U.S. patent application number 14/996145 was filed with the patent office on 2016-07-14 for locating features in the heart using radio frequency imaging.
The applicant listed for this patent is Kyma Medical Technologies Ltd.. Invention is credited to Assaf Bernstein, Eyal Cohen, Vered Cohen Sharvit, Uriel Weinstein.
Application Number | 20160198976 14/996145 |
Document ID | / |
Family ID | 44069436 |
Filed Date | 2016-07-14 |
United States Patent
Application |
20160198976 |
Kind Code |
A1 |
Weinstein; Uriel ; et
al. |
July 14, 2016 |
LOCATING FEATURES IN THE HEART USING RADIO FREQUENCY IMAGING
Abstract
Diagnostic apparatus (20) includes an antenna 32, which is
configured to direct radio frequency (RF) electromagnetic waves
into a living body and to generate signals responsively to the
waves that are scattered from within the body. Processing circuitry
(36) is configured to process the signals so as to locate a feature
in a blood vessel in the body.
Inventors: |
Weinstein; Uriel; (Mazkeret
Batia, IL) ; Bernstein; Assaf; (Givat Nilly, IL)
; Cohen; Eyal; (Ariel, IL) ; Sharvit; Vered
Cohen; (Modiin, IL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kyma Medical Technologies Ltd. |
Tel Aviv |
|
IL |
|
|
Family ID: |
44069436 |
Appl. No.: |
14/996145 |
Filed: |
January 14, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13513396 |
Sep 12, 2012 |
9265438 |
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PCT/IB2009/055438 |
Dec 1, 2009 |
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14996145 |
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12127544 |
May 27, 2008 |
8352015 |
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13513396 |
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Current U.S.
Class: |
600/425 |
Current CPC
Class: |
A61N 1/36578 20130101;
A61B 2562/0217 20170801; A61B 8/085 20130101; A61B 2560/0412
20130101; A61B 2562/14 20130101; A61B 5/6823 20130101; A61B
2562/166 20130101; A61B 5/0507 20130101; A61B 8/4254 20130101; A61B
5/061 20130101 |
International
Class: |
A61B 5/05 20060101
A61B005/05 |
Claims
1-58. (canceled)
59. A radio-frequency (RF) monitoring and/or diagnostic apparatus,
comprising: a planar antenna configured to direct RF
electromagnetic waves into the body of a patient and to generate
signals responsively to the waves that are scattered from within
the body, wherein a first surface of the antenna comprises a
printed circuit board; and processing circuitry configured to
process the signals so as to determine a feature in the body
responsively to a difference in a dielectric constant of the
feature relative to surrounding tissue; wherein, the antenna is
configured to be positioned in close proximity to a surface of the
body of the patient.
60. The apparatus of claim 59, wherein close proximity to a surface
of the body of the patient comprises the antenna being in contact
with the skin of the patient.
61. The apparatus of claim 59, further comprising a first material
configured for arrangement between the body of the patient and the
antenna.
62. The apparatus of claim 61, wherein the first material comprises
at least one of an adhesive, a gel, and apparel.
63. The apparatus of claim 59, wherein determining the feature in
the body comprises identifying and tracking a movement of the
feature in the body.
64. The apparatus of claim 63, wherein tracking the movement of the
feature in the body includes imaging the feature and estimating a
trajectory of the feature based on analysis of the image of the
feature.
65. The apparatus of claim 59, wherein the feature comprises a
tissue in the body.
66. The apparatus of claim 59, wherein the RF electromagnetic waves
are in a frequency range of from about 400 MHz to about 4 GHz.
67. A radio-frequency (RF) monitoring and/or diagnostic apparatus
comprising: a planar antenna configured to direct RF
electromagnetic waves into the body of a patient and to generate
signals responsively to the waves that are scattered from within
the body, wherein a first surface of the antenna comprises a
printed circuit board; processing circuitry configured to process
the signals so as to determine a feature in the body responsively
to a difference in a dielectric constant of the feature relative to
surrounding tissue; and a first material configured for arrangement
between the antenna and the skin of the patient during use.
68. The apparatus of claim 67, wherein the first material comprises
at least one of an adhesive, a gel and apparel.
69. The apparatus of claim 67, wherein the first material is
configured so as to substantially affix the antenna in close
proximity to the skin of the body.
70. The apparatus of claim 67, further comprising a cavity
proximate the antenna and configured to at least increase overall
gain of the signals.
71. The apparatus of claim 70, wherein the cavity comprises a
waveguide.
72. The apparatus of claim 71, wherein the waveguide includes
dimensions configured such that a cutoff frequency of the lowest
propagating mode in the waveguide exceeds an upper band frequency
limit of the antenna.
73. The apparatus of claim 67, further comprising a case for
sealing the planar antenna against the first material and/or a
liquid.
74. The apparatus of claim 67, wherein the RF electromagnetic waves
are in a frequency range of from about 400 MHz to about 4 GHz.
75. A radio-frequency (RF) monitoring and/or diagnostic apparatus
comprising: a planar antenna configured to direct RF
electromagnetic waves into the body of a patient and to generate
signals responsively to the waves that are scattered from within
the body, wherein a first surface of the antenna comprises a
printed circuit board; processing circuitry configured to process
the signals so as to determine a feature in the body responsively
to a difference in a dielectric constant of the feature relative to
surrounding tissue; and a wearable element comprising at least one
of the planar antenna and the processing circuitry, the wearable
element configured to abut skin of the body of the patient.
76. The apparatus of claim 75, wherein the planar antenna is a
multi-element antenna, and further comprising a switching matrix
configured to switch elements of the multi-element antenna between
a transmitter and a receiver.
77. The apparatus of claim 75, wherein determining a feature in the
body comprises identifying and tracking a movement of the feature
in the body.
78. The apparatus of claim 75, wherein tracking the movement of the
feature in the body includes imaging the feature and estimating a
trajectory of the feature based on analysis of the image of the
feature.
79. The apparatus of claim 75, wherein the planar antenna includes
a plurality of antenna elements, the plurality of antenna elements
positioned about the body of the patient such that a first antenna
element receives reflected waves and a second antenna element
receives the scattered waves.
80. The apparatus of claim 79, further comprising one or more
position sensors configured to determine respective positions of
the plurality of antenna elements.
81. The apparatus of claim 75, wherein the RF electromagnetic waves
are in a frequency range of from about 400 MHz to about 4 GHz.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation-in-part of U.S. patent
application Ser. No. 12/127,544, filed May 27, 2008, which is
assigned to the assignee of the present patent application and
whose disclosure is incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates generally to methods and
systems for medical diagnostic imaging, and specifically to radio
frequency (RF)-based imaging.
BACKGROUND OF THE INVENTION
[0003] Narrowing of the coronary arteries due to atherosclerosis is
commonly treated by implantation of a stent, using a catheter, to
hold the artery open. In a large fraction of cases, however, the
treated artery closes up again due to in-stent restenosis,
necessitating further treatment. Accurate assessment of such
restenosis generally requires re-catheterization. A number of
non-invasive techniques have been proposed, such as in U.S. Pat.
No. 6,729,336, in which an electromagnetic wave transmitter is used
to excite a stent, and an acoustic sensor detects stent acoustic
oscillations.
[0004] RF imaging is best known in the context of radar systems,
but RF diagnostic imaging systems have also been developed for
medical applications. For example, U.S. Patent Application
Publication 2008/0169961, whose disclosure is incorporated herein
by reference, describes computerized tomography using radar, which
may be used for generating an image of living tissue. As another
example, U.S. Pat. No. 7,454,242, whose disclosure is incorporated
herein by reference, describes tissue-sensing adaptive radar
imaging for breast tumor detection.
[0005] Various antenna designs have been proposed for RF imaging of
body tissues. For example, U.S. Pat. No. 6,061,589, whose
disclosure is incorporated herein by reference, describes a
microwave antenna for use in a system for detecting an incipient
tumor in living tissue, such as that of a human breast, in
accordance with differences in relative dielectric characteristics.
A composite Maltese Cross or bow-tie antenna construction is
employed to irradiate the living tissue and to collect backscatter
or other scatter returns.
SUMMARY
[0006] Embodiments of the present invention that are described
hereinbelow provide improved devices and methods for detecting
features inside a living body using RF imaging techniques. Although
some of these embodiments are directed specifically to detection of
features in the heart, and specifically in the coronary arteries,
the principles of these embodiments may similarly be applied in
imaging, detection and tracking of features elsewhere in the
body.
[0007] There is therefore provided, in accordance with an
embodiment of the present invention, diagnostic apparatus,
including an antenna, which is configured to direct radio frequency
(RF) electromagnetic waves into a living body and to generate
signals responsively to the waves that are scattered from within
the body. Processing circuitry is configured to process the signals
so as to locate a feature in a blood vessel in the body.
[0008] In disclosed embodiments, the apparatus includes an
ultrasound transducer, and the processing circuitry is configured
to guide the ultrasound transducer to direct an ultrasonic beam
toward the feature. In one embodiment, the feature located by the
processing circuitry includes a stent, and the ultrasound
transducer is configured to generate a Doppler signal responsively
to a flow of blood through the stent.
[0009] Additionally or alternatively, the apparatus includes a
tracking unit, which is configured to track respective coordinates
of the antenna and of the ultrasound transducer, and the processing
circuitry is configured to guide the ultrasound transducer
responsively to the respective coordinates. The apparatus typically
includes position transducers fixed respectively to the ultrasound
transducer and to the antenna, wherein the tracking unit is
configured to track the respective coordinates responsively to
position signals exchanged between the position transducers and the
tracking system.
[0010] Further additionally or alternatively, the apparatus
includes a display, wherein the processing circuitry is configured
to guide the ultrasound transducer by driving the display to
present to an operator of the ultrasound transducer an indication
of a direction in which the ultrasound transducer should be
aimed.
[0011] In a disclosed embodiment, the blood vessel is a coronary
artery. The processing circuitry may be configured to track a
cyclical motion of the feature over multiple cycles of the
heart.
[0012] Typically, the processing circuitry is configured to locate
the feature responsively to a difference in a dielectric constant
of the feature relative to surrounding tissue.
[0013] There is also provided, in accordance with an embodiment of
the present invention, diagnostic apparatus, including an antenna,
having a front surface configured to brought into contact with an
outer surface of a living body so as to direct radio frequency (RF)
electromagnetic waves into the body and to generate signals
responsively to the waves that are scattered from within the body.
A dielectric gel is adapted to be spread between the outer surface
of the body and the front surface of the antenna. Processing
circuitry is configured to process the signals so as to locate a
feature in the body.
[0014] Typically, the body has a first dielectric constant, and the
gel has a second dielectric constant that is chosen to match the
first dielectric constant. In disclosed embodiments, the gel has a
dielectric constant that is between 30 and 75. The gel may be
adhesive.
[0015] In one embodiment, the gel is water-based and includes an
additive selected from a group of additives consisting of an
alcohol, a salt, a sugar, and glycerin. Alternatively, the gel
includes silicone and an additive having a dielectric constant
greater than 70.
[0016] There is additionally provided, in accordance with an
embodiment of the present invention, diagnostic apparatus,
including an antenna, which has a front surface and is configured
to direct radio frequency (RF) electromagnetic waves from the front
surface into a living body and to generate signals responsively to
the waves that are scattered from within the body, and which
includes an array of antenna elements, each antenna element
including a planar element at the front surface of the antenna and
a cavity behind the planar element. Processing circuitry is
configured to process the signals so as to locate a feature in the
body.
[0017] In disclosed embodiments, the front surface of the antenna
includes a printed circuit board, and the planar element of each
antenna element includes a conductive radiator printed on the
printed circuit board. The printed circuit board may include
multiple conductive vias surrounding the radiator for isolating the
antenna elements from one another.
[0018] There is further provided, in accordance with an embodiment
of the present invention, diagnostic apparatus, including an
antenna, including an array of antenna elements, which are
configured to direct radio frequency (RF) electromagnetic waves
into a living body and to generate signals responsively to the
waves that are scattered from within the body. Excitation circuitry
is coupled to apply a RF excitation waveform at multiple different
frequencies to different transmitting antenna elements, selected
from the array, according to a predetermined temporal pattern.
Processing circuitry is coupled to receive the signals from
different receiving antenna elements, selected from the array, and
to process the signals at the different frequencies due to the
different transmitting and receiving antenna elements so as to
locate a feature in the body.
[0019] In some embodiments, the excitation circuitry includes a
driver circuit, which is configured to generate the RF excitation
waveform with a variable frequency, and a switching matrix, which
is configured to select sets of the antenna elements in
alternation, each set including at least one transmitting antenna
element and one receiving antenna element, and for each selected
set, to couple the driver circuit to excite the at least one
transmitting antenna element at a selected frequency while coupling
the processing circuitry to receive the signals from the at least
one receiving antenna element. In a disclosed embodiment, the
driver circuit and the switching matrix are coupled to select pairs
of one transmitting antenna element and one receiving antenna
element, and to excite the transmitting antenna in each pair at
each of a plurality of frequencies in accordance with the
predetermined temporal pattern.
[0020] In some embodiments, the apparatus includes a signal
conditioning unit, which is configured to cancel a background
component of the signals that arises from direct coupling between
the transmitting and receiving antenna elements before the
processing circuitry receives the signals. The signal conditioning
unit may include an amplitude and phase modulator, which is coupled
to receive the RF excitation waveform from the driver circuit, to
modify a phase and amplitude of the received waveform so as to
generate an anti-phased signal matching the background component,
and to add the anti-phased signal to a signal received from the at
least one receiving antenna element in order to cancel the
background component.
[0021] In disclosed embodiments, the processing circuitry is
configured to transform the signals received at the different
frequencies due to the different transmitting and receiving antenna
elements into a three-dimensional (3D) image, and to process the 3D
image in order to find a location of the feature. In one
embodiment, the processing circuitry is configured to compute a
weighted sum of the signals received at the different frequencies
due to the different transmitting and receiving antenna elements,
using respective weights provided for a plurality of voxels in the
3D image, to determine values of the voxels in the 3D image.
[0022] There is moreover provided, in accordance with an embodiment
of the present invention, a method for diagnosis, including
directing radio frequency (RF) electromagnetic waves into a living
body and generating signals responsively to the waves that are
scattered from within the body. The signals are processed so as to
locate a feature in a blood vessel in the body.
[0023] There is furthermore provided, in accordance with an
embodiment of the present invention, a method for diagnosis,
including spreading a dielectric gel between an outer surface of a
living body and a front surface of an antenna. The front surface of
the antenna is brought into contact, via the dielectric gel, with
the outer surface of the living body so as to direct radio
frequency (RF) electromagnetic waves into the body and to generate
signals in the antenna responsively to the waves that are scattered
from within the body. The signals are processed so as to locate a
feature in the body.
[0024] There is also provided, in accordance with an embodiment of
the present invention, a method for diagnosis, including providing
an antenna, which has a front surface and which includes an array
of antenna elements, each antenna element including a planar
element at the front surface of the antenna and a cavity behind the
planar element. Radio frequency (RF) electromagnetic waves are
directed from the antenna elements via the front surface of the
antenna into a living body and generating signals, using the
antenna elements, responsively to the waves that are scattered from
within the body. The signals are processed so as to locate a
feature in the body.
[0025] There is additionally provided, in accordance with an
embodiment of the present invention, a method for diagnosis,
including defining a temporal pattern specifying a sequence of
multiple different frequencies and spatial channels associated with
an array of antenna elements. Radio frequency (RF) electromagnetic
waves are directed at the multiple different frequencies into a
living body from multiple different transmitting antenna elements
that are selected from the array in accordance with the temporal
pattern. Signals are generated responsively to the waves that are
scattered from within the body and are received at multiple
different receiving antenna elements that are selected from the
array in accordance with the temporal pattern. The signals from the
different receiving antenna elements at the different frequencies
are processed so as to locate a feature in the body.
[0026] The present invention will be more fully understood from the
following detailed description of the embodiments thereof, taken
together with the drawings in which:
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 is a schematic, pictorial illustration of a system
for tracking and assessment of a feature in a human body, in
accordance with an embodiment of the present invention;
[0028] FIG. 2 is a block diagram that schematically shows elements
of a system for tracking and assessment of a feature in a human
body, in accordance with an embodiment of the present
invention;
[0029] FIG. 3 is a schematic, pictorial illustration of an antenna
array, in accordance with an embodiment of the present
invention;
[0030] FIG. 4 is a schematic, exploded view of an antenna element,
in accordance with an embodiment of the present invention;
[0031] FIG. 5 is a block diagram that schematically illustrates a
feature detection subsystem, in accordance with an embodiment of
the present invention;
[0032] FIG. 6 is a timing diagram that schematically illustrates an
excitation pattern that is applied to an antenna array, in
accordance with an embodiment of the present invention; and
[0033] FIG. 7 is a flow chart that schematically illustrates a
method for measuring blood flow through a stent, in accordance with
an embodiment of the present invention.
DETAILED DESCRIPTION OF EMBODIMENTS
Overview
[0034] Embodiments of the present invention that are described
hereinbelow use radar imaging techniques to identify and locate
features in the body. Features are thus identified based on the
difference in their complex dielectric constant (referring to both
permittivity and conductivity) relative to the dielectric constant
of the surrounding tissue. These techniques are particularly useful
in detecting and tracking metal objects in the body, but they may
also be used to locate features of other kinds, including both
introduced objects, such as plastic objects, and
naturally-occurring features, such as calcifications, and even
pockets of air or other gases. The term "feature," as used in the
context of the present patent application and in the claims, should
therefore be understood as referring to any item or location in the
body having a distinct dielectric constant.
[0035] Some embodiments of the present invention are directed to
locating features in the heart, and particularly in the coronary
blood vessels. In these embodiments, an antenna directs RF
electromagnetic waves toward the heart and receives the waves that
are scattered from within the body. Processing circuitry processes
the signals generated by the antenna due to the received waves in
order to locate the feature or features of interest, and possibly
to track the movement of such features over the course of the heart
cycle.
[0036] The radar-based location of a feature may be used in guiding
the beam of an ultrasound transducer toward the feature. In one
such embodiment, the antenna and processing circuitry find the
location of a stent in an artery and guide the ultrasound
transducer to direct its beam toward the stent. The ultrasound
transducer may operate in Doppler mode in order to measure the flow
of blood through the stent and thus non-invasively assess possible
restenosis in the stent.
[0037] In the embodiments that are described hereinbelow, the
antenna comprises an array of antenna elements, with a front
surface that is brought into contact with the outer surface (i.e.,
the skin) of the patient's body. A dielectric gel may be spread
between the body surface and the front surface of the antenna in
order to match the dielectric constants and thus improve the
penetration of the RF waves into the body. Additionally or
alternatively, the antenna elements may comprise a cavity and
possibly other features to enhance the efficiency of coupling of
electromagnetic energy from the antenna elements into the body
while reducing loss and crosstalk between the elements.
[0038] In the disclosed embodiments, excitation circuitry applies a
RF excitation waveform at multiple different frequencies to
different transmitting antenna elements in the array. Meanwhile,
the processing circuitry receives signals from different receiving
antenna elements. The selection of transmitting and receiving
antennas, as well as the selection of excitation frequency, follows
a predetermined temporal pattern, which may be implemented by a
switching matrix connected to the antenna elements.
[0039] As a result of this scheme of excitation and reception, the
processing circuitry receives and processes signals from multiple
spatial channels (corresponding to different pairs of antennas) at
multiple different frequencies for each channel. Taken together in
the time domain, these multi-frequency signals are equivalent to
short pulses of RF energy. To reconstruct a three-dimensional (3D)
image of the interior of the body and find the location of a
feature or features, the processing circuitry applies a spatial
transform to the set of received signals. The transform may, for
example, comprise an inverse spherical Radon transform or an
algebraic approximation of such a transform.
[0040] Despite measures that are taken to reduce coupling between
antenna elements within the array, this sort of direct coupling
still generates a strong background component, which tends to mask
the signals due to scattered waves from the body. (The term "direct
coupling," as used in the context of the present patent application
and in the claims, refers to short-range passage of RF waves
between antenna elements by paths other than through the region of
interest in the patient's body, including coupling that occurs
within the array and near-field reflections.) In order to reduce
this masking and enhance the dynamic range of the signals, in some
embodiments a signal conditioning unit is used to adaptively cancel
the background component out of the signals that are passed to the
processing circuitry. To improve visibility of moving features,
such as features in the heart, the signal conditioning unit or
another element of the processing circuitry may even be configured
to cancel all parts of the signals that do not vary over time.
System Description
[0041] FIG. 1 is a schematic, pictorial illustration of a system 20
for tracking and assessment of a feature in a body of a patient 26,
in accordance with an embodiment of the present invention. In this
embodiment, an operator 22, such as a physician, directs an
ultrasonic beam from an ultrasound transducer 24 into the chest of
patient 26. The probe containing transducer 24 operates in Doppler
mode, as is known in the art, in order to measure the velocity of
blood flowing through a coronary artery of the patient, and
specifically through a stent (not shown) that is implanted in one
of the patient's coronary arteries. A console 28 drives transducer
24 and processes the signals that are output by the transducer in
order to extract the Doppler information and displays the results
on a monitor 30. The operator steers the probe toward the location
of the stent under guidance from system 20, as explained in detail
hereinbelow.
[0042] Aiming the ultrasound probe correctly under these
circumstances is difficult: The stent is small and is typically
embedded in clutter in the ultrasound image due to other anatomical
features; and the stent and surrounding features of the heart are
in continual motion. Therefore, system 20 uses radar imaging in
order to find the location of the stent and guide operator 22. For
this purpose, an antenna 32 directs RF waves into the patient's
chest. For good dielectric matching, in order to enhance the
penetration of the RF waves into the body, a layer of a dielectric
gel 33 is spread between the front surface of the antenna and the
patient's skin. The gel may also have adhesive qualities, in order
to aid in holding the antenna in place during the procedure.
[0043] Typically, gel 33 has a dielectric constant that is between
30 and 75. This value defines the effective dielectric constant of
the antenna in its near-field. It is chosen to be close to the
effective dielectric constant of the tissue layers in the path to
the target region in the body. The desired dielectric constant may
be achieved by increasing or decreasing the concentration of
certain additives in the gel. For example, if a water-based gel is
used, the additive may be an alcohol (such as ethanol), salt,
sugar, or glycerin. Alternatively, a silicone gel may be used with
an additive such as barium, having generally a dielectric constant
greater than 70.
[0044] Antenna 32 is coupled by a cable or wireless link to a radar
control unit 34. The control unit comprises processing circuitry
36, which drives the antenna to emit the RF waves into the body and
processes the signals generated by the antenna due to reception of
scattered waves from the body. Based on the received signals,
circuitry 36 forms a 3D radar image of the interior of the body,
and specifically, in the present embodiment, finds the location of
the stent. These functions of circuitry 36 are described in detail
hereinbelow.
[0045] In order to guide the direction of ultrasound transducer 24,
the position coordinates (location and orientation) of the
transducer and of antenna 32 are registered in a common coordinate
frame. For this purpose, system 20 includes a tracking subsystem,
comprising a tracking transmitter 38, which generates a field that
is detected by sensors 40 and 42 on the antenna and on the
ultrasound transducer, respectively. Transmitter 38 may, for
example, generate a magnetic field, and sensors 40 and 42 may be
magnetic sensors, as in the trakSTAR.TM. system distributed by
Ascension Technology Corporation (Milton, Vt.). Alternatively,
sensors 40 and 42 may be replaced by transmitting elements, which
generate fields that are detected by a fixed sensor.
[0046] Further alternatively, other types of tracking devices may
be used, such as optical, ultrasonic or mechanical position sensing
devices, as are known in the art. For the sake of generality, the
term "position transducer" is used herein to refer to the elements
that are attached to ultrasound transducer 24 and antenna 32, such
as sensors 40 and 42, for the purpose of finding their coordinates,
regardless of the specific choice of position sensing technology.
Additionally or alternatively, ultrasonic transducer 24 and antenna
32 may be mechanically fixed in a common frame of reference. A
variety of alternative configurations are described in the
above-mentioned U.S. patent application Ser. No. 12/127,544 and may
similarly be used with the elements of system 20.
[0047] In the system configuration shown in FIG. 1, sensors 40 and
42 output signals to processing circuitry 36. The processing
circuitry processes the signals to find location and orientation
coordinates of the sensors, and hence of antenna 32 and transducer
42, in a common coordinate frame. Based on these coordinates,
processing circuitry 36 registers the ultrasonic images formed by
transducer 24 with the radar images formed by antenna 32. The
processing circuitry finds the location of the stent in the radar
image, and may also estimate its orientation. On this basis, the
circuitry guides operator 22 to aim transducer 24 toward the stent
along the stent axis, i.e., along the direction of blood flow, in
order to maximize the Doppler component in the ultrasound signals.
For this purpose, the processing circuitry drives a guidance
display 44, which indicates to the operator how to aim the
ultrasound transducer toward the target.
[0048] In an alternative embodiment (not shown in the figures),
ultrasound transducer 24 may be held and manipulated by a robot
arm, which is guided automatically by processing circuitry to aim
the transducer in the desired direction.
[0049] Although FIG. 1 shows a particular type of antenna and mode
of coupling the antenna to the patient's body, other antenna types
and configurations may also be used for the purposes described
herein. For example, the antenna may mounted in a cushion below the
patient's back, in a wearable element that fits over the patient's
body, or in any other suitable mount. Some alternative
configurations of this sort are shown in the above-mentioned U.S.
patent application Ser. No. 12/127,544.
[0050] FIG. 2 is a block diagram that schematically shows key
elements of system 20, and particularly of processing circuitry 36,
in accordance with an embodiment of the present invention. Some of
these elements are described in greater detail hereinbelow. Antenna
32 is driven by and outputs signals to a feature detection
subsystem 50. The antenna typically comprises an array of antenna
elements 48, which are connected to a switching matrix 54 in a
front end 52 of subsystem 50. The switching matrix selects
different sets of the antennas to transmit and receive signals at
different, respective times and frequencies, in a predetermined
temporal pattern. Typically, the sets comprise pairs of
antennas--one transmitting and one receiving--although other
groupings may also be used. The pattern of antenna control is
described in detail hereinbelow.
[0051] A driver circuit 58 generates signals, at multiple different
frequencies, for exciting the transmitting antennas and demodulates
the signals received by the receiving antennas. Typically, the
signals are in the range of about 400 MHz to about 4 GHz, although
higher and lower frequencies outside this range may also be used. A
signal conditioning unit 56 between the driver circuit and
switching matrix 54 amplifies the outgoing and the incoming signals
and also cancels background components in the received signals.
This functionality is also described below.
[0052] Front end 52 outputs the demodulated received signals (as
intermediate-frequency or baseband signals) to a digital data
acquisition unit 60, which samples and digitizes the signals. Unit
60 typically comprises a high-resolution analog/digital converter,
such as a 14-bit converter, with suitable sampling circuits as are
known in the art.
[0053] A target detection, measurement and tracking unit 62
receives and processes the digital samples. Unit 62, as described
in detail hereinbelow, processes the sampled signals in order to
generate a 3D radar image of the interior of the chest of patient
26. Within this image, elements having a dielectric constant that
is different from that of the surrounding tissue, such as a metal
stent in a coronary artery, stand out. On this basis, unit 62
identifies and measures the location coordinates of the stent
relative to antenna 32.
[0054] Since the heart is in constant motion, unit 62 may also
track and model the motion of the stent in order to more precisely
guide ultrasound transducer 24. The direction of motion of the
stent during the heart cycle also gives an indication of the
direction of the stent axis (along which the ultrasound transducer
should be aimed): Since the axis of the stent is oriented along the
coronary artery in which the stent is implanted, and the coronary
artery runs along the heart wall, the stent axis will typically be
perpendicular to the direction of motion of the heart wall, and
hence to the axis of motion of the stent in the radar image. As
noted above, operator 22 is guided to aim ultrasound transducer 24
toward the stent in a direction along, or at least close to, the
stent axis.
[0055] Although the present embodiment relates specifically to
identification and tracking of a stent, the techniques and circuits
that are described here may be used, by the same token, in locating
and tracking other features in the coronary blood vessels, such as
calcifications, as well as features elsewhere in the body.
[0056] A tracking unit 64 communicates with tracking transmitter 38
and receives position signals from position sensors 40 and 42. The
tracking unit processes these signals in order to compute the
coordinates of ultrasound transducer 24 and antenna 32 in the frame
of reference of transmitter 38. The tracking unit may be a
commercially-available device, such as in the above-mentioned
FASTRAK system.
[0057] A guidance processor 66 receives the position (location and
orientation) coordinates from tracking unit 64 and the position
coordinates of the stent from feature detection subsystem 50.
Guidance processor 66 registers the coordinates of the stent in the
coordinate frame of the tracking unit or, equivalently, registers
the coordinates of ultrasound transducer 24 in the coordinate frame
of antenna 32, in which the stent coordinates have been found. The
guidance processor is then able to compute the geometrical skew and
offset between the present viewing axis of the ultrasound
transducer and the desired viewing axis, which will intercept the
stent along (or close to) the stent axis. Based on the computed
skew and offset, the guidance processor may drive guidance display
44 to show operator 22 the required correction. For this purpose,
the guidance display may show, for example, target crosshairs and
directional arrows, or any other suitable sort of directional
indication. Alternatively, console 28 may use the computed skew in
adjusting the Doppler velocity readings to account for the angle of
measurement relative to the flow.
[0058] Processing circuitry 36 typically comprises a combination of
dedicated hardware circuits (such as in front end 52 and digital
data acquisition unit 60) and programmable components. The front
end circuits are described in detail hereinbelow. Target detection,
measurement and tracking unit 62 and guidance processor 66
typically comprise programmable processors, such as a
general-purpose microprocessor or a digital signal processor, which
are programmed in software to carry out the functions that are
described herein. Alternatively or additionally, these elements of
circuitry 36 may comprise dedicated or programmable digital logic
units such as an application-specific integrated circuit (ASIC) or
a field-programmable gate array (FPGA). Although units 62 and 64
and processor 66 are shown, for the sake of conceptual clarity, as
separate functional blocks, in practice at least some of the
functions of these different blocks may be carried out by a single
processor. Alternatively, the functions of a given block may be
divided up among two or more separate processors.
Antenna Design and Operation
[0059] FIG. 3 is a schematic, pictorial illustration of antenna 32,
in accordance with an embodiment of the present invention. Antenna
32 is a planar ultra-wideband, unidirectional antenna, comprising
an array of antenna elements 48. The antenna is designed for
high-permittivity surroundings, enabling transmission and reception
of ultra-wideband signals to and from the human body with minimal
loss. In the pictured embodiment, the antenna comprises twelve
antenna elements 48, which are spread in a rectangular plane to
allow Cartesian acquisition of an image. Alternatively, the antenna
may comprise a larger or smaller number of antenna elements, in a
rectangular or non-rectangular array.
[0060] Each antenna element 48 comprises a planar element
comprising a conductive radiator 70, which is printed on a circuit
board 72. This circuit board serves as the front surface of
antenna, which is brought into contact with the patient's body.
Circuit board 72 comprises multiple conductive vias 74 surrounding
each radiator 70 for isolating antenna elements 48 from one
another. The antenna elements are enclosed from behind by a case
76.
[0061] FIG. 4 is a schematic, exploded view of one of antenna
elements 48, in accordance with an embodiment of the present
invention. Each antenna element is constructed as an ellipse-shaped
slotted antenna, excited electrically at its center feed point.
Circuit board 72 comprises a dielectric substrate, such as an FR4
laminate, with a component (front) side that includes radiator 70
and a ground plane 80. The radiator shape is optimized with an
elliptical template to maintain a low voltage standing wave ratio
(VSWR), with high antenna gain and flatness at boresight. This
flatness assures good coverage of the entire region of interest
(ROI) in the patient's body with constant antenna gain.
[0062] The printed (rear) side of board 72 includes an excitation
transmission line 82 feeding the center point of radiator 70
through a conductive via. Transmission line 82 comprises a
fifty-ohm microstrip, with a micro-miniature coaxial (MMCX)
connector (not shown) for connecting to front end 50.
Alternatively, other types of radiator shapes and feed lines may be
used.
[0063] A conductive cavity 84 is attached to the component side of
board 72 behind each radiator 70 in order to reduce antenna
reverberations from back-lobe scattering and to increase the
overall gain. (Antenna element 48 as shown in FIG. 4 has a nominal
gain of 7 dBi at boresight.) Cavity 84 comprises a hollow waveguide
86, with dimensions designed such that the cutoff frequency of the
lowest propagating mode (TE10) in the waveguide is higher than the
upper band frequency limit of antenna 32,
i . e . , f cutoff = C 0 / r 2 a , ##EQU00001##
wherein C.sub.0 is the speed of light, .di-elect cons..sub.r is the
permittivity of the interior of the waveguide, and a is the largest
transverse dimension of the waveguide. In the present example, with
a frequency limit of 4 GHz, the depth of waveguide 86 is 15 mm. The
waveguide creates an imaginary characteristic impedance, causing
back-lobe radiation from radiator 70 to reflect from the cavity in
phase with the back-lobe waves. This reflection enhances the
external buffering of the antenna and attenuates non-TE and TM
modes, and therefore reduces interference and noise.
[0064] Cross-coupling between antenna elements 48 can cause
interference, which reduces the dynamic range and may saturate the
receiver circuits. This cross-coupling is reduced in antenna 32 by
appropriately setting the distance between the antenna elements in
the array and by surrounding radiators 70 with conductive vias 74,
as noted above. The vias serve as an electric wall that prevents
internal waves from traveling between elements. They also create a
conductive continuity between ground plane 80 on the component side
of board 72 and the top conductive transverse plane of waveguide 86
located on the print side.
[0065] Antenna 32 is sealed against liquids and gels, thus
preventing unwanted materials from reaching the print side and
cavities of the antenna elements. Case 76, including cavities 84,
can be constructed from a molded plastic with a suitable conducting
coating. Additionally or alternatively, the antenna elements may be
printed on the molded plastic after coating. Although switching
matrix 54 is shown and described herein as a part of processing
circuitry 36, it may alternatively be incorporated into antenna 32
or mounted adjacent to the antenna, thereby performing the
switching alongside the patient and reducing the weight and
rigidity of the cable from the antenna to control unit 34.
[0066] Although antenna 32 is shown here as a unitary assembly
containing antenna elements 48, the antenna elements (of similar
design to that shown in FIG. 4) may alternatively be used singly or
as dual- or multi-element panels, which can be attached to
different body locations. Multiple position sensors can be used to
compute and register the respective positions of the antenna
elements. In such embodiments, system 20 may be configured to
measure and analyze both waves reflected from the region of
interest of the body and waves transmitted through the region and
scattered by the target.
Signal Switching and Processing
[0067] FIG. 5 is a block diagram that schematically shows details
of feature detection subsystem 50, in accordance with an embodiment
of the present invention. As noted earlier, switching matrix 54
connects antenna elements 48 to the other circuits of front end 52.
Each antenna element connects to a respective single-pole
double-throw (SPDT) switch 90, which determines whether the switch
is to transmit or to receive waves at any given time. The transmit
antenna element is selected, from among the multiple antenna
elements, by a transmit switch 92, while the receive antenna
element is selected by a receive switch 94. The switching matrix
thus permits any pair of the antenna elements to be selected as the
transmitter and receiver at any given time.
[0068] Switching matrix 54 is designed for high isolation between
channels, typically better than 40 dB over the entire frequency
range of antenna 32. Switches 90, 92 and 94 are digitally
controlled by a digital output control module 104 and allow fast
(non-mechanical) switching. This fast switching is required in
order to allow the entire waveform sequence of different antenna
pairs and frequencies to be completed in a short frame time, as
described hereinbelow. For this purpose, matrix 54 is typically
configured to achieve a switching time of less than 1 .mu.s.
[0069] Driver circuit 58 comprises a broadband signal generator 98,
which generates the RF excitation waveform to drive the
transmitting antenna elements, and a receiver 114, which receives
and demodulates the signals generated by the receiving antenna
elements. Signal generator 98 and receiver 114 are both
synchronized and sweep their frequencies according to a predefined
frequency plan, which is shown in FIG. 6, based on a shared local
oscillator 96. The frequency plan specifies the frequencies and
power levels to be generated by the signal generator, in
synchronization with an external trigger. The driving waveform
entering signal conditioning unit 56 from signal generator 98 is
sampled by a broadband coupler 100, amplified by a power amplifier
102 according to the required transmit power level, and transferred
to switching matrix 54.
[0070] Receiver 114 is a tuned super-heterodyne receiver, which is
able to adjust its bandwidth and gain according to the received
signal. The receiver demodulates the received signals coherently,
in synchronization with local oscillator 96, in order to extract
both the amplitude and the phase of the signals. The complex ratio
between the transmitted and received signals, as measured by
detection, measurement and tracking unit 62 for each antenna pair
at each selected frequency, indicates the frequency response along
corresponding paths through the region of interest. This region
includes the chest, thoracic cavity, beating heart and the stent
itself.
[0071] Despite the measures described above for reducing coupling
between different antenna elements 48 in antenna 32, the signals
received from antenna 32 by signal conditioning unit 56 may still
include a strong background component due to the direct coupling
between the transmitting and receiving antenna elements. This
background component raises the noise level due to transmitter
nonlinearity and impurities in the transmitted signal and can even
cause receiver 114 to saturate. It is therefore desirable to reduce
the level of the background component that reaches the receiver in
order to enhance the dynamic range of the radar image.
[0072] For this purpose, signal conditioning unit 56 comprises an
amplitude and phase modulator, referred to here as an IQ modulator
108, which receives the sampled RF excitation waveform from coupler
100. The IQ modulator modifies the phase and amplitude of the
sampled signal, under the control of an analog output control
module 110, so as to generate an anti-phased signal matching the
background component that is to be canceled. The amplitude and
phase values of IQ modulator 108 are periodically updated and are
then kept constant per frequency and per channel until coupling
values change significantly and need updating. In other words, IQ
modulator 108 outputs a signal that is equal in amplitude to the
background component but 180.degree. out of phase. A coupler 106
adds this anti-phased signal to the received signal from switching
matrix 54 and thus cancels the background component without
degrading the actual radar signal from the body. An amplifier 112
amplifies the signal following background cancellation for input to
receiver 114.
[0073] FIG. 6 is a timing diagram that schematically illustrates a
temporal excitation pattern that front end 52 applies to antenna
32, in accordance with an embodiment of the present invention. The
front end generates a sequence of frames 120. Each time a radar
measurement is triggered (ten times per second, for example), the
frame defines a sweep of the excitation signal both in frequency
and over spatial channels (antenna pairs). Each frame 120 comprises
multiple frequency sub-frames 122 according to the number of
frequencies to be used in image reconstruction. In the example
shown in FIG. 6, there are 128 such sub-frames, each lasting 750
.mu.s. The frequencies in this example, as noted above, span the
range between 400 MHz and 4 GHz.
[0074] Each sub-frame 122 begins with a settling time 126
(typically a few hundred microseconds) for locking the amplitude
and phase of signal generator 98. Following this initial delay,
switching matrix 54 selects different channels 124 in sequence.
Each channel uses one transmitting antenna element and one
receiving antenna element, up to a total of n channels (for
example, one hundred such channels in the example shown in FIG. 6,
each open for 5 .mu.s). During each channel period, detection,
measurement and tracking unit 62 collects samples of the received
signal from receiver 114 for subsequent use in
multi-frequency/multi-channel radar image reconstruction.
[0075] In alternative embodiments (not shown in the figures), other
sorts of channel configurations may be used. For example, in
monostatic configurations, a selected antenna element may serve as
both transmitter and receiver, as opposed to bistatic or
multistatic configurations, in which each antenna either transmits
or receives. As another option, antenna elements may simply
transmit and receive broadband RF pulses, rather than multiple
narrowband pulses as in the embodiment described above.
Method of Operation
[0076] Based on the collected samples of the received signals,
detection, measurement and tracking unit 62 detects small
reflecting volumes within the region of interest (ROI) in the
patient's body. As noted above, the corresponding reflections arise
at the boundaries of media having different dielectric properties.
The information provided by coherent detection of the signals over
the broad range of frequencies covered in each frame is equivalent
mathematically to the temporal information that would be provided
by reflection of a single short pulse. The locations of the
reflectors may be found by integrating over the propagation paths
of the reflected waves, using an inverse spherical Radon
transformation, for example.
[0077] In an embodiment of the present invention, detection,
measurement and tracking unit 62 implements a first-order
approximation of the inverse spherical Radon transform: For each
voxel (x, y, z) in the ROI and for each frequency f and pair of
antenna elements, a complex weight W(x,y,z,f,pair) is
pre-calculated, either using an empirical calibration procedure or
mathematical modeling. The weight is, in effect, the normalized
complex amplitude (with conjugated phase) of the reflection that
would be received at the receiving antenna in the pair from a point
object at location (x,y,z) when irradiated by the transmitting
antenna with a wave of frequency f. Because the body tissue through
which the waves propagate is inhomogeneous, the weights may be
adjusted, either empirically or by model calculation, to account
for the specific tissue layers (skin, fat, muscle, lungs, etc.)
through which the waves pass.
[0078] The set of weights thus derived defines a sort of matched
filter. Detection, measurement and tracking unit 62 applies this
filter to the matrix of complex signals Sig(f,pair) that it
receives in any given frame in order to compute the reflection
intensity V for each voxel, as a weighted sum over the received
signals:
V ( x , y , z ) = pair f W ( x , y , z , pair , f ) Sig ( pair , f
) ( 1 ) ##EQU00002##
The inventors have found that this simplified approximation of the
inverse spherical Radon transform is both robust and
computationally efficient.
[0079] FIG. 7 is a flow chart that schematically illustrates a
method for measuring blood flow through a stent, in accordance with
an embodiment of the present invention. The method is described
hereinbelow, for the sake of clarity, with reference to the
elements of system 20 that have been described above, but the same
techniques may similarly be implemented in other system
configurations. Furthermore, the elements of this method that
relate to locating the stent in the body of patient 26 may likewise
be applied, mutatis mutandis, for locating other features, both
natural and artificial, in the coronary blood vessels, as well as
elsewhere in the body.
[0080] Front end 52 drives antenna 32 to emit and receive RF waves
over multiple frequencies and spatial channels (antenna pairs), at
a scanning step 130, as described above. Detection, measurement and
tracking unit 62 collects samples of the received signals and
applies the weights defined in equation (1) to transform the signal
values to voxel intensities, at an image reconstruction step 132.
To improve the clarity of the image, the processing circuitry may
apply additional image processing operations, such as subtracting
the mean voxel value from all voxels in the image. The mean value
may be smoothed over multiple successive images using a recursive
filter. Unit 62 then identifies the coordinates of the target
feature, i.e., the stent or another strong reflector, in the 3D
image, at a target identification step 134.
[0081] Features in the coronary arteries (or elsewhere in the
heart), such as the stent, move regularly with the heart rhythm, as
well as with chest movement due to respiration. In order to guide
the ultrasound transducer, detection, measurement and tracking unit
62 tracks the motion of the target in the successive images, at a
target tracking step 136. For example, unit 62 may apply a Kalman
filter, as is known in the art, to estimate the motion trajectory
of the target.
[0082] Guidance processor 66 registers the coordinates of
ultrasound transducer 24 with antenna 32, at a coordinate
registration step 138. The processor uses the position coordinates
provided by position sensors 40 and 42 at this step, as explained
above. Based on these coordinates, the processor registers the
ultrasound beam in the coordinate frame of the 3D image that was
reconstructed at step 132.
[0083] Guidance processor 66 drives guidance display 44 to guide
operator 22 in aiming the ultrasound beam toward the target, at an
aiming step 140. It could be possible but would probably be
impractical for a human operator, to move ultrasound transducer 24
continually back and forth in synchronism with the motion of the
heart. (Such tracking could be feasible for a robot driven by the
processing circuitry.) To alleviate this difficulty, the guidance
processor selects a single location within the trajectory of motion
found at step 136 and guides the operator to aim at the selected
location. Console 28 measures the flow through the stent at this
location, at a flow measurement step 142, and thus provides an
indication of the extent of any restenosis.
[0084] To select the target location at step 140, guidance
processor 66 may, for example, find the center of mass of the
trajectory found at step 136, and then choose a point that is
displaced from the center toward the end of the trajectory that has
the greater dwell time, which is the diastolic end. Blood flow
through the coronary arteries occurs mainly during diastole, so
that the diastolic end of the trajectory will give the strongest
Doppler signal. Furthermore, aiming the ultrasound transducer
toward the end of the trajectory with the greater dwell permits the
ultrasonic beam to capture the stent for a longer part of each
heart cycle and thus improves the signal/noise ratio.
[0085] It will be appreciated that the embodiments described above
are cited by way of example, and that the present invention is not
limited to what has been particularly shown and described
hereinabove. Rather, the scope of the present invention includes
both combinations and subcombinations of the various features
described hereinabove, as well as variations and modifications
thereof which would occur to persons skilled in the art upon
reading the foregoing description and which are not disclosed in
the prior art.
* * * * *