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Description  |
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FIELD OF INVENTION
This invention relates to medical treatment and, more particularly, to the
control of a laser output in such a way as to limit the dwell time on any
particular portion of the tissue, thus permitting target specific energy
deposition and damage which is selective and controllable.
BACKGROUND OF THE INVENTION
There are many medical conditions, the treatment of which is substantially
improved by being able to control the deposition of laser energy in a
specific target tissue in order to damage that target tissue while sparing
the adjacent tissue. While those in the past have utilized lasers,
particularly in the port wine stain (PWS) syndrome, to destroy blood
vessels, the problem associated with such systems is that the dwell time
of the laser over the target produces significant thermal diffusion which
damages not only the abnormal PWS vessels which are ectatic, i.e., dilated
and filled, and strongly absorb the radiation, the ones producing the wine
stain, but also damages a significant depth of the dermis such that
scabbing and sloughing occurs as a consequence of treatment. Additionally,
the use of an anesthetic is prescribed because of the amount of energy
imparted to the target area which is painful to the patient. It will be
appreciated that the desired treatment for port wine stains is to necrose
only those vessels producing the stain while leaving most of the
surrounding collagen and the normal vessels undamaged. This translates
into control of thermal diffusion, which up until the present time has
been difficult either because of the relatively long pulse lengths of the
shuttered CW lasers utilized or because of the relatively short pulse
lengths of the dye lasers which do not completely destroy the abnormal
vessels. Consequently, no successful control has heretofore been exercised
to limit the volume affected by thermal diffusion. The result of the lack
of control is that lasers which dwell on a given target area for 20
milliseconds or more produce so much thermal diffusion that scabbing and
sloughing of the epidermis and portions of dermis are produced regardless
of wavelength, assuming any kind of therapeutic levels are introduced to
the target area.
By way of further background, if it is desirable to destroy abnormal tissue
contained within the volume of normal tissue and spare the overlying
normal layer, differential absorption of the light is required. This can
be obtained by either intrinsic optical qualities of the target or tagging
with some exogenous chromophore. In the latter case, the wavelength or
color of the laser must be selected on the basis of the absorption spectra
of both the target and the surrounding tissue. In other words, it should
be a wavelength where the target tissue is a good absorber and the
surrounding tissue is a poor absorber. Moreover, the irradiation time must
be selected on the basis of the thermal properties of the target and
surrounding tissue and the geometric shape and dimensions of the tissue
structure.
Thus, in some cases the target tissue is distinguished from surrounding
tissue by a difference in absorption spectra either due to a
naturally-occurring chromophore (absorbing molecule) or due to the
selective deposition of some dye used in the treatment regimen. One
important example of such a target tissue present throughout the body is
the vasculature which contains erythrocytes. The erythrocytes contain
hemoglobin, a naturally-occurring chromophore with a broad usable
absorption band in the visible. The entire range of visible wavelengths
shorter than approximately 600 nm (nanometers) and extending into the
ultraviolet is available to purposely inflict damage to target tissues
containing this chromophore. The specific wavelength selected depends on
the relative effects of scattering, which varies with wavelength; the
presence of other chromophores, such as melanin, in the adjacent or
overlying tissues; and the availability of light sources.
In selecting the exposure time, one is limited by the time which will
confine thermal damage due to heat transport to an acceptable distance
from the target. For the treatment of port wine stains, it is desirable to
be in the regime where the dominant mechanism for heat transport is
conduction. A characteristic thermal diffusion length for heat conduction
is given by
L.sup.2 =4Kt
where L=distance that heat diffuses; K=thermal diffusivity coefficient; and
t=time allowed for diffusion.
This formula varies slightly with the geometry of the irradiated target in
surrounding media, but the variations are not significant. A typical
thermal diffusivity coefficient for biological tissues is 0.0015 cm.sup.2
/second.
Considering, for example, the treatment of port wine stains, a type of
hemangioma that consists of hypertrophic capillaries in the dermis causing
a pink, red or purple coloring of the skin, the pink and red lesions are
high in erythrocytes carrying oxygenated hemoglobin (HbO.sub.2), while the
purple lesions contain large quantities of deoxygenated hemoglobin (Hb).
The lesions are characterized as consisting of abnormal capillary
structure with the capillaries varing in diameter, the mean diameter being
about 50 micrometers. The wall of the vessel, however, is only a few
micrometers thick. The average vessel spacing is 100 micrometers. In order
to damage the vessel containing the target hemoglobin, its wall, and a
small portion of collagen surrounding the wall, the latter two being
relatively free of chromophore, it is important to select an exposure time
corresponding to thermal diffusion over a characteristic length slightly
greater than the wall thickness. This, in general, refers to the delivery
of radiation to a given target area of less than one millisecond. Those
prior art devices which deliver 20 milliseconds or more cause damage due
to thermal diffusion of heat to a distance even greater than the vessel
spacing. Thus the entire tissue bulk is heated by the vessel network
embedded within it and the damage is not at all selective.
J. L. Finley, S. H. Barsky and D. E. Geer in an article entitled "Healing
of Port Wine Stains After Argon Laser Therapy," Archives of Dermatology,
1981, Volume 117, pps. 486-489, and J. L. Ratz, P. L. Bailin, and H. L.
Levine in an article entitled "CO.sub.2 Laser Treatment of Port-Wine
Stains: A Preliminary Report," J. Dermatol. Surg. Oncol. 1982, Vol. 8, No.
12, pps 1039-1044, describe both argon lasers and carbon dioxide lasers
used in the clinical treatment of port wine stains. Neither of these
provide an optimal treatment modality as the dwell time is not limited to
prevent thermal diffusion.
It will be appreciated that the CO.sub.2 laser radiation whose wavelength
is approximately 10 micrometers is very strongly absorbed in water and
most proteins. In port wine stains, both the abnormal vasculature and the
surrounding dermal tissues are approximately 90% water and consequently
absorb the incident laser radiation and are heated to the point of thermal
necrosis. Thus this treatment does not involve any specificity of damage.
The necrotic tissue eventually sloughs off and is replaced via the normal
healing process by scar tissue formation. Since the scar tissue formed is
usually flat and white, it is often more acceptable to the patient that
the original dark and, sometimes, hypertrophic lesion.
Present treatment of port wine stains with the argon laser is performed
using comparatively long pulse times. Because of this, heat has time to
diffuse to the surrounding tissue, and the effect observed is the same as
for the CO.sub.2 laser in which radiation is uniformly absorbed in both
vascular and surrounding tissue. The similarity of clinical results with
the CO.sub.2 and the long pulse argon lasers has been noticed and
documented in an article by J. W. Buecker, J. L. Ratz and D. F. Richfield
entitled "Histology of Port Wine Stains Treated with CO.sub.2 Laser,"
Fifth International Conference of Laser Medicine and Surgery, Detroit
1983, in an abstract. Thus the nonspecificity of prior art laser treatment
of port wine stains is both documented and explainable by the relatively
long irradiation times causing massive long-distance thermal diffusion for
argon lasers and the non-specific absorption for CO.sub.2 lasers.
By contrast ultrashort laser pulses have been used. Studies by R. R.
Anderson and J. A. Parish entitled "Microvasculature Can Be Selectively
Damaged Using Dye Lasers: A Basic Theory in Experimental Evidence in Human
Skin," Lasers in Surgery and Medicine, 1981, Volume 1, pps. 263-276, show
that when utilizing pulse dye lasers with fluence level on the order of 3
to 5 Joules/cm.sup.2 and exposure times of approximately 300 nanoseconds
(ns) target specific damage may be produced in normal blood vessels. The
wavelength used was 577 nanometers. While the above pulse width was short
enough to restrict thermal diffusion to a small portion of the individual
erythrocytes having typical dimensions of 7-15 micrometers carrying the
hemoglobin, and thermal diffusion subsequent to the pulse could have
allowed heating of the containing vessel without damage to the surrounding
tissues, the short 300-nanosecond duration caused the vessels to burst and
to spew forth blood. It is possible that a shock wave produced by the
ultrashort pulse ruptured the blood vessels causing formation of purpura.
Since the vessels are 50.mu. in diameter and the wall is about 1.mu.
thick, the pulse is so short that only the hemoglobin itself (which is the
optical absorber) and any spot on the inner edge of the wall which happens
to be in intimate thermal contact with the hemoglobin bearing portion of
an erythrocyte are heated during the pulse. After the pulse, the peak
temperature achieved within the vessel decays as heat diffuses away. While
regions outside the vessel are in fact heated as this diffusion occurs, it
is not possible to achieve thermal damage to an adequate depth to insure
permanent vessel necrosis.
Note that at the present time several microseconds is the longest pulse
time available from commercial pulsed dye lasers. This is still too short
to achieve the desired effect.
In summary, it will be appreciated that the difficulty in the prior art
methods of utilizing argon laser treatment lies not in the wavelength, at
least for low melanin skins, but in the exposure time utilized. The argon
lasers are CW lasers which are mechanically shuttered to provide
pulsewidths which may vary from 20 milliseconds to 100 milliseconds or
more. Even the shortest of these exposure times, 20 milliseconds, results
in thermal diffusion to a length of 100 micrometers which is equal to the
average spacing between targets. Thus, even if the laser power is
initially absorbed only in the target volumes, thermal diffusion during
the laser pulse itself provides nearly uniform heating of the entire
irradiated area. In order to achieve true specificity, damaging only the
target vessels, the exposure time must be limited to about one millisecond
or less, which is too short to be achieved with the mechanical shutters
presently in use. Additionally, nanosecond pulses from dye lasers cause
blood vessel rupture and causes only partial necrosis. Thus these
techniques are not optimally useful in treating port wine stains.
Note the following U.S. patents deal with scanning lasers: U.S. Pat. Nos.
3,362,007; 3,642,007; 4,069,823; and 4,316,467; whereas U.S. patents
dealing with coaxial bilaser beams include U.S. Pat. Nos. 3,456,651;
3,710,798; 3,769,963; 3,906,953; 3,910,276; 4,240,431 and 4,408,602.
Finally, U.S. Pat. No. 3,434,476 deals with a plasma arc scalpel.
SUMMARY OF THE INVENTION
In order to limit thermal diffusion, apparatus is provided which moves the
focused laser beam in a circle or other path which controls dosimetry of
radiation applied to a target area, with the scanning rate being adjusted
to limit thermal diffusion from the irradiated target site for selective
target specific energy deposition. In one embodiment, a hand piece is used
in which the scanning is provided by rotating optics within the hand
piece, with the speed of rotation determining the scanning rate. In an
alternative embodiment the scanning may be provided by a rigidly mounted
head containing the scanning optics. When used for dermatologic purposes,
the adjustable scanning mechanism prevents radiation from impinging on
tissue for more than about one millisecond in one embodiment for the
selective necrosis of highly-filled port wine stain blood vessels, while
leaving adjacent tissue undamaged. The dwell time of the laser beam is
designed to match the thermal diffusion time for destruction of the wall
of the abnormal vessels, and some surrounding collagen with the dwell time
adjusted by the scanning rate. Neither the small, empty normal vessels nor
the collagen adjacent the normal vessels are attacked by the impinging
radiation since they contain no absorbing chromophore. In one embodiment,
a CW laser is used, with time per scan being maintained at less than 60
milliseconds to give the visual impression of a continuous ring of light
to the operator, with a rotary scan diameter of 2.8 millimeters and a
focal spot size of 0.14 millimeters. The wavelength of the laser is held
below 600 micrometers so that the hemoglobin in the erythrocytes absorb
sufficient radiation for the necrosis of the vessels containing the
erythrocytes. While the subject invention will be described in connection
with CW lasers, pulsed lasers may be used and are within the scope of this
invention.
In one embodiment the simple hand piece for a CW laser is provided with
cooling apparatus involving a cooling ejectant line and a cooling suction
line on opposite sides of the nose of the hand-held device. In a further
embodiment, bleachable indicators are painted onto the skin prior to usage
to indicate to the operator what areas of the epidermis have been
irradiated.
It has also been found that to reduce the chances for regeneration of a
lesion it is possible to slightly lengthen the pulse which is delivered to
the given target area extending the thermal damage into the collagen
immediately surrounding the abnormal vessel. An exposure time of about one
millisecond allows heating as far as 20 micrometers from the edge of the
abnormal vessel. This is adequate to insure total vessel destruction but
is still small compared with the spacing between vessels, insuring sparing
of a significant volume of the dermis. The exact exposure time involves
the correlation between the length of thermal diffusion damage and
redevelopment or final destruction of the lesion. Other considerations
include the fluence level of the focused light required to produce the
desired damage effect and the depth of field, it being understood that the
target tissue must be raised to an adequate temperature for a sufficient
length of time to produce thermal necrosis.
In order to achieve adequate fluence levels from available laser systems,
the laser beam diameter and depth of field are controlled by any of a
variety of focusing techniques. The depth of field is made large enough to
compensate for any variation in the distance between the target and the
delivery system whether this distance is maintained by a mechanical
fixture or simply by the operator's hand. Note that a larger depth of
field makes the control of the distance between the delivery system and
the skin less difficult. The larger depth of field is also useful because,
when the skin is depressed by as little as one millimeter by the nose of
the tool, this can cause a 1 millimeter bulge inside the nose of the
instrument.
Other uses for the subject invention include the treatment of other
cutaneous vascular lesions such as telangiectasias, nevus araneus,
strawberry nevus, cavernous hemangiomas, cherry hemangiomas, and venous
lakes. The treatment of deep, cavernous hemangiomas is expected to require
multiple treatments since these lesions are so hypertrophic that vessels
nearest to the surface will optically shield underlying vessels. By
causing thermal necrosis of the surface vessels and permitting adequate
time for phagocytic removal of the necrotic tissue, the next layer of
vessels is made vulnerable to the laser irradiation. It should be noted
that laser wavelength may also be a more important consideration in
treating very deep lesions since very strongly absorbed wavelengths will
provide shallower depth of necrosis per treatment. Another use for the
invention is in the treatment of psoriasis. Psoriasis is characterized by
an abnormal, ectatic and hypertrophic vasculature, along with other
abnormal features. Selective destruction of the vascular component permits
destruction of the lesion without subsequent tissue inflammation and
psoriatic regeneration.
The device is also useful in the treatment of various forms of
neovascularization of the eye including diabetic retinopathy and senile
macular degeneracy. For this type of lesion a microscopic rather than hand
held delivery system is required. In the case of retinal disorders, where
melanin absorption in the retinal pigment epithelium could cause
undesirable remote heating, a line scan, operator defined and computer
controlled, allowing the scan to exactly follow the vascular line would be
preferable. However, even full area scanning would destroy less tissue
than present modalities for treating each of these diseases. The currently
accepted modality calls for intentional destruction of large tissue
volumes to reduce the production of angiogenic substances. The scanner
described herein, however, has the potential to limit damage to such small
volumes that frequently repeated treatments, with greater preservation of
visual acuity may be possible.
Another use for the device is in the treatment of structures bearing
melanosomes, including actinic keratoses, lentigo, malignant melanomas,
and freckles. In this case the dwell time (i.e., scanning rate) is
adjusted to destroy the melanosome bearing cells, leaving adjacent cells
unharmed.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other features of the subject invention will be better understood
in connection with the detailed description taken in conjunction of which:
FIG. 1 is diagrammatic representation of the utilization of the scanning
hand piece in the treatment of a typical port wine stain;
FIG. 2 is a diagrammatic illustration of the operation of the hand piece of
FIG. 1 illustrating a focal spot which moves around in a circular scan
within the nose of the hand piece while the hand piece is moved across the
area to be irradiated, liquid cooling and suction pipes being utilized to
provide fluid which cools the irradiated area;
FIG. 3A is a cross-sectional and diagrammatic illustration of one
embodiment of the hand piece in which a rotting offset lens is utilized to
scan the focused beam in a circular pattern;
FIG. 3B is a diagrammatic and cross-sectional illustration of an
alternative device utilized for the scanning of the beam by providing a
rotating optical wedge interposed in the optical path;
FIG. 4 is a top view and schematic diagram of the circular scan produced by
the hand pieces of FIG. 3A and FIG. 3B;
FIG. 5 is a diagrammatic representation of the result of irradiation of a
port wine stain with the prior art ultrashort laser pulses, illustrating
the rupturing of the port wine stain blood vessels causing the spewing
forth of blood;
FIG. 6 is a diagrammatic representation of the result of irradiation of a
port wine stain with a prior art CW or long pulse laser showing complete
necrosis of all irradiated areas, and;
FIG. 7 is a diagrammatic representation of the result of utilizing the
subject scanning system in which thermal diffusion is limited, causing
necrosis of the port wine stain blood vessel, its wall and a very small
portion of the collagen immediately adjacent the vessel wall, without
disruption of the majority of the normal tissue within the port wine stain
area.
DETAILED DESCRIPTION
Referring now to FIG. 1, in one embodiment of a patient 10 having a port
wine stain 12 is being treated by laser radiation from a laser source 14
which is channeled by fiber optic cable 16 to a hand-held unit 20 which
has therein internal optics utilized to provide scanning of a beam within
the nose portion 22 of the tool. This is accomplished in one embodiment
through drive motor 24 utilized to control the scan speed by rotating
cable 26 which drives a hollow cylindrical barrel in the tool that carries
the focusing optics. Alternatively, air drive motors or small electrical
motors may be used in the hand piece to drive the rotating optics. Hollow
shaft motors (either electrically or pneumatically driven) may incorporate
the optical path within the hollow shaft, supporting the rotating element
on the end of the shaft. Solid shaft motors must be used with gear or
other coupling mechanisms to drive the rotating element. An optional
source of cooling liquid 28 is applied to the hand tool which is channeled
to the nose portion 22 and is removed by a suction unit 30 such that the
area of the target adjacent the nose of the tool is cooled.
The operation of the hand tool can be better seen in conjunction with FIG.
2 in which like reference characters are utilized between FIGS. 1 and 2.
In FIG. 2 an optical system 32 is utilized to focus and rotate a focal
spot 34 such that in the illustrated embodiment the spot rotates in a
circle 36 as illustrated by the dotted arrow. Cooling fluid is delivered
at one side of nose 22 by a delivery tube 38 and is removed by a suction
tube 40 as illustrated.
It is the purpose of the rotating optics within the hand tool to scan the
focal spot such that it resides over a target for no longer than about one
millisecond in one embodiment. The control of the scan rate controls the
time with the focused spot resides at a given location within the target
area and is readily adjustable by the scan rate. While a circular scan is
illustrated in the embodiment of FIG. 2, it will be appreciated that
raster scan, Rosette type, orbital, elliptical, or other scan patterns
scans may be performed by optics to prevent the focal spot from residing
at any given location for longer than a predetermined period of time. In
one embodiment the depth of field is made greater than two millimeters by
virtue of the focal system aperture utilized. In this embodiment, a focal
spot size of 0.14 millimeters, a scan pattern diameter of 2.8 millimeters,
and a scan rate of less than 60 milliseconds per cycle are used. For port
wine stains, the wavelength of the laser is held below 600 micrometers so
that the hemoglobin in the erythrocytes absorbs sufficient radiation to
provide for the necrosis of the vessels containing the chromophores.
In one operative mode, the hand piece is moved in a serpentine fashion as
illustrated by dotted arrow 33 across an area 46 which corresponds to the
area of the port wine stain.
As mentioned before, a dye may be first applied to the affected area which
changes color upon irradiation by focal spot 34 such that the treated area
may be ascertained with a high degree of certainty. This aids the operator
who may be unable to "see" which areas have been treated since the
treatment is so gentle as to provide minimal visible color change of the
lesion. The actual lightening of the lesion occurs slowly, over a period
of days to weeks as the body phagocytizes the necrotic tissue. Instead of
a hand held unit, the same treatment may be provided by a programmed
scanner. In such a case the indicating dye would not be necessary.
Referring to FIG. 3A, hand tool 20 may take on a configuration in which a
body or housing 50 includes a hollow cylindrical barrel illustrated at 52
to be rotated via cable 26 or other means about an axis 54 which is
typically the central axis of the hand tool. The barrel is supported via a
bearing system generally indicated by bearings 56.
Laser radiation is transmitted to the hand tool via fiber optic cable 16
which is coupled via a cable termination 60 to a lens 62 which collimates
the light generally along axis 54. A lens 64 having a convex surface 66
focuses the parallel light as indicated by dotted lines 70 to a point 72
on the surface of skin 74. Since the optical axis of the lens is offset
from centerline 54, its rotation via a barrel 52 causes the focal spot 72
to rotate on the surface of the skin 74. As can be seen, cooling liquid
may be introduced through tube 38 such that liquid proceeds across the
irradiated areas illustrated by 76 to suction tube 40.
The same system may include as a scanning means an optical wedge 80 which
is shown in FIG. 3B in which like apparatus is given like reference
characters vis-a-vis FIG. 3A. Note that power to rotate each of the
barrels of FIGS. 3A and 3B is delivered by line 26, be it mechanical,
electrical, hydraulic, or pneumatic. Note also that a fixed lens 82 is
provided in nose 22 of the hand tool to achieve focusing.
Indeed, any focusing optics which is moved so as to provide a scanning
beam, be it a raster scan, a circular scan, a line scan, or an elliptical
scan, or some combination of these is within the scope of this invention.
It is only important that the focal spot 72 not remain over any point
within a target area 74 for any longer than is necessary for the
particular purpose intended. For port wine stains this means that the
dwell time for the spot should be on the order of one millisecond in order
to prevent the type of damage which will now be described.
Prior to describing the damage done by thermal diffusion for radiation
impinging upon the skin for too long a period of time and referring now to
FIG. 5, when using ultrashort pulses to treat port wine stains, it will be
appreciated that the abnormal, ectatic vessels are those illustrated | | |
