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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and apparatus for non-contacting
identification of the temperature distribution in a non-uniform
examination subject which may use of phase and amplitude measurements of
attenuated microwave radiation directed at the examination subject.
2. Description of the Prior Art
The temperature distribution of an examination subject is of particular
interest in the medical field, wherein the examination subject is a human
patient. It is possible to analyze the temperature distribution of a
patient to identify areas exhibiting a pathological condition because
diseased tissue exhibits a different heat dissipation from healthy tissue.
It is also possible using a hyperthermia method to undertake local heating
of an area affected by a nidus, for example, a tumor, in order to achieve
decomposition of diseased cells, particularly in combination with
radiation therapy. Under such treatment methods, the heating cannot exceed
a critical limit value, and must be topically within the diseased area so
that no healthy tissue is damaged. In a hyperthermia method, the
temperature distribution is also of interest to assist in precisely
localizing the area of heat application, and monitoring the success of the
therapeutic measures.
As a consequence of the slight temperature differences which occur in the
human body, methods for identifying the temperature distribution in humans
must do so with a high precision. This requirement is difficult to meet,
however, because, given a dielectrically non-uniform examination subject,
such as the human body, the distribution of the complex dielectric
constant in the examination subject enters significantly into the measured
result, not only for measuring the characteristic radiation of the
subject, but also measuring the actual temperature.
In the human body, for example, the dielectric constant varies between a
value .epsilon.=10 for fat tissue up to a value .epsilon.=60 for bone.
Because of the different dielectric constants , the characteristic
radiation which emanates from a point in the examination subject is
differently attenuated and refracted in the tissue, so that it is not
possible to obtain an unambiguous result outside of the examination
subject. Given the same temperature, a region within the examination
subject having a high dielectric constant radiates more strongly than a
region having a low dielectric constant. The intensity of the
characteristic region outside of the examination subject is thus a
function of the distribution of the dielectric constant and of the
temperature. Making the assumption that the dielectric constant
corresponds, for example, to an average value, causes incorrect results.
For example, it is possible that a first area in the examination subject
measured at 40.degree. C. on the basis of the characteristic radiation is
in reality colder that a second area measured at 37.degree. C. If the
first area measured at 40.degree. C. has an extremely high dielectric
constant, its characteristic radiation in comparison to the area measured
at 37.degree. C. will be incorrectly evaluated, the latter area having a
low dielectric constant in accordance with the assumption.
A method and apparatus of the type described above are disclosed in the
article "Aperture Synthesis Thermography:" Haslam et al, IEEE Transactions
on Microwaves, Vol. MTT-32, No. 8, August 1984. The characteristic thermal
radiation of an examination subject in this method and apparatus is
received by an antenna. The received signal is divided in terms of
amplitude and phase in a computer, these values serving for calculating
the temperature distribution. The complex relationship with the dielectric
constant and the problems resulting therefrom with respect to the
reliability of the measured results are not discussed in detail in this
publication.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method and apparatus
for non-contacting identification of the temperature distribution in a
non-uniform examination subject which permits the temperature distribution
to be identified with high precision.
The above object is achieved in accordance with the principles of the
present invention wherein a three-dimensional distribution of the complex
dielectric constant is measured in the examination subject and this
three-dimensional distibution of the complex dielectric constant is then
taken into consideration in a second step for deriving the temperature
distribution. In this manner, the temperature distribution is identified
taking the non-uniform attenuation and scatter of the characteristic
thermal radiation into consideration.
An apparatus for practicing the above method has a microwave transmitter
and a microwave receiver, which may be in the form of a single detector or
first and second detectors. If two detectors are utilized, a first
microwave detector is connected to an evaluation circuit which identifies
the three-dimensional distribution of the dielectric constant in the
examination subject and supplies information regarding this distribution
to the first input of a calculating means. The second microwave detector
is used to acquire the characteristic thermal radiation of the examination
subject, and information regarding this characteristic thermal radiation
is supplied to the second input of the calculating means in terms of
magnitude and phase. A signal representing the three-dimensional
temperature distribution in the examination subject is then obtained at
the output of the calculating means.
In an embodiment using only one microwave detector, the detector is
connected to both evaluation circuits and is switched therebetween.
By obtaining the three-dimensional distribution of the dielectric constant
in the examination subject, and taking this distribution into
consideration in calculating the three-dimensional temperature
distribution, it is assured that the characteristic radiation emitted from
an area within the examination subject is processed with the weighting
appropriate thereto. On the basis of the measured distribution of the
dielectric constant, the attenuation and scatter of the characteristic
radiation in the non-uniform examination subject is incorporated into the
final calculation, so that the characteristic thermal radiation measured
outside of the examination subject is corrected in accordance with its
path through the examination subject. The correction is undertaken with
reference to the distribution of the dielectric constant present along the
propagation path between the subject area of interst and the reception
means. The method and apparatus thus enable even minute temperature
differences to be identified with high three-dimensional resolution in a
non-uniform examination subject.
In a further embodiment of the invention, a heating source is provided for
heating a prescribed volume in the examination subject.
If the examination subject is a patent and the prescribed volume is a
diseases area of the patient, the heating source can be utilized under
control of the measurement apparatus so that only the diseased area, for
example, an ulcer or cancer cells, is elevated in temperature and a high
destruction rate of the diseased cells is achieved. For effective tumor
treatment, the temperature in the tumor and in the surrounding normal
tissue, should be brought as close as possible to the maximum temperature
of 42.degree. through 43.degree. C. which is permitted for the normal
tissue. During this hyperthermia in the prescribed volume, temperature
supervision with a precision of fractions of one degree Celsius and with a
three-dimensional resolution of at least one centimeter is required. Such
precise temperature supervision can be achieved with the method and
apparatus disclosed herein.
DESCRIPTION OF THE DRAWINGS
The single FIGURE is a schematic block diagram for an apparatus for
undertaking a non-contacting identification of the three-dimensional
temperature distribution in an examination subject, which practices the
method described herein.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
An examination subject schematically represented at 1, such as a human
body, is disposed between a radiator 2 of a microwave transmitter 3 and
the reception antenna 4 of a first microwave reception means 5. For better
coupling, the intervening space between the radiator 2, the subject 1 and
the reception antenna 4 may be occupied by a coupling medium. The
transmission frequency of the microwave transmitter 3 may be at a value in
the range from about 0.1 through about 10 GHz. A range between about 1 and
about 3 GHz is preferable. The microwave transmitter 3 also includes a
transmission amplifier 7 and an HF oscillator 9 connected to a control
unit 11. The transmission amplifier 7 may be adjusted such that a power of
about 10 mW per square centimeter is available at the examination subject
1. The HF oscillator 9 operates with a fixed frequency which is
adjustable. The adjusted frequency corresponds to that frequency which is
desired at the microwave transmitter 3. The HF oscillator 9 receives start
and stop instructions from the control unit 11. Operation of the microwave
transmitter 3 is controlled by the control unit 11 in this manner.
A first microwave reception means 5 includes the reception antenna 4 which
may, for example, be a two-dimensional array, a first multiplexer 15, a
first reception amplifier 17, a first phase detector 19, a first
analog-to-digital converter 21, and a first memory 23. In accord with a
control signal, the first multiplexer 15 connects a selected reception
element 13 of the reception antenna 4 to the first reception amplifier 17.
The received signal amplified in the first amplifier 17 is divided in
terms of amplitude and phase in the first phase detector 19. For that
purpose, the first detector 19 is supplied with a reference signal R taken
from the output of the transmission amplifier 7. The amplitude and phase
are each converted into a digital signal in the first analog-to-digital
converter 21 and are entered into the first memory 23 true to position.
The output of the first memory 23 is connected to a computer 25 which
calculates the distribution of the dielectric constant in the examination
subject 1 from the stored amplitude and phase data of the received
microwave signals. This three-dimensional distribution is stored in an
intermediate memory 27. The output of the intermediate memory 27 is
connected to a first input of a further computer or calculating means 29.
The control unit 11 activates the microwave transmitter 3 by the
aforementioned start instructions. The examination subject 1 is irradiated
in accordance with the preselected transmission frequency in the microwave
range. The radiation is attenuated and scattered in the examination
subject 1, and the transmitted radiation is detected by the first
microwave reception means 5. The reception signals of the reception means
5, in terms of amplitude and phase, are supplied to an evaluation circuit
consisting of the computer 25 and the intermediate memory 27 wherein the
three-dimensional distribution of the dielectric constant is the
examination subject 1 is identified.
A method and apparatus for identifying the three-dimensional distribution
of the dielectric constant are described in German OS No. 35,31,893,
corresponding to copending U.S. application Ser. No. 903,999, filed Sept.
4, 1986. The method disclosed therein is suitable for use as one step in
the method disclosed herein.
As described in the aforementioned German OS No. 35 31 893 and U.S.
application Ser. No. 903,999 , the three-dimensional distribution of
dielectric constants in an object such as a patient can be accomplished by
emitting radiation directed at the object, receiving radiation which is
transmitted and scattered by the object in a selected volume, measuring
the amplitude and phase of the received radiation, calculating the
three-dimensional distribution of the dielectric constants in the object
from the amplitude and phase of the received radiation in the selected
volume, and, if desired, visually displaying the result of the
calculation. The following equations can be used to calculate the
three-dimensional distribution of the dielectric constants:
V(r)=.omega./c(.sqroot..epsilon..mu.-.sqroot..epsilon.(r).multidot..mu.(r)
and
V=G.sub.S.sup.- .psi..sub.S ( .psi..sub.o +G.sub.S G.sub.T.sup.-1
.psi..sub.S).sup.-1,
wherein V(r) is the scatter potential, .omega.=2.pi.f is the radian
frequency for the emitted radiation, c is the speed of light, .epsilon. is
the dielectric constant of any radiation coupling medium, surrounding said
object, .mu. is the permeability of said coupling medium .mu.(r) is the
permeability of the object, G.sub.S is the Green's function for
propagation of the radiation from the object to a measuring location
within the selected volume, G.sub.T is the Green's function for
propagation of radiation between two locations in the object,
G.sub.S.sup.-1 is the inverted Green's function G.sub.S, .psi..sub.o is
the incoming radiation field at the object, .psi..sub.S is the scatter
field at the measuring location, is a unit operator, and .epsilon.(r) is
the distribution of dielectric constants in the object.
A second microwave reception means 31 is also directed at the examination
subject 1. The second microwave reception means 31 includes a second
reception antenna 33, which may also be in the form of a two-dimensional
array, a second multiplexer 35, a second reception amplifier 37, a second
phase detector 39 supplied with a reference signal R' from another
oscillator 56, a second analog-to-digital converter 41, and a second
memory 43. The output of the second analog-to-digital converter 41 is
supplied to the second memory 43, which has an output connected to a
second input of the calculating means 29. The manner of functioning of the
second reception means 31 is analogous to that of the first reception
means 5. Allocated to the reception location, the received microwave
signals, divided in terms of amplitude and phase, are available at the
output of the second memory 43. The reception antenna 33 is disposed at a
right angle relative to the emission direction of the radiator 2.
With the assistance of the microwave transmitter 3 and the first reception
means 5 the distribution of the dielectric constant in the examination
subject 1 is identified in accordance with the teachings of aforementioned
copending application Ser. No. 903,999. If the temperature distribution in
the examination subject 1 exhibits only extremely small differences and is
chronologically constant, the distribution of the dielectric constant
.epsilon. remains substantially unaltered over the entire examination
timespan. In such a case, the characteristic radiation is preferably
measured by the second microwave reception means 31 with an inactivated
microwave transmitter 3, because a renewed measurement of the distribution
of the dielectric constant .epsilon. is not necessary. The first and
second reception means 5 and 31 need not be simultaneously operated. It is
also possible to tune the reception amplifiers 17 and 37 to the same
frequency of, for example, 3 GHz.
If the examination is undertaken in conjunction with a hyperthermia
treatment, i.e., heating at a prescribed temperature within the
examination subject 1, and if a modification in the distribution of the
dielectric constant .epsilon. in the examination subject 1 is anticipated,
measurement of the distribution of the dielectric constant .epsilon. must
be undertaken again during the examination timespan. If this is to be
undertaken in parallel with identification of the characteristic radiation
of the examination subject 1, the first and second reception amplifiers 17
and 37 are preferably tuned to different frequencies. The first reception
amplifier 17 may be tuned to a transmission frequency of the microwave
transmitter 3 of, for example, 3 GHz, and the second reception amplifier
may be tuned to a frequency of, for example, 1 GHz. Errors in the
identification of the characteristic radiation of the examination subject
1 due to the microwave radiation attenuated in the examination subject 1
can be suppressed in this manner. A reference signal R' of the phase
detector 39 is then a fixed reference signal having the same frequency of
1 GHz.
The calculating means 29 has an output at which the three-dimensional
temperature distribution in the examination subject 1 is available after
the calculation has been completed. The algorithm which is used in the
calculating means 29 will be discussed in detail below. The output of the
calculating means 29 is connected to a further memory 45 for storing the
three-dimensional temperature distribution. The output of the memory 45 is
connected to a display unit 47. The display unit 47 may, for example, be a
picture screen on which the temperature distribution of desired planes or
slices in the examination subject 1 can be displayed. The output of the
memory 47 is also connected to a heat source control unit 49, which
operates a heating source 51 including a heat radiator 53. The heat
radiator 53 is directed at the examination subject 1, and can heat a
prescribed volume therein.
The heat radiator 53 may be any suitably shaped baffle or other means for
locally directing heated air from the heat source 51 toward the
examination subject 1.
The heat source control unit 49 identifies the temperature maximum in the
three-dimensional temperature distribution in the examination subject 1.
In the temperature maximum exceeds a prescribed limit value, the heating
source 51 will be disconnected by the control unit 49. For a human patient
as the examination subject 1, this prescribed limit value is approximately
43.degree. C.
The coordinates in the examination subject 1 identifying the location at
which the temperature maximum exists are also identified in the control
unit 49. These coordinates are compared to prescribed coordinates of a
volume in the examination subject 1 which are entered into the control
unit 49 via an input 55. The prescribed coordinates may, for example, have
been identified with the assistance of computer tomography or NMR
exposure, and may be the coordinates, for example, of the center of a
tumor. Dependent upon a comparison of the prescribed coordinates to those
for the identified temperature maximum, the control unit 49 automatically
guides the emission direction of the heat radiator 53 such that the
coordinates of the maximum of the temperature distribution coincide with
the prescribed coordinates, i.e., with the center of the tumor. It is
assured in this manner that the temperature maximum, given treatment using
a hyperthermia method, will be accurately subjected to heat for destroying
the diseased tissue. The calculating means 29, the memory 45 and the heat
source control unit 49 are all controlled by the main control unit 11 by
respective control lines 57, 59 and 61.
In a further embodiment, the first and second microwave reception means 5
and 31 can be combined to form a common reception means. For example, only
the components of the reception means 5 need then be provided from the
antenna 4 through the memory 23. The reception means 5 then serves both
for identifying the distribution of the dielectric constant .epsilon. and
for identifying the characteristic radiation of the examination subject 1.
In this embodiment, it is necessary that the microwave transmitter 3 be
disconnected when the characteristic radiation of the examination subject
1 is identified. This operating mode is undertaken by the control unit 11.
In this embodiment, a connection 58, indicated by dashed lines, runs from
the output of the memory 23 to the second input of the calculating means
29. The amplitude and phase of the characteristic thermal radiation are
thus supplied to the calculating means 29 ia this connection 67.
In this embodiment, only that which is referred to as "off line" operation
is possible. This means that identification of the temperature
distribution from the dielectric constant and identification of the
characteristic thermal radiation ensue subsequently rather than
simultaneously.
That which is referred to as "on line" operation, by contrast, is possible
by using the two microwave reception means 4 and 31. As described above,
the two reception amplifiers 17 and 37 are tuned to different frequencies,
of, for example, 3 and 1 GHz respectively, for this type of operation. The
intermediate memory 27 may be omitted when the apparatus is operated in
this manner. The output of the computer 25 is then directly connected to
the first input of the calculating means 29. The calculating means 29 thus
respectively simultaneously receives at its two inputs the value of the
dielectric constant .epsilon. the value of the amplitude and phase of the
characteristic thermal radiation for a three-dimensional point in the
examination subject 1. The calculating means 29 can identify the
temperature T at that location from this information.
The following intergral equation is solved in the calculating means 29:
S(r).varies..intg.d.sup.3 r'.epsilon.(r')T(r')G(r-r') (1)
In the above equation, S(r) denotes the characteristic thermal radiation of
the examination subject 1 at a location r. The characteristic radiation
S(r) is known on the basis of the amplitude and phase of the microwave
radiation incident on the antenna 33. The functions .epsilon.(r') is the
distribution of the dielectric constant in the examination subject 1. This
is identified, for example, in accordance with the aforementioned U.S.
Application Ser. No. 903,999, and is supplied to the first input of the
calculating means 29, either from the memory 27 or directly from the
computer 25. The function T(r') is the desired temperature distribution
for which the above integral equation (1) is to be solved. The function
G(r-r') is Green's function, which describes the propagation of the
microwave radiation from the location r' of a point in the examination
subject 1 to the location r of a point in the reception antenna 33. The
terms r and r' are thus location vectors respectively from a point of
origin to the reception antenna 33 or to the examination subject 1.
Green's function G(r) is defined by the integral equation (2):
G(r)=G.sub.o (r)+.intg.d.sup.3 r'G.sub.o (r-r').multidot.(k.sub.o.sup.2
-k.sup.2 (r')).multidot.G(r') (2)
wherein in G.sub.o is Green's function for the propagation of microwave
radiation in the surrounding coupling medium. G.sub.o is defined by the
following differential equation (3):
(.DELTA.+k.sup.2)G.sub.o (r-r')=.differential.(r-r') (3)
In the above equation k.sub.o is the wave number of the microwave radiation
in the coupling medium, and k(r) is the wave number of the microwave
radiation in the examination subject 1. k.sub.o=
.omega./c.multidot..sqroot..epsilon..sub.o .mu..sub.o, and k(r)=.omega./c
.sqroot..epsilon.(r).mu.(r), wherein .epsilon..sub.o and .mu..sub.o are
respectively the dielectric constant and permeability of the coupling
medium, and .epsilon.(r) and .mu.(r) identify the three-dimensional
distribution of the corresponding quantities in the examination subject 1.
.DELTA. is the Laplacian differential operator
##EQU1##
the function and the .differential.(r-r') is the Dirac
.differential.-function. The analytic form of G.sub.o are the known
speherical waves
##EQU2##
the value .omega. is the radian frequency of the microwave radiation and c
is the speed of light. When .epsilon.(r) or .epsilon.(r).multidot..mu.(r)
are known from the microwave imaging, the required Green's function G can
be calculated from the above integral equation (2). Because the integral
equation (2) for G contains the non-uniform distribution of the dielectric
constants of the examination subject, G(r) describes the propagation of
the radiation from a point in the examination subject to the detector
taking the attenuation and scatter in the examination subject into
consideration.
Integral equation (1) is solved by transformation into Fourier space, by
the following relationship, wherein * denotes the Fourier convolution:
(.epsilon.*T)(k).multidot.G(k).alpha.S(k) (5)
and
##EQU3##
In the above, k is the three-dimensional spatial frequency vector. After
back-transformation into the local space by inverse Fourier
transformation, equation (6) can be rewritten as:
.epsilon.(r).multidot.T(r).varies.FT.sup.-1 [(S/G)]=R(r) (7)
Wherein FT.sup.-1 denotes the inverse Fourier transformation. The
temperature distribution T(r) can be identified therefrom in accord with
the following equation:
##EQU4##
The method for identification of the temperature distribution T(r) in the
examination subject 1 is undertaken in the following sequence. The
microwave transmitter 3 is activated and emits microwaves in a direction
toward the examination subject 1. These mirowaves are three-dimensionally
attenuated and scattered to differing degrees in the non-uniform
examination subject 1. The microwave radiation energing from the
examination subject 1 is acquired by the first reception means 5 and is
divided in terms of amplitude and phase. The distribution of the
dielectric constant .epsilon.(r) is calculated in the computer 25 and is
stored in the intermediate memory 27 for each spatial point in the
examination subject 1. The three-dimensional distribution of the
dielectric constant .epsilon.(r) in the examination subject is then
subsequently available in the intermediate memory 27.
The microwave transmitter 3 is then shut off. The characteristic thermal
radiation of the examination subject is then identified by the second
microwave reception means 31. The temperature distribution in the
examination subject 1 is calculated in the calculating means 29 in
accordance with the algorithm described above, using the distribution of
the dielectric constant. The result is entered in the memory 45.
In an examination subject 1 in whom a tumor or other diseased tissue is to
be destroyed by thermal decomposition, temperature elevation of the
prescribed volume of the examination subject 1 follows. For this purpose,
the heating source 51 is energized for a defined timespan. Subsequently,
the characteristic radiation in the examination subject 1 is again
detected by the second microwave reception means 31 and is processed to
form a three-dimensional temperature distribution, in combination with the
distribution of the dielectric constant .epsilon.(r) which is still
available in the intermediate memory 27. Heating of a localized area of
the examination subject 1 is then undertaken from the new temperature
distribution.
Alternatively, it is possible to first re-activate the microwave
transmitter 3 and measure a new distribution of the dielectric constant
.epsilon.(r) by the first reception means 5. The characteristic radiation
is then identified and the new temperature distribution is calculated.
This operation can be repeated until the critical temperature of about
43.degree. C. for the tissue is reached. This method of temperature
identification can be utilized for monitoring the hyperthermia treatment.
The frequency of the heating source 51 should be selected so as to be
outside of the bandwidth of the frequencies of the first and second
reception means 5 and 31.
Although modifications and changes may be suggested by those skilled in the
art it is the intention of the inventors to embody within the patent
warranted hereon all changes and modifications as reasonably and properly
come within the scope of their contribution to the art.
* * * * *
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