Patent Application: US-94294504-A

Abstract:
a tissue - sensing adaptive radar method of detecting tumours in breast tissue uses microwave backscattering to detect tumours which have different electrical properties than healthy breast tissue . the method includes steps for reducing skin reflections and for constructing a three - dimensional image using synthetic focusing which shows the presence or absence of microwave reflecting tissues .

Description:
the present invention provides for a method and apparatus for tissue sensing adaptive radar imaging of breast tissue . when describing the present invention , all terms not defined herein have their common art - recognized meanings . as used herein , “ microwave ” means non - ionizing electromagnetic radiation which has a wavelength between about 10 − 4 to 10 − 1 m , and frequencies in the range of about 10 8 to about 10 10 hz . the term “ radar ” refers to a method of detecting the presence and location of an object by detecting reflections of microwave radiation from the object . microwave imaging in medical situations is well described in a publication entitled “ medical applications of microwave imaging ”, edited by l . e . larsen and j . h . jacobi , ieee press 1986 , the contents of which are incorporated herein by reference . the term “ tissue sensing ” means a process of radar imaging which may distinguish between various tissues of the breast such as skin , glandular tissue , and tumors . the term “ adaptive ” means a process of radar imaging which incorporates signal manipulation steps to remove non - tumor reflective signals using localized or signal - specific information . the system of the present invention may be described as a tissue - sensing adaptive radar system . the physical basis for breast tumor detection with microwave imaging is the contrast in dielectric properties of normal and malignant breast tissues . in general terms , the system includes a plurality of wideband antennas for illuminating the breast and collecting the reflections as well as a computer system for acquiring the data and synthesizing an image from the accumulated data . the system is able to isolate reflections coming from a specific location within a three - dimensional volume , which in the present instance , is defined by the volume of the breast . the following description is of a preferred embodiment of the invention , which is not intended to be limiting of the claimed invention . the preferred method of radar scanning in the present invention is referred to herein as “ cylindrical ” scanning . in a cylindrical scan , the patient lies face down on a modified patient bed which has a well for the breast to fall into . the well may include a gel or a liquid to better conform the breast to the surface of the well and the antennas . the antennas may be integrated into the surface of the well or may be moved around the well as shown in fig1 . in one embodiment , the antennas may be conformal antennas integrated by printing onto the surface of the well . the antennas preferably comprise wideband antennas such as standard tem horn antennas which are well known in the art . they may be adapted to effectively operate in a dielectric similar to either skin or fat tissue . the present invention proceeds with data acquisition in two steps . first , an initial scan is performed to locate the breast in the imaging volume , and second , a tumor - sensing scan is performed to locate reflecting structures ( such as a tumor ) within the breast . the scans may be performed with a single antenna which is moved from location to location or a plurality of antennas in an array . it is not necessary that the initial scan be performed with microwaves as it is a boundary sensing step . it is possible to perform the initial scan with an alternative method such as using higher frequency signals which have shallower penetration or laser light . the initial scan may be performed along one two - dimensional path or a set of two - dimensional paths as shown in fig2 a . the antenna may be moved ( or multiple antennas provided ) to a plurality of locations along the z direction , then a plurality of locations in the x direction , and finally a plurality of locations again in the z direction . with antennas integrated into the scanning bed , a subset of the antennas may be used for this scan . the tumor - sensing scan is preferably done to form a synthetic conical array . the antenna may be moved to a plurality of locations along a row ( x - y plane ), with multiple rows spanning from the nipple to the chest wall , as shown in fig2 b . as most microwave measurement equipment is intended for use in the frequency domain , the measured data are in the frequency domain . in order to obtain reasonable resolution for the images and maintain compatibility with the image formation algorithms described herein , conversion to time domain signals is required . a weighting window is applied to the measured data to produce the desired time - domain pulse . this pulse may be a differentiated gaussian pulse with maximum frequency content near 5 . 24 ghz and full - width half - maximum ( fwhm ) bandwidth from 1 . 68 to 10 ghz . any pulse with ultra - wideband frequency content in the range of 0 . 1 to 10 ghz may be suitable for use . the weighted signals are transformed to the time domain either with inverse fourier transforms or , more preferably , inverse chirp - z transforms , both of which are well known in the art . the latter provide flexibility in selection of the time step and smaller time steps may assist in clutter reduction . the recorded signals have early and late time content . the early time content is dominated by the incident pulse , reflections from the skin and residual antenna reverberations . the late time content contains tumor backscatter and backscatter due to clutter . the signal processing goals are to reduce the early - time content , which is of a much greater amplitude than the tumor response , and to selectively enhance the tumor response while suppressing the clutter to permit reliable detection of tumors in the reconstructed images . the images are reconstructed using the image formation steps described below . first , the signals are calibrated by removing the response of the antenna , which is done by subtracting the signal received at the antenna without any scattering object present . next , an image is formed by synthetically scanning the focal point through the region inside the array . the resulting image indicates the location of the skin , and the imaging region for detection is defined using this information . the results of a simulated two - dimensional skin sensing scan is shown in fig3 . thresholding or filtering of the image is used to identify the skin , edges are identified by the largest group of connected pixels and all other pixels are removed from the resulting image of the skin . the pixels located closest to and farthest away from each antenna are assumed to represent the first and second skin interfaces . the initial image of the skin formed from the initial skin - sensing scan is then used to determine an appropriate time - gate for the sensing scan data . in order to limit data to reflections from within the volume defined by the skin , the calibrated data arriving before the first skin reflection and after the second skin reflection are set to zero . a two - step process may be used to reduce the reflection from the skin . these steps are applied to the signal recorded at each antenna or each location . the first step estimates the skin reflection using signals recorded at a number of antennas near the current antenna . the signal recorded at the current antenna is referred to as the target signal . the signals at neighboring antennas are matched to the target signal by time - shifting and scaling each signal in turn . this process is repeated several times in order to obtain the best match . the estimate of the target is obtained by taking the average of the shifted and scaled set of signals . a separate estimate is calculated for each antenna . the estimates are likely to be different , as each target signal requires different time - shifts and scaling . the estimate for each antenna is subtracted from the target signal . the resulting signal is likely to contain imperfectly cancelled reflections . this first step estimates the large reflection from the interface between the immersion liquid and the skin . a second reflection is generated from the interface between the skin and the interior of the breast . these reflections are likely similar at neighboring antennas , as the underlying tissues are expected to be somewhat similar . the second step in the skin subtraction process provides an estimate of the reflections remaining in the signal after the first subtraction , which are related to local tissue variations . the estimate in this step is formed with the subtracted signals at the neighboring antennas . again , the signals are scaled and time - shifted to match the target signal and the estimate is the average of the set of signals . this estimate is subtracted from the target signal . this two - step process may be referred to as “ adaptive estimation ” of the skin reflection . the technique of adaptive estimation may also be used to estimate and subtract reflections from other strongly scattering objects , such as blood vessels and glandular tissue . the calibrated signals , with greatly reduced skin and other non - tumor reflections , may then be adjusted such that the value of the signal at its midpoint in time corresponds to the maximum amplitude of the signal . for example , a differentiated gaussian excitation signal has a zero - crossing at its centre point in time . the backscattered signal that would follow after a specific time delay corresponding to the round - trip distance between the antennas and the scattering object ( tumor ) would also have a zero - crossing at its centre point . the processed signals are synthetically focused at a plurality of specific points in the breast . first , distances from each antenna to each focal point are computed and converted into time delays . focusing includes a weighting based on relative distance between the focal point and antennas . in a preferred embodiment , path loss compensation is not required . the time delays are used to identify the contribution from each processed signal . all contributions are summed and the squared value of this sum is assigned to the pixel value at the focal point . an estimate of the velocity of propagation of the signal in the medium is used . the focal point is scanned to a new location in the region of interest , and this process is repeated . the focal point coordinates are defined with respect to the defined volume within the skin . focusing may include a weighting based on relative distance between the focal point and antennas . this gives greater emphasis to data recorded at antennas located near the focal point . it may also be preferable to filter the image to emphasize pixels nearer the center of the scanned volume . in one embodiment , the microwave imaging of the present invention may be combined with other imaging methods such as magnetic resonance ( mr ) techniques . mr images provide excellent definition of various tissue types , and blood vessels larger than about 1 mm may be imaged with time - of - flight angiography , as is well known in the art . the comparison or co - registration of mr and microwave images provides complementary information for image interpretation . for example , clutter in a microwave radar image may be identified as arising from tissue structures in the breast , or a lesion that enhances in a mr scan may be shown to be a strongly scattering object in a microwave image . as will be apparent to those skilled in the art , various modifications , adaptations and variations of the foregoing specific disclosure can be made without departing from the scope of the invention claimed herein . the various features and elements of the described invention may be combined in a manner different from the combinations described or claimed herein , without departing from the scope of the invention . the following references are incorporated herein as if reproduced in their entirety . p . m . meaney , k . d . paulsen and m . w . fanning , “ microwave imaging for breast cancer detection : preliminary experience ,” proceedings of spie , vol . 3977 , 2000 , pp . 308 - 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