Lasers in urology

Introduction• The word “LASER” is an acronym for light amplification by stimulated emission of

radiation. • Laser technology is one of the consequences of Nobel Prize winning developments in

quantum mechanics. • Laser is an energy source, which provides large quantities of energy rapidly to

remote locations. • Lasers are being used as a standard tool of the research lab, communications,

surveying, manufacturing, diagnostic medicine and surgery.• Various applications of lasers in medicine have been sought, since the creation of the

laser. • Lasers have become well established in urology, particularly in the treatment of

urolithiasis and benign prostatic hyperplasia (BPH). • Laser use in these areas is becoming standard and accepted practice and now it is

being investigated in laparoscopic urology. • In this chapter, we will first discuss the history and basic mechanisms of laser and

then various types of lasers, and their applications in the field of urology.

History• Albert Einstein (1879–1955) was the first, who proposed the basic concept

of laser technology. • He showed that light is not in the form of continuous waves, but it is in the

form of packets of energy called “photons.”• In 1960, first visible light laser was produced by TH Maiman, which

consisted of synthetic ruby crystal with silver-coated ends surrounded by a helicoidal flash tube.

• Silver-coated ends improved the performance of laser by creating an optical Resonator.

• In 1966, Parsons was the first to use similar ruby laser in a pulsed mode in canine bladders, followed by CO2 laser for condyloma in 1980, photodynamic therapy (PDT) for carcinoma bladder in 1982 and the pulsed dye laser for urolithiasis.

• In 1990, modern era of lasers in urology began with their application in BPH.

BASIC PRINCIPLE OF LASER• The term “light” is generally accepted

to be electromagnetic radiation ranging from 1 nm to 1000 μm in wavelength.

• The visible spectrum (what we see) ranges from approximately 400–700 nm.

• The wavelength ranges from 700–10 μm is considered the near infrared and anything beyond that is the far infrared.

• Conversely, 200–400 nm is called ultraviolet (UV) and below 200 nm is the deep UV.

• Einstein innovation was based on two basic principles

• of physics:

• 1. Light travels in packets of energy, known as photons.• 2. Most atoms or molecules exist naturally in a ground or low-energy state. • Some atoms also exist at any given time at a higher discrete energy level. • Their energy level can be raised by adding electricity, heat or light energy to

atoms in their ground state. • Electrons in atoms remain in a specific orbital patterns and radii. • Each orbital has a specific energy level. • An electron can move from one orbital to another of higher or lower energy

and will absorb or emit an amount of energy exactly equal to the difference in energy between the two orbitals.

• The process by which a photon’s energyis captured by an electron, which leads the electrons to ahigher-energy orbital, is called stimulated absorption .

• The process by which an electron jumps to a lower level orbital and releases the energy in the form of photon is called “spontaneous emission” .

• In 1917, Einstein proposed that stimulated emission may be the key to laser action.

• So, a photon of a particular wavelength (i.e. energy) must collide with an atom ready for spontaneous emission at that wavelength (i.e. having an electron already in the higher-energy orbital), a situation known as a population inversion, leading to an immediate release of photon and electron descents to a lower orbital.

• The colliding photon leaves the electron with the emitted photon, with both photons having the identical wavelength, phase and direction.

• Energy must be supplied to this population. • In a laser, the energy source is usually electric or flash lamp driven. • The population of atoms or molecules that become excited are the lasing

medium.

• A photon hits an electron, transferring its energy to the electron and electron jumps into a higher energy orbital. This is known as absorption

An electron in an orbit higher than the ground state may spontaneously lose its energy in the form of an emitted photon

• A photon may hit an electron that is already in a highenergy orbit with the result that two perfectly coherent and collimated photons leave the electron.

• This is known as stimulated emission

Fig. 33.5: Components of laser

COMPONENTS OF LASER SYSTEM• All laser devices have three components (Fig. 33.5) :• 1. Optical resonating chamber (cavity).• 2. Two mirrors.• 3. Space between these mirrors is filled with a lasing• medium, such as argon, neodymium: yttriumaluminium-• garnet (Nd:YAG) or carbon dioxide (CO2 ).• The lasing material can be molecules, ions, atoms, semiconductors

or even free electrons in an accelerator.• An external energy source, such as an electric current excites the

lasing medium within the optical cavity. • This causes many atoms of the lasing medium to be raised to a

higher-energy state.

• Spontaneous emission occurs in all directions. Light (photons) emitted retained within the resonating cavity by multiple reflections from the well-aligned mirrors.

• One mirror is completely reflective and the other partially transmissive. • Stimulated emission occurs when a photon acts over another excited atom

in the optical cavity, producing pairs of identical photons of equal wavelength, frequency and energy.

• These all remain in phase with each other.• The mirrors act as a positive feedback mechanism for the stimulated

emission of radiation by reflecting the photons back and forth.

• The partially transmissive mirror emits some of the radian energy as laser light, which passes through a lens that focuses the laser beam to a very small beam diameter or spot size ranging 0.1–2.0 mm.

• The characteristics differentiate laser light from natural• light are:• • • Coherence—the photons are all in phase • • • Collimation (they travel parallel with no divergence)• • • Monochromaticity (they all have the same wavelength).• Different types of lasers have different lasing mediums(which

can be solid, liquid or gas).

Coherence

Collimated light

• So, there is release of photons in different wavelengths of the electromagnetic spectrum, which determines the characteristics of a particular laser.

• Other factors that affect laser performance include the power output and the mode of emission (e.g. continuous wave, pulsed or Q-switched). Pulsed lasers

• have more precise control and less lateral heat conduction than a continuous output laser.• • • Continuous wave lasers emits a steady-state and• uninterrupted beam.• • • Pulsed lasers have the following different types:• – – A gated pulse laser has a timed interrupted output• with a peak power lower than continuous mode.• – – A true pulse mode in which the power output is built-up between pulses leading to

higher peak power than a continuous mode.• – – A superpulse in which beam appears to be continuous because of fast frequency of

pulses per second (about 300–1000/second).• • • Q-switched mode produces very high peak power outputs (in the order of tens of

millions of watts) for very short durations (a few nanoseconds).

• Surgeon can control three variables of laser:• 1. Power (measured in watts)• 2. Spot size (measured in millimeters)• 3. Exposure time (measured in seconds).• Out of these three variables, power is the least useful as a

parameter and may be kept constant with widely varying effects, depending on the spot size and the duration of exposure.

• Power density (PD) is a more useful measure of the intensity of the beam at the focal spot than power because it takes into account the surface area of the focal spot.

• PD (watts per square centimeter) = (power in the focal spot)/(area of the focal spot)

LASER TISSUE INTERACTION• The laser tissue interaction involves certain physical• principles:• •• Absorption: Necessary for the conversion of light to• thermal energy• •• Reflection: The back scattering of energy away from the• tissue surface• •• Scattering: The internal reflection of energy by various• tissue components, resulting in diffuse absorption• •• Transmission: When energy is not fully absorbed, but rather

penetrates through the tissues, resulting in less transfer of energy to the tissue.

• Quality of the laser-tissue interaction depends upon the wavelength of laser used and local tissue properties including the density, degree of opacity (e.g. quantity of pigments), water content and blood supply of the tissue.

• The more dense or opaque a tissue is, greater the degree of absorption of light energy and more is the heat production.

• Molecules, proteins and pigments may absorb light only in a specific range of wavelengths.

• Hemoglobin, for example, absorbs light energy that has a wavelength as high as 600 nm and is translucent to light beyond this range.

• The CO2 laser produces light in the far infrared spectrum at 10,600 nm. • This is heavily absorbed by water contained in tissue and therefore

does not penetrate deeply.

• Local blood circulation also affects the degree of laser energy absorption because of the hemoglobin and circulating blood acts as a heat sink by transporting absorbed thermal energy away from the site of delivery.

• The wavelength of laser light can be proportional to the depth of penetration into specific tissues.

• The longer is the wavelength, the deeper will be the expected penetration.

• The Nd:YAG laser, for example, produces light in the near infrared region (1064 nm) and penetrates to a depth of approximately 4–6 mm in most tissues (at its wavelength, Nd:YAG is not absorbed by hemoglobin or water in any significant quantity).

• The CO2 laser with a wavelength of 10,600 nm penetrates only to a depth of less than 0.1 mm because its wavelength is highly absorbed by water present in the tissues.

• The laser tissues interaction involves one or more of• the following effects:• 1. Thermal effect: Absorbed energy is transformed• leading to increase in temperature of the tissue, with• various biological effects. These effects are described• as follows:• Temperature Effects on tissues• > 40ÅãC Protein denaturation• > 60ÅãC Protein coagulation• 100ÅãC Vaporization of tissue water• > 250ÅãC Carbonization• > 300ÅãC Tissue vaporization• The thermal effect depends upon the wavelength• of the laser light, the PD and tissue properties, e.g.• CO2 -laser have wavelength of 10,600 nm (in the• infrared region of the spectrum). So, there is almost• complete energy absorption leading to carbonization• and vaporization.

• 2. Mechanical effect: Very high PD over stone surface results in freeing of a column of electrons and the formation of a “plasma bubble.”

• The rupture of this plasma bubble changes the ultrastructure of the stone, leading to fragmentation of stone.

• 3. Photochemical effect: Photoactivation of a specific drug and its transformation to a toxic compounds resulting in cellular death through the generation of the free radicals and peroxides that damages DNA and mitochondria.

• 3. Photochemical effect: Photoactivation of a specific drug and its transformation to a toxic compounds resulting in cellular death through the generation of the free radicals and peroxides that damages DNA and mitochondria.

• 4. Tissue-welding effect: It is the tissue reapproximation through the application of focused thermal energy.

• This energy induces the interdigitation of collagen(biological glue) with a solder (50% human albumin).

TYPES OF LASERS

Ruby Laser• It is a solid-state laser using synthetic ruby crystal as its lasing

medium. • Theodore Harold “Ted” Maiman at Hughes Research Laboratories

on May 16, 1960 made the first working ruby laser.• It produces pulses of visible light at a wavelength of 694.3 nm,

which is a deep red color.• The active part of the ruby is the dopant, which consists of

chromium ions suspended in a sapphire crystal.• The dopant is responsible for all of the absorption and emission of

radiation. • Depending on the concentration of the dopant, synthetic ruby

usually comes in either pink or Red.

• Use of ruby laser has decreased with invent of other lasers.

• But they are still used in a number of applications where short pulses of red light are required. Holographers are using it for making holographic portraits with ruby lasers, in sizes up to a meter square.

• Ruby lasers were used extensively in tattoo and hair removal, but are being replaced by alexandrite lasers and Nd:YAG lasers in this application.

CO2 Laser

• CO2 lasers operate in the invisible far infrared portion of the electromagnetic spectrum at 10,600 nm.

• Special materials are necessary for their construction. • Typically, the mirrors are silvered, while windows and

lenses are made of either germanium or zinc selenide. It is highly absorbed by water, therefore it vaporizes water-dense tissues to a superficial depth of less than 1 mm.

• Depth of thermal coagulation is about 0.5 mm. • It can coagulate vessels smaller than 0.5mm effectively.

Neodymium:Yttrium-Aluminum-Garnet Laser

• Neodymium:yttrium-aluminum-garnet laser emits light at a wavelength of 1,064 nm (close to infrared portion of the electromagnetic spectrum) and its active medium consists of neodymium atoms in an yttrium-aluminium-garnet rod.

• In early 1990, this laser was first used for BPH. • This is poorly absorbed by water and body pigments. So, it can penetrate the tissues

more deeply (4–6 mm). • The laser-tissue interaction of Nd:YAG laser comprises of light energy absorption and

conversion to heat or thermal energy. • The relatively slow and nonselective absorption of Nd:YAG laser energy by soft tissue

proteins leads to a relatively gradual transfer of thermal energy to tissues, producing slow tissue cooking or coagulation.

• The Nd:YAG laser thus became an inherently excellent tissue coagulator. • Subsequently, the coagulated tissue sloughs with passage of time• leading to prolonged dysuria and catheterization following laser prostatectomy, but

with time, this laser was superseded by high energy lasers with vaporization properties.

• High power Nd:YAG has also been tried, but that led to coagulation and tissue charring causing sloughing of tissues and prolonged dysuria.

• It has good hemostatic (coagulates blood vessels as much as 5 mm in diameter) and cutting properties.

• It can be delivered in a continuous, pulsed or Q-switched mode. • It is also used for lithotripsy in Q-switched mode. The frequency-

doubled, double-pulse Nd: YAG• (FREDDY) laser is a solid-state laser with wavelengths of 1064 and 532

nm, which can fragment stones by generation of a plasma bubble. • The FREDDY laser is not able to fragment cystine stones and is also not

able to coagulate, incise or vaporize tissue; however, the FREDDY laser can be considered as a low-cost alternative for laser lithotripsy for noncystine stones.

Dye Lasers• Pulsed-dye laser was the first laser lithtrite available, which used a

coumarin green dye as the liquid laser medium.• Coumarin dye emitted wavelength of 504 nm and it was delivered

through optical quartz fibers. • This laser was first tried for stones, but both calcium oxalate

monohydrate and cystine stones were resistant to this laser. • Ureteral perforations occurred with this laser. • Other limitations of the coumarin pulsed-dye laser include the initial

high cost of the device, requires about 20 minutes before it is ready to function and the required eye protection (amber glass), interfering with visualization of stone.

• As opposed to a solid-state laser, the dye in the lasing chamber requires replacement, which is also inconvenient and expensive.

Alexandrite Laser

• It contains chromium-doped mineral known asalexandrite (BeAl2 O4 ).

• The wavelength range is 380–830nm and is strongest at 700–830 nm.

• It is well absorbed by pigment like melanin. So, it is used for cutaneous lesions and lithotripsy of pigmented stones.

• It has been tried for tissue welding.

Semiconductor Diode Laser• Laser light is produced using light-emitting diodes between reflecting mirrors in

a resonator tube. • With conventional lasers, less than 5% of the electrical input is converted into

laser light. • So, conventional lasers require high-energy cooling devices and radiators, thus

increasing the size of the machine. • The diode laser allows the more efficient use of the photons that are generated. • Thus, the available diode lasers used for medical purposes are small and

portable. • The 980 nm diode laser uses a wavelength that has the highest simultaneous

absorption in hemoglobin and water, which is theorized to provide both hemostatic and ablative properties.

• The 1470 nm wavelength is well absorbed by both hemoglobin and water. • Diode lasers are currently used for tissue coagulation and its use has been

established in BPH.

Potassium-Titanyl Phosphate/Lithium Triborate Laser

• Potassium-titanyl phosphate (KTP) crystal acts as a lasing medium in this laser. • It produces double the frequency of an Nd:YAG laser and produces a 532 nm wavelength

leading to intermediate level of coagulation and vaporization. • Depth of tissue penetration is half as compared to Nd:YAG laser, but higher energy per unit

tissue volume is produced, which increases tissue vaporization and desiccation.• Initially low power KTP lasers were used as hybrid with older Nd:YAGlasers, which combined

the cutting, sculpting or incising capabilities of low power KTP and coagulative necrosis property of Nd:YAG lasers.

• Later on, high power KTP 60 W and 80 W were used with rapid vaporization properties. • The KTP and lithium triborate (LBO) lasers produce the same 532 nm light beam within the

visible green region of the electromagnetic spectrum with different maximal average power 80 W and 120 W, respectively.

• Lithium triborate is built up of a continuous network of B3 O7 groups with lithium cations located in the interstices.

• Lithium triborate laser is also known as GreenLight high-performance system (HPS) .

• The KTP or LBO lasers are selectively absorbed by hemoglobin within prostatic tissue, thus permitting photoselective vaporization of prostatic tissue (PVP) by rapid photothermal vaporization of heated intracellular water.

• With a short optical penetration of 0.8mm, the resulting coagulation zone is limited to 1–2 mm,which leads to a more focused and effective vaporization when compared with the 4–7 mm coagulation zone of Nd:YAG.

• GreenLight HPS 120 W, 532 nm LBO laser, PVP has substantially more vaporization efficiency and speed with equally favorable tissue interaction as compared to 80 W KTP.

Holmium:Yttrium-Aluminum-Garnet Laser

• The holmium (Ho) wavelength (2143 nm, nonvisible/infrared) is absorbed by water .

• The depth of penetration is 0.5 mm. • It is a pulsed solid-state laser, which leads to a shorter absorption length. • Both Ho and YAG have excellent hemostatic properties and can be used

in normal saline, so decreases incidence of hyponatremia and operative time.

• Holmium energy is delivered through small, flexible fibers.• Energy travels along the laser fiber through internal reflection. • Specialized clear lens protective eyewear is recommended for the

operating physician and those working with the fiber. • For the patient having the eyes closed or blocked by a sheet is sufficient.• The machine is key-controlled and easy to use

(A) GreenLight high-performance system (AMS,Minnetonka, MN); (B) Potassium-titanyl phosphate side firing

laser fiber

(A) Holmium laser; (B) Enucleation of prostatewith end firing fiber

Thulium Laser

• The thulium laser has a tunable wavelength between 1.75and 2.22 μm, mid infrared (invisible).

• Its characteristics are similar to Ho laser, but it is a continuous wave laser and has high absorption in water and low absorption in oxyhemoglobin.

• Its optical penetration is 0.4 mm and coagulation depth is less than 1 mm.

• This results in high absorption of the laser energy into the prostate and tissuevaporization.