WO2002025773A1

WO2002025773A1 - Microwave antenna and imaging system for imaging human tissue

Info

Publication number: WO2002025773A1
Application number: PCT/SE2001/001978
Authority: WO
Inventors: Johan Landelius; Hans SKÖLD
Original assignee: Medipeda Ab; H. Sköld Elektrokonsult
Priority date: 2000-09-19
Filing date: 2001-09-17
Publication date: 2002-03-28
Also published as: SE0003333D0; AU2001290411A1

Abstract

Microwave antenna unit 2 adapted to direct a non-ionizing electromagnetic input wave energy, within a pre-selected wide band frequency range, to tissue and to detect wave energy reflected by said tissue. The microwave antenna is a multi-layer arrangement comprising first layers 10,12,14 provided with directional couplers, second layers 16 provided with coupling means and a third layer 18 provided with a predetermined number of antenna patch elements.

Description

Microwave antenna and imaging system for imaging human tissue .

Field of the invention The invention relates to a non-invasive medical system according to the preambles of the independent claims.

Background of the invention

Medical microwave imaging devices uses microwave radiation to image an object by detecting the effects the object had on the microwave beam after it has interacted with the object. With microwave radiation, it is the dielectric permittivity and conductivity properties of the tissues of the object being imaged that determines the nature of the interaction. The dielectric permittivity and conductivity properties of an object are expressed together as a complex permittivity.

Microwaves, as a component of the electromagnetic radiation spectrum, are in the frequency range between approximately 0, 1 Giga Hertz (GHz) to 300 GHz. This corresponds to the wavelength range between 300 mm and 1 mm. The microwave range useful for microwave .imaging of biological tissues is in the range from about 0,5 to about 3 GHz, but other ranges of the microwave spectrum can be used as well. The quantum energy of the photons in this range of the electromagnetic spectrum comprises non-ionizing radiation.

There are two basic categories of microwave imaging. The first category is static imaging based on forming images by determining the absolute permittivity values of the microwave radiation after its inter action with the object. The second category is dynamic imaging, which is based on variations in permittivity within the object occurring at the time of incidence of the microwave radiation. This second form of imaging is extremely useful in applications for imaging biological tissues to monitor ongoing physiologic change. It must be understood, however, that both static imaging and dynamic imaging still require an active imaging process whereby a microwave scanner employs moving or scanning incident radiation and detects the changes in the microwave radiation based on interaction with the object being imaged. US-5, 829,437 relates to a microwave method and system to detect and locate cancers in heterogeneous tissues.

The system includes a microwave source that generates a frequency sweep as a linear function of time. Such a sweep can be processed to generate a synthetic pulse. The output power from the source is applied to an input port of a directional coupler or circulator via a wave-guide. The output power from a port of the circulator is directed to an illumination/ collector device via a flexible wave-guide. The illuminator consists of an array of wave-guides wherein a time delay of the power injected or collected by each guide can be electronically controlled or processed. The distal end of the wave-guide serves as an antenna that both radiates and collects power.

The primary use of the system disclosed in US-5, 829, 437 is to locate cancers in heterogeneous tissues, e.g. human breast. The equipment is primarily stationary and has rather large physical dimensions.

One of the arrangements described in the US-patent relates to a transrectal arrangement including a cylindrical rectal insert container used to house an antenna array. A fluid surrounds the container, hold by a balloon-like plastic film, that has a dielectric constant similar to that for the rectum wall.

One drawback with the system described in the US-5,829,437 is that it is, due to its size, primarily a stationary system.

Therefore, one object of the invention is to achieve a hand-held system.

Another object of the invention is to achieve a system provided with an antenna that is less voluminous but obtains a higher resolution than prior art systems.

Still another object of the invention is to achieve a non-invasive microwave system with high resolution.

Summary of the invention

The above-mentioned objects are achieved by a system according to the characterizing portions of the independent claims. Preferred embodiments are set forth in the dependent claims.

By integrating the directional coupler within the multi-layer arrangement of the antenna unit fewer electrical connections is required in the cable connecting the antenna unit with the processing unit. Because of that the physical dimensions of the cable is reduced that in turn renders the measurement system much easier to handle.

The general principle that has been followed during the construction of the antenna unit according to the present invention, in order to achieve a unit which is optimized with regard to the physical dimensions, is the following: The materials used for the multi layer arrangement and also for the adapter means, e.g. the filter mat, are chosen such, that the average dielectric constant for the arrangement and adapter means corresponds to the dielectric constant of human tissue when using a predetermined frequency.

This results in that the volume or size of the frequency dependent details of the antenna units may be reduced to less than 20% the size for corresponding elements adapted to be used in air.

This in turn results in that an antenna unit may include more patch elements, giving better measurement accuracy, per volume unit compared to conventional antenna units without being too heavy and cumbersome for the user to handle. The total number of patch elements in the antenna unit is dependent of the intended use.

Portable equipment must have low weight, which means smaller antenna units having lower resolution compared to stationary equipment. The only parameter that limits the number of patch element in a stationary equipment is the processing capacity of the controlling computer.

Short description of the appended drawings

Figure 1 is a cross-sectional side view of an antenna unit according to the invention.

Figure 2 is a schematic illustration of an antenna unit according to the invention. Figure 3 is a schematic illustration of patch elements in an antenna unit according to the invention.

Figure 4 illustrates a multi-layer configuration used in an antenna unit according to the invention.

Figure 5 illustrates the principle of applying pulses to patch elements.

Figure 6 illustrates schematically the control of a patch element according to the present invention

Figure 7 shows a block diagram of a non-invasive medical imaging system for imaging tissue using the antenna unit according to the present invention.

Figures 8a and 8b schematically illustrate different microwave lobes.

Figure 9 shows different pulse forms.

Detailed description of preferred embodiments of the invention The propagation velocity of microwaves in a specific material is dependent of the dielectric constant (epsilon, ε). Below is a table giving examples of different materials and its dielectric constants using a 2 GHz microwave signal.

The value of epsilon for human tissue is to some extent dependent of the frequency. At 14 GHz there is a limit where the epsilon value for some reason decreases and does not reach the "normal" level until 22 GHz. The center frequency of the microwaves used in the present invention may be chosen to any value in the interval 2,0 - 10 GHz.

Principally, a lower propagation velocity implies, in a predetermined position, a higher frequency. A higher frequency in turn means smaller physical dimensions of details in environments with a higher epsilon value. Antenna unit

The antenna unit comprises patch elements arranged in a phased array matrix. Figures 1-4 illustrates schematically different aspects of the antenna unit according to the present invention.

The patch elements are arranged in a predetermined pattern (see fig. 3) on one layer of a multi-layer arrangement. The layers in the arrangement comprise substrates with a dielectric constant that nearly corresponds to the dielectric constant of water (ε is 30 - 80).

The geometry for the substrate in the antenna unit is arranged so that the microwave radiation constantly emerges through the environment (said substrate) almost having the same dielectric properties as water and the geometry may therefore be adapted to the lower velocity of wave-propagation applicable in that environment.

Figure 1 is a cross-sectional side view of an antenna unit according to a preferred embodiment of the present invention.

The antenna unit 2 comprises a housing 4, preferably made from a metal, e.g. sheet metal or cast aluminum. The housing protects and holds the layer arrangement inside of the antenna unit and is also a handle for the antenna unit. Furthermore it is an electrical shield for the circuitry in the unit. The housing is provided with an opening to receive a cable 6 for connecting the antenna unit to a signal processing unit (not shown in figure 1).

The multi-layer arrangement comprises a printed circuit card 8, layers 10,12,14 provided with directional couplers, diodes and filters, layer 16 with connections to patch elements, patch element layer 18 and a soft adapter means 20, e.g. a mat, with a dielectricity constant similar the rest of the arrangement, e.g. epsilon (ε) is between 30 and 80. ' .

The printed circuit card 8 comprises low surface mounted components 22 that e.g. includes control circuits to reduce the number of electrical conductors in the cable 6. According to a preferred embodiment of the invention the layers 10,12,14,16 and 18 are arranged in a so called semi rigid multi-layer where the number of layer is e.g. dependant of the number of patch element and the design of e.g. the directional couplers.

In order to keep the low ε-value throughout the whole measurement arrangement, from the microwave generator (patch elements) to tissue to be radiated, the soft adapter means 20 (wave coupling means) is placed between the multi-layer arrangement and the skin. The adapter means consists of a very soft, ca. 10 mm thick, mat made from a material having a dielectric constant, which is the same, or very close to that of the layer arrangement. The purpose of the adapter means 20 is to achieve an airtight contact to the skin in order to avoid applying some gel between the antenna surface and the skin. This feature is important e.g. when use in acute health care.

Alternately, in some case e.g. where the equipment is stationary, also a gel or even water might be used as adapter means.

Figure 4 illustrates the principle design of a semi rigid multi-layer arrangement used in the antenna unit where numeral 24 refers to a rigid layer, 26 to a flexible layer including e.g. a conductive pattern and 28 to holes with conductors through the rigid layer to connect different layers to each other.

The layers, made from e.g. Microwave Laminates Non- woven Glass Ceramic Filled PTFE Composites and the intermediate adhesive ( e.g. a Thermoplastic

Sheet Adhesive/Bonding Film) have the same high dielectricity constant. Another example of used laminate material is a so called: "Titanum Oxide laminate with prepeg sheets HT 1.5".

The thickness of each layer is between 0, 1 and 0,4 mm and the total height of the multi-layer arrangement including layers 8 (including surface mounted components 22), 10, 12, 14, 16 and 18 is then preferably lower than 10 mm. The arrangement may, however, have other heights up to e.g. 50 mm without departing from the scope of the present invention which is only defined in the appending claims. Figure 3 is a schematic illustration of patch elements 30 in the patch element layer 18 seen from below (with references to figure 1) in an antenna unit according to the invention. The number of patch elements in the matrix is determined in dependence of the required resolution when presenting the measurement result. In the figure 20 patch elements are arranged in a matrix in 5 rows, where each row comprises 4 elements. However, a much larger number of patch elements may be arranged, e.g. 49 patch elements arranged in a 7x7 matrix or even higher number of patch elements. The size of one patch element is e.g. less than 15 X 15 mm.

The symmetric shape gives the opportunity to perform beam sweep in both X- and Y-directions.

Figure 2 schematically illustrates the antenna unit according to a preferred embodiment of the present invention. In order to illustrate further the present invention the different layers are shown unfolded in figure 2.

Using the same reference signs as in figure 1 the unit shown in figure 2 comprises the printed circuit card 8 with connections for the computer, PIN diodes and filters, the layer 10 provided with directional couplers, a grounding layer 32 (the figure only discloses one of many grounding layers, not shown in figure 1), the layer 16 provided with connections to the patch elements, the patch element layer 18 and adapter means 20.

Figure 5 illustrates the procedure of applying pulses to patch elements. Only one row of patch elements 30 is shown. Each element is provided with a directional coupler 34, here in the form of a three-port circulator.

The directional couplers prevent that a generated pulse edge 36 is coupled directly to the electronics of a receiving part of a processing unit (not shown).

In short the procedure is the following: A pulse edge is first filtered and then fed via the directional couplers to the patch elements in a controlled order. In this illustration the feeding line to each patch element has a predetermined length

(illustrated by "1" for the top element) in order to induce a specific phase shift for that patch element. According to a preferred embodiment of the invention the different length of different feeding lines are achieved by arranging PIN-diodes in a so-called hair nail pattern 48 (which is described in detail below).

The different patch elements generate microwave radiation towards tissue in response of an applied pulse. Echo from tissue and transition parts between tissue is then detected by the patch elements and returned via the directional couplers to the receiving part (indicated by an arrow).

The direction of the scanned area is in the illustrated embodiment fixed and then also the direction of the echoes. This circumstance is the basis for the image evaluation/processing performed in the processing means (described below).

The power used by the micro wave antenna units according to the present invention is very low e.g. average power less than 1,0 mW/cm², the reason is that the receiver part is sensitive and that each pulse only comprises a quarter of a wavelength. Energy absorption is low since polarizing molecules (e.g. water) do not have enough time to complete physical movement (rotation) that should have been transformed into heat. Because the energy not is transformed into heat the pulse will reach a larger depth without requiring more power compared to the situation when the energy is transformed into heat.

Figure 6 illustrates schematically the control of a patch element according to the present invention. In figure 6 only one patch element 30 is disclosed coupled to a directional coupler 34. A pulse 36 generated by a pulse generator (not shown) is applied to the antenna unit. A PIN-diode 38 is controlled via a first control circuit 42 by control signals from a control means (not shown) from one of a predetermined number of control busses 40A, 40B, 40C and 40D (generally depicted 40) . The PIN-diode is thus activated to be opened and the pulse is then filtered in a band-pass filter 44 controlled via a second control circuit 46 by control signals from the control means from another of a predetermined number of control busses 40.

According to a preferred embodiment of the invention the different lengths of connecting means are achieved by using PIN-diodes in a so called hair-nail- pattern 48, which in principle is a loop in the form of a hair-nail where the PIN- diodes are rungs of a ladder. Control signals from the control means control via one of the control busses 40 the connection and disconnection of the PIN-diodes according to a predetermined scanning pattern. The PIN-diodes short-circuit the conductor at predetermined distances in order to achieve a certain length of the feeding conductor to the patch elements. This configuration involves a possibility to change the length of the feeding conductor by activating one of the PIN-diodes resulting in a short circuit.

Detected signals from the patch element is coupled, via a PIN-diode 50 to a processing means (not shown). The PIN-diode 50 is also controlled by control signals from one of the control busses 40.

It should be understood that the PIN-diode naturally could be replaced by any component adapted to achieve the same function as the PIN-diode, e.g. conventional transistor.

Medical imaging system

Figure 7 shows a block diagram of a non-invasive medical imaging system for imaging tissue using the antenna unit according to the present invention. The system comprises a medical device 100 connected to the antenna unit 2 via the cable 6. The medical device comprises a signal generator 102 arranged to generate an input signal within a pre-selected wide band frequency range, processing means 104 that processes a detected wave energy signal received from the antenna unit and a presentation unit 106 connected to the processing means. A control means 108 is also arranged in the medical device controlling the generator 102, the processing means 104 and the antenna unit 2 via cable 6.

By using the antenna unit according to the present invention the microwave radiation may scan predetermined areas in a controlled way and also be focused at a specific distance from the antenna.

The scanning is achieved by phase-shifted activation of the different elements, the elements are activated one after another in order to achieve a phase-shifted activation. The phase-shifts and thus the target area may be considered fixed between specified borders/thresholds. The phase-shifts are achieved by arranging feeding lines with different length and are achieved according to a preferred embodiment of the invention by arranging and controlling the above-mentioned hair-nail pattern. The length is directly proportional to the desired phase-shift.

Figures 8a and 8b schematically illustrates that different lobes may be achieved by suitable control of the phase shifts between patch elements.

In order to scan the lobe of the antenna the patch elements are activated sequentially and regularly.

A total phase shift of 180° between the first and the last patch element in one row results in a lobe scanning of about 180°. This is schematically illustrated in figure 8a where consecutive phase shifts in one row (indicated by the arrow) result in that a larger area is scanned. Figure 8b illustrates a more narrow lobe (more concentrated) where the phase shift pattern is more complicated (exemplified by smaller arrows in the figure).

Alternating activation from the outer rows towards the center rows results in a concentration to one point, i.e. a focused lobe.

Thus, a controlled matrix of patch elements allow a fully controlled lobe to be scanned in different patterns over a surface and may be focused at different distances from the antenna unit.

The complex changes of phase shifts regarding activation of the patch element are achieved both by the physical construction of the antenna unit and by a controlling computer program arranged in the control means 108.

By use of the controlling computer program each PIN-diode of the antenna unit may be individually activated during a measurement procedure. This involves activation prior detection or scanning, in dependence of the object to be measured. Corresponding information is applied to the processing means in order to be able to register the resulting information from detected reflections and attenuation of the applied radiation lobe in the same interval as the radiation was generated. The controlling computer program enables the user to control the spreading, the scanning angle and the focus depth of the generated radiation and at the same time the feedback of the information forming the basis for the image processing of the measurement results.

The controlling computer program controls via the control busses 40 all the above-mentioned measurement parameters. More specifically control bus 40A is adapted to enables measurements from chosen patch elements. Control bus 40B controls the phase shift for each patch element by activation or non-activation of one of the PIN diodes in the hair-nail pattern 48. Control bus 40C controls the band pass filter for each patch element via an eight bit addressing procedure.

Control bus 40D opens for activation of chosen patch elements.

The operator of the system controls the system by setting certain parameters to desired values. Among those parameters may be mentioned: the morphology of the scanning lobe, the number of activated patch elements, the number of patch elements used for measurements and the border frequencies for the band pass filter.

The reflection for transition between different types of tissue may be relatively high, e.g. between muscle and fat the reflection factor is 0,5 (where 1,0 means total reflection). Detection of echoes from these transitions is used to determine the distance between the transition and the antenna unit and is used by the processing means to visualize dynamic events with large accuracy. The computer program in the processing means includes inter alia capabilities, in the form of filter modules, to reduce the amount of information when showing a static image so that the resolution may be increased when presenting dynamic events.

The leading edge of a pulse has a very short rising time and is generated by the signal generator 102 that preferably is a saw tooth generator. A band-pass filter in connection with the generator only allows pulses (and then its corresponding pulse edge) in predetermined frequency range (e.g. XX-XX kHz) to pass. The starting point for each measurement sequence is applied both to the generator and the processing means.

According to a preferred embodiment of the invention a method called Time

Domain Reflection (TDR) is used that is based upon the fact that a generated pulse edge, which in this case is a quarter of a wave length of the corresponding base frequency (in the range 2,0-10 GHz), retransmits echoes to the generator

(or in this case to the processing means). The retransmitted echoes show both the distance to a change of impedance in a conductor (in this case the tissue) and also the magnitude of the change.

Figure 9 shows the a conventional radar pulse in the form of a burst (above) and a pulse (below) used for the antenna unit according to the present invention.

Only the fast leading edge of the pulse passes the filter means used in connection to the patch elements, and the trailing edge is therefore not considered when perforrriing the measurements.

No blanking of the receiver unit of the processing means during the generation of the pulse is required due to the short pulse and the low power used. This should be compared to radar applications where this blanking is necessary and involves a blind area close to the antenna.

In order to further optimize the performance of the antenna unit according to the present invention and in particular to optimize the signal to noise ratio of the microwave antenna unit the pulse width is made variable. The reason is that the antenna unit provided with patches has an inherent narrow bandwidth that is related to the pulse width. In this alternative embodiment the band-pass filter described in the above-described preferred embodiment may be omitted. The maximum pulse width that is possible to use is chosen such that interference with reflected pulses is avoided. The minimurn pulse width is related to the quality of the used electrical circuitry.

The present invention is not limited to the above-described preferred embodiments. Various alternatives, modifications and equivalents may be used. Therefore, the above embodiments should not be taken as limiting the scope of the invention, which is defined by the appending claims.

Claims

1. Microwave antenna unit adapted to direct a non-ionizing electromagnetic input wave energy, within a pre-selected wide band frequency range, to tissue and to detect wave energy reflected by said tissue, characterized in that said microwave antenna is a multi layer arrangement comprising first layers (10,12,14) provided with directional couplers, second layers (16) provided with coupling means and a third layer provided with a predetermined number of antenna patch elements (30).

2. Microwave antenna unit according to claim 1, characterized in that said layers exhibits dielectric characteristics similar that of normal tissue.

3. Microwave antenna unit according to claim 1 or 2, characterized in that said patch elements are arranged in a predetermined number of rows.

4. Microwave antenna unit according to claim 3, c ar act erize d in that the patch elements in one row may be activated in a predetermined order.

5. Microwave antenna unit according to claim 4, characterized in that said predetermined order is dependent of a phase-shift induced by said coupling means.

6. Microwave antenna unit according to any preceding claim, characterized in that said directional couplers are three port circulators.

7. Microwave antenna unit according to any preceding claim, characterized in that said coupling means is a hair-nail pattern including PIN-diodes.

8. Microwave antenna unit according to any preceding claim, characterized in that said patch elements are activated by an input signal with a frequency having a center frequency in the interval 2,0 - 10,0 GHz.

9. Microwave antenna unit according to any preceding claim, characterized in that each layer has a maximal thickness of 1,0 mm or less.

10. Microwave antenna unit according to any preceding claim, characterized in that each layer is flat and has a rectangular shape.

11. Non-invasive medical imaging system for imaging tissue, comprising a signal generator to generate an input signal, processing means that processes a detected wave energy signal, characterized in that said system comprises a microwave antenna unit according to any of claims 1-9, wherein said antenna unit is connected to the signal generator, adapted to direct input wave energy to the tissue and to detect wave energy reflected by said tissue, wherein said detected wave energy signal is applied to said processing means.

12. Imaging system according to claim 11, characterized in that said processing means is connected to a presentation unit adapted to generate an image of said tissue.