referring now to the drawings , which are provided by way of example and not limitation , there is shown a system for detecting lesions within biological tissue . the disclosed system is applicable to various locations of the body . in one particular application , the system can be employed to detect and treat lesions found in breast tissue . the presence of an elevated temperature in a small volume of tissue relative to the temperature of adjacent volumes of tissue can be an indication of a malignant lesion . the absolute temperature of a sample is of less interest than the local differential temperature . since all matter above absolute zero kelvin temperature radiates energy , it is possible , in principle , to measure that temperature through remote sensing . the energy - density of radiation in the microwave region can be accurately modeled by the rayleigh - jeans law . however , since human tissue has conductivity and dielectric properties differing from free space , there are sizable attenuations of signal strength and refractions when regions deep within the tissue are being examined . consequently , passive microwave radiometry has not been successful previously except for exploring very shallow depths within tissue . passive and non - invasive breast cancer detection can be accomplished by a scanning microwave radiometer . as stated , one particular application would be to the detection of breast cancer . since lesions experience a more rapid rate of mitosis than normal tissue and exhibit angiogenesis , they have relatively elevated temperatures that can indicate the presence of lesions . the rayleigh - jeans radiation law can be employed to predict accurately the radiation as a function of temperature at microwave frequency . that temperature can be measured in small volumes and compared to the temperature of adjacent small volumes . it has been noted that the resting metabolic rate for humans is about 1 mw / cm 3 . further , it has been shown that the specific heat production rate associated with lesion growth increases with the doubling time of tumor volume ( see fig1 and 2 ; from k . l . carr , december 1989 , revised aug . 6 , 2002 , ieee transactions on microwave theory and techniques ). a tumor that doubles in volume in two years has a metabolic rate of about 5 mw / cm 3 and a corresponding increased temperature of about 1 ° c . ; a tumor that doubles in 60 days has a metabolic rate of about 40 mw / cm 3 . the corresponding higher temperature of the more aggressive lesions makes microwave radiometry a desirable mode of detection , particularly where scanning procedures can be performed at frequent intervals to monitor the progress of the more virulent lesions without the risk of repeated x - ray exposure . with reference to fig3 , there is shown one approach to a passive tissue scanning system . the system can be embodied in an antenna assembly 100 which can be configured to define a radiometer that passively measures electromagnetic energy . the radiometer can be a microwave radiometer that measures energy emitted at microwave wavelengths . the system can further be configured to characterize temperature differences between separate cell units of tissue . unfortunately , a single ½ - reflector does not have sufficient signal - to - noise ratio ( snr ) to differentiate temperature of the tissue of interest from normal tissue with approximately the same temperature . the unwanted signal is considered “ clutter ” in radar parlance . the solution to this problem lies in employing two ½ - reflectors with independent antennas . various angles between the reflectors are contemplated as is the use of three or more reflectors . by utilizing coherent detection it is possible to eliminate all signals emanating from tissue outside the common focus region . very large improvements in snr can be realized by this technique with signals becoming detectable well below the noise floor . thus , as shown in fig3 , the antenna system 100 includes a first ellipsoidal antenna assembly 110 and a second ellipsoidal antenna assembly 120 . each of the ellipsoidal antenna assemblies 110 , 120 include an ellipsoidal reflector 130 , 140 . as will be developed further below , the ellipsoidal reflectors 130 , 140 each have associated therewith , an interior or internal focal point 150 , 152 and an exterior or external focal point , respectively 154 , 156 . mounted within the first ellipsoidal reflector 130 is a first antenna 160 . the first antenna is positioned at the interior focal point 150 of the first ellipsoidal reflector 130 . mounted at or near the interior focal point 152 of second reflector 140 is a second antenna 162 . the second antenna can be asymmetrically affixed to a rotating structure 170 , such as a turntable , positioned at the interior focal point 152 of the second reflector 140 . in this way , as the turntable rotates , the second antenna 162 will move through a defined path . the system 100 further includes a tissue receptacle 180 . the receptacle 180 can be sized and shaped to receive various tissues , but as shown in fig3 , one application can be configured to receive breast tissue . the receptacle is also contemplated to define structure which is flexible but capable of securely retaining tissue for examination . the receptacle 180 is intended to remain stationary while the scanning system 100 moves to complete a scan . further , mounted within the first reflector 130 can be a light source 190 and a photocell or photo detector 192 . the light source 190 is configured to project light energy onto an exterior of receptacle 180 . the photo detector 192 receives light energy reflected off of the receptacle . in one embodiment , the reflectors 130 , 140 define dual ellipsoidal half - reflectors , that are nominally orthogonal with independent antennas 160 , 162 located at their respective internal foci 150 , 152 and with common external foci 154 , 156 . other angles between the reflectors 130 , 140 are also contemplated as are other partial ellipsoidal sections . here , a half - ellipsoidal shape is employed to maximize the opening to an internal cavity defined by the reflectors . the generally orthogonal angle is selected also to enhance operational characteristics . as shown in fig4 a and 4b , the scanning system 100 can be mounted to a three dimensional mechanical stage or transport assembly 200 . although any mounted staging can be employed , one such 3 - d stage can be a modified form of an epson three axis module . such an arrangement would permit raster scanning of external common foci 154 , 156 of the reflectors 130 , 140 positioned within a volume of tissue held with a receptacle 180 . the external focal positions 154 , 156 of each reflector 130 , 140 may be altered by the differential dielectric properties of the intervening tissue each signal encounters as it travels to its respective antenna so that only an approximate coincidence of external focus may occur . the effective position of the external focal point 156 of a second reflector 140 is “ dithered ” in the “ x ” and “ z ” directions by the turntable 170 to seek coincidence with the external focal point of the first antenna 154 through the mechanical displacement of the second antenna 162 within its reflector 140 . by measuring the varying impedance of the coupled antennas during the raster scan due to the “ dithered ” position of the second antenna 162 , the instantaneous position of the second antenna 162 within its reflector 140 that corresponds to peak coupling of the antennas 160 , 162 as a function of the “ x ”, “ y ”, and “ z ” position of the first antenna 160 is stored in a “ lookup ” table for subsequent positioning of the second antenna 162 . by this method , coincidence , or near - coincidence , of the common focal points 154 , 156 is assured resulting in maximum sensitivity of the correlation process independent of the dielectric properties of the tissue involved . a resonant cavity tuned to 10 ghz may be incorporated in the antenna termination of either antenna 160 or 162 while the other antenna is terminated in a characteristic impedance , typically 75 ohms to enhance determination of the optimum coupling position of the antenna 162 . consequently , by taking this approach , both first and second antennas 160 , 162 receive signals that are comprised of coherent signals from the common focal point and incoherent signals from tissue outside the common region . a flow chart describing one approach to processing of signals of the scanning system 100 is provided in fig5 . such signal processing can be accomplished using off the shelf circuitry which is commonly available . it is also to be recognized that employing conventional approaches , the external focal point 156 of the second reflector 140 can be “ dithered ” or otherwise moved in the “ y ” direction as well as in the “ x ” and “ z ” directions . further , a monte carlo method , one where the external focal point 156 of the second reflector 140 is randomly dithered or moved in multiple directions , is contemplated to achieve peak coupling of the antennas 160 , 162 . upon taking this random approach , an angle associated with peak coupling is stored for subsequent positioning of the second antenna . this angle can in certain circumstances be identified for a first tissue volume examined and then used initially when beginning an examination of a second adjacent tissue volume . thus , as shown in fig5 , a scanning system 100 is electronically connected to signal processing circuitry . in this regard , the first antenna 160 associated with the first reflector 130 is immediately connected to a low noise amplifier ( lna ) 210 . first antenna 160 and photo detector 192 are co - located at focal point 150 . similarly , the second antenna 162 is electronically connected , to another low noise amplified ( lna ) 210 . the signals amplified by the lna &# 39 ; s are transmitted to mixers 220 where they are down - converted by oscillator 260 to a nominal 30 mhz signal and then through intermediate frequency ( if ) amplifiers 230 . from there , the separate signals are further amplified in automatic gain control ( agc ) circuits 230 and then multiplied in a multiplier 240 , and integrated 250 . it is also noted that the reflection that occurs at the skin surface of a tissue volume being examined due to the discontinuous dielectric constant of the air / skin interface is diffused since the diverging rays are out - of - focus at those sites . notably , the signals received by the antennas are processed in a cross - correlation detector that eliminates signals from regions outside the coherent common focal region , thereby significantly increasing the detection signal - to - noise ratio . to increase the sensitivity of the temperature measurement it is necessary to employ some focusing of the radiation into the detecting antenna . as stated , this can be accomplished by utilizing the properties of an ellipsoidal ½ - reflector that has two conjugate foci . ellipsoidal reflectors have the property of focusing radiated energy from one focal point to the other focal point . by placing a detecting antenna within a reflector at an interior focal point , rays of energy emanating from within the tissue at an exterior focal point of the reflectors , will be concentrated at the antenna site . further , as stated , the tissue may be “ scanned ” in three dimensions in a raster - scan fashion by moving the reflector / antenna assembly such as by employing a transport assembly ( fig4 a and 4b ), and thereby the exterior focal point , to effect a volumetric survey of the entire area of tissue to be examined ( i . e ., breast ). employing the signal processing approach outlined above , when the two external foci of the ellipsoidal reflectors are co - located , the signals propagating to the internal foci in the reflectors 130 , 140 will be comprised of two compound signals each : f ( t )+ g ( t ) into one reflector and f ( t )+ h ( t ) into the other reflector . the signal f ( t ) represents the signal into each detector antenna 160 , 162 from the tissue in the immediate vicinity of the common external foci 154 , 156 while the signals from the tissue outside the focus vicinity generate signals into the two detector antennas of g ( t ) and h ( t ), respectively . these signals are amplified in the low noise amplifiers ( lna ) 210 and as mentioned , down - converted to a lower frequency in mixers 220 driven by a common local oscillator . in a presently contemplated approach , a 10 ghz local oscillator 260 and 30 mhz intermediate amplifier ( if ) 230 can be used to convert the signal at around 10 ghz to a signal from zero to 30 mhz . the signals are amplified further in the agc circuit 230 to nominal equal voltages . again , the two outputs are then multiplied together in a four - quadrant linear multiplier 240 and integrated 250 . the first three integrals vanish since the signals g , h , and f are not coherently related and are equally likely to have instantaneous values that are (+) or (−). the squared signal from the tissue in proximity to the external foci , however , is always positive since the functions are squared , i . e ., (−)·(−)=(+) and (+)·(+)=(+). evaluated between 0 and t is t / 2 plus a very small oscillating term that can be ignored . the result is that the signal from the tissue of interest at the external mutual focus point is not contaminated by signals from other tissue that contribute only background clutter noise . such signals are eliminated for a dramatic increase in signal - to - noise ratio ( snr ). it is essential that both exterior foci 154 , 156 of the two antenna assemblies coincide electromagnetically throughout the scan in order to utilize the very large snr enhancement that comes from canceling the signal from “ uninteresting ” tissue . since the radiation rays from the tissue at the nominal scan site will encounter differing dielectric materials as they travel toward each antenna at the internal focal points , the effective points of external focus will be refracted and displaced and may not coincide with the result that there is reduced cross correlation between the two signals . in order to ensure correlation , the second antenna is directed into a search mode around the position of focus of the first antenna . it is the turntable 170 to which the second antenna 162 is affixed ( see fig3 ) which facilitates ensuring this correlation . thus , the depth of focus of the ellipsoidal reflectors permits some misalignment of their common focus points , however , small “ x ” and “ z ” displacements of the antenna site within the second reflector 140 will permit relatively large sweeps of the external focal point of that antenna . by mounting the second antenna 162 on the rotating platform or turntable 170 in the “ x ”-“ z ” plane and rotating that antenna rapidly relative to the scan rate , e . g ., 3600 rpm , it is possible to determine the optimum spatial displacement of the second antenna 162 to effect peak coincidence with the focal point of the first antenna 160 . a standard dimensional set for a breast scan is taken from a conventional mammography cassette : 24 cm × 18 cm . depth of scan is 10 cm , or more . much of this volume is free space , however , and should be excluded from the scanning process to reduce the scan time . to do that , the surface contour of the breast must be established . a similar approach can be taken to reduce or control scanning volumes of other tissues as well . this is accomplished by optically servoing the external focal point of the first ellipsoidal reflector 130 during a preliminary scan to track the surface contour of the tissue being scanned and store its coordinates in a look - up table . the subject tissue ( i . e . breast ), will be restrained in the receptacle 180 , for example a brassiere that is made of cotton and contains no metallic fasteners . the cotton fabric has imprinted on it a black and white pattern of equal black and white areas for focusing purposes . the brassiere holds the breasts firmly against the chest to reduce the depth of scan required . an opening at a base end of the first reflector 130 containing the light source 190 , such as a flood lamp , is used to illuminate the brassiere white / black surface . the photo detector 192 mounted at the antenna site ( internal focal point ) of the first reflector 130 receives the reflected light from the brassiere surface as the scan volume is being traversed by the external focal point 154 . since the resistance of the photo detector 192 is highly nonlinear in response to the light level ( illuminated areas create a lower resistance than dark areas ), and since the average illumination of an out - of - focus image is comprised of light contributed equally by white and dark areas , the net resistance of a focused image is much lower than that of an unfocused image ( see fig6 and 7 ). this facilitates servoing the “ z ” axis of the first reflector 130 so that the lowest resistance is located and maintained throughout the “ x ”-“ y ” scan . the “ x ”, “ y ”, “ z ” coordinates are stored in a look - up table . thus , in one particular approach , the scanning procedure can involve four stages : 1 ) locating and mapping the tissue ( i . e ., breast ) surface and determining the scan boundaries , 2 ) scanning the volume of the tissue while locating and storing the second antenna offset for coincidence of focus with the first antenna , 3 ) finding the differential temperature of each tissue volume relative to its near - neighbors , and 4 ) displaying tomographically , in false color , 3 - d thermal images of the tissue ( e . g . 3 - d isometric , false - color format or in tomogoraphic slices ). accordingly , in a presently contemplated approach to scanning tissue , the two approximately orthogonal reflectors 130 , 140 ( see fig3 ) are mounted on a common three - axis linear actuator assembly 200 ( fig4 a and 4b ) and aligned so that their external foci 154 , 156 are coincident . the reflectors can be moved en masse to accomplish a raster - scan by the common foci in the tissue of interest . typically the scanned volume may be 24 cm × 18 cm × 10 cm ( a canonical measurement volume will be considered to be 1 cm 3 ). the assembly is positioned relative to the body of the subject to effect maximum coverage of the tissue to be examined , such as a breast including the axillary region . the subject wears a special brassiere 180 that facilitates focusing the external foci 154 , 156 onto the surface of the brassiere 180 . the brassiere or receptacle 180 can be formed from flexible material ( e . g . cotton without metallic fasteners ) which retains tissue without substantial compression of the tissue . during the scan of the parallelepiped volume in “ x ” and “ y ”, the “ z ” axis is driven by a servo - mechanism to achieve peak focus on the brassiere 180 surface and the coordinates of each surface site are stored . a typical scan - speed in the “ x ” direction will be 30 cm / sec with ½ sec allowed for step and reversal . at about 1 . 5 sec / line and 18 lines in the “ y ” direction ( see fig9 references a - g ), the duration of the breast surface mapping and data storage will require 27 seconds . having determined the surface coordinates of the tissue being examined ( typically reducing the scanned volume by about 60 % relative to the rectilinear volume ), the tissue volume is rescanned at the original 30 cm / sec rate with the scan interrupted at the tissue boundary and stepped to the next line . a typical 3 - d scan will require about 1 . 8 minutes to completion . with reference to fig8 and 9 , each cellular volume 300 of 1 cm 3 will be traversed by the first reflector exterior focal point in about 33 . 3 msec . during that time , the interior antenna in the second reflector will be rotating in the “ x ”-“ z ” plane with about a 2 cm offset at a 3600 rpm rate and will make two complete revolutions during the cellular transit time . the offset of the interior antenna site will cause an offset of the external focal point . when the effective position of the exterior focal point of the second antenna is closest to the exterior focal point of the first antenna , a maximum of coupling occurs between the antennas and may be detected through impedance measurements , e . g ., by terminating the second antenna by its inherent impedance and monitoring the impedance of the first antenna at 10 ghz . the angular position for peak coupling of the rotating second antenna ( see fig3 ) is noted and stored for use in a subsequent measurement scan to maximize coincidence of the two detectors . in the measurement scan of the tissue , the stored data in the look - up table is used to correct continuously the position of the second antenna during the scan of the two - reflector assembly to achieve maximum sensitivity . as each site is addressed , the second antenna displacement is adjusted for maximum coupling . as previously described , the rayleigh - jeans law signal from each antenna is amplified in a lna and mixed with a common reference local oscillator to beat the signals down to a 30 mhz if amplifier with agc . the signals are then multiplied together in a 4 - quadrant multiplier and integrated to remove all uncorrelated “ clutter ” signal leaving only the signal from the common volume in the vicinity of the coincident focal point . that integrated output will rapidly increase since the integral of the squared coherent signal is integrated at 30 , 000 cycles / msec . when a pre - determined integration level is reached , the integration is interrupted and the duration of integration noted as a surrogate for the temperature of that region ; the shorter the integration time , the higher the temperature . that number is stored for comparison later with adjacent near - neighbor cells . for example , the integration time “ t ” can be stored as a surrogate for the local temperature that is compared to the six nearest - neighbor temperatures . again , the duration of the measurement scan is about 1 . 8 minutes . the procedure described above is adequate to construct a tomographic image of the breast with those sites exhibiting temperatures above the average temperature of adjacent near - neighbors highlighted through false color , or other methods . areas of potential interest may be revisited with a new scan to refine the resolution and temperature gradually down to about 3 mm and 0 . 1 ° c . differential temperatures . thus , for higher resolution , the sites with elevated temperature and their immediate neighbors may by scanned again at a slower rate to increase the thermal resolution and the spatial resolution . steps as small as 10 microns with increased integration time will allow much higher thermal and spatial resolution than is possible with the basic 1 . 8 minute scan . such hot spot high - resolution re - scanning is contemplated to take approximately 20 seconds per site . once a lesion is detected , a biopsy can be taken or other further analysis or scan conducted to characterize the lesion . where it is determined that the lesion is cancer or should otherwise be removed or treated , the detecting antennas are reconfigured or replaced with energy projecting antennas . such energy projecting is then employed to treat or otherwise ablate the subject lesion . this is known as hyperthermia . raising the lesion temperature to 113 ° f . is adequate to destroy it . accordingly , the present disclosure is intended to address passive , non - invasive tomography . the presently disclosed system thus employs a completely passive scanning of tissue , one where no radiation is generated . there is also no compression of tissue being examined and scanning can be repeated at short intervals to track identified lesions . the system is also capable of early detection of lesions (& lt ; 3 mm detection threshold ) and functions to minimize false negatives and false positives . accordingly , it will be apparent from the foregoing that , while particular forms of the contemplated approaches have been illustrated and described , various modifications can be made without parting from the spirit and scope of the invention .
a passive , noninvasive tomography apparatus and method is disclosed for in - depth tissue imaging and lesion detection . in one particular approach , the disclosed apparatus and method is adapted for use in breast imaging .