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Description  |
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BACKGROUND OF THE INVENTION
Surgical methods are being developed to ablate tissue by inserting an
optical fiber catheter into a body lumen and passing laser light through
the fiber optics onto the surgical site. Laser catheters employing optical
fibers to deliver laser radiation to ablate tissue are coupled to pulsed
Xe:Cl excimer or other suitably powered lasers. However, there are many
problems associated with using excimer lasers with a wavelength of 308 nm,
for example. Noxious gases used with excimer lasers must be vented. The
laser is extremely large and bulky and generates an excessive amount of
electrical noise that may affect other hospital equipment. As a
consequence, excimer lasers require heavy shielding. Moreover, the laser
beam quality is so poor that optical processing of the beam is difficult.
Also, the excimer laser generates light in the UV-B range resulting in the
potential for mutagenicity of the irradiated tissue. The use of such a
system at a wavelength of 308 nm is known to cause cataracts. In view of
the dangers associated with excimer lasers and other problems associated
with other existing medical lasers, a need exists for the development of a
surgical laser system more suitable for a hospital environment, that will
provide a radiation source suitable for tissue ablation and provide a more
convenient and reliable laser source for a variety of medical
applications.
SUMMARY OF THE INVENTION
To avoid the problems associated with excimer lasers, we have chosen a
solid state ND:YAG laser, frequency tripled to 355 nm, to ablate tissue.
The pulses produced by currently available lasers such as ND:YAG, doubled
alexandrite, or Ti:sapphire are short (approximately 100 ns or less) and
at the energies required for ablation the use of optical fiber waveguides
to deliver the radiation to tissue can result in damage to the optical
fibers. Other laser host materials can be used, such as YALO, or yttrium
aluminate. Also, other types of solid state lasers such as holmium doped
lasers can be used. By providing a laser pulse of sufficient duration and
energy from a solid state laser, tissue can be ablated without damaging
the optical fiber used to deliver radiation to the surgical site. The
laser output may be a single pulse or a plurality of pulses with a
fluence, or energy/cm.sup.2, sufficient to remove tissue. Moreover, the
laser wavelength is preferably in the range of 320 to 400 nm.
The laser surgery techniques can be extended to all tissues of the body.
For example, skin lesions can be excised by direct application of a laser
beam without transmission through optical fibers. Similarly, a solid state
laser beam can be used for the removal of cancerous or precancerous
material during surgery. Also, a catheter can be used to apply the solid
state laser beam to calcified material or soft tissue within a body lumen.
Laser revascularization of coronary arteries utilizing solid state lasers
can thus remove calcified plaque. Spectroscopic diagnostics are utilized
to determine what tissue is to be removed. Moreover, in vitro applications
are useful for biopsy and autopsy purposes.
In the preferred embodiment, the short pulse output of the solid state YAG
laser is transmitted to an optical "extender" for "stretching" the initial
pulse into a train of a selected number of pulses, each delayed by a given
time interval, the train of pulses resulting in the "filling" in of
overlapped pulses. Thus, pulses having a desired waveform can be
lengthened by overlapping several pulses. When the interpulse delay is
less than the initial, or seed pulse duration, the train of pulses overlap
into a single pulse. When a plurality of initial, or seed pulses are
received by the filler/multiplexer, the separation between the resulting
output pulses are such that the pulse trains from the respective initial
pulses do not overlap.
The optical extender utilizes beam splitters and mirrors to produce the
desired pulse trains. Similarly, a pulse filler/multiplexer utilizes a
selected number of different optical delay paths which are combined by
beam splitters to produce the desired number of outputs. The positions of
the pulse extender and pulse filler can be reversed to optimize particular
operating conditions. Pulse duration is set such that tissue is removed
without damaging the transmitting optical fibers.
In another preferred embodiment a solid state laser per se can be
constructed to produce pulses having the desired duration, power and shape
to remove tissue. For example, a slow Q switched, mode locked laser can be
used to provide such a pulse. Also, two or more lasers can be used to
produce pulses with the desired time separation between pulses from the
respective lasers. Thus, with our invention, a system is provided for the
precise laser machining of all human tissues.
A further embodiment of the present invention relates to the use of two or
more colors or wavelengths of light to further increase the effectiveness
of the tissue ablation process. In this procedure the removal of a
selected region of tissue is initiated using a pulse of light of one
wavelength and then completing the removal of the selected region by
irradiating the same region with a second pulse of light of another
wavelength.
The first pulse can produce transient or non-transient changes in the
tissue. In the case of transient changes, the second pulse is delivered to
the tissue within a period of time that is short enough to work in
combination with the first pulse. It is by coupling the effects of two
pulses of different wavelengths that has resulted in a substantial
increase in ablation effectiveness.
The first pulse operates to prepare the tissue by thermal, photochemical,
vibrational or electronic excitation mechanisms that either alone, or in
combination, alter the absorption and/or scattering characteristics of the
tissue relative to the second pulse of different wavelength. The precise
mechanism by which the absorption and/or scattering characteristics of the
tissue is altered is dependent upon the type of tissue, the pulse
duration, temporal separation of pulse pairs, the wavelengths of the
pulses and the energy delivered by each pulse to the tissue. Note that the
first pulse can also produce some permanent non-reversible change that can
be used to prepare the tissue. In this embodiment the temporal separation
of the pulses is not as critical for effective two color ablation.
Generally, this type of tissue preparation is photochemical in nature.
In one embodiment a ND:YAG laser produces single pulses each of which is
divided into a first subpulse that is tripled (to 355 nm, for example) and
delivered to arterial tissue, and a second pulse, at the fundamental
frequency (1066 nm), that is delivered to the tissue through an optical
delay to provide the desired pulse separation, within one microsecond of
the first pulse. The first 355 nm pulse alters the propagation of light
through the irradiated tissue such that the second pulse is more fully
absorbed by the tissue and results in the removal of the desired amount of
tissue. For the purposes of the present application, it is the use of the
first pulse to increase the rate of attenuation of the second pulse as it
propagates through the tissue that is critical to effective ablation.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates the output pulse of a typical solid state laser relative
to a preferred rectangular pulse.
FIG. 2 illustrates the rate of tissue ablation and the probability of
optical fiber survival as a function of pulse separation.
FIG. 3 is a sectional view of a laser catheter embodiment disposed in the
bend of an artery and illustrating the device in operation.
FIG. 4 is a block diagram of a system in accordance with the invention for
removal of unwanted deposits in an artery.
FIG. 5 shows an optical extender for converting a short pulse into a train
of 3 to 4 pulses.
FIG. 6 illustrates another optical extender which can transform a single
short pulse into a train of several pulses delayed by a given time
interval.
FIG. 7 shows a pulse broadening multiplexer which generates a plurality of
overlapping pulses.
FIG. 7a shows the pulse outputs generated by the multiplexer.
FIG. 8(a)-(e) and 9(a)-(c) show various laser pulse outputs which are
effective for removing tissue.
FIG. 10 shows a further embodiment of the pulse broadening multiplexer.
FIG. 11 illustrates a further embodiment of the laser subsystem to be
employed in a sequenced pulse ablation procedure.
FIG. 12 illustrates the fluence thresholds that are necessary to provide
ablation using various lasers.
FIG. 13 illustrates the particular wavelengths and energy levels of a
particular two color ablation process.
FIG. 14 is a three-dimensional graphical representation illustrating crater
depth in a tissue sample as a function of fluence of the two colors or
wavelengths being employed.
FIG. 15 is a graphical illustration of crater depth in bone tissue versus
pulse separation of sequenced pulse ablation using one and two
wavelengths.
FIG. 16 is a graphical illustration of crater depth in bone tissue for
selected pulse pairs.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 illustrates the typical pulsed output 12 of a solid state laser. The
rectangular pulse 14 represents an optimal pulse configuration that can be
approximated by the methods described below. The amplitude of pulse 14 is
much less than the peak amplitude of output 12. Also, the peak height of
pulse 14 is small relative to its time length duration. However, the areas
under the respective curves 12 and 14 are comparable. When the duration of
pulse 14 with the necessary fluence is sufficiently long and has the
necessary energy density it provides an improved pulse configuration for
the ablation of tissue. A wavelength of 355 nm is advantageous in that it
produces a desirably small penetration depth in body tissue. Thus, by
controlling the penetration depth with the appropriate selection of
wavelength, damage to underlying healthy tissue is reduced. Moreover,
ND:YAG lasers can be used safely in hospitals, have good beam quality, and
operate at non-mutagenic wavelengths. However, Nd:YAG lasers generally
produce only frequency tripled short pulses having the necessary ablation
energy (of a duration approximately 10 ns) efficiently.
Solid state lasers can produce long duration pulses by using long, high-Q
cavities or by appropriate Q switching techniques. However, it is
difficult to produce a long duration pulse with a frequency tripled solid
state laser. Since, it is often desirable to use optical fiber waveguides
to deliver radiation to a surgical site, particularly at locations
accessible through body lumens, long duration pulses are used with
sufficient energy to remove tissue and which can pass through without
damaging the bulk material or surface of that fiber.
FIG. 2 illustrates the two conditions which must be met to transmit a
satisfactory output pulse. First, sufficient energy must be transmitted to
the body area to remove tissue. Second, the energy must be below the
threshold of damage for the optical fiber. The respective materials were
tested using two short pulses (7.5 ns) at a wavelength of 355 nm laser
light of equal energy. The pulses were delayed by an amount varying from 1
nanosecond to 1 second.
The effect of various pulse delays on body tissue is shown by the dotted
line in FIG. 2. The tissue effectively "remembers" that it has been
irradiated for times in excess of several microseconds i.e., energy
deposited in tissue can be stored for several microseconds and the energy
needed to reach the ablation threshold is cumulative over this period. It
has been found that the rate of ablation or crater depth is a maximum
under these conditions for a pulse separation less than 100 nsec and
decreases steadily to zero at 100 msec separation. However, effective
ablation can generally occur at a pulse separation of less than 1 msec.
The selection of an appropriate combination of laser wavelength, pulse
energy and pulse duration is critical to properly remove body tissue. The
proper choice of laser pulse parameters is especially important for the
removal of calcified plaque. It has been found that the most efficient
removal of "hard" biological tissue occurs when wavelengths below 400 nm
and intensities in the range of MW/cm.sup.2 to GW/cm.sup.2 are used.
Accordingly, within these limits, the amount of tissue ablated depends
primarily on fluence (energy/cm.sup.2) and the same removal amount be
obtained with long, low intensity pulses as with short, high intensity
pulses.
We have determined that hard body tissue such as bone, tooth enamel and
calcified plaque can be removed efficiently by vaporizing the soft tissue
component inside the hard tissue which entrains and removes the hard
component particles which are not vaporized. The boiling point of the soft
component in the hard tissue is approximately 300.degree. C. A fluence of
10.sup.6 -10.sup.9 Watts/cm.sup.2 is required to vaporize the soft tissue.
By comparison, the melting point of hard tissue is 1600.degree. C. To
vaporize the hard component would require a fluence which is substantially
greater than 10.sup.9 Watts/cm.sup.2. Thus, much less power is required to
remove hard tissue using our method. Moreover, less damage is incurred to
areas surrounding the surgical site by using our method of vaporizing the
soft component of hard tissue without vaporizing the hard components. With
our technique, plasmas are not created and less heat is transmitted to the
areas surrounding the surgical site.
Also, ablation proceeds at a energy rate which is greater than the rate of
thermal diffusion into the surrounding tissues. Thus, the fluence is
maintained above the threshold required for tissue removal. Since the
vaporization of soft tissue entrains heat away from the surgical site, the
process results in a "cold cut" at the surgical site.
The determination of the requisite laser pulse parameters is extremely
difficult in view of the previous lack of understanding of the nature of
tissue response to laser light. The laser intensity should be great enough
to vaporize the soft component in the hard tissue at a sufficiently rapid
rate to entrain and remove the hard components which do not vaporize.
Thus, the process requires less absorbed laser energy to remove a given
amount of tissue, compared to ablation in which both tissue components are
vaporized. Also, since the hot material is rapidly removed, much of the
deposited heat is carried out of the tissue before it can be transferred
to the adjacent tissue via thermal diffusion. The irradiance, fluence, and
wavelength of a laser beam represent the fundamental controllable
parameters governing the ablation process.
The effect on optical fibers involves a converse relationship. The exact
path of curve 18 illustrating the probability of optical fiber survival is
dependent on the power level employed. The instantaneous power in the
optical fiber at any given time must be below the threshold which will
cause damage. Note that the energy transmitted is directly related to the
pulse duration. The damage to the optical fiber may occur when adjacent
pulses overlap and breakdown thresholds are exceeded. However, optical
fiber damage is rapidly reduced to a tolerable level at a pulse separation
in the range of 100-200 nanoseconds. Two breakdown mechanisms
predominate: self focusing and surface breakdown. Self focus damage limits
peak power. Front surface breakdown restricts the pulse energy profile for
a pulse energy to do ablation. Note also that the amount of energy which
can be transmitted through a fiber is limited by damage of the fiber core
due to electron avalanche breakdown. For shorter pulses or larger optical
fiber cores, damage is caused by self focusing within the fiber core. For
longer pulses or smaller optical fiber cores, damage is caused by front
surface breakdown. Thus, optical fibers can only transmit high energy
pulses at moderate power levels which require longer pulses.
To summarize, three requirements must be met. First, each fiber must
transmit enough energy to ablate a required cross-sectional area exceeding
the core diameter of the fiber. Second, peak power on each fiber must be
below self focussing breakdown. Finally, the pulse energy profile on each
fiber must be below the threshold for front surface breakdown. The energy
required for ablation of tissue must be greater than thermal relaxation or
heat dissipation at the removal site. FIG. 2 illustrates that pulse
separation between 100-200 nanoseconds and 1 millisecond fulfill these
requirements. By stretching the solid state laser pulse, we simply and
effectively solve the mutual problems of tissue removal and optical fiber
transmission. Various pulse trains and sequences of pulses with variable
delays, in addition to continuous pulses can be utilized. A variety of
pulse shapes can be used to increase the energy transmitted. For example,
a weak first pulse with an energy below the ablation threshold can be
transmitted to the surgical site, followed by a second pulse with much
greater energy than the first, such that the cumulative fluence of the two
pulses is sufficient to ablate tissue.
Our invention uses a pulse separation which is greater than the relaxation
time of the avalanche process in the optical fiber. Thus, electron
avalanche breakdown is avoided. Also, the pulse separation is less than
the tissue memory, i.e., the time period during which the energy is
stored. As a result, tissue is removed without damaging the optical fiber
which transmits the laser pulse.
FIG. 3 shows the typical catheter environment utilized to apply the laser
light at a desired body part. This, catheter is described in detail in
U.S. Pat. No. 4,913,142 to Kittrell et al. which is incorporated herein by
reference. As noted in Kittrell et al., spectroscopic analysis is utilized
to accurately position the catheter. The laser catheter 10 contains a set
of optical fibers, consisting of a central optical fiber 20a, a first ring
of optic fibers represented by 20b, b', and a second ring of optical
fibers 20c, c'. Note that a central lumen can also be provided in another
embodiment in which a guidewire is used to position the catheter. Each
optical fiber is composed of a core, a cladding with a lower index
material than the core, and a protective buffer. In the preferred
embodiment the-core and cladding are fused silica or glass or fluorite
glass, so as to withstand high laser power. Catheter 10 is shown removing
plaque 34 from within an artery 32. Laser light is applied to the optical
fibers to remove overlapping nibbles 35a, b, b', and c as needed.
FIG. 4 is a block diagram for the system of the invention which utilizes
spectroscopic diagnosis for removal of plaque in an artery. The pulse
stretcher 229 and the pulse filler/multiplexer 231, 233 produce an output
sufficient to remove tissue. The pulse stretcher optically regenerates
pulses. The pulse filler/multiplexer fills the space between the pulses.
Again, note that the term "extender" refers to either "stretching" pulses
or "filling" pulses or both. These elements can be interchanged in this
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