Patent Publication Number: US-2019175095-A1

Title: A Medical Imaging System and Method

Description:
BACKGROUND 
     The invention relates to a medical imaging system and method. 
     Various medical imaging techniques are known for examining the human body. Such imaging techniques may be used to interrogate tissues and organs in order to identify abnormalities, such as tumors or lesions. For example, X-ray (mammography), microwave imaging, ultrasound, MRI are all common imaging modalities. Such techniques are commonly used to examine breast tissue, but may also be used in other areas of the body, such as the liver, pancreas, prostate, thyroid, lungs, ovaries and lymph nodes. 
     The quality of the image produced by an imaging modality is a key factor in its uptake. In particular, the spatial resolution and presence or absence of artefacts in the image are fundamental properties which may be considered when selecting an imaging modality. As a result, there is generally a desire to improve the quality of the image produced using an imaging modality. 
     The invention seeks to provide a system which improves the quality of the image obtained using a microwave antenna array. 
     SUMMARY 
     In accordance with an aspect of the invention, there is provided a medical imaging system comprising: a microwave antenna array comprising a plurality of antennae which are spaced from one another, the plurality of antennae defining a transmitting antenna and receiving antennae, wherein the transmitting antenna is configured to transmit microwave signals so as to illuminate a body part of a patient and the receiving antennae are configured to receive the microwave signals following scattering within the body part; wherein the microwave antenna array has a plurality of configurations defining positions of the plurality of antennae relative to the body part, wherein in each configuration the plurality of antennae are located at positions which are unoccupied in the other configurations; the medical imaging system further comprising: an actuator configured to move the microwave antenna array between the plurality of configurations so as to position the plurality of antennae at previously unoccupied positions; and a processor configured to obtain a data set for the microwave signals produced in each of the plurality of configurations of the microwave antenna array and to generate an output indicative of the internal structure of the body part from a concatenation of the data sets. 
     The microwave antenna array may be formed on a substrate, wherein the substrate is contoured to conform to the body part. 
     The substrate may be hemispherical. 
     The actuator may be configured to rotate the microwave antenna array relative to the body part. 
     The physical spacing between the antennae may be greater than half the wavelength of the microwave signal and an effective spacing between the antennae at the positions defined by the plurality of configurations may be less than half the wavelength of the microwave signal. 
     The antennae may be positioned in a formation formed from a regular tessellation of a polygon which is tilted with respect to a direction of movement of the antennae. 
     The polygon may be a square. 
     The formation may be tilted at an angle which is between 20 degrees and 35 degrees, and preferably at an angle of 27 degrees or more specifically 26.565 degrees in order to maximize the spacing between the antennae. In particular, with this arrangement the antennae are spaced at uniform spacings aligned parallel to the axis of rotation. 
     In accordance with an aspect of the invention, there is provided a medical imaging method comprising: (a) illuminating a body part of a patient with microwave signals emitted by a transmitting antenna of a microwave antenna array; (b) receiving the microwave signals following scattering within the body part at a plurality of receiving antennae of the microwave antenna array to obtain a data set; (c) moving the positions of the antennae relative to the body part so that they are located at positions which were previously unoccupied and repeating steps (a) and (b); (d) concatenating the data sets obtained and generating an output indicative of the internal structure of the body part from the concatenated data sets. 
     The antennae may be moved using an actuator connected to the microwave antenna array. 
     The actuator may be configured to rotate the microwave antenna array relative to the body part. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:— 
         FIG. 1  is a system diagram of a medical imaging system according to an embodiment of the invention; 
         FIG. 2  is a schematic view of the medical imaging system; 
         FIG. 3  is a flowchart depicting a sampling method; 
         FIG. 4  is a plan view of an antenna array of the medical imaging system in a first configuration; 
         FIG. 5  is a plan view of the antenna array in a second configuration; 
         FIG. 6  is a plan view of the antenna array in a third configuration; 
         FIG. 7  shows the positions of the antennae for each configuration shown in  FIGS. 4 to 6  overlaid on one another; and 
         FIG. 8  is an illustrative example showing a translation of a tilted tessellation. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a medical imaging system  2  according to an embodiment of the invention. The medical imaging system generally comprises a processor  4 , a microwave antenna array  6  in communication with the processor  4 , and an actuator  8  connected to the microwave antenna array  6  and in communication with the processor  4 . 
     As shown in  FIG. 2 , the antenna array  6  comprises a plurality N of antennae  16  which are arranged over the surface of, or within, a shell substrate  18 . The shell  18  has a curved profile as shown. In particular, the shell  18  is part or hemi-spherical and is configured to approximate the shape of a breast. The antennae  16  are arranged over the shell  18  such that they all point to a common focal point. 
     The antennae  16  are each electrically connected to a switching matrix  20 . The switching matrix  20  is in turn connected to both a transmit path and a receive path. The transmit path comprises a signal generator  22  coupled to an amplifier  24 . The receive path comprises an amplifier  26  coupled to a detector  28  and the processor  4 . 
     The switching matrix  20  selectively couples each of the antennae  16  to either the transmit path or the receive path. 
     The antenna array  6  is operated in a multi-static fashion. Specifically, the switching matrix  20  is controlled so as to connect one of the antennae  16  to the transmit path and the remaining antennae  16  to the receive path. The signal generator  22  generates a stepped frequency continuous wave (CW) signal which is amplified by the amplifier  24  and then transmitted by the antenna  16  connected to the transmit path. The stepped frequency continuous wave signal is a sequential series of pulses of continuous wave energy, where each pulse has its frequency stepped up across a range of frequencies, typically within the 3-8 GHz range. The other antennae  16  receive the transmitted signal and the received signal is detected and then recorded by the processor  4 . 
     As shown in  FIG. 2 , the shell  18  receives a cup  30 . The cup  30  has a complementary shape to the shell  18  such that if fits snugly within the shell  18 . A layer of coupling fluid (dielectric constant controlled fluid) may be inserted in the gap  31  between the shell  18  and the cup  30  so as to improve the coupling between the antennae  16  and the cup  30  in order to minimize signal loss and thus improve transmission of the microwave signal. 
     As described previously, an actuator  8 , such as a motor, is connected to the microwave antenna array  6 . The actuator  8  is configured to move the microwave antenna array  6  relative to the cup  30  which remains stationary against the breast  36 . Specifically, the actuator  8  causes the microwave antenna array  6  to rotate relative to the cup  30  about the breast  36 . The microwave antenna array  6  rotates about the center of the shell  18  (i.e. its axis of symmetry). This may be enabled by a threaded engagement between the cup  30  and the shell  18 . In particular, the outside of the cup and the inside of the shell  18  may have threaded portions which engage to allow the shell  18  and the antenna array  6  to be rotated relative to the cup  30 . This may also allow the cup  30  to be quickly and easily removed so as to enable the coupling fluid to be replaced. 
     The antennae  16  may be as described in WO 2009/060181. In particular, the antennae  16  may comprise a slot  16  formed in a conductive element, the slot having a rectangular external boundary defined by a substantially closed internal edge of the conductive element. A microstrip feed line may be spaced from the conductive element by a dielectric substrate with the distal end of the line positioned at the geometric center of the slot. Although such an antenna is effective at launching microwaves into human tissue, its size may prevent the antennae from being positioned sufficiently close to one another. The system  2  uses the actuator  8  to rotate the antenna array  6  so that the antennae  16  assume positions which were previously unoccupied. Although the positions of the antennae  16  may overlap to a certain extent, the term “unoccupied” is used herein to refer to new sampling positions. The rotation of the antenna array  6  therefore creates a “virtual” array which has a higher density of antennae  16 . 
     In use, a patient lies in a prone position such that their breast  36  sits in the cup  30 . A layer of coupling fluid may also be provided in the gap  35  between the cup and the breast  36  in order to improve coupling between the antennae  16  and the breast  36 . 
     Although not shown, one or more inserts may be placed inside the cup  30  so as to enable a better fit between the internal surface of the cup  30  and the breast  36 . For example, a plurality of such inserts may be provided, each having different shapes and sizes, to enable the system to be better adapted to breasts of different shapes and sizes. The inserts may be made from the same material as the cup (e.g. ceramic). 
       FIG. 3  shows a flowchart of a data acquisition method. As shown, the switching matrix  20  connects an antenna m of the N antennae to the transmit path. All other antennae  16  (n=1 . . . N, n≠m) are connected to the receive path. The antenna  16  connected to the transmit path illuminates the breast  36  with the microwave signal. The signal is scattered by the breast tissue and the scattered signal is received at each of the non-transmitting antennae  16  where it is detected (possibly in a time-sharing arrangement) and recorded. If m#N, the switching matrix  20  steps to the next antenna  16  (m=m+1) to be connected to the transmit path. This is repeated until all antennae  16  have been connected to the transmit path. 
     If additional data sets are required, then the actuator  8  is used to rotate the array  6  relative to the breast  36  while retaining the breast in position. The acquisition process is then repeated with the antenna array  6  in the new configuration. 
     The processor  4  may record the relative difference between the measured phase and amplitude of the transmitted signal as compared to the phase and amplitude of the scattered signal, recorded as a complex number (having real and imaginary parts). 
     The signal detected at each antenna  16  is affected by scattering arising from objects within the imaged volume (i.e. the breast  36 ). In particular, tumors can generate significant reflections as they exhibit much higher dielectric properties than adipose and connective tissues due to their significant water content and so can be identified in the acquired data. 
     The acquired data sets may be used by the processor  4  to construct an image of the internal structure of the breast  36 . Specifically, the data sets acquired for each configuration of the antenna array  6  are concatenated (i.e. combined) and the concatenated data set is used to generate the image. The resolution of the image is therefore improved by combining the data sets for each configuration of the antenna array  6 . The Data reconstruction may be performed using Phased Array (frequency domain), Delay and Sum (DAS—time domain) techniques or any other suitable technique, such as 3D Fourier Transformation, Back Projection, etc. From this, the processor  4  is able to identify (possibly, with additional user input or confirmation) a region of interest (if present) in which a possible tumor or other pathology may exist. 
     As described above, the rotation of the antenna array  6  relative to the breast  36  during the image capture process moves the antennae  16  to previously unoccupied positions. The effective spacing of the antennae  16  (i.e. the spacing between the occupied positions across all configurations) in the virtual array is therefore smaller than the physical spacing of the antennae  16  on the shell  18 . This may therefore improve the resolution of the image. The effective spacing of the antennae  16  in the virtual array is preferably less than half the wavelength of the microwave signal in order to prevent grating lobes from occurring in the image. Owing to the size of the antennae  16 , such spacing may not otherwise be physically achievable. As described previously, the frequency of the microwave signal may be stepped up across a range of frequencies, typically within the 3-8 GHz range. Accordingly, the wavelength referred to here is based on the center frequency, i.e. 5.5 GHz (which at εr≅9, λ≅2 cm). 
     Although the antennae  16  may be positioned over the shell  18  in any arrangement which causes them to be moved to previously unoccupied positions,  FIGS. 4 to 6  show a preferred arrangement which maximizes coverage.  FIG. 4  shows the antenna array  6  in a first configuration and  FIGS. 5 and 6  show the antenna array  6  in second and third configurations where the array has been rotated about an axis of rotation which is preferably aligned with the axis of symmetry of the antenna array  6 . 
     The arrangement shown in  FIGS. 4 to 6  is based on a projection into a hemispherical form of a tilted regular tessellation (tiling) of polygons, where the corners of the tiles define the positions of the antennae  16 . 
     For a simple tessellation of unit squares, a horizontal translation by 0.5 will generate a virtual array which has a reduced effective spacing in the horizontal direction, but which retains the same vertical spacing. In contrast, a diagonal translation (movement along two axes of motion—vertical and horizontal) will reduce the effective spacing in both the horizontal and vertical directions. This same effect can be achieved by instead tilting the tessellation of squares and then performing a translation along a single axis (e.g. horizontal), as shown in  FIG. 8 . 
     By tilting the tessellation the corners of the squares move along paths which are all offset from one another, rather than moving along common paths. A uniform vertical spacing between the corners may be achieved by tilting the square tessellation by 26.565 degrees (=tan−1(0.5)). This maximizes the vertical separation between the corners. 
     As described above, the arrangement shown in  FIGS. 4 to 6  is a projection of the tilted tessellation into a hemispherical form. The tilted tessellation may be transformed into a hemispherical form using an inverse Mercator projection or other map projection. Rotation of the antenna array  6  about its axis thus causes the antennae  16  to occupy new positions which are spaced between the original positions of the antennae  16  so as to define a virtual array which has a substantially uniform density with an effective spacing which satisfies the half wavelength criterion.  FIG. 7  shows the positions of the antennae  16  for each configuration overlaid on one another and thus demonstrates the density of the virtual array. 
     Typically in a hemispherical array where the antenna are separated by a single angular distance A, the angular rotation for each increment will be A/3, thereby increasing the virtual array density by a factor of three. 
     The density of the positions shown in  FIG. 7  is sufficient to produce a concatenated data set which is completely determined. In other words, from the concatenated data set, the interference pattern that is the scattered field may be calculated at any spatial position, including at positions other than those of the samples. Thus from a set of discrete samples complete information may be obtained about the field at all spatial positions. This avoids artefacts such as grating lobes from occurring within the image. The movement of the antennae  16  reduces the effective spacing between the antennae  16 . This therefore allows larger and/or fewer antennae to be used while still obtaining an effective spacing which is less than half the microwave wavelength. 
     Although the antenna array  6  has been described as having a plurality of discrete configurations/positions, the data may instead be acquired from a continuously moving array. Further, the entire antenna array  6  need not rotate and the individual antennae  16  may instead assume different positions within the shell  18  in each configuration of the antenna array  6 . 
     Although the system has been described with reference to the imaging of a breast, it will be appreciated that it may be adapted for other areas of the body. The antenna array  6  therefore need not be hemispherical and may be contoured to conform to other body parts. Where the antenna array  6  does not have rotational symmetry, the antennae  16  may be translated rather than rotated. 
     The actuator  8  may cause movement of the antennae  16  in any suitable way. In other arrangements, the movement of the antennae  16  may be manually actuated. 
     Where manual actuation is used, the antenna array  6  may be arranged so as to ensure repeatability of the measurements. In particular, the antenna array  6  may have discrete positions and a suitable form of feedback for confirming to the user when the antenna array  6  is in the proper position. The antenna array  6  may be biased towards discrete positions using a suitable indexing mechanism or the like. 
     It will be appreciated that the processor  4  need not display, or indeed, generate, an image of the internal structure of the breast in order to identify a region of interest. The region of interest may instead be determined based on the raw data. 
     The system may employ multiple transmit and receive paths, such that data can be recorded from multiple antennas simultaneously, and may even comprise a number of transmit and receive paths corresponding to the number of antennas, such that data can be recorded from all antennas simultaneously. In an alternative topology, the switching matrix could be removed thus allowing each antenna to be connected to a transmitting/receiving device. 
     To avoid unnecessary duplication of effort and repetition of text in the specification, certain features are described in relation to only one or several aspects or embodiments of the invention. However, it is to be understood that, where it is technically possible, features described in relation to any aspect or embodiment of the invention may also be used with any other aspect or embodiment of the invention. 
     The invention is not limited to the embodiments described herein, and may be modified or adapted without departing from the scope of the present invention.