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FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to a system for causing ablation of a target
material of living tissue while not causing damage below a predetermined
depth. The novel system is particularly useful using a carbon dioxide
laser.
When using a laser for ablating tissue it is desirable to deliver maximum
power density to the tissue to be ablated while minimizing temperature
rise in adjacent tissue, particularly in the tissue underlying the tissue
to be ablated preventing necrosis in such underlying tissue. Such a
temperature rise in underlying tissue may cause thermal damage or
carbonization, which generally results in increased scarring and healing
time. For this purpose, surgical lasers used for tissue ablation are
usually operated with short pulses to deliver high energy in short periods
of time. Various pulsing techniques have been developed for this purpose,
in which the energy applied for ablation is varied by changing the pulse
repetition rate, pulse duration, and/or pulse energy.
Generally, it is desirable to provide power density of at least 40
watts/mm.sup.2 in order to obtain ablation. This power density must be
provided, however, for a short enough period of time so the ablation is
without carbonisation, and to minimize thermal damage below a depth of 50
micrometers. At the same time, it is desirable to have a spot diameter on
the tissue of at least 3 mm to allow for controllable ablation, since a
smaller diameter is more likely to produce holes rather than uniform
tissue removal. In the pulse technique for operating a laser, however,
these desirable characteristics oppose each other.
In this regard it is generally desirable to expose the tissue to pulses of
less than 1 msec to minimize the depth of thermal damage, and to provide
at least 0.1 sec between pulses to allow the tissue to cool down, while at
the same time to provide an average power of not less than 20-30 watts to
reduce the surgery time. However, in the pulse technique for operating a
laser, these desirable characteristics also oppose each other.
Various prior art techniques are known is which a target material is
scanned with laser radiation to selectively cause necrosis of the target
material. Such prior art uses lasers that are absorbed nonuniformally by
the target material so as to cause the selective necrosis. One such prior
art teaching is U.S. Pat. No. 4,733,660 which issued to Irving Itzkan on
Mar. 29, 1988 and is entitled Laser System for Proving Target Specific
Energy Deposition and Damage.
A second such prior art teaching is in an article entitled Hexascan: A New
Robotized Scanning Laser Handpiece by D. H. McDaniel et al, which appeared
in Volume 45 of CUTIS page 300 in May of 1990.
OBJECTS AND BRIEF SUMMARY OF THE INVENTION
An object of the present invention is to provide a novel laser system
having advantages in the above respects when used for ablating a surface.
A system is provided for causing ablation of a target material of living
tissue while not causing damage below a predetermined depth which includes
a laser which generates a beam of laser radiation to be uniformly absorbed
by the target material; a scanner for moving the beam of laser radiation
in a predetermined pattern on the target material so that the "elements"
of the target material are sequentially irradiated; and the rate at which
the scanner moves the beam of laser radiation in the predetermined pattern
is controlled so that ablation is caused uniformly on the target material
but only to a predetermined depth.
In one embodiment of the system the scanning rate and predetermined pattern
is such that each of the "elements" of the target material experiences a
predetermined minimum time interval between applications of radiation
thereto. In the preferred embodiment the predetermined minimum time
interval is 0.1 seconds and the laser beam has an average power of at
least 40 watts/mm.sup.2.
In the preferred embodiment of this invention the scanner causes the beam
of laser radiation to trace Lissajous figures over said target material.
In a further embodiment of this invention wherein the scanned laser beam
defines a solid cone with a circular base projected onto the surface of
said target material wherein the circular base projected onto the target
material has a radius of at least 1.5 mm and the laser beam is focussed to
a radius of no larger than 0.25 mm on the target material.
In the preferred embodiment of this invention the laser for generating the
beam of laser radiation is a carbon dioxide laser.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is herein described, by way of example only, with reference
to the accompanying drawings, wherein:
FIG. 1 illustrates one form of laser system constructed in accordance with
the present invention;
FIGS. 2-7 are diagrams helpful in explaining the Lissajous figures produced
by the scanning system in the laser of FIG. 1;
FIG. 8 illustrates a hand-held laser apparatus constructed in accordance
with the present invention for use in free-hand surgery; and
FIG. 9 illustrates another laser apparatus constructed in accordance with
the present invention particularly useful for microsurgery.
DESCRIPTION OF PREFERRED EMBODIMENTS
The Overall Method and System
FIG. 1 illustrates the main components of a laser system constructed in
accordance with the present invention for use in ablating tissue, shown at
T. Thus, the illustrated system includes a laser 2 which produces a
continuous laser beam 4. In the preferred embodiment of this invention the
laser 2 is a carbon dioxide laser. The continuous laser beam is applied to
a laser scanner system, generally shown by box 6, which cyclically scans
the beam along two orthogonal axes to cause the beam to trace Lissajous
figures, shown generally at 8 in FIG. 1, over the tissue T to be ablated.
The laser beam leaving the scanning system 6 first passes through a
focussing lens 10 which focusses the laser beam on tissue T.
The scanning system 6 includes two mirrors 12, 14, each rotated by a motor
M.sub.1. M.sub.2. These mirrors are so located with respect to the laser
beam 4 and also to each other to cyclically scan the laser beam along two
orthogonal axes, and to cause the beam to trace the Lissajous FIGS. 8 over
the tissue T to be ablated.
Production of the Lissajous Figures
The manner in which the Lissajous FIGS. 8 are produced by the scanning
mirrors 12, 14 will now be described particularly with reference to FIGS.
2-7.
FIG. 2 illustrates a system including motor M1 rotating at angular velocity
.OMEGA.1 about an axis defined by the normalised vector B1. A mirror
(e.g., 12, FIG. 1) is fixed to the motor such that its normal, defined by
N1, lies at an angle of .theta./2 to the rotating axis B1. As the motor
rotates, the vector N1 defines a cone of half angle .theta./2. The axis of
symmetry of the cone is defined by the vector B1. A ray, defined by vector
A, impinging on the mirror at an angle of 45.degree. to axis B1 will,
according to the laws of reflection, produce reflected rays described by
the time dependent vector C1(t). This vector C1(t) traces an envelope of a
cone with an elliptical base. The vector Z1, which represents the axis of
this cone, lies in the plane defined by vectors A and B1. The angle
between vectors Z1 and B1 is also 45 degrees.
A Cartesian coordinate system based on the three vectors X1, Y1 and Z1 may
now be defined. The origin of this coordinate system is represented by
0.sub.1 in FIG. 2. The vector X1 lies in the plane containing vectors A,
B1 and Z1, and is perpendicular to vector Z1. The direction of vector Y1
is perpendicular to vectors X1 and Z1.
The projections of the reflected rays can now be described by the following
equations:
a.sub.x1 (t)=.THETA. cos (.OMEGA..sub.1 t+.delta..sub.1) Eq. 1
a.sub.y1 (t)=.THETA./.sqroot.2 sin (.OMEGA..sub.1 t+.delta..sub.1)Eq. 2
where:
a.sub.x1 (t) is the angle of the projection C1(t) in the plane X1-Z1;
a.sub.y1 (t) is the angle of the projection of C1(t) on the plane Y1-Z1;
and .delta.1 is an arbitrary phase which defines the angles a at time t=0.
The relatively large displacement associated with amplitude .THETA. in
equation (1) lies in the plane containing vectors A and B1. The smaller
amplitude .THETA./.sqroot.2 of equation (2) is in the direction of vector
Y1.
Now can be added a second motor M2 (FIG. 3), whose axis is defined by
vector B2 rotating with angular velocity .OMEGA.2. A mirror (e.g., 14,
FIG. 1) whose normal is N2(t) is fixed to motor M2 forming an angle of
.THETA./2 between normal N2(t) and vector B2 (as in motor M1). Motor M2
will be aligned such that the axis of vector B2 lies at 45.degree. to the
axis of vector Z1. Vector B2 also lies in the plane defined by vectors Z1
and Y1. As a result, there is obtained reflected rays C2(t) which form a
solid cone with a circular (not elliptical) base. The axis of symmetry Z2
of this cone lies at 45.degree. to the axis B1 of the motor M1 and in the
plane defined by vectors Z2 and B2.
A new Cartesian coordinate system may now be defined having an origin at
0.sub.2 (see FIG. 3). Vector X2 is perpendicular to vector Z2 and lies in
the plane defined by vectors B2 and Z2. Vector Y2 is perpendicular to
vectors X2 and Z2.
The larger amplitude always exists in the X direction and the smaller
amplitude in the Y direction. The two motors M1, M2 are aligned in such a
way that the X direction of motor M1 combines with the Y direction of
motor M2, and the Y direction of motor M.sub.1 combines with the -X
direction of the second motor. In this way amplitude compensation is
obtained, resulting in a cone with a circular (not elliptical) base.
All the rays C2(t) exiting from the second mirror (e.g., 14, in FIG. 1) are
defined by the following equations:
##EQU1##
Assuming .delta.1=-90.degree. and .delta.2=0 then:
a.sub.x2 (t)=(.THETA./.sqroot.2) cos (.OMEGA.1.multidot.t)+.THETA.
cos(.OMEGA.2t) Eq. 5
a.sub.y2 (t)=.THETA. sin (.OMEGA.1.multidot.t)+(.THETA./.sqroot.2) sin
(.OMEGA.2.multidot.t) Eq. 6
The angle of the exiting rays formed with axis Z2 can exist between zero
and (.THETA.+.THETA./.sqroot.2). Thus the rays fill the whole area of the
base of the cone whose half angle is defined by
(.THETA.+.THETA./.sqroot.2).
A ray which is focussed by a lens of focal length "f" (e.g., lens 10, FIG.
1), will be displaced at the back focal plane of the lens by an amount
a.f, where a is the angle subtended by the ray and the optical axis of the
lens (see FIG. 4).
If a lens is placed perpendicular to axis Z.sub.2 (FIG. 5), a time
dependent ray pattern will be produced at the focal plane of the lens (of
focal length f), given by the following equations (see FIGS. 5 and 6):
.sub.x2 (t)=fa.sub.x2(t) =f(.THETA./.sqroot.2) cos (.OMEGA..sub.1
*t)+f.THETA. cos (.OMEGA..sub.2 *t) Eq. 7
.sub.y2 (t)=fa.sub.y2(t) =f.THETA. sin (.OMEGA..sub.1
*t)+f(.THETA./.sqroot.2) sin (.OMEGA..sub.2 *t) Eq. 8
For example, the lens may be of f=125 mm; the mirror wedge angle may be
.THETA.=2.34 mRad; and the angular velocities may be .OMEGA.1=600 rad/sec
and .OMEGA.2=630 rad/sec. Let A=.THETA.f/.sqroot.2=0.207;
B=.THETA.f=0.293; and C=.OMEGA.2/.OMEGA.1=1.05. The ray exiting from the
lens will scan at the focal plane an area whose limits are defined by a
circle of radius 0.5 mm (see FIG. 6). Every 20 revolutions the ray
completely scans the whole area and starts anew. The 20 revolution scan
period is about 0.2 seconds. The resultant ray pattern can be seen in FIG.
6.
Example of Laser for Free-Hand Surgery
FIG. 8 illustrates the invention in one form of laser apparatus used for
free-hand surgery. The laser apparatus illustrated in FIG. 8, therein
designated 20, outputs a laser beam via an articulated-arm system 22 and a
handpiece 24 grasped by the surgeon for directing the laser beam to the
appropriate locations of the tissue T to be ablated in accordance with the
present invention, the laser of FIG. 8 includes a scanner system,
generally designate 26, as described above for cyclically scanning the
continuous laser beam along two orthogonal axes and thereby to cause the
beam to trace Lissajous figures over the tissue T to be ablated. In the
apparatus illustrated in FIG. 8, the focussing lens (10, FIG. 1) is in the
hand-held handpiece gripped and manipulated by the surgeon.
Following is one example of the parameters of a hand laser apparatus such
as illustrated in FIG. 8.
1. Lens focal length 125 mm
2. Scan Radius r=2.0 mm (A=0.828 mm, B=1.172 mm)
3. Laser Power P=20 watts
4. Raw Beam Radius (before lens) W1=4 mm
5. Rotation speed of motors
.OMEGA.1=600 rad/sec
.OMEGA.2=630 rad/sec
C=.OMEGA.2/.OMEGA.1=1.05
6. Laser Wavelength =0.0106 mm
7. From FIG. 7 we see that
Vavg.=808 mm/sec
Vmin.=44 mm/sec
These two velocities, Vavg. and Vmin., are four times that shown in FIG. 7
because FIG. 7 represents a scan radius of 0.5 mm, whereas in the above
example the radius is four times greater.
The above parameters produce the following results:
1. Spot radius at focus
##EQU2##
2. Power density at focus P.D.=P/S=P/.pi..multidot.w0.sup.2 =637 watts/mm'
where S is the area of the focussed spot.
At this power density the thermal damage is minimal, and there are no signs
of carbonisation. Assuming no scanning, the rate of evaporation Ve would
be:
##EQU3##
At such a large speed there is no way of controlling the homegeneity of
tissue removal. As a result, deep holes and valleys are formed.
If the scanner is operated at a scan radius of r=2 mm, the average power
density on the tissue within the scanning area is:
P/.pi..multidot.r.sup.2 =20/.pi.19 4=1.6 watts/mm.sup.2
The rate of evaporation of the scanned area is:
Ve=KXP.multidot.D.=0.4.times.1.6=0.64 mm/sec.
At this speed it is easy to control the rate of tissue removal causing
minimal damage.
Because of the scan speed, each element of the tissue feels the equivalent
of a short time pulse. The pulse duration is given by the ratio of the
spot diameter at the focus (2WO) to the linear scan speed (Vs) (see FIG.
7).
The average pulse duration (Tavg.) is given by:
Tavg.=2WO/Vavg.=2.times.0.1/808=250 .mu.sec.
Pulses of this duration give very low thermal damage. The time between
successive pulses is 0.2 sec. This is the ideal time for the tissue to
cool down. This is a further reason for low thermal damage.
Example of Laser for Microsurgery
FIG. 9 illustrates the invention included in another form of laser
apparatus particularly useful for microsurgery. The laser, generally
designated 30 in FIG. 9, outputs a laser beam via a system of articulated
arms 32 and a micro-manipulator 34, such as described in U.S. Pat. No.
4,228,341, to the tissue T to be ablated. Micro-manipulator 34 includes a
joystick 35 enabling the surgeon to manipulate the laser beam as desired,
and also an eyepiece and microscope (not shown) to permit the surgeon to
view the working area containing the tissue to be ablated. The scanner
system, generally designated 36, corresponds to the scanner system 6 in
FIG. 1, and is effective to cyclically scan the continuous laser beam
along two orthogonal axes as described above to cause the beam to trace
Lissajous figures over the tissue to be ablated.
Following is one example of the parameters of a gynecological colposcope
constructed as illustrated in FIG. 9 and having a working distance of 400
mm.
1. Focal length f=400 mm
2. Scan Radius r=2 mm
3. Laser Power 60 watts
4. Raw Beam radius w1=4 mm
5. Rotational speed of motors
.OMEGA.1=600 rad/sec
.OMEGA.2=630 rad/sec
6. Laser wavelength =0.0106 mm
7. From FIG. 8
Vavg.=808 mm/sec
Vmin.=44 mm/sec
The laser apparatus illustrated in FIG. 9 and constructed in accordance
with the foregoing parameters produces the following results:
##EQU4##
2. Power Density at focus P.D.=P/S=P/.pi.wO.sup.2 =165 watts/mm.sup.2. At
this power density the thermal damage and carbonisation is minimal.
3. Assuming no scanning, the rate of evaporation would be:
Ve=0.4.times.P.D.=0.4.times.165=66 mm/sec
This represents a speed too great for controlled work.
With the scanner, P.D.=P/.pi.r.sup.2 =60/.pi.2.sup.2 =4.78 watts/mm.sup.2,
and the rate of evaporation Ve=0.4.times.P.D.=0.4.times.4.78=1.9 mm.sec.
This represents an evaporation rate which is very convenient for efficient
working conditions.
4. Pulse duration Tavg=2Wo/Vavg=2.times.0.34/808=840 .mu.sec.
Pulses of this duration create very low thermal damage.
While the invention has been described with respect to several preferred
embodiments, it will be appreciated that these are set forth merely for
purposes of example, and that many variations may be made. For example,
the scanning need not trace Lissajous figures; in fact, only one scanning
mirror is needed since the movement of the laser by the surgeon will cause
the beam to scan the surface to be ablated. Also, more than two mirrors
could be used. Many other variations, modifications and applications of
the invention may be made.
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