Laser Physics and Tattoo Removal

LASER

Laser is an acronym: LASER

Light

Amplification

Stimulated

Emission

Radiation

LASER means “Light amplification by the stimulated emission of radiation”, which means spatially coherent light emitted after the simulation of something.

The substance (lasing medium) that is stimulated ranges widely from gases to solids to liquid. The light that is emitted can be a range of visible colors or invisible wavelengths. Lasers are diverse in their application, power, and sophistication. Some may be purchased for a few dollars (a laser pointer). Tattoo removal lasers generally cost around $100,00 for a new, top-of-the-line system. Scientific lasers can be as large as a room, mounted on an aircraft, or tiny components of a cutting-edge research project.

What Is a Laser

A device that emits light (electromagnetic radiation) through a process called stimulated emission

Laser light is usually spatially coherent so that the light is emitted in a narrow, low-divergence beam

Typically, lasers emit light with a narrow wavelength spectrum (“monochromatic light”)

Key Components of a Laser

Gain Medium (material with properties that allow it to amplify light created by the flashlamp, examples include dye, gases, solids such as crystals, and semiconductors; the gain medium determines the wavelength of light that will be emitted by the laser)

Pump Source/Flashlamp (provides energy to the laser system; they discharge high current pulses and are filled with Krypton gas, Xenon, or a combination)

High Reflector (used to reflect light through the gain medium to amplify this light)

Output Coupler (a partially reflective mirror allowing for extraction of a portion of the laser beam)

Laser Beam (the “business-end of the laser”; this will be used to create an effect in the skin)

Q-Switching

Extremely high peak powers can be achieved by allowing the photons inside a laser cavity to build by using fully reflective mirrors at both ends.

A mechanically, optically, or electronically-gated mirror within a laser cavity can release photons in a fraction of a millisecond.

This process is called Q-switching and can generate a high energy pulses with pulse widths in the nanosecond range.

Why Extremely Short Pulses are Useful

Keeping the energy input constant, a shorter pulse allows greater power to be delivered.

A continuous wave or millisecond laser are not able to generate the peak power that can be condensed into a nanosecond (billionth of a second)

These ultra-short pulses can produce 300 degree centigrade tissue temperatures in nanoseconds.

This causes a rapid thermal expansion that shatters the target (e.g. tattoo pigment) into extremely small particles that can be removed by macrophages in the patient’s tissue.

The shock wave to tissue that results from this rapid thermal expansion is also referred to as the photoacoustic effect.

Two Key Mechanisms Shatter Tattoo Ink

PhotoAcoustic Effects

The photoacoustic effect is a conversion between light and acoustic waves due to absorption and localized thermal excitation.

When rapid pulses of light are incident on a sample of matter, they can be absorbed and the resulting energy will then be radiated as heat.

This heat causes detectable sound waves due to pressure variation in the surrounding medium.

One the key ways tattoo removal lasers break up tattoo ink is through shattering the ink through these high-intensity waves.

Selective Absorption of Chromospheres

Chromophore: the part of a molecule responsible for its color

The human skin has many chromophores, including melanin, hemoglobin, etc.

When using a tattoo removal laser, one of the goals is the have the tattoo ink absorb wavelengths of light that will destroy the ink, but leave other chromophores undamaged.

Remember, color is reflected wavelengths that were not absorbed by a substance. An red apple absorbs all colors except red, which is reflected to the viewer. A black tire absorbs all colors. A white piece of paper reflects back all colors.

Certain laser wavelengths are used that can be absorbed by tattoo ink that we are removing.

Goals of Laser Tattoo Removal

We are seeking to shatter ink in the skin but leave the skin as undamaged as possible.

The Theory of Selective PhotoThermolysis

One of the goals of laser therapy is to confine damage to microscopic sites of selective light absorption in the skin, such as blood vessels, pigmented ink, and unwanted hair with minimal damage to adjacent tissue. To achieve this selective effect, laser would need to fulfill three requirements:

They should emit a wavelength that is highly absorbed by the targeted structure

They should produce sufficiently high energies to inflict thermal damage to the target

The time of tissue exposure to the laser should be short enough to limit the damage to the target without heat diffusion to the surrounding tissues. This is known as the thermal relaxation time (TRT).

The Gold Standard: Nd:YAG Lasers

Q-switched Nd:YAG Lasers (532nm and 1064nm)

The Q-switched Nd:YAG laser system overcomes the obstacle of excessive melanin absorption and is used to remove blue and black ink and tattoos in darker skin types (1064nm), or red pigment (532nm).

The clinical endpoint following laser treatment is whitening of the skin with occasional mild pinpoint bleeding. Current models offer a spot size range of 1.5 to 8mm.

Pulse length as low at 5ns up to 20ns, which allows significant power while meeting Thermal Relaxation Time constraints – damaging tattoo ink particles while minimizes damage to the skin.

1064nm

The long 1064nm wavelength has the deepest penetration and carries the least risk of hypo-pigmentation; however, it is also the least effective in removing brightly colored pigments.

Of all the laser systems, it is the preferred system for use in darker skin types. This wavelength may also be useful when residual, more deeply placed ink particles are all that remain.

532nm

The 532nm wavelength (green light) is absorbed by hemoglobin, and as a result, purpura lasting 1 week to 10 days frequently occurs after treatment.

This wavelength is also effective for red, orange, and occasionally yellow ink.

Some reports have detailed the paradoxical darkening of red tattoo pigment as well as other skin-toned, yellow, and pink tattoos.

This occurs as the laser pulse reduces ink from rust-colored ferric oxide (Fe2O3) to jet black ferrous oxide (FeO).

Similarly, bright colors may contain white ink made up of titanium dioxide (TiO2, T4+) that is reduced to TiO2 or blue Ti3+ upon laser treatment.

Other Tattoo Removal Lasers

Alexandrite lasers (755nm): Designed with goal of removing blue and green tattoos that are difficult to completely remove with 1064nm and 532nm wavelengths from Nd:YAG lasers. Alexandrite lasers can be highly effective at removing darker colors such as black and blue, have some difficulty with green tattoos, and do a very poor job addressing red, yellow, and other lighter colored ink. Also, these lasers often operate at a slower rate that makes treatments take longer and may discourage their use.

Ruby Lasers (694nm): The earliest lasers used a Ruby crystal. Early tattoo removal lasers were ruby lasers, but were associated with significant scarring. Newer q-switched Ruby lasers are uncommon, but they are very effective at removing green ink, as well as a range of other colors. Of course, ruby lasers are ineffective at removing red ink tattoos because the red ink is a reflection (rather than absorption) of the wavelength of light associated with red. Without the required absorption of energy, shattering the tattoo ink is almost impossible.

Hypopigmentation occurs in many patients with Ruby lasers, which resolves within 3 to 4 months of treatment

Future Tattoo Removal Lasers

A recent study from Lawrence-Livermore labs sought to discover the optimal process to break up tattoo ink. It investigated by computer simulation the actual mechanism of the breaking up of tattoo ink.

It found that if the laser pulse length is sufficiently short, the strong acoustic waves created were able to exceed the fracture threshold for graphite tattoo pigment. The shorter the pulse the better.

Although the tattoo particles would never reach the melting point, a cavitation bubble would be formed. The steam generated would get into the cracked particle and cause a stream-carbon reaction.

Found that the optimal pulse length was 10-100 picoseconds to minimize laser fluence and collateral damage.

New research is focused on the optimal pulse duration for fragmenting tattoo particles. This is thought be to 10 picoseconds to 100 picoseconds.

Picosecond: one-thousandth of a nanosecond; Fetosecond: one-thousandth of a picosecond, one-millionth of a nano-second.

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  6. Medical Components of Laser Tattoo Removal
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