U.S. patent application number 10/986223 was filed with the patent office on 2006-05-18 for apparatus for determining physical parameters in an object using simultaneous microwave and ultrasound radiation and measurement.
This patent application is currently assigned to Frigoscandia Equipment AB. Invention is credited to Harald Merkel.
Application Number | 20060101917 10/986223 |
Document ID | / |
Family ID | 36272085 |
Filed Date | 2006-05-18 |
United States Patent
Application |
20060101917 |
Kind Code |
A1 |
Merkel; Harald |
May 18, 2006 |
APPARATUS FOR DETERMINING PHYSICAL PARAMETERS IN AN OBJECT USING
SIMULTANEOUS MICROWAVE AND ULTRASOUND RADIATION AND MEASUREMENT
Abstract
The present invention relates to an apparatus for determining a
dielectric function in an object. The apparatus comprises one
transmit antenna for transmitting microwave radiation through said
object, and one receive antenna for receiving the transmitted
microwave radiation, one ultrasound transmitter for emitting
ultrasound radiation through said object to generate a density
variation in the object, means to analyse the microwave radiation
transmitted through the density variation to determine the
acousto-electric interaction .delta. in the object, and means to
calculate the dielectric function in the object from the
acousto-electric interaction. The invention also relates to a
method for determining the dielectric function in an object.
Inventors: |
Merkel; Harald; (Lindome,
SE) |
Correspondence
Address: |
BIRCH STEWART KOLASCH & BIRCH
PO BOX 747
FALLS CHURCH
VA
22040-0747
US
|
Assignee: |
Frigoscandia Equipment AB
Helsinborg
SE
|
Family ID: |
36272085 |
Appl. No.: |
10/986223 |
Filed: |
November 12, 2004 |
Current U.S.
Class: |
73/601 ;
324/639 |
Current CPC
Class: |
G01N 2291/044 20130101;
G01N 29/2456 20130101; G01N 2291/048 20130101; G01N 29/0672
20130101; G01N 33/02 20130101; G01N 2291/102 20130101; G01N 22/00
20130101; G01N 29/07 20130101 |
Class at
Publication: |
073/601 ;
324/639 |
International
Class: |
G01N 22/00 20060101
G01N022/00; G01N 29/00 20060101 G01N029/00 |
Claims
1. An apparatus for determining a dielectric function in an object,
said apparatus comprising: at least one transmit antenna for
transmitting microwave radiation through said object, at least one
receive antenna for receiving the transmitted microwave radiation,
at least one ultrasound transmitter for emitting ultrasound
radiation through said object to generate a density variation in
the object, means to analyse the microwave radiation transmitted
through the density variation to determine the acousto-electric
interaction in the object, and means to calculate the dielectric
function in the object from the acousto-electric interaction.
2. The apparatus according to claim 1, wherein said at least one
transmit antenna is connected to a first microwave generator
generating a transmit signal having a first fixed microwave
frequency which is transmitted from said first antenna.
3. The apparatus according to claim 1, wherein apparatus further
comprises a means to determine the attenuation comprising: a mixer
producing an intermediate frequency signal by mixing the received
transmitted microwave radiation from said receive antenna with a
local oscillator signal having a second fixed microwave frequency,
said local oscillator signal is generated by a second microwave
generator, and an evaluation unit determining the acousto-electric
interaction by evaluating phase and amplitude of the IF signal.
4. The apparatus according to claim 1, wherein said emitted
microwave radiation and ultrasound radiation are arranged to be
moved in relation to said object.
5. The apparatus according to claim 4, wherein said apparatus is
stationary and the object is moved on a conveyor means.
6. The apparatus according to claim 4, wherein the apparatus is
moved in relation to a stationary object.
7. The apparatus according to claim 1, wherein said ultrasound
radiation is an ultrasound signal having a third fixed frequency,
generated by an ultrasound generator.
8. The apparatus according to claim 1, wherein the apparatus
further comprises at least one ultrasound receive antenna for
receiving ultrasound radiation to determine an ultrasound runtime
and damping mapping, referred to as metric for the object, which is
used to determine the acousto-electric interaction in the
object.
9. The apparatus according to claim 8, wherein the apparatus
further comprises means to determine the phase of the ultrasound
radiation for each focal point that is a part of the ultrasound
metric.
10. A method for determining a dielectric function in an object
comprising the steps of: transmitting microwave radiation through
said object from at least one transmit antenna, receiving the
transmitted microwave radiation in at least one receive antenna,
emitting ultrasound radiation, from at least one ultrasound
transmit antenna, through said object to generate a density
variation in the object, analysing the microwave radiation
transmitted through the density variation to determine the
acousto-electric interaction in the object, and calculating the
dielectric function in the object from the acousto-electric
interaction.
11. The method according to claim 10, wherein the step of analysing
the microwave radiation to determine the acousto-electric
interaction in the object comprises the step of obtaining an
ultrasound metric of the object.
12. The method according to claim 11, wherein the ultrasound metric
is obtained by: a) focusing emitted ultrasound radiation to a point
in the object b) adjusting the phase of the ultrasound radiation
while measuring an acousto-electric efficiency signal to obtain a
maximum of the acousto-electric efficiency signal, c) storing the
value of the phase together with position of the focal point in a
memory, and d) repeating steps a)-c) until the metric for the
object is completed.
13. The method according to claim 11, wherein the step of
calculating the dielectric function in the object comprises the
steps of: selecting at least one point inside the object, focusing
the ultrasound on the point, determining the damping of the
received transmitted microwave radiation, and determining the
dielectric function using the ultrasound metric.
14. The method according to claim 11, wherein the step of
calculating the dielectric function in the object comprises the
steps of: selecting at least one pair of point inside the object,
focusing the ultrasound on both points, determining the damping of
the received transmitted microwave radiation for both points, and
determining damping and the dielectric function between the two
points using the ultrasound metric.
15. An apparatus for determining local distribution of temperature
in a food product, said apparatus comprising: at least one transmit
antenna for transmitting microwave radiation through said food
product, at least one receive antenna for receiving the transmitted
microwave radiation, at least one ultrasound transmit antenna for
emitting ultrasound radiation through said food product to generate
a density variation in the food product, means to analyse the
microwave radiation transmitted through the density variation to
determine the acousto-electric interaction (.delta.) in the food
product, and means to calculate the dielectric function in the food
product from the acousto-electric interaction and to calculate the
local distribution of temperature in the food product based on the
calculated dielectric function.
Description
TECHNICAL FIELD
[0001] The present invention relates to an apparatus for
determining physical parameters, such as temperature or density,
inside an object by determining the dielectric function of the
object as defined in claim 1. The invention also relates to a
method for determining the dielectric function inside an object as
defined in claim 10, and an apparatus for determining the local
distribution of temperature in a food product as defined in claim
15.
BACKGROUND TO THE INVENTION
[0002] In order to obtain information regarding temperature,
density and other interior parameters of arbitrary objects without
destroying, invading or dissecting the object, radiation(s) of
various types are available to provide information that allow(s) to
reconstruct the desired parameters.
[0003] Choosing a specific type of radiation, there are four
distinct cases that incorporate their proper implications on the
choice of method of analysis. These are classified by two question
areas: [0004] transparency of the object to the radiation chosen
[0005] resolution in the object required with respect to the
wavelength of the chosen radiation. Case 1A
[0006] (The object is transparent or weakly absorbing to the
radiation used for measurement, and the resolution to be achieved
is equal or smaller than a radiation wavelength)
[0007] The only source of information is obtained by probing the
near field using e.g. [0008] Atomic Force Microscopy (APM) [0009]
reading out the force on a sub-wavelength-size stencil being
positioned with high precision on the surface of a material reading
out the structure on the surface of the object under test, [0010]
Raster Tunnel Microscopy (RTM) [0011] where instead of the force
one measures the tunneling current from a sub-wavelength sized
probe being positioned close to the surface of the object under
test generating information on the electronic state of the surface
of the object, or [0012] optical Near Field Microscopy [0013] where
electromagnetic radiation passes through microscopically small
holes requiring the hole to be much smaller than a wavelength of
the radiation used generating surface images of the optical
properties at sub wavelength resolution on thin probes. [0014]
Impedance tomography [0015] Where a set of electrodes is attached
to the object under test and the impedance between all the probes
is measured. This method allows calculating some properties of the
interior of the object under test but resolution is generally poor.
This method has been used with success in differential
approaches--measuring the impedance of the cardiac region prior and
after medication to evaluate the influence of, e.g. anti-clogging
drugs.
[0016] As a general feature, the high resolution of the methods
mentioned above are not due to the intrinsic wavelength of the
chosen radiation but due to another constraint (mostly mechanical
as diaphragms, stencils) that provides sub-wavelength resolution. A
general shortcoming is given by the thickness requirement of the
object under test--the above methods generate either only surface
information or interior information at a very limited depth without
losing resolution.
Case 1B
[0017] (The object is transparent or weakly absorbing to the
radiation used for measurement, and the resolution is much larger
than a radiation wavelength.)
[0018] This case is covered by all direct imaging and optical
transmission methods. Using electromagnetic radiation in this
regime, there are [0019] LIDAR [0020] X-ray
[0021] As a means of analysis, ray tracing and one-to-one mapping
methods are appropriate since scattering does not play a role--it
can be assumed without loss of resolution that each pixel
information taken at a given position is only affected by the
object's volume located in between the radiation source and the
receiver.
[0022] A recent development in this area is the passive radar where
the thermal emission inherent to all bodies in the environment
around a receiver is measured and imaged. This radar method does
not require any transmitted signal and is therefore not
traceable.
[0023] Among non-electromagnetic methods there are commercially
available [0024] Ultrasound tomography and [0025] Nuclear Magnetic
Resonance (NMR) Case 2A
[0026] (The object is moderately absorbing to the radiation used
for measurement, and the resolution is equal or smaller than a
radiation wavelength)
[0027] The fact that the object is moderately absorbing to the
radiation used for measurement puts a thickness limit to the probes
that can be investigated.
[0028] For this case no feasible method is available today
regarding the state of the art.
Case 2B
[0029] (The object is moderately absorbing to the radiation used
for measurement, and the resolution is much larger than a radiation
wavelength)
[0030] In this case, most radio frequency and microwave frequency
applications are found (especially when the object under test is
lossy and it is embedded in a non-lossy environment) and microwave
tomography is available. Among these methods the most popular one
is [0031] (active) radio detection and ranging (RADAR) [0032] where
the signal runtime between a source and a target and back to a
receiver is measured either by putting the receiver at the same
place as the transmitter (monostatic radar) or by putting the
receiver at a different location than the transmitter (bistatic
radar) and the frequency change due to the relative velocity of the
source and target are evaluated (Doppler radar).
[0033] There is thus a need to develop an apparatus for determining
physical parameters, such as temperature, density, composition, for
an object that is modestly absorbing to the radiation used for
measurement, and where the desired resolution is much larger than a
radiation wavelength.
SUMMARY OF THE INVENTION
[0034] The purpose of the present invention is to provide an
apparatus for determining the dielectric function of an arbitrarily
formed object.
[0035] The purpose is achieved by an apparatus according to claim
1, and a method according to claim 10, using ultrasound waves to
create a controllable variation in product density. The apparatus
then uses microwave radiation to read out the density variation and
to relate it to a spatial distribution of the dielelectric
function. This may in turn be used for determining the object's
temperature, water content and density.
[0036] An advantage with the present invention is that the
resolution of the spatial distribution not is limited to the
microwave wavelength, but rather determined by the wavelength of
the ultrasound.
[0037] Another advantage with the present invention is that a
contact free measurement of physical properties, such as
temperature, water content, etc, may be established applying the
invention as virtual probes.
[0038] Other objects and advantages will be apparent for a skilled
person from the detailed description of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0039] FIG. 1 shows a system according to the invention.
[0040] FIG. 2 illustrates the emitted radiation into a product
under test.
[0041] FIG. 3 shows a flow chart for determining a physical
property, such as temperature, inside a product under test.
[0042] FIG. 4 shows a flow chart illustrating the process for
obtaining an ultrasound metric.
[0043] FIGS. 5a and 5b show flow charts illustrating two
embodiments of the process for determining the spatial distribution
of the dielectric function within a product under test.
[0044] FIG. 6 shows a principal function of a first use of the
present invention.
[0045] FIGS. 7a-7d show a principal function of a second use of the
present invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0046] Prior to this invention there exist as tools for
reconstructing the interior properties of materials (where
diffraction and scattering are predominant) only [0047] microwave
tomography [0048] ultrasound tomography.
[0049] In both cases, resolution is determined by the wavelength of
the used radiation.
[0050] In this invention, ultrasound and microwave methods are
combined. Object reconstruction can be done by pure microwave
inverse scattering methods and by pure ultrasound tomography
methods with their respective limitations. Here ultrasound is not
used as an object reconstruction tool but as a tool to generate a
density variation in the object to be investigated. This said
density variation creates a change of phase and frequency in the
transmitted microwave radiation that is used for object
reconstruction. Therefore the available resolution of this method
is determined by the resolution of the ultrasonic wave (smaller
than a millimeter for typical medical ultrasound frequencies). The
density readout is performed using microwave radiation (at a
frequency where attenuation still allows reasonable penetration
depths e.g. S, ISM5.8 or X band), This method avoids the
fundamental difficulty of microwave tomography approaches that a
millimeter resolution requires millimeter wavelengths.
Unfortunately millimeter radiation is absorbed by most objects of
interest within some wavelengths therefore not allowing any
interior parameters to be extracted. In the above classification,
this invention covers areas 1B, 2A and 2B. Such a method is not
known prior to this invention.
[0051] The system described by this invention is preferably to be
used in the food industry. In the food industry, it is often
important to accurately control the temperature of food products.
For example, when food products are to be freezed, it is important
that the entire product is freezed. When it cannot be ensured that
the entire product, e.g. a chicken fillet, has been freezed, one
may have to discard products or deliver products with short shelf
life. Therefore, there is a need for a non-destructed and
non-contact control of the freezing of products. This problem may
be solved by means of measuring the dielectric function and
converting it to a distribution of temperature, as will be
described in the following.
[0052] However, the system is by no means limited to this type of
industry. Potential other applications are: [0053] concrete
hardening (construction industry) [0054] glue hardening (airplane
construction) [0055] medical imaging (functional brain tomography,
spinal tomography) [0056] ground survey, tracking pipes and
underground tubes [0057] save and rescue equipment (detecting
persons under rubble) [0058] mine sweeping (especially plastic
mines in overgrown areas)
[0059] In the following the preferred embodiment is summarized. The
modifications required to the geometry in order to adapt this
method in the above other application areas are small.
[0060] In the following a continuous wave (CW) microwave and pulse
wave train ultrasound based system is described for sake of
simplicity. The method described is not limited to this case. Other
modulation schemes for both, electromagnetic waves and ultrasound
waves such as amplitude modulation (AM), frequency modulation (FM)
frequency modulated continuous wave (FMCW), pulse code modulation
(PCM), phase modulation (PM) and wavelet based modulation
techniques (WM) are applicable and are optimal for certain other
applications.
[0061] FIG. 1 describes a apparatus 40 according to the invention.
The system is placed close to a conveyor means 11, which transports
the products under test 12 through the sensor measurement gap 13.
The system 40 consists of a microwave part 50, an ultrasound part
70 and an evaluation unit 60. The system comprises in this
embodiment two fixed-frequency microwave generators 51 and 52 and a
fixed frequency ultrasound generator 71. The first microwave
generator 51 has a first fixed microwave frequency f.sub.1 (e.g.
5.818 GHz) and is coupled to at least one transmit antenna 42, and
the second microwave generator 52 has a second fixed microwave
frequency f.sub.2 (e.g. 5.8 GHz) and is preferably coupled to a
down converter 54, such as a mixer. The down converter shifts the
transmitted microwave signal, which is collected by at least one
receive antenna 43, and the received microwave signal from the
second microwave generator 52 to a low intermediate frequency IF.
This allows the microwave signal transmitted through the product
under test 12 to be evaluated in amplitude and phase. It
furthermore comprises a filter unit 59, an analog to digital
converter ADC 55, a set of signal processors 56 and an evaluation
processor 60 that contains necessary algorithms to control the
system and to evaluate the data. The result is submitted to a
display unit 65. The system 40 also comprises a set of transducers
72 (only one shown for sake of clarity), in addition to the
transmit antenna 42 and receive antenna 43, all grouped around the
measurement gap 13. The transducers emit an ultrasound signal
having an ultrasound frequency f.sub.US (e.g. 4.5 MHz) through the
product under test 12. This causes a density displacement traveling
at ultrasound speed. At the same time a microwave signal from the
first microwave generator 51 is emitted from the transmit antenna
42. This signal also travels through the product under test 12. The
microwave signal exhibits damping and phase delay by traveling
through the product leaving the microwave frequency unchanged. In
those volumes of the product under test 12 where the ultrasound
wave creates a density displacement, a part of the microwave signal
is shifted in frequency and upper and lower sidebands are created.
The transmitted microwave signal is collected using the microwave
receive antenna 43. The received signal is down converted using the
down converter unit 54. The low frequency signal is then filtered
using a filter unit 59 and analog-digital converted using the ADC
55. The digital signal is evaluated using a receive signal
processor 56. The receive signal processor converts the incoming
digital signal to zero frequency using standard state-of-the-art
digital filters.
[0062] The outcome of this filtering corresponds to the S.sub.21
parameter, which is not shifted in frequency, between the transmit
42 and receive 43 antenna as well known to a person familiar with
the art. In the above we refer to the receive antenna 43 as
microwave port 2 and the transmit antenna 42 as the microwave port
1.
[0063] In the system described by this invention there is a second
set of bandpass filter 58, another ADC 55 and a second digital
signal processor 57 in parallel to the first signal path 59, 55,
56.
[0064] The bandpass filter 59 is tuned to the difference frequency
between the both microwave generators 51 and 52, which in the
present embodiment is 5.818 GHz-5.8 GHz=18 MHz. The second bandpass
filter 57 is tuned to the difference frequency between the
microwave generators (e.g. 18 MHz) added the center frequency (e.g.
4.5 MHz) of the ultrasound signal generator 71. Therefore this
second digital signal processor path, containing 58, 55 and 57,
converts the incoming signal to zero frequency that has been
shifted in frequency by the ultrasound frequency. The measurement
result is therefore limited to the cross section between the
ultrasound and the microwave signal.
[0065] The IF bandwidth of the first 59, 55, 56 and second 58, 55,
57 digital receivers are chosen to be half the ultrasound frequency
f.sub.US generated by the ultrasound generator 71. This is required
to optimize the frequency shift by varying the ultrasound
transducer phases.
[0066] During the first stage of obtaining an ultrasound metric of
the product 12, an ultrasound receiver 73 has to be present which
collects the ultrasound radiation emitted from the transducers 72
and evaluate the damping, T.sub.56, and runtime as described in
more detail below. In the above we refer to the ultrasound receiver
73 as microwave port 6 and the transducers 72 as the microwave port
5. The damping and runtime is evaluated in a ultrasound evaluation
unit 74, but this may naturally be integrated in the evaluation
unit 60.
[0067] FIG. 2 illustrates the emitted radiation into a product
under test. The transducers 72 emit, in this example, an ultrasound
pulse 91 through the product under test 12. This causes a density
displacement traveling at ultrasound speed. At the same time a
microwave signal 90 is emitted from the transmit antennas 42,
travels through the product 12 and exhibit damping and phase delay
with unchanged microwave frequency except in the area 95, where the
ultrasound wave cause density displacement. In this area a part of
the microwave signal is shifted in frequency, as described above,
and upper and lower sidebands are created. The transmitted
microwave signal 90 is collected using the receive antenna 43.
[0068] The ultrasound wave 91 is collected in a receiver 73 during
the process of obtaining the ultrasound metric which is used during
the next stage of determining the spatial distribution of the
dielectric function.
[0069] FIG. 3 show a flow chart describing the measurement
principle according to the invention using a system as described in
connection with FIG. 1.
[0070] Basically, the method of this invention is a
microwave-ultrasound combination measurement method of the
dielectric and the acousto-electric properties of matter where the
resolution is inherited from the ultrasound wavelength.
[0071] The measurement procedure consists of three phases as
described below.
[0072] Phase 1
Obtaining the Ultrasound Metric
[0073] In this phase a map of the local ultrasound runtime and
damping properties are established which is henceforth referred to
as the ultrasound metric.
[0074] By varying the phases between the ultrasound transducers 72
using a phase programming logic, any desired phase form of the
ultrasound field can be generated. It is possible to control the
phases of all ultrasound transducers in a way to focus the
ultrasound power to a point with a geometrical size of the order of
a half wavelength of the ultrasound wave. Focusing the ultrasound
wave in the medium on the smallest possible volume causes the
frequency displacement of the transmitted microwave signal to reach
a maximum. Therefore, the phase of the ultrasound transducers is
varied to optimize the microwave signal. Evaluating the delay time
between the ultrasound pulse and the achieved maximum frequency
shift allows determining at what distance from the antenna the
focus point is located inside the product under test 2. This
measurement is repeated for a set of points covering the whole
product under test with a predetermined resolution.
[0075] As a result, a table comprising the phases to be chosen for
each independent focus point and the location with respect to the
antenna is obtained. At the same time, the strength of the maximum
signal is obtained from each of these measurement points from all
over the measurement object which allows to map the local
ultrasound damping.
[0076] The local strength of the ultrasound signal is calculated by
measuring runtimes and damping values between all ultrasound
transducers. (Of course, any choice of phase is optimized by
maximising the microwave signal for each point in this layer).
Assuming these delay time and damping values for the layer of the
product close to the transducers, the phase for the closest focus
points are obtained.
[0077] Tuning the phases for transmission to focus the ultrasound
power in one focus point and tuning the phases for reception to
focus on another focus point, the runtime between the two focus
points of the first layer is obtained.
[0078] Assuming these values to be valid around the focus points
and also close to the next layer of points, phase and amplitude
values for one after the other point of the next layer are
obtained. (Of course, any choice of phase is optimized by
maximizing the microwave signal for each point in any layer).
[0079] This process is repeated until the whole product under test
is scanned.
[0080] The result is a table of the local damping of the ultrasound
signal and the local phase delay of the ultrasound signal between
all scanned focal points, the "ultrasound metric" together with the
microwave signal strength for all the focal points.
[0081] The ultrasound metric may be obtained on a reference object,
which is representative to the objects that are to be analysed.
Thereafter, measurements may be made on such objects without the
need of obtaining an ultrasound metric for each of the objects.
[0082] The metric by itself can also be considered as a substantial
result of the invention and can be used as autonomous applications.
Furthermore, metrics obtained on reference objects may be used as
means to speed up measurements according to phase 1.
Phase 2:
[0083] Evaluating the Microwave Interaction
[0084] Based on the above generated ultrasound metric and the
microwave response the acousto-electric interaction is obtained in
a layer-by-layer wise starting from the layer closest to the
microwave antennas. It is not required to proceed this analysis in
a layer by layer way but it proves convenient for a subsequent 3D
image processing to do so.
[0085] The strength of the microwave signal measured in each focal
point is determined by the product of the
(a) local strength of the ultrasound signal and
(b) the compressibility and
(c) the dielectric function of the material in the focus point.
[0086] Since the local strength of the ultrasound signal in all
focal points is known from the metric, the interaction between the
incident and the frequency-shifted transmitted microwave signal on
the layer closest to the microwave antenna is obtained by applying
a Green's function theorem resulting in the dielectric function at
this focal point. No other point interaction than the interaction
of this specific focal point is possible because the microwave
sideband response must originate in the region where the ultrasound
focus has extended during the measurement. Therefore the resolution
of the method is given by the wave packet resolution of the
ultrasound signal (down to 250 micrometers) and not by the
microwave wavelength (of the order of several centimeters) in a
non-disturbing way. Nevertheless the incident microwave signal is
influenced by the neighboring elements on the way from the transmit
antenna to the focal point and also on the way to the receive
antenna. The microwave signal at the focal point depends on all the
dielectric points in the product under test and is represented by a
linear form in the contrasts and the incident field amplitudes. The
field collected in the receive antenna is also described by a
linear form containing all unknown contrasts. For each measurement,
a bilinear form containing all unknown contrasts is obtained. For
each measurement, a new equation is generated. Since there is an
equation for each focal point, the equation system can be solved in
a one-to-one way without iteration.
[0087] The result is a map of the acousto-electric and the
dielectric properties of the product under test with the same
underlying special structure as the ultrasound metric.
Phase 3:
Calculating the Acousto-Dielectric Properties
[0088] The ultrasound damping is not significantly temperature
dependent. In contrast the ultrasound runtime and the dielectric
function together with the compressibility of the product exhibit a
strong temperature dependence.
[0089] The ratio between compressibility and dielectric function
yields a function of temperature. Using the dielectric and
acousto-electric maps, the temperature of the measurement object is
obtained.
[0090] Further details of the third phase are described in
connection with FIGS. 6 and 7a-7d.
[0091] Having described the three phases in detail, the measurement
will now be further described with reference to FIG. 3.
[0092] The flow starts at step 100, which means that a microwave
signal at the first frequency .omega..sub.transmit=2.pi.f.sub.1 is
sent out from the transmit antenna 42 and a microwave signal at a
mix of frequencies .omega..sub.transmit and .omega..sub.receive is
received at the receive antenna 43. A damping S.sub.21 and a
frequency offset .delta. and a signal generation at the offset
frequency S'.sub.21 between the two signals is measured in step
101, and in the following step 102 the measured damping S.sub.21 is
compared to a previously recorded reference damping S.sub.21,0,
which corresponds to the measured damping with an empty measurement
gap 13, i.e. no object under test 12 is present in the gap. If the
measured damping is equal to the damping with no object under test
present in the gap, the flow is fed back to point 103 and the
damping is measured again in step 101.
[0093] When an object is introduced in the measurement gap 13 the
flow continues to step 104 where an ultrasound metric is obtained.
This step is described more closely in connection with FIG. 4.
[0094] The spatial dielectric properties of the object is
thereafter measured and calculated using the metric obtained in
step 104. This procedure is described in more detail in connection
with FIG. 5.
[0095] When the dielectric properties of the object is determined
other physical properties may be determined, step 106, such as
temperature, water contents density, etc., using the spatial
distribution of the dielectric properties (based on predetermined
.epsilon.(T) models). Such models are known in the prior art, such
as described in the published PCT-application WO02/18920, assigned
to the present applicant.
[0096] FIG. 4 shows a flow chart disclosing the process of
obtaining the ultrasound metric. The flow starts at step 120, where
the ultrasound radiation is focused to a point in the object. The
ultrasound will generate a signal in the sideband path, which
corresponds to the frequency displacement measured by the microwave
signal, denoted .delta. and an acoust-electric efficiency signal,
which is measured in step 121 and in step 122 a check is made to
determine if the acousto-electric efficiency signal is at maximum,
if not the flow is fed back through step 123, where the value of
the phase of the ultrasound signal is updated, to step 120. The
process is repeated until the maximum frequency displacement is
obtained. When the flow continues to step 124, the phase of the
ultrasound signal together with information regarding the position
of the focal point as described above, is stored in a memory. In
step 125 it is determined if there are another point that should be
measured to obtain the ultrasound metric of the product under test
12. If not, the process for obtaining the metric ends in step 127,
or the flow is fed back via line 126 to step 120.
Measurement of the Dielectric Function Based on a Known Ultrasound
Metric (c.f. FIG. 4)
[0097] FIG. 5a shows a first embodiment for determining the
dielectric function in an object, such as a food product, to
determine a physical property in the object, such as internal
temperature without physically probing the object, during
preparation of the object.
[0098] The flow starts in step 110, where a point in the object is
selected. It is advantageous to select a point that has been used
during the process of obtaining the ultrasound metric. The selected
point corresponds to point 3 in equations 1-17.
[0099] The ultrasound radiation is thereafter focused on this point
in step 111 and in step 112, the S-parameters S.sub.31 and S.sub.23
are measured, as described in more detail in connection with FIG.
6.
[0100] In step 113, a decision is made whether another point should
be selected or not. If another point should be selected the flow is
fed back to step 110, where a new point is selected before steps
111 and 112 are repeated. If not, the flow continues to step 114
where the matrix with the measured S-parameters is inverted to
solve either S.sub.31 for virtual receivers or S.sub.32 for virtual
transmitters.
[0101] The dielectric function .epsilon.(x) for each selected point
x is thereafter calculated in step 115 using prior art algorithm.
The temperature in the selected point is thereafter calculated as
indicated by step 106 in FIG. 3.
[0102] FIG. 5b shows a second embodiment for determining the
dielectric function in an object, such as a food product, to
determine a physical property between two locations in the object,
such as material properties, e.g. the presence of a brain tumor,
without physically probing the object.
[0103] The flow starts in step 210, where a pair of points in the
object is selected. It is advantageous to select points that have
been used during the process of obtaining the ultrasound metric.
The selected points correspond to point 3 and 4 in equations
1-17.
[0104] The ultrasound radiation is thereafter focused on both
points in step 211 and in step 212, the S-parameters S.sub.31,
S.sub.23, S.sub.41, S.sub.24, S.sub.4'1, S.sub.24', S.sub.3'1 and
S.sub.23' are measured, as described in more detail in connection
with FIG. 7.
[0105] The S-parameter S.sub.43, i.e. the damping between the
selected points, is calculated in step 213. Point 3 acts as a
virtual transmitter and point 4 functions as a virtual receiver in
this embodiment.
[0106] The mean value of the dielectric function {overscore
(.epsilon.)}(x,y) between the selected points x and y (i.e. points
3 and 4 in equations 1-7, is thereafter calculated in step 214.
[0107] In step 215, a decision is made whether another pair of
points should be selected or not. If another pair of point should
be selected the flow is fed back to step 210, where a new pair is
selected before steps 211 to 214 are repeated. If not, the flow
continues to step 106 in FIG. 3, where the desired physical
properties are calculated.
First Use of the Invention
[0108] FIG. 6 shows a schematically the function of a first use of
the present invention. If an ultrasound metric u(x,t) is obtained
for all points x within a product it is possible to calculate the
dielectric constant in every point by applying the following
steps:
[0109] 1) Focus the ultrasound on one of the points 3. It is known
that the ultrasound only affects the focal point concerning
frequency shift of the microwave signal sent from the transmit
antenna 1 to the receive antenna 2, thus generating a signal in the
sidebands, i.e. microwave base frequency (f.sub.1).+-.ultrasound
frequency (f.sub.US).
[0110] 2) Measure the signal strength in at least one of the side
bands. If the signal strength in both side bands is measured, a
more reliable result from the measurement is obtained. The signal
strength measured in the receive antenna 2 may be expressed as:
V.sub.2(t)=S.sub.21V(t)=S.sub.23.alpha..sub.3u.sub.3(x,t)S.sub.31V.sub.1(-
t),
[0111] Where S.sub.21 is the damping caused by the product 12
present in the measurement gap, V.sub.2(t) is the measured signal
strength in the side band and V.sub.1(t) is the signal strength of
the signal sent from the transmit antenna 1. S.sub.23 is the
damping between point 3 to the receive antenna 2, .alpha..sub.3 is
a factor that determines the efficiency in point 3 at which an
ultrasound wave is converted into a microwave sideband signal
(referred to as acousto-electric gain), u.sub.3(x,t) is the
ultrasound metric in point 3 and S.sub.31 is the damping between
the transmit antenna 1 and point 3.
[0112] In a first approximation the efficiency .alpha. can be
expressed as: .alpha. = .DELTA. .times. .times. y ##EQU1## where
.DELTA..epsilon. is the change of dielectric constant due to the
pressure wave cause by the ultrasound radiation, y. With the
compression module .kappa., the relation .DELTA. .times. .times. -
1 = .kappa. .times. .times. y ##EQU2## is established. The value of
K is known to a skilled person in the arts and will not be
discussed in more detail,
[0113] 3) Repeat the process for all desired points, denoted 3 in
FIG. 6, in the product 12.
[0114] 4) Use all measurement data in an inverse scattering
algorithm and calculate the spatial distribution of the dielectric
function in the product.
[0115] If an object moves at a relative slow speed, and fulfilling
the relationship below, in relation to the measurement apparatus,
no compensation of the emitted ultrasound and microwave radiation
needs to be taken into consideration. v obj t meas < v US f US =
d Focal , ##EQU3## v.sub.obj is the speed of the objects movement
in the measurement gap 13, t.sub.meas is the measurement time for
the complete process, v.sub.US is the speed of ultrasound in the
object, f.sub.US is the ultrasound frequency and d.sub.Focal is the
diameter of the focal point.
[0116] If the relative speed is high, the focusing of the
ultrasound must include an adjustment of the ultrasound radiation,
to maintain the focal point in the object during the measurement
steps, to compensate for the movement. In addition v obj v US
.times. << 1 ##EQU4## to avoid Doppler shift. Second Use of
the Invention
[0117] FIGS. 7a-7d show a principal function of a second use of the
present invention when calculating the dielectric constant between
two points 3 and 4 in a product. A first point 3 may be considered
to be a source and the second point 4 may be considered to be a
receiver.
[0118] The principal function is very much the same as described in
connection with FIG. 6, but with the exception that two upper and
two lower sidebands are generated since two focal points 3 and 4
simultaneously generated by the ultrasound radiation. The first
upper and lower side bands are the same as described in connection
with FIG. 6, and the second upper and lower side band have the
double ultrasound frequency, i.e. microwave base frequency
(f.sub.1).+-.2*ultrasound frequency (2f.sub.US). If the same
ultrasound frequency is used for this purpose, it is possible to
choose two different ultrasound frequencies to generate second
order sideband. The apparatus described in connection with FIG. 1
needs in this example to be added with an extra sideband path
adjusted for the second upper and lower sideband.
[0119] The following relationships can be established for point 3
and 4, each as a single virtual source:
V.sub.2(t)=S.sub.23.alpha..sub.3u.sub.3(x,t)S.sub.31V.sub.1(t)
(solid line) 1:
V.sub.2(t)=S.sub.24.alpha..sub.4u.sub.4(x,t)S.sub.41V.sub.1(t)
(dashed line) 2:
[0120] By displacing the focal point from 3 to 3' and the focal
point from 4 to 4' according to FIG. 7b new relationships can be
expressed:
V.sub.2(t)=S.sub.23'.alpha..sub.3'u.sub.3'(x,t)S.sub.3'1V.sub.1(t)
(solid line) 3:
V.sub.2(t)=S.sub.24'.alpha..sub.4'u.sub.4'(x,t)S.sub.4'1V.sub.1(t)
(dashed line) 4:
[0121] From FIG. 7a a relationship including the sought damping
between point 3 and 4 may be expressed:
V.sub.2(t)=S.sub.24.alpha..sub.4u.sub.4(x,t)S.sub.43.alpha..sub.3u.sub.3(-
x,t)S.sub.31V.sub.1(t) (double arrow 3=>4) 5:
V.sub.2(t)=S.sub.23.alpha..sub.3u.sub.3(x,t)S.sub.34.alpha..sub.4u.sub.4(-
x,t)S.sub.41V.sub.1(t) (double arrow 4=>3) 6:
[0122] Equation 6 is not used in solving the 7.times.7 problem and
is replaced by a suitable approximation, see equations 16 and
17.
[0123] FIG. 7c illustrates the relationship of the double source
corresponding to 3 and 4.
V.sub.2(t)=S.sub.23.alpha..sub.3u.sub.3(x,t)S.sub.3'3.alpha..sub.3'u.sub.-
3'(x,t)S.sub.3'1V.sub.1(t) (solid line) 7:
V.sub.2(t)=S.sub.24'.alpha..sub.4u.sub.4'(x,t)S.sub.4'3.alpha..sub.3u.sub-
.3(x,t)S.sub.31V.sub.1(t) (dashed line) 8:
[0124] The relationship between point 3' and 4' may be expressed:
V.sub.2(t)=S.sub.24'.alpha..sub.4'u.sub.4'(x,t)S.sub.3'4'.alpha..sub.3'u.-
sub.3'(x,t)S.sub.3'1V.sub.1(t) (double arrow 3'=>4') 9:
V.sub.2(t)=S.sub.23'.alpha..sub.3'u.sub.3'(x,t)S.sub.3'4'.alpha..sub.4'u.-
sub.4'(x,t)S.sub.4'1V.sub.1(t) (double arrow 4'=>3') 10:
[0125] Equation 10 is not used in solving the 7.times.7 and
8.times.8 problem and is replaced by a suitable approximation, see
equation 15 for the 8.times.8 problem and equations 16 and 17 for
the 7.times.7 problem.
[0126] The following relationships are evident from FIGS. 7a-7c:
S.sub.41=S.sub.43'S.sub.3'1 11: S.sub.24=S.sub.44'S.sub.24' 12:
S.sub.23'=S.sub.33'S.sub.23 13: S.sub.4'1=S.sub.4'3'S.sub.31
14:
[0127] Equations 11-14 are used to eliminate S-parameters, which
results in the S-parameters as illustrated in FIG. 7d. There is one
S-parameter that is sought S.sub.43 and one S-parameter that is
completely uninteresting S.sub.3'4', together with several unknown
S-parameters that require 10 equations to solve the problem, i.e.
equations 1-10.
[0128] It is possible to reduce the number of equations needed to
find the damping between point 3 and point 4 by applying a trick
introduced by Zienkiewicz for Finite Elements.
[0129] Equation 10 is not used and an approximation is used
instead: 15 .times. : S 4 ' .times. 3 ' .apprxeq. 1 2 .function. [
S 4 ' .times. 3 .times. S 33 ' + S 4 ' .times. 4 .times. S 43 ' ]
##EQU5##
[0130] It is even possible to reduce the number of equations needed
to only 8 equations by applying Zienkiewicz tric twice, which
eliminates the need of equations 6 and 10. The approximation used
instead of the equations are: 16 .times. : S 4 ' .times. 3 '
.apprxeq. 1 2 .function. [ S 4 ' .times. 3 .times. S 33 ' + S 44 '
.times. S 43 ' ] 17 .times. : S 43 .apprxeq. 1 2 .function. [ S 43
.times. S 33 ' + S 44 ' .times. S 34 ' ] ##EQU6##
[0131] The damping S.sub.43 between point 3 and 4 and between point
3' and 4' can be calculated by turning the needed equations to
logarithms, Equations 1 through 10 become a inhomogenous linear
system of equations with as many unknowns as equations where a
solution is always available as long as the analysis points are
chosen properly. One has to solve the system for S.sub.43 in order
to obtain the microwave runtime between point 4 and point 3
illustrating the role of these points as "virtual probes".
[0132] The above described system uses a "virtual transmitter"
(i.e. point 3) and a "virtual receiver" (i.e. point 4). One can
easily place one of these point to coincide with a real transmit or
receive antenna respectively arriving at the first usage of the
invention. Placing both virtual probes at the place of the physical
probe antennas will result in the traditional microwave measurement
technique known prior to this invention.
[0133] Depending on the physical problem to be solved, one utilizes
a single (virtual receiver or virtual transmitter) or both virtual
probe concepts. It is also possible to use sets of probes (e.g.
virtual probe arrays) to create a specific beam pattern
generated/received by the virtual probes.
[0134] Different probe configurations may be used for applications
as mine sweeping, material analysis, mineral exploration, medical
applications etc.
Shorthand Mathematical Derivation of the Method:
[0135] Electromagnetic radiation is governed by Maxwell's equations
where the vectorial electric field E is easily cast into a
Helmholtz-form that is written in three dimensional space x and
time t dependent coordinates as: .DELTA. 2 .times. E - 0 .times. r
.times. .mu. 0 .times. .mu. r .times. .differential. .differential.
t 2 .times. E = 0 ##EQU7## Where .DELTA. is the Laplace operator,
.epsilon..sub.0 the dielectric constant of vacuum, .epsilon..sub.r
the local relative dielectric function of the material at a given
location (being a 3.times.3 tensor), .mu..sub.0 stands for the
permeability of vacuum and .mu..sub.r for the local relative
permeability of the material under test. In this shorthand
derivation, .mu..sub.r is set to be the unit tensor 1 (3.times.3).
To a skilled person it is obvious that a similar method can be
derived by solving for .epsilon..sub.r and .mu..sub.r
simultaneously.
[0136] At the same time, ultrasonic waves with a tensorial
3.times.3 stress amplitude y and a local sound speed of the medium
v can also be cast in a similar form .DELTA. 2 .times. y - v
.times. .differential. .differential. t 2 .times. y = 0
##EQU8##
[0137] The solutions of both differential equations are performed
taking the location of the radiation sources into account. Focusing
on the key point of the process, any ultrasonic wave with a
non-vanishing amplitude creates a stress in the material (being of
compression or shear type). This stress is reflected by a local
compression of the material: By this compression, the density of
polarized charge is affected--as a known fact, any compression of a
dielectric object changes the relative dielectric function tensor
.epsilon..sub.r as:
.epsilon..sub.r.apprxeq..epsilon..sub.r0+.alpha.y
[0138] This relation creates a coupling between ultrasonic wave
propagation and electromagnetic waves exploited in this invention.
The strength of the interaction is determined by the
acousto-optical interaction a being a 3.times.3.times.3 tensor. For
a complete picture of the physics involved one has to mention that
the above relation only holds for comparably small ultrasound waves
where e.g. cavitation and other nonlinear effects can be
neglected.
[0139] The complete system to be solved for electromagnetically is
then given by: .DELTA. 2 .times. E .function. ( x , t ) - 0
.function. [ r .times. .times. 0 + .alpha. y .function. ( x , t ) ]
.times. .mu. 0 .times. .mu. r .times. .differential. .differential.
t 2 .times. E .function. ( x , t ) = 0 ##EQU9##
[0140] To a person skilled in the art it is obvious that this type
of differential equation becomes a convolution in frequency space
a) when Fourier transform in time t is applied:
.DELTA..sup.2E(x,.omega.)+.omega..sup.2.epsilon..sub.0[.epsilon..sub.r0+.-
alpha.y(x,.omega.)].mu..sub.0.mu..sub.r.sym.E(x,.omega.)=0
[0141] And where the circled times operator E(x,.omega.) denotes a
frequency convolution integral (e.g. found in "Anleitung zum
praktischen gebrauch der Laplace transformation" by G. Doetsch,
1988) that becomes in full form (omitting eventual normalization
constants in front of the convolution integral): [ .DELTA. 2 +
.omega. 2 .times. 0 .times. r .times. .times. 0 .times. .mu. 0
.times. .mu. r ] .times. E .function. ( x , .omega. ) + .alpha.
.omega. 2 .times. 0 .times. .mu. 0 .times. .mu. r .times. lim Q
.fwdarw. .infin. .times. .intg. .xi. = - Q + Q .times. y .function.
( x , .omega. - .xi. ) .times. E .function. ( x , .xi. ) .times.
.times. d .xi. = 0 ##EQU10## Therefore assuming a single frequency
ultrasound excitation and a single frequency microwave signal
incident to the object, the received microwave signals contain a
part in the incident microwave frequency but also sidebands at the
difference and sum of ultrasound and microwave frequencies created
by the convolution integral.
[0142] The above relation offers a whole new world to extract
information from a microwave field--by properly phase-controlling
the ultrasound and by using pulsed wave trains.
Single Virtual Probe
[0143] One applies the method to solve along a path involving a
single virtual probe. This corresponds to either a virtual
transmitter or a virtual receiver depending on what transmission
parameter one solves the upcoming linear equation system that has
been described above where all relations to either point 3 or 4
vanish. The wave propagation mechanisms are identical for this
case. For the ideal (homogenous, boundary condition free) case, one
arrives at the following propagation relations:
[.DELTA..sup.2+.omega..sup.2.epsilon..sub.0.epsilon..sub.r.mu..sub.0.mu.-
.sub.r],
E(x,.omega.)+.alpha..omega..sup.2.epsilon..sub.0.epsilon..sub.r.m-
u..sub.0.mu..sub.rE(X,.omega.-.xi.)=0
[.DELTA..sup.2+(.omega.-.xi.).sup.2.epsilon..sub.0.epsilon..sub.r.mu..sub-
.0.mu..sub.r], E(x,.omega.-.xi.)=+qE(X,.omega.-.xi.)=0 Double
Virtual Probe
[0144] In addition one can apply the method to solve along a path
through two virtual probes. This corresponds to either a virtual
transmitter or a virtual receiver depending on what transmission
parameter one solves the upcoming 9.times.9 linear equation system
that has been described above where all equations are present. For
the ideal (homogenous, boundary condition free) case, one arrives
at the following propagation relations
[.DELTA..sup.2+.omega..sup.2.epsilon..sub.0.epsilon..sub.r.mu..sub.0.mu..-
sub.r],
E(x,.omega.)+.alpha..omega..sup.2.epsilon..sub.0.epsilon..sub.r.mu-
..sub.0.mu..sub.rE(X,.omega.-.xi.)=0
[.DELTA..sup.2+(.omega.-.xi.).sup.2.epsilon..sub.0.epsilon..sub.r.mu..sub-
.0.mu..sub.r]E(x,.omega.-.xi.)=+qE(X,.omega.-.xi.)
[.DELTA..sup.2+(.omega.-.xi.-.eta.).sup.2.epsilon..sub.0.epsilon..sub.r.m-
u..sub.0.mu..sub.r]E(x,.omega.-.xi..eta.)=+q'qE(Y,.omega.-.xi.-.eta.)
[0145] The first two equations denote the generation of a sideband
at the analysis point X taking the role of a virtual transmitter.
The third equation denotes the generation of a second sideband on
top of the first by focussing at another analysis point Y which
takes the role of a virtual receiver. The frequency offsets are
denoted .eta. at point X and .eta. at point Y determined by the
frequency of the ultrasound used to accomplish focusing. Please
note that these may not be the same frequencies for both points X,
Y in certain applications.
[0146] The first equation states the generation of a sideband at a
predetermined location .xi. with the sideband offset x. The second
equation states the propagation of the sideband through the whole
object under test when a source with strength q is placed a
position X. The method allows therefore to "probe" the object by
synthesizing a microwave source at arbitrary positions inside the
object. One measures then the radiation generated from this source
when moving this source around.
* * * * *