattering medium is soft or liquid. Such a transparent medium  36  must be as thin as possible in order to avoid delays that could change the time-dependent transmission. Nevertheless, a minimal spacing between the optical fiber  34  and the scattering medium  24  of approximately 2 mm is beneficial to avoid too high an intensity of the laser beam  20  for applications such as breast optical scanning. 
     Considering FIG. 5, the size of the reflector  28  can be limited to approximately 5 mm in diameter. A larger reflector  28  would increase transmission only slightly and the additional detected photons would be less interesting as they would come from a source point much farther from the injection point and would be detected with larger time delays. 
     In the case of a detection limited to a small area, the same re-injection strategy can be used, as illustrated in FIG.  12 . The use of a doughnut shaped re-injection reflector  38  increases the effective surface of the detector. The effective detection surface is not equal to the reflector diameter since the probability of a re-injected photon reaching the detector is not equal to one. The re-injection reflector  38  will increase the number of detected photons but will also contribute to a slight degradation of the image resolution. There is thus a trade-off between the increase in the detected light and the decrease in the image quality. The benefit of a re-injection reflector  38  at the output is strongly dependent on the intrinsic detected area. The smaller the detector is, the higher the gain from using the re-injection reflector will be. 
     As discussed previously, another way to alleviate weak transmission of light is to use parallelism. Parallelism is complicated by the cross-talk that exists between the different ports in a multi-port geometry. FIG. 13 shows an example of a two-port geometry using optical fibers  34 , and FIG. 14 shows the corresponding cross-talk effects on the detected signals. 
     The laser beam  20  enters an all-fiber light divider  31  which divides the laser beam into input optical fibers  34 , which are fed to port  1  and port  2 . Optical fibers  34  transmit the received light to the detectors  29 . 
     In the example shown in FIG. 14, theoretical calculations are given for a 40 mm thick slab having a scattering coefficient μ s ′=1 mm −1 , an absorption coefficient μ a =0.002 mm −1  and a refractive index n=1.34, the port separation is 40 mm. The calculation corresponds to the total temporal distribution of the transmission over a small area (solid line)  42  which is the sum of the useful signal (dashed line)  44  and cross-talk (dotted line)  46 . The cross-talk limits the spacing between two adjacent ports to about a separation equal to the thickness of the scattering slab. 
     In a multi-port scanning device as illustrated in FIG. 13, time dependent transmissions through the different ports can be measured together and at the same time using a fast detecting system such as a streak camera. Most of the time, it is useful to adjust the fiber lengths L 1  and L 2  of the different ports to equal sizes so that the different light signals are synchronized and arrive simultaneously at the detectors  29 . 
     Another strategy can be used to synchronize the useful signals while eliminating the cross-talk. The strategy is to use unequal pathlengths at both the input and output side of the medium  24  as illustrated in FIG.  15 . FIG. 16 shows the detected signals corresponding to FIG.  15 . The useful signals are shown in solid line  50  while the cross-talks are shown in dotted lines  52 . Once again the theoretical calculations are given for a 40 mm thick slab having a scattering coefficient μ s ′=1 mm −1 , an absorption coefficient μ a =0.002 mm −1  and a refractive index n=1.34, the port separation is 40 mm and the fiber length difference ΔL is 50 cm. FIG. 16 a  shows the signal detected at port  1  and FIG. 16 b  shows the signal detected at port  2 . 
     As can be seen in FIGS. 16 a  and  16   b , the two useful signals arrive at the detector simultaneously since the light travels along the same total length of fiber L 1 +L 2 . The cross-talk arrive at the detector before or after the useful signal: the cross-talk from port  2  to port  1  travels along a length of fiber equal to L 1 +L 2 −2ΔL and the cross-talk from port  1  to port  2  travels along a length of fiber equal to L 1 +L 2 +2ΔL. Using this strategy, it is possible to eliminate the cross-talk since it is not temporally superimposed on the useful signal. Such a strategy can be used only if the repetition rate of the input pulses is sufficiently low: the time interval between the time windows at which the useful signal occurs must be sufficiently large to accommodate the delayed cross-talks. 
     The same strategy can also be applied to multi-port geometries that contain more than two ports. Different pathlengths must then be used with always the same goal of keeping the same total pathlength (input and output fibers) for the useful signals while having different pathlengths for the cross-talks. 
     Using the foregoing strategy to eliminate the cross-talk makes it possible to relax the constraint for the scanning configuration. The multi-port geometry discussed previously is limited by the cross-talk effects. If the cross-talk is eliminated, any kind of parallelism can be used and adjacent ports can be as close as desired. The scanning area of each port is not limited anymore by the minimal fiber spacing and more dense parallelism can be used. 
     Having described several embodiments of the scanning module for imaging through scattering media in accordance with the present invention, it is believed that other modifications, variations and changes will be suggested to those skilled in the art in view of the description set forth above. It is therefor to be understood that all such variations, modifications and changes are believed to fall within the scope of the invention as defined in the appended claims.