Download Medical Applications of Lasers: Diversity is Key to Success

Laser in der Medizin Medical Applications of Lasers:
Diversity is Key to Success
Therapy, clinical tests or device fabrication:
Diverse lasers ensure optimum results
Medical applications for lasers are
incredibly diverse, encompassing three
broad areas: therapeutic procedures, clinical testing/diagnostics, and medical device manufacturing. Supporting such varied applications in turn requires a wide
array of laser technology. This article discusses representative applications from all
of these areas, and reviews key laser attributes and recent technological developments for each.
Therapeutic Procedures
With wavelengths optimized for human
tissue interactions, lasers are now found in
many therapeutical segments. This wavelength selectivity allows to address particular
parts of the biological tissue, leaving others
unaffected. This is important for many applications in, e.g., dermatology. In ophthalmology it was crucial, that laser radiation can
be transmitted by the vitreous body. That
enabled the development of an entire new
class of none invasive treatments with much
less pain for the patient.
Figure 1: A 577 nm OPSL laser
delivers superior photocoagulation
for wet-form AMD sufferers because
this wavelength matches a strong
absorption peak in oxy-hemoglobin.
32 LTJ November 2010 Nr. 6
AMD Treatment
Age-related macular degeneration (AMD) is
the leading cause of blindness in the world.
There are two distinct forms of AMD, both of
which affect older patients. Dry form AMD is
the more common and is characterized by a
steady and usually slow decline of the visual
field of view often beginning near the center
of the eye. Until recently, there was no drug
or interventional treatment for this disorder,
but an ultrafast laser based method is now
showing excellent promise.
Wet form AMD is an episodic disorder
characterized by blood vessels periodically
bursting in the retina causing a sudden and
dramatic loss in vision. Early stages of wet
AMD can be treated by lasers in two possible ways. Where the problem blood vessels
are grouped behind the fovea (yellow spot),
a type of Photodynamic Therapy (PDT) is
preferred. If the blood vessels are outside
the fovea and are allowing blood leakage
into the eye, photocoagulation is usually a
recommended treatment. In photocoagulation, the laser produces controlled, local
the author
matthias schulze
Matthias Schulze is Director Marketing OEM
Components & Instrumentation for Coherent
Inc. He joined Coherent in 1995 as a sales
engineer in Germany and subsequently was
holding various positions in marketing. First
at Coherent in Luebeck and later in companywide responsibilities. He holds a PhD in
physics from the Technical University in Berlin, Germany.

Dr. Matthias Schulze
Coherent Inc.
5100 Patrick Henry Drive
Santa Clara, CA 95054
Tel. +493030100786
[email protected]
www.coherent.com
cauterization, destroying the tiny culprit vessels and preventing further bleeding. Photocoagulation is also used to treat diabetic
macular edema (DME).
A key to successful photocoagulation is
tissue selectivity; that is, closing the target
vessels without damaging surrounding tissue in any way and with minimum discomfort to the patient, i.e. with minimal thermal
loading of the eye. The main differentiator
between the leaking vessel and other tissue
is blood, so this can best be achieved by using a laser wavelength that is preferentially
absorbed by blood. Photocoagulation also
needs a visible laser in order to permit the
beam to pass benignly through the transparent front of the eye. The main component
of blood with visible absorption is oxy-hemoglobin, and until recently, the most commonly used laser wavelength has been 532
nm which is close to a weaker absorption
© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Laser in der Medizin
peak in oxy-hemoglobin. But the absorption
of oxy-hemoglobin actually peaks at 577 nm
(see Figure 1).
A laser that emits precisely at this wavelength can be constructed using optically
pumped semiconductor laser (OPSL) technology. So to better treat wet-form AMD,
Coherent developed the Genesis laser family,
which delivers sufficient cw output power at
577 nm. First released in 2008, this yellow laser is enabling improved vessel closure with
reduced thermal loading on the eye.
Besides wavelength scalability, OPSL
technology offers another advantage for
photocoagulation. Specifically, the short
upper state lifetime of OPSL gain medium
enables these lasers to be fast pulsed directly
(at tens of kilohertz and beyond) simply by
switching the current supply to the pump diodes on and off. The development of pulsed
OPSLs for this application has enabled an innovative photocoagulation approach using
microsecond pulses without the expense or
complexity of an external modulator. This
fast pulsing provides extreme dosing control that appears to enable laser treatment to
initiate a wound-healing response without
actually causing any trauma.
LASIK
Laser in Situ Keratomileusis (LASIK) is arguably
the single most popular elective medical procedure to date – see Figure 2. Here a deep UV
(193 nm) excimer laser is used to reshape the
cornea and thus correct a patient’s eyesight
for near-sightedness and/or astigmatism.
Figure 2: Guided by a computer program, the excimer laser reshapes the cornea to
correct nearsightedness. The laser trims the cornea’s center, making it flatter. For farsightedness, a doughnut shaped-ring of tissue is removed. (Photo: VSDAR)
The first step in LASIK is to measure the
shape of the cornea and determine a pattern
to be ablated. In the actual procedure, the
patient typically is mildly sedated. An incision is made in the cornea using a scalpel
or sometimes an ultrafast laser, to enable
a thin (typically < 160 microns) flap to be
lifted from the center of the cornea, directly
exposing the bulk material. This is then carefully ablated according to the predetermined
pattern. The flap is then replaced.
For many years, the most important laser
parameters for this application were pulse
to pulse energy stability and excellent beam
uniformity; spatial hot spots or erratic pulse
energies can lead to uncontrolled ablation
and a poor outcome. But recently, there has
been equal emphasis placed on an increase
in pulse repetition rate for two reasons. First
this minimizes the duration for the procedure and thereby minimizes patient discomfort. Second, although the LASIK apparatus
uses an eye-tracking system to follow inevitable natural movement of the eye, minimizing the overall treatment time minimizes any
possibility of errors due to eye movement.
To support this need, the latest generation
of excimers for LASIK such as the Coherent
ExciStar now provide pulse repetition rates
up to 500 Hz.
the company
Coherent Inc.
Coherent designs and manufactures a
broad selection of lasers and supplies
electro-optic instruments for laser test
and measurement. The company‘s
products include laser diodes and laser
diode systems, carbon dioxide (CO2) lasers, excimer lasers, ion lasers, CW and
Qswitched DPSS lasers and systems,
ultrafast lasers and amplifiers. The company provides worldwide service and
applications support.
Contact:
Petra Wallenta, PR Manager Europe
Coherent GmbH
phone: ++49 - 89 - 5892746-25
E-Mail: [email protected]
www.coherent.de
© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 3: Scheme of the Cellvizio system, a fibered confocal microscope (FCM) to
obtain real-time fluorescent images. (Photo: Mauna Kea Technologies)
www.laser-journal.de LTJ 33 Laser in der Medizin Skin Resurfacing
Another elective medical application is
fractional skin resurfacing. Here the goal is
to modify the appearance of facial skin to
restore a more youthful appearance. Laser
treatments for this purpose date back many
years but fractional skin resurfacing is a much
less traumatic procedure than earlier methods, with less patient discomfort and a much
shorter healing period. For example, physicians who offer this procedure typically claim
only five days of healing are necessary.
While generally considered to be primarily a cosmetic procedure, recently, this type
of fractional skin resurfacing also has been
used to successfully reduce the appearance
of scars on burn victims. The laser of choice is
a carbon dioxide laser with a power output of
a few tens of watts. The output is chopped as
the laser is scanned across the target skin in
order to produce a closely spaced pattern of
small holes with a typical depth of 2 mm. As
these holes heal, the skin is tightened. In addition, the laser treatment promotes new collagen growth. The end result is a reduction in
both fine and crease wrinkles. This treatment
is now also used to reduce scar disfiguration
in burn victims and to reduce acne scars.
ing through a positive lens matched to the
numerical aperture of a fiber bundle. This
bundle contains up to 30,000 individual
fibers with a core diameter of 1.9 microns.
With this arrangement, the beam is rastered
over the input facet of the bundle at up to 12
frames per second. Because this is a coherent
fiber bundle, the spatial mapping of the laser
across the individual fibers is completely preserved at the distal end, which can be configured for surface or sub-surface confocal
images – see figure 4. Returned fluorescence
follows the same path via the scanning mirrors. It is then spectrally filtered to eliminate
laser backscatter before being focused on to
an avalanche photodiode through a confocal aperture. The majority of applications require blue excitation, provided by an OPSL,
the Coherent Sapphire 488 nm laser.
Retinal Scanning
Retinal scanning is used to survey the back
of the eye for damage, evidence of disease
or other problems. It is a much-preferred alternative to the fundus camera whose use
typically causes patients discomfort due to
the intense flash light source, and temporary
over-sensitized vision. Moreover, a fundus
photograph requires the eye to be dilated
which is not possible for some patients. In
contrast, a retinal scanner obtains an image by rastering a collimated visible laser
across the back of the eyeball and detecting
backscatter using an avalanche photodiode
(APD). Early instruments used a single laser,
either green or near-IR. But recently, these
instruments have incorporated three lasers
(red, green and blue) in order to create a fullcolor image of the eye. These instruments
have benefitted from the availability of
economical OPSLs at arbitrary visible wavelengths, and their excellent (TEM00) beam
quality and low noise characteristics.
In cases where the scan or other evidence
suggests a vascular type problem, another
type of laser scanner may be used to perform a FA (fluorescein angiography) scan.
For this test, the patient’s blood is fluorescently labeled with fluorescein dye. The FA
retinal scanner then uses a 488 nm laser to
provide the same rastered field of view and
acquires detailed fluorescent images of the
blood vessels at the back of the eye, including any leaking blood.
Clinical Testing and Diagnostics
In many cases, the use of lasers in this market
segment takes advantage of technologies
originally developed for other markets. An
example is how laser based confocal microscopy paved the way towards endoscopic
and retinal scanning applications.
Endoscopic Confocal Microscopy
Traditional endoscopy is widely used to examine both the gastrointestinal (GI) and
respiratory tracts. During the exam, the
physician may excise 30 or more different
samples for subsequent analysis by microscope. Not only is this a time-consuming
process, but sadly, this approach often fails
to spot precancerous problems and small
cancerous growths. In response companies such as Mauna Kea Technologies, have
pioneered the development of the fibered
confocal microscope (FCM) to obtain realtime fluorescent images during the actual
procedure. Some products are already FDA
approved for use in both bronchoscopy and
GI endoscopy.
Figure 3 schematically illustrates the main
components of the Cellvizio FCM system
from Mauna Kea. The laser beam is deflected
with a pair of scanning mirrors before pass-
34 LTJ November 2010 Nr. 6
Figure 4: A selection of single frame images acquired with the Cellvizio. (a) In vivo
mouse colon after instillation of acriflavine (Photo: D. Vignjevic, S. Robine, D. Louvard,
Institut Curie, Paris, France). (b) In vivo reflectance imaging of human mouth mucosa.
(c) Ex vivo Autofluorescence imaging in human lung (Photo: Dr. P. Validire, Institut
Mutualiste Monsouris, Paris, France). (d) Microcirculation of the peritubular capillaries
of a live mouse kidney. (Photo: I. Charvet, P. Meda, CMU, Geneva, Switzerland and L.
Stoppini, Biocell Interface, Geneva, Switzerland).
© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Laser in der Medizin
Figure 5: This low density polyethylene
(LDPE) medical fluid container has been
welded with a diode laser system.
Medical Device Manufacturing
Medical Device Manufacturing is a very diverse applications space that requires a similarly diverse range of laser solutions. Plastics
welding and Micromachining are good examples illustrating this breadth.
Plastics Welding
A fast growing application area is laser welding of plastics using direct diode systems,
usually fiber-coupled, with powers typically
in the tens of watts range. Laser welding offers
many advantages over other plastic-welding
techniques, such as hot plate and ultrasonics. It can be used on many types of plastics
that are commonly employed in the medical
products, including polyethylene, polypropylene, acrylic, nylon and even Teflon (see
Figure 5). In most cases laser welding of plastics relies on the so-called lap weld, where
two sheets of material are overlapped. This
creates a large area surface-to-surface join,
rather than an edge-to-edge join achieved
in a butt weld. The lap weld geometry yields
a very strong joint, and can be implemented
in a process known as transmission welding.
With 100 watts of direct diode output, weld
speeds can exceed 2000 mm/min even for
weld widths of several millimeters.
mia. In this case, ablation catheters or mapping catheters are directed to the heart, via
a femoral entrance. Different technologies
may then be inserted to selectively kill tissue
by heat or cold. Here, excimer-processing
tasks may include stripping polymer insulation from the metal wires of cauterization
devices to provide electrical access. Laser
thinning (outer layer stripping) to increase
the flexibility of devices is another technique
that relies on Coherent excimer lasers.
One emerging medical device application is the use of very high pulse energy excimers, such as the LambdaSX, to produce
complete miniaturized circuits with a single
laser pulse for use in disposable blood analyzers, Another developing process is the use
of solid state ultraviolet lasers, such as the
Coherent AVIA, series to produce miniature
marks on medical products enabling suppliers to form unique device identification
(UDI) marks. For example, individualized
marks can be made on high value products,
such as titanium surgical screws and plates,
to help stem the growing problem of counterfeit and illegally transshipped products
(see Figure 6).
The main, overarching trend throughout
medical device fabrication is for increased
miniaturization, either to reduce product
size or to increase the functionality/size ratio. Laser micromachining has proved to be
a key technology supporting this by drilling,
cutting, scribing and contouring materials
as diverse as silicon, metals and plastics, with
features as small as a few tens of microns.
The goal of micromachining is spatial
selectivity: producing micron-scale features
while avoiding peripheral thermal damage.
Until recently, precision micromachining has
been powered by pulsed lasers with nanosecond pulse durations, either DPSS (diode-
pumped solid-state) lasers or excimer lasers.
But now, the availability of fiber-based ultrafast lasers such as the Coherent Talisker
is enabling even higher resolution and superior edge quality as well as virtually eliminating peripheral thermal effects (see Figure
7). The reason is that ultrafast (picosecond
and femtosecond) lasers remove material
by a combination of thermal and optical
(multiphoton absorption) effects. The latter
is a relatively cold ablative process. Moreover, multiphoton absorption allows the machining of several high bandgap materials
(e.g. glass, certain polymers) that have low
linear, optical absorption and so are difficult
to process with existing, commercially available lasers. In addition, ultrafast lasers also
provide greater control of depth and greater
selectivity with respect to the bottom layer
(in the case of thin films) because more material is removed as vapor and there is less
thermal diffusion in the vertical direction.
Figure 6: UV lasers can be used to create small encrypted identifying marks on
parts as small as this titanium surgical
screw head. (Photo: Stryker Micro Implants)
Figure 7: This hole was laser drilled
with an ultrafast industrial fiber laser
(Coherent Talisker) in 0.2 mm thick stainless steel. The edge quality would be
near-impossible with any other commercial laser type.
Conclusion
Lasers are utilized in medical applications
only when they provide superior value and
results; a better outcome, faster healing, a
lower cost test, or a less invasive procedure.
The fact that diverse medical applications
now utilize nearly every type of commercial
laser technology, (including DPSS, OPSL,
fiber, excimers, direct diode, ultrafast, and
carbon dioxide) proves that laser manufacturers have universally risen to the challenge
of delivering improved performance, reliability and value.
Micromachining
The breadth of laser machined medical devices is amazing, including stents, bioabsorbable stents, stent grafts, embolic filters,
precision drug delivery devices, electrophysiology/ neurological devices, many types of
catheters, intravascular radiation delivery
devices, angioplasty balloons, and femoral
closures. One of the most dynamic areas for
these devices is electrophysiology, particularly to treat cardiac disorders such as aryth-
© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.laser-journal.de LTJ 35