U.S. patent application number 11/436584 was filed with the patent office on 2007-11-22 for automatic atherectomy system.
Invention is credited to Daniel Gelbart, Samuel Victor Lichtenstein.
Application Number | 20070270688 11/436584 |
Document ID | / |
Family ID | 38712836 |
Filed Date | 2007-11-22 |
United States Patent
Application |
20070270688 |
Kind Code |
A1 |
Gelbart; Daniel ; et
al. |
November 22, 2007 |
Automatic atherectomy system
Abstract
An automatic atherectomy system uses the rotary burr at the tip
of a catheter as a sensing device, in order to measure both the
electrical conductivity and permittivity of the surrounding tissue
at multiple frequencies. From these parameters it is determined
which tissue lies in the different directions around the tip. A
servo system steers the catheter tip in the direction of the tissue
to be removed. In non-atherectomy applications the rotary tip can
be replaced with any desired tool and the system can be used to
automatically steer the catheter to the desired position. The
steering is done hydraulically, by pressurizing miniature bellows
located near the catheter tip.
Inventors: |
Gelbart; Daniel; (Vancouver,
CA) ; Lichtenstein; Samuel Victor; (Vancouver,
CA) |
Correspondence
Address: |
DAN GELBART
4706 DRUMMOND DR
VANCOUVER
BC
V6T-1B4
US
|
Family ID: |
38712836 |
Appl. No.: |
11/436584 |
Filed: |
May 19, 2006 |
Current U.S.
Class: |
600/427 |
Current CPC
Class: |
A61B 2017/00026
20130101; A61B 2017/00084 20130101; A61B 5/053 20130101; A61B
2017/003 20130101; A61B 2017/00539 20130101; A61B 17/320758
20130101; A61B 2017/320004 20130101; A61B 2017/00022 20130101; A61B
5/0538 20130101 |
Class at
Publication: |
600/427 |
International
Class: |
A61B 5/05 20060101
A61B005/05 |
Claims
1. A method of automatically controlling a tool for removing
non-desired tissue from the body without harming desired tissue,
said method comprising the steps of: discriminating between desired
and non desired tissue based on sensing the properties of the
tissue adjacent to the tool tip; automatically steering said tool
in the direction of the non-desired tissue; and eliminating the
non-desired tissue.
2. A method for automated removal of plaque from blood vessels by
the use of a rotary tool, said method comprising the steps of:
discriminating between blood vessel walls, blood and plaque based
on sensing the properties of the tissue adjacent to the tool tip;
automatically steering said tool in the direction of the plaque;
and removing the plaque by rotary abrasion.
3. A method of automatically steering a catheter in a body lumen,
said method comprising the steps of: discriminating between the
wall of said lumen and the inside of said lumen based on sensing
the properties of the tissue adjacent to tip of said catheter;
automatically steering tip to avoid damaging said wall.
4. A method as in claim 1 wherein said discrimination is based on
electrical properties.
5. A method as in claim 2 wherein said discrimination is based on
electrical properties.
6. A method as in claim 3 wherein said discrimination is based on
electrical properties.
7. A method as in claim 1 wherein said discrimination is based on
mechanical properties.
8. A method as in claim 2 wherein said discrimination is based on
mechanical properties.
9. A method as in claim 3 wherein said discrimination is based on
mechanical properties.
10. A method as in claim 1 wherein said discrimination is based on
thermal properties.
11. A method as in claim 1 wherein said discrimination is based on
the electrical properties of conductivity, permittivity and changes
of permittivity with frequency.
12. A method as in claim 2 wherein said discrimination is based on
the electrical properties of conductivity, permittivity and changes
of permittivity with frequency.
13. A method as in claim 1 wherein said steering is done
hydraulically.
14. A method as in claim 2 wherein said steering is done
hydraulically.
15. A method as in claim 3 wherein said steering is done
hydraulically.
16. A method as in claim 1 wherein said steering is done by using a
magnetic field external to the body.
17. A method as in claim 2 wherein said steering is done by using a
magnetic field external to the body.
18. A method as in claim 3 wherein said steering is done by using a
magnetic field external to the body.
19. A method as in claim 2 wherein said discrimination is based on
the fact that plaque has lower electrical conductivity than blood
or blood vessel wall, and the electrical permittivity of blood
vessel walls falls faster with frequency than the permittivity of
blood.
20. A method as in claim 2 wherein the rotary action of the tool is
used to scan the different tissues surrounding the tool.
Description
FIELD OF THE INVENTION
[0001] The invasion relates to medical devices and in particular to
procedures in which an undesired tissue has to be removed without
harming an adjacent desired tissue, such as in atherectomy.
BACKGROUND OF THE INVENTION
[0002] In many medical procedures an undesirable tissue is adherent
or touching a desired tissue and the removal of the undesired
tissue has to be done with extreme caution in order not to harm the
desired tissue. A well known example is atherectomy, the process of
removing plaque from blood vessels. The most common method of
atherectomy is based on the use of a high speed rotary burr,
mounted at the end of a very flexible catheter. The burr pulverized
the plaque into such fine particles that they can be left in the
blood stream. A well known system is manufactured by the Boston
Scientific company (www.bostonscientific.com) under the name
Rotablator.TM.. No further data is given here about this system as
it is a well known commercial system. Other potential uses of the
invention are removal of tumors, such as prostate cancer,
liposuction, dental work and more. Today most these procedures are
performed by a surgeon is manipulating a surgical tool (directly or
remotely) while observing the tool position using means such as
fluoroscopy or ultrasound, or by tactile feel. In some procedures
there is no need to remove tissue but there is still a need to
navigate within the body, such as directing a catheter through the
blood system. The present invention can save the majority of the
surgeon's time and operating room expenses
[0003] The present invention provides an automated way to navigate
within the body and remove undesired tissue without doing any harm
to desired tissue, even in situations that the undesired tissue is
adherent. The same invention can be used for just navigation,
without tissue removal. The preferred embodiment shown is
atherectomy. In atherectomy there is a need to differentiate
between plaque and blood vessel wall. It is well known that plaque
has different electrical properties than blood vessel wall; however
the blood vessels are full of blood which has electrical properties
similar to the vessel wall. In order to automate atherectomy a
discriminator between vessel wall, plaque and blood is required.
Also, it is desired to sense proximity to a vessel wall, not just
contact. The present invention provides exactly this capability. A
similar situation exists in some tumor removal procedures: some
tumors have different electrical properties than healthy tissue but
the in-situ measurement of these properties is complicated by the
fact that the voids left in the process of tissue removal are being
filled with fluids which affect the measurements. Prior at attempts
to automate atherectomy relied on a guide wire (which can not be
used in case of complete occlusion) or on devices to help the
surgical tool glide in the correct trajectory within the blood
vessel. Since the plaque can be softer or harder than the vessel
wall, it is very difficult to rely on such "self steering" methods.
The current invention identifies the different materials
surrounding the rotary burr at the tip of the atherectomy catheter
and automatically steers the burr to remove the undesired tissue,
such as plaque.
SUMMARY OF THE INVENTION
[0004] The invention uses the tip of a catheter as a sensing
device, in order to measure both the electrical conductivity and
permittivity of the surrounding tissue at multiple frequencies.
From these parameters it is determined which tissue lies in the
different directions. A servo system steers the catheter tip in the
direction of the tissue to be removed. In non-atherectomy
applications the rotary tip can be replaced with any desired tool
and the system can be used to automatically steer the catheter to
the desired position. The steering is done hydraulically, by
pressurizing miniature bellows located near the catheter tip.
[0005] In general, the invention can be used for a broad range of
applications as the invention does not rely on the type of
procedure used. It can be used with rotary burrs, stents, guide
wires, suction, electro-surgery etc.
[0006] In atherectomy there is a need to differentiate between at
least three types of tissue: vessel wall, plaque and blood. Both
vessel wall and blood have high conductivity and high permittivity,
while plaque has low conductivity and permittivity. The key for
differentiating blood from vessel wall is the change in
permittivity with frequency: the permittivity of the vessel wall
falls much faster as the frequency increases.
[0007] Other features and advantages of the invention will become
apparent by studying the description of the preferred embodiment in
conjunction with the drawings.
DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is a schematic view of the invention.
[0009] FIG. 2 is an isometric close-up view of the catheter
tip.
[0010] FIG. 3 is a cross section of the catheter tip showing the
steering method.
[0011] FIG. 4 is a schematic diagram of the tissue
discriminator.
[0012] FIG. 5 is a graph of the different waveforms produced by the
discriminator.
[0013] FIG. 6 is an isometric view of the actuation mechanism.
[0014] FIG. 7 is an isometric view of an alternate sensing
method.
DETAILED DESCRIPTION OF THE INVENTION
[0015] Referring to FIG. 1, a blood vessel 1 having a wall 2
contains undesired plaque 3 as well as blood 4. A. atherectomy tool
5 is introduced using catheter tube 6. The tool is driven by air
motor 8 via flexible rotating cable 7. No further details of the
atherectomy system are given, as these are well known commercial
systems such as the Rotablator.TM. system. An electrical contact 9
measures the electrical impedance between cable 7 and the return
path which is ground (the patient is electrically grounded). The
discriminator 10 measures the complex impedance to ground by
measuring the In-Phase current (I) and the Quadrature, or 90 deg
out of phase current (Q). From these measurements the conductivity
and permittivity of the tissue can be computed, based on the well
known methods of electrical impedance measurements. A full
explanation is given later. Based on the measured value, the type
of tissue is determined by computer 12 and the catheter is
automatically steered by hydraulic actuator 13 (via tubes 14) to
remove the undesired plaque 3. As it approaches the wall 2, the
electrical properties start changing allowing precise and gentle
steering and removal up to the wall 2 but without actually touching
the wall. This is possible as the measured properties are also a
function of tissue thickness, so when the plaque becomes very thin
the properties of the underlying layer are showing through. In
order to determine the rotational orientation of burr 5 a sense
wire 11 is used in conjunction of a conductive strip on burr 5.
[0016] Referring now to FIG. 2, the rotary burr 5 has a standard
diamond powder coating 15 and is rotated at high speed by cable 7.
It is made from electrically insulating material such as ceramic,
with the exception of metallized strip 16. Alternatively, it can be
made of metal and coated with a hard ceramic coating except for
strip 16. A sense wire 11 terminates with tip 21 very close to burr
5. Once per rotation strip 16 comes close to tip 21. This point can
be detected by measuring the electrical impedance between wire 11
and cable 7. A sharp drop signifies this point, which is used as a
rotational reference point. Catheter 6 contains five channels on
top of the central channel used by cable 7. Channel 18 is used for
sense wire 11. Channels 17 are used to steer burr 5 in the desired
direction by inflating sections of bellows 20. Referring now to
FIG. 3, bellows 20 is divided into four separate sections 19
connected to channels 17. Pressurizing a section will cause it to
elongate, bending bellows 20 in the opposite direction. Fluid 22 is
a saline solution or pure water. For lower electrical noise the
outside of catheter 6 is metallized with a very thin coating 23. A
sub-micron thickness, deposited by sputtering or evaporation, is
sufficient. Such a thin coat does not affect flexibility.
[0017] The discrimination of tissue types is shown in FIG. 4. To
discriminate plaque from the wall of a blood vessel by electrical
properties is relatively easy and well known in the medical
literature, as plaque has a higher electrical impedance (both lower
conductivity and lower permittivity). It is more difficult to
differentiate the wall from the blood filling the vessel, as both
have high conductivity and high permittivity. However, the
permittivity of the wall falls much faster (by about a factor of
1000 faster) with frequency. This can be seen from the following
table. While the cited values differ between studies, all studies
show that permittivity of blood falls much slower than permittivity
of the vessel wall as frequency increases.
TABLE-US-00001 log.sub.10(Freq) 3 5 6 7 8 Conductivity (S/m) Blood
0.70 0.70 0.70 1.00 1.49 Fat 0.025 0.025 0.030 0.040 0.060 Muscle
0.40 0.40 0.40 0.40 0.75 Fibrous 0.24 0.24 0.24 0.29 0.33 Material
Calcium 0.08 0.08 0.10 0.12 0.17 Vessel Wall 0.58 0.58 0.58 0.67
0.83 Relative Permittivity Blood 4100 4000 2000 300 75 Fat 20000
100 50 30 12 Muscle 400000 10000 8000 200 70 Fibrous 2000 500 50 5
3 Material Calcium 10500 500 250 70 30 Vessel Wall 100000 5000 4000
100 30
[0018] The impedance of the tissue to ground (the patient is
grounded) is shown schematically as impedance 24. A current is sent
from oscillator 26 via resistor 25, contact 7, cable 7 and burr 5
to the tissue impedance 24. The lower the impedance 24 the lower
the voltage at contact 9 will be. Both the in-phase component I and
the quadrature component Q is measured buy any one of the standard
methods of AC impedance measurement. By the way of example, the I
component is found by multiplying output f1 of oscillator 26 with
the voltage senses at contact 9 using an analog multiplier 30. The
Q component is found by multiplying the same voltage with the
output of f1 shifted by 90 degrees by phase shifter 29. The output
of the multipliers are filtered by capacitors 31 and converted to
digital by A/D converters 38. This is the standard sine and cosine
separation method for finding the conductivity and permittivity
components of a complex impedance. For frequencies below a few MHz,
the voltage at contact 9 can be digitized and the derivation of the
conductivity and permittivity can be done completely via digital
signal processing. In order to generate the rotational reference
pulse, the point when the voltage of sense wire 11 drops each
revolution has to be found. The actual voltage can vary over a wide
range, depending on the surrounding tissue, but the dip is always
when the conductive strip 16 (see FIG. 2) is nearest to tip 21. By
comparing the average voltage at wire 11 to the instantaneous
voltage, the reference point is found independent of voltage.
Signal f1 is fed to sense wire 11 via resistor 32. The sine wave
envelope is detected by diode 33 and capacitor 34. The average is
derived by resistor 35 and capacitor 36. Comparator 37 generates a
positive output when the instantaneous value is below the average
value. Again, the reference pulse generation can also be digital if
the signal on wire 11 is digitized from the start.
[0019] Since the rate of change of the permittivity with frequency
is required, at least two frequencies have to be used, three would
be even more accurate. These are generated by oscillators 26,27 and
28. For each frequency the circuit shown has to be replicated. It
is also possible to use a single variable frequency source and
single detection circuit and multiplex the detection process.
[0020] A typical discriminator output is shown in FIG. 5. Graph 43
is the amplitude of the sinewave at sense wire 11, used to generate
the rotational reference. Graph 39 shows the conductivity, derived
from the I component (the higher the conductivity the lower the I
component will be). Graph 40 shows the permittivity, derived from
the Q component (the higher the permittivity the lower the Q
component will be). Graph 41 shows the permittivity at a much lower
frequency. The horizontal scale is in degrees relative to the
reference pulse, which is created when tip 21 is nearest to
conductive strip 16. In this example tip 21 is drawn close to the
vessel wall. Using just the data at f1, it is difficult to tell the
wall (0-90 degree range) from blood (above and below burr 5, at
90-180 degree range and 270-360 degree range). At the second
frequency f2, the permittivity in the wall area increases much
faster than in the blood area. The plaque is easy to spot as it has
much lower conductance and permittivity. Using the values of table
1 for frequencies of 1 KHz and 10 MHz, the conductivity and
permittivity of plaque (fat+calcium+fibrous material) are below 0.1
S/m and 50, while blood is above 0.7 S/m at both frequencies and
wall is above 0.58 at both frequencies. The permittivity of the
wall is much higher than blood at 1 KHz (100,000 vs. 4100) but
falls much faster at 10 MHz, dropping a factor of 1000 for the wall
but only a factor of 14 for blood. This example shows that by using
just three factors: conductivity, permittivity and ratio of
permittivity at 10 MHz to 1 KHz the three tissues can be
discriminated with a large margin. Adding a third frequency f3
increases the accuracy. Note that the rotational speed of the burr
5 is about 1-3 KHz. For oscillator frequencies below that, the
results will have to be sampled and integrated over many rotations.
This is not a problem, as the steering is done at a much lower
bandwidth than the measuring. An alternative is use a frequency of
about 100 KHz as the lowest oscillator frequency. A second
alternative, shown in FIG. 7, is to replace the rotary tissue
sensing by four sense wires 11 instead of a single one, and have
each one connect to a discriminator. Each one of the wires
corresponds to one actuator direction. The advantages are:
1. A completely standard burr can be used, however sensing does not
extend tip of burr.
2. System can be used for applications not requiring rotary
burrs.
[0021] 3. Only low frequency processing is required, as processing
can be done at the steering bandwidth instead of the rotation
speed. Steering bandwidth is below 100 Hz.
[0022] The catheter has four actuation channels 17 and four sense
wires 11 terminating in four tips 21. If a burr is used, tips can
protrude to partially envelope burr.
[0023] The computer 12 in FIG. 1 performs the discrimination
between tissues based on the rules shown above and steers the burr
5 into the undesired tissue, in this case plaque. Clearly the
decision rules and parameters will change with the application and
the tissue used. A large data base of impedance data for a large
number of tissues is posted on the Italian National Research
Council website at:
http://niremf.ifac.cnr.it/cgi-bin/tissprop/htmlclie/uniquery
[0024] There are similar data bases available on the internet for
properties of malignant tumors versus healthy tissue.
[0025] The hydraulic actuators 13 are shown in FIG. 6 in
conjunction with FIG. 1. A motor, such as a stepper motor, 48 is
driven from computer 12 via a standard interface. A piston 45 is
moved in a cylinder 44 via the action of a thread 46 and a mating
female thread 47. The pressure is transmitted via hypodermic tubing
14 to channel 17 (not shown) in catheter 6. Four identical units
are used for +X, -X, +Y and -Y.
[0026] By the way of example, burr 5 is a standard burr with an
external diameter of between 1.5 to 2.5 mm. Because the system is
automated a single small burr can be used for all blood vessel
sizes, as the computer will steer the bar in all radial directions
to clean a large vessel. Catheter 6 is slightly smaller than burr
5. Diameter of piston 45 is 1-2 mm and stroke is about 10 mm.
Piston 45 and cylinder 44 are made of very hard material, such as
alumina, ruby or tungsten carbide, with a lapped fit. The pressure
of the fluid is fairly high, typically 50-100 Kg/cm2. Typical
component values for the discriminator 10 are: frequencies in the 1
KHz to 1 GHz range, time constants of filter 31 of about 10-100 uS,
time constant of capacitor 34 of 10-100 uS, time constant of
capacitor 36 and resistor 35 of 10-100 mS.
[0027] An alternate way of steering is by using push-wires in
channels instead of a liquid. The actuators and catheter are very
similar to the ones discussed earlier.
[0028] Still another way of steering is use catheter tips made of
ferromagnetic material and have a controlled external magnetic
field. A variation is a system having a fixed external field and a
catheter tip carrying three orthogonal coils to generate a force in
any desired direction. This is available as a commercial system
under the trademark Niobe. It is sold by the Stereotaxis
corporation (vwww.stereotaxis.com).
[0029] While the preferred embodiment relates to atherectomy and
used electrical impedance sensing other applications and other
sensing methods are part of this invention. By the way of example,
different tissues can be discriminated by their mechanical
properties such as stiffness, hardness and damping. This can be
sensed by a vibrating tip. Tissues can also be discriminated by
thermal properties. A tip similar to FIG. 7 can carry four
temperature dependent resistors instead of sensing tips 21. A
constant current is passed through resistors and their temperature
is measured by the voltage drop across them. Different tissues have
different heat conductivities: plaque will conduct less than blood
vessel wall while blood will conduct heat rapidly, as convection
exists.
[0030] Also, the word "automatically" in this disclosure and claims
should be broadly interpreted, from a simple assist to the surgeon
in operating surgical systems to fully unattended operation of such
a system. In the minimal version the surgeon fully controls the
system; the tissue discriminator just assists the surgeon in the
decision and operation of the atherectomy or other system. In a
fully unattended operation the catheter can also be automatically
advanced into the body and can be programmed to enter the correct
blood vessel when coming to a junction point where there are
multiple choices of routes. In the same manner, the "tool" or
"catheter tip" should be broadly interpreted to include both
contact tools (burrs, rotary wires, blades, suction,
electro-surgery etc) as well as non contact tools (lasers,
water-jet, gas jet etc).
* * * * *
References