System for causing ablation of irradiated material of living tissue while not causing damage below a predetermined depth

A system for causing uniform ablation of irradiated material of living tissue while not causing damage below a predetermined depth where laser radiation is provided sequentially and continuously in a predetermined pattern is disclosed.

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.

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: 
EQU a.sub.x1 (t)=.THETA. cos (.OMEGA..sub.1 t+.delta..sub.1) Eq. 1 
EQU 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: 
EQU a.sub.x2 (t)=(.THETA./.sqroot.2) cos (.OMEGA.1.multidot.t)+.THETA. 
cos(.OMEGA.2t) Eq. 5 
EQU 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): 
EQU .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 
EQU .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: 
EQU 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: 
EQU 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: 
EQU 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: 
EQU 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.