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Description  |
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BACKGROUND OF THE INVENTION
A. Field of the invention
This invention constitutes a bridging of two commercially important fields
in modern medical technology, namely magnetic resonance imaging (MRI), and
extracorporeal shock wave lithotripsy (ESWL) to open up a new domain of
nonsurgical treatments. The invention is the use of the static magnetic
field of an MRI device for purposes other than imaging, specifically as a
component of electromagnetic transducers and the use of the MRI device for
automated control of procedures. The transducers may be used for the
generation of shock waves for extracorporeal lithotripsy or for other
applications of shock wave therapy or, in general for producing motion
which may be useful in medical applications.
Related Art
Destroying various targets inside a human body with shock waves requires a
specially designed instrument which is able to:
a) locate the target inside the human body;
b) generate a special acoustic pressure wave;
c) focus the acoustic pressure wave onto the target.
In the field of Extracorporeal Shock Wave Lithotripsy (ESWL) the usual
targets are renal or gall-stones. These targets first must be located and
positioned in the focal area of the shock wave generator. Then, by
applying a set of powerful acoustic shock waves through the surface of the
human body in such a way that the pressure increases at the target, the
stones can be fragmented. Weak acoustic waves travel through the soft
tissues without any damage as long as the pressure remains below a certain
level. Focusing acoustic waves produces pressure above a given threshold
to destroy targets.
Current commercial lithotripsy systems use X-ray or acoustic imaging
techniques (ultrasound) to locate the target. X-ray imaging exposes the
patient to ionizing radiation, and non-calcified stones which are most
effectively treated with ESWL are not seen. Ultrasound is limited by poor
image quality, including artifacts produced when imaging stones and stone
fragments.
There are three common methods currently employed for shock wave
generation: the spark generator, the piezo-electric array, and the
electromagnetic acoustic generator.
Spark generators are used to create powerful electric sparks to generate
shock waves. The poor focusing ability of the spark generator results in
soft tissue damage around the stone. Another drawback of this technique
stems from the rapid burn out rate of the electrodes of the spark
generator, requiring replacement after each procedure. Piezo-electric
generators build up shock waves by the displacement of a mosaic array of
piezo-electric crystals. Even with large array size and good focusing, the
shock waves achieve only a moderate pressure at the focal point, and
therefore, are unable to break up larger calculi within a reasonable time.
Electromagnetic acoustic shock wave generators have been developed for
implementation in this field. The early stage of this development is
disclosed by Reichenberger et al. in their Siemens Research and
Development Report, titled "Electromagnetic Acoustic Source for the
Extracorporeal Generation of Shock Waves in Lithotripsy." (1986, vol. 15,
187-194). The electromagnetic acoustic source lithotripter includes a
discharge capacitor as a power supply. An enameled copper wire slab coil
is suspended by a ceramic support. The coil is separated from a metallic
membrane by a thin insulating film. The coil and the conductive membrane
act as the primary and secondary windings of a transformer. Upon
application of a current to the coil via the discharge power supply, eddy
currents are induced in the membrane which result in a repulsive force
between the coil and the membrane. The membrane is thus caused to emit an
acoustic pulse which is then focused on the target by an acoustic lens.
SUMMARY OF THE INVENTION
This invention adapts the MRI device to function with a lithotripter. The
invention provides a novel solution for target localization and for
acoustical pressure wave generation. The invention uses computer-aided
real-time feedback from an image of the target to furnish positioning and
focusing information to facilitate target destruction.
The magnetic and radio frequency fields of the MRI device are utilized to
determine position or potential motion of a target (stones, tumors or
other destructible objects) by using existing MRI imaging techniques. This
provides excellent contrast of the target. Unlike known MRI devices, the
present invention includes a transducer which converts electrical energy
to mechanical energy in a form of motion or acoustical pressure involving
the static magnetic field of the MRI device. Shock waves, special
acoustical pressure waves, are generated when a charged capacitor, or
equivalent energy supply, is connected to a conductive plate located
within the MR imager's high static magnetic field. The discharge of the
capacitor starts a current flow, which interacts with the static magnetic
field and results in a force to act on the current carrier. The current
carrier is attached to an acoustically transparent material which
transmits the pressure wave, generated by the force, toward the target
located in the human body, for example. The conductive plate may be shaped
for only geometrical focusing, or alternatively, a set of plates may be
arranged to make a phased-array. In a preferred embodiment, a combination
of both methods can be used to generate a strong, well-focused acoustic
pressure wave aimed at the target.
Information available from the images is used for the control of the
procedure. Besides the localization of the target, damage to the
surrounding tissues and the mechanical effect of the acoustic pressure
wave at and around the focal point can be obtained from the images.
Variables such as repetition rate, peak pressure amplitude, size of the
focal area, the position of the focus point, and the position of the
target itself can be changed to optimize the procedure. All these control
functions can be performed by a human operator; however, the preferred
embodiment utilizes a real-time computer-controlled system.
The adaptation of an MRI device to function as a lithotripter has a number
of advantages. First, the fields of the MR imager are non-ionizing.
Second, the transducer in the MRI device can furnish large amounts of
energy in the form of acoustic pressure waves for breaking up hard targets
such as renal and gall stones or for decimation of large sized tumors and
other soft tissue targets. Other advantages of the MRI acoustic pressure
generator include the large area available within the MRI for placement of
transducers and the high magnetic field strength, thus enabling higher
energy per pulse to be generated. Better accuracy of focusing with phased
arrays under continuous computer control, and the potential to match the
size of the target with the beam area are also achieved. The tumor
decimation effect, causing cell death through cavitation, is important
because it allows the present invention to be applied to the non-surgical
treatment of tumors or other abnormal soft tissues. It may be possible to
adapt this technique to the destruction of non-tumor cells, for example
nerve cells or endocrine glands.
The use of the MRI device's static magnetic field as an alternative source
of the magnetic component of the electromagnetic transducers has other
practical applications as well. Transducers can function in a variety of
ways. For example, implanted transducers can run pumps, electric motors or
vibrators which can be housed in the human body or within catheters.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is best understood by reading the following detailed
description in conjunction with the following drawings, in which:
FIG. 1 is a schematic diagram of the way an MRI device's magnet produces
acoustic pressure waves or motion according to the present invention.
FIG. 2 shows a transducer comprised of a plurality of plates covering a
flat surface according to the present invention.
FIG. 3 shows a transducer comprised of one spherically shaped plate
according to the present invention.
FIG. 4 shows a transducer comprised of a plurality of angled plates
according to the present invention.
FIG. 5 is a diagrammatic representation of a lithotripsy system according
to the present invention.
FIG. 6 is a diagrammatic illustration of a preferred embodiment of the
present invention depicting the phased array method and a real-time
computer control system therefor.
DETAILED DESCRIPTION
In a preferred embodiment, the MRI device is modified for extracorporeal
lithotripsy. Referring to FIG. 1, a static magnetic field B.sub.O of a
magnetic resonance imaging device is represented by an arrow 102. A
transducer plate 120 is electrically connected to a local energy storage
device (a capacitor 104, for example) via a control device 106 (a
controlled switch). The patient cradle 108 is movable through all three
coordinate directions (x, y and z) and carries the patient 110 with
hardened accumulation targets 112. An acoustically transparent medium 114
acoustically connects the surface of the human body to the transducer
plate 120. The force F (116) acting on a conductor 120 of length 1
carrying current I (118) in a magnetic field of flux density B will be:
##EQU1##
In a homogenous static magnetic field, when the magnetic flux density is
B.sub.O (102) and the angle between the direction of B.sub.O and the
direction of the current I (118) is .phi., the force which creates the
acoustic pressure wave APW 122 will be:
F=B.sub.O lIsin.phi.
In the case of an incompressible fluid (e.g., water) as the acoustically
transparent medium 114, the acoustic pressure wave APW's amplitude P
produced by the plate 120 (of width w) carrying current perpendicular to
the direction of the magnetic field (.phi.=.pi./2) is:
P=B.sub.O I/w
Focusing this acoustic pressure wave may be done by forming the transducer
plate into a section of a sphere as shown in FIG. 3. Alternatively,
focusing may be achieved by cutting the plate into small pieces, driving
them with a different time-delayed current and placing them on a flat
surface (see FIG. 2 for example) or any angled surface (see FIG. 4). These
later examples are considered phased-arrays.
Suppose that in each case the total surface area covered by the source
plates is the same (A.sub.s) and the acoustic pressure wave arrives at the
target surface (of area A.sub.t) in phase. The pressure P.sub.t will be
the sum of pressures from the source plates:
##EQU2##
where P.sub.ij represents the pressure generated by the force acting on
the plate t.sub.ij, n is the number of rows and m is the number of
columns. When the number of plates N=nm, the total source surface area is:
A.sub.s =Na (if a is the unit surface area). The representation of the
loss from the source to the target c.sub.ij depends upon the distance
between the surface of the plate t.sub.ij and the target; the composition
of the medium through which the pressure waves are transmitted (i.e.,
water and tissue); and the angle between the tangent of the surface and
the direction from the source to the target. To achieve summing of the
acoustic pressure wave at the target, the phase of the arriving waves must
be the same.
In FIG. 3, only one spherically shaped plate generates the pressure waves.
The focusing is purely geometrical. The surface is perpendicular at any
point to the center of the sphere which becomes the focal point. The
pressure amplitude loss, due to angular displacement if the tangent of the
source plate is not perpendicular to the direction toward the target, is
zero when the target is positioned precisely at the center. The distance
between the source and the target is equal to the radius of the sphere.
Therefore, there is no need for time-delayed firing of multiple plates.
Another embodiment is presented in FIG. 2, where the plates are arranged to
create a flat surface. The focusing is based exclusively upon the
phased-array method. The focal point is determined by the phase of the
arriving waves, e.g., the firing sequence of the plates. The focal point
can be calculated and partially moved, but in the case of most of the
periphery plates of the array, there will be a loss due to the
non-perpendicular direction of the plates' tangent. A time delay may be
added to the firing time of plate A as compared to the firing time of the
farthest plate B. This additional time delay can be calculated from the
time necessary for the pressure wave to travel from the farthest plate B
to the target. If the wave travels everywhere with the same speed, the
time delay will only depend on the difference of distances from the i-th
plate to the target and from the farthest plate to the target. Hence,
because of the losses, the area where the focal point can be moved is
limited. The angular loss is equal to the sine of the angle between the
tangent of the plate and the direction of wave from the plate to the
target. Hence, placing the plates in a tilted position can improve
performance.
The combination of the previous methods is shown in FIG. 4. This embodiment
enjoys higher efficiency and flexibility. The plates are arranged to be on
or close to a surface of a sphere. Therefore, there is no significant loss
of power due to the inappropriate angulation of the plates. The distance
between individual surface points and the target may vary. In this case,
independent firing of the plates may be necessary to achieve a
phased-array. This enables spatial movement of the focal point and permits
the focal point to remain on the target throughout the procedure.
FIG. 5 depicts an MRI system adapted with the necessary tools for
production and control of shock waves on a target in a human patient. The
MRI system comprises a magnet 502 with gradient and RF coils, a patient
cradle 504 on which the patient is positioned, a spectrometer 506 executes
the MRI procedure, and computer system 508 for the control of the
procedure, image reconstruction and display. The components of the system
necessary for shock wave generation and targeting comprise plates 520 each
with its driver(s), firing control unit 516, with a power supply 518 and a
coupling balloon 514.
The treatment procedure may be better understood with reference to FIG. 6.
In FIG. 6 reference numerals 606, 608, 610, 612, 614 and 620 correspond to
reference numerals 506, 508, 510, 512, 514 and 520 in FIG. 5,
respectively. Two driver control units U.sub.i and U.sub.j (630 and 632,
respectively) are also shown in FIG. 6. Driver control units 630 and 632
represent sections of the firing control unit 516 of FIG. 5, which, in one
embodiment, are used to drive individual plates 620. The computer system
608 invokes an MRI procedure with the spectrometer 606 under control of an
operator. The basis of the procedure is the following: all MRI systems
apply the same principles (reference General Electric Medical Systems
Signa System operator manual OMS2 Rev. 12); MRI systems employ magnetic
field gradients along all three orthogonal axes (x, y, z) in a sequence.
The magnetic field gradient slightly alters the strength of the main
magnetic field. Each type of nucleus has a unique gyromagnetic ratio
(.gamma.). This ratio, times the flux density of the magnetic field (B),
determines the nucleus precessional frequency (f). The main magnetic field
is altered by the field of the gradients, as the precession frequency is
altered. Thus spatial location of a nucleus can be determined from the
response to the radio frequency excitement, under a sequence of gradients.
The coordinates of the target T(x,y,z) 612, available from the image
display 630, directly correspond to the real spatial coordinates of target
T(xyz) 612. The position of the transducer plates (with the driver
circuits) 620, is also known by the computer system. The position of the
transducer plates is either measured previously or determined from images,
as would become apparent to those working in the art.
Due to the inhomogeneities of the main magnetic field and/or nonlinearity
of the gradients, some correction is necessary when calculating the image
coordinates to correspond to the spatial coordinates. This correction is
either provided by the manufacturer of the magnet or can be determined by
measurements. An article titled "Correction of Spatial Distortion in MR
Imaging: A Prerequisite for Accurate Stereotaxy" authored by Schad, Lott,
Schmitt, Sturm and Lorentz, in the May/Jun. 1987 Journal of Computer
Assisted Tomography, 11(3):499-505 discusses the correction commonly used
in the MRI-guided stereotactic surgery. Its disclosure is incorporated
herein by reference as if reproduced in full below.
Once the target is identified with an electronic pointer on the image
display, the spatial coordinates of the target will be available after
computer-aided calculations. Repeated measurements can reveal the motion
of the target, and the different travel path of the acoustic pressure wave
from each of the elements of the transducer can be compensated in the
firing sequence. A plate p.sub.i 622, for example, is a distance d.sub.i
far away from the target. If the average speed of the shock wave of this
travel path be v.sub.i, and another plate p.sub.j 624, with parameters
d.sub.j and v.sub.j, the time t.sub.i for the shock wave to travel from
the i-th plate to the target will be:
t.sub.i =d.sub.i /v.sub.i
and for the j-th:
t.sub.j =d.sub.j /v.sub.j
Thus the time difference t.sub.ij between the firing of these plates is:
t.sub.ij =t.sub.i -t.sub.j
and the simultaneous arrival of the shock wave fronts at the target can be
achieved.
Cradle motion in MRI systems is also electronically controlled and a target
organ or area found on localizing images can therefore be positioned in a
desired area delimited by the shock wave generator. The supply voltage
from the power supply 518 charges the capacitors, found in the driver
circuits 520 between firings. A switch element closes the charged
capacitor to the plate at the command of the firing control unit 516. The
generated acoustic pressure waves must first pass through coupling balloon
514, to reach the surface of the human body 510. At this point the
pressure amplitude must be below the pain threshold. On the surface and/or
inside of the target 512, the acoustic pressure wave forms a shock wave
due to waves arriving in phase. The effect of each and/or a set of shock
waves alters the target and for the next firing changes can be implemented
based on images made during, interleaved, or after the firings. Specially
designed software running on the MRI system computer (and/or on a separate
computer) can make these calculations and perform real-time system control
as realized by practitioners in the field.
Table 1 has been included to show technical parameters for an experimental
model and a clinical system. These values are set forth as exemplary and
are not meant to limit the practical scope of the invention.
TABLE 1
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Technical Parameters
Experimental model
Clinical system
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Power supply voltage:
6000 V (2 .times. 20) * 6000 V
Transducer resistance:
1 Ohm (2 .times. 20) * 0.1 Ohm
Transducer 1 mH <0.1 mH
inductance:
Current peak 4000 A 50000 A
amplitude:
MRI field strength:
0.8 T 1.5 T
Transducer surface
area (A.sub.s);
length: 0.7 m 20 .times. 0.05 = 1 m
width: 0.1 m 2 .times. 0.05 = 0.1 m
Target surface 0.01 m .times. 0.01 m
8 area (A.sub.t):
A.sub.s /A.sub.t :
700 1000
Calculated pressure
amplitude
at the transducer:
32 kPa 750 kPa
at the target
22.4 MPa 750 MPa
Measured pressure at
3.2 +/- 50% MPa
the theoretical focal
point:
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While the invention has been described with reference to particular
embodiments thereof, those skilled in the art will be able to make various
modifications to the described embodiments of the invention without
departing from the true spirit and scope thereof. It is intended that MRI
systems with electromagnetic transducers and methods which are equivalent
to those described herein in that the various elements or steps perform
substantially the same function in substantially the same way to
accomplish the same result are within the scope of the invention. For
example, the transducer design may comprise single-turn wire coils of a
flat design. This provides a very low inductance-resistance ratio. A low
inductance/resistance ratio means the current in the coils can be turned
on very quickly. Fast turn-on means acoustic shock waves can be generated
with very short wavelengths. Short wavelengths permit focusing on small
objects by optimizing rise time, duration and amplitude of the waves. Such
switching may be accomplished by high-voltage, high-amperage solid-state
devices.
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