L 36 Modern Physics [2]

How lasers workMedical applications of lasersApplications of high power lasers

Medical imaging techniquesCAT scansMRI’s

How lasers work• First we must understand

the difference between incoherent and coherent radiation

• Ordinary light sources (light bulbs, fluorescent lights, etc) produce incoherent light

• lasers produce coherent light all atoms radiate in the same manner

Spontaneous vs Stimulated Emission • Coherent radiation is

produced when an atom undergoes stimulated emission.

• Spontaneous emission occurs when an electron makes an unprovoked transition to a lower energy level

• Stimulated emission occurs when an incoming photon induces the electron to change energy levels amplification

Ei (larger energy)

Ef (smaller energy)

Spontaneous emission

Stimulated emission

Incomingphoton

photon

Simulated emission• With stimulated emission, one photon

comes in and its vibrations cause the electron to fall to the lower energy level, emitting another photon (the bridge)

• Thus, one photon goes in and 2 come out, moving in step and in the same direction

• To get a system of atoms to undergo lasing action, we must arrange to have many atoms in the excited state this is called population inversion.

Population Inversion• In a normal situation (a)

more atoms are in the lower state than the upper state

• If an external energy source is provided to excite electrons into a higher energy state, a population inversion can be created as in (b)

• this is called pumping

The laser medium

• The atoms are pumped into an excited state

• The excited electrons fall into an intermediate state (upper laser state) and stay there until a photon comes along and causes them to fall down to the lower laser state.

He Ne Laser (633 nm red)• A HeNe laser is a gas laser which uses a

medium of 15% Helium and 85% Neon.• A high voltage discharge is produced in this gas

mixture and this produces the population inversion.

• The lasing action starts when one atom emits a photon which then induces another atom to emit and so on.

• The partially silvered mirror keeps most of the photons in.

• Photons which are not moving horizontally do no become part of the laser beam.

A Helium-Neon (HeNe) Laser

Medical Applications of Lasers

Laser surgery to correct for(a) nearsightedness, and(b) farsightedness

Applications of High Power Lasers

Using lasers toCut metals

Laser Fusion

Multiple beams of a powerful laser are focusedon a tiny pellet containing fusion fuel. The laser energy compresses the pellet producing a mini-hydrogen bomb that produces energy

pellet

Solid State Laser Diodes

small

Come in a variety ofdifferent colors

Lasers Diodes

• Diode lasers use semiconductor materials (tiny chips of silicon) as the lasing media

• When current flows through the silicon chip it emits an intense beam of coherent light.

• Diode lasers are used to read the information embedded in the pits in CD’s and DVD’s, and also to read UPC’s in bar code scanners and in laser pointers!

Medical Imaging Techniques

• x-rays

• CT and CAT scans (Computerized Tomography)

• MRI’s (Magnetic Resonance Imaging)

X-rays

• very short wavelength (0.01 – 0.1 nm) electromagnetic waves

• produced when energetic electrons slam into a metal target

• able to penetrate soft tissue, but not bone

• produces a two dimensional shadow image

A pineapple and a bananna• A shadow image can be

misleading• two shadows taken from

different angles provides a better picture

• shadows taken at multiple angles gives a more complete picture

• this is what a CT or CAT scan does

CAT Scans

X ray images are taken at many different anglespassing through the patient. Some of the cuts overlap. A full three dimensional image can be reconstructed using computers. this procedure is called tomography.

Computerized Tomography

• A computerized tomography or CT scan image is formed by analyzing x-ray shadow images taken at many different angles and positions

• an x-ray source and an array of electronic detectors rotates around the patient as the patient slowly moves through the ring.

Is there a better medical diagnostic?

• A CAT scan does a good job of imaging bones, but it does not provide as good an image of soft tissue

• Also, it requires that the patient receives a big dose of x-rays, which can be harmful in themselves it is an invasive diagnostic

• Magnetic resonance imaging (MRI) is a better method of imaging soft tissue

MRI- how does it work?• MRI works by locating the hydrogen atoms

inside the body. Since the body is mostly water, there are lots of hydrogen atoms

• the nucleus of a hydrogen atom is a single protons. Protons behave like tiny bar magnets with a north pole at one end and a south pole at the other end.

• If you put a bar magnet in a magnetic field, it will try to align itself with the field.

A bar magnet in a magnetic field

N

S

magnetic field

NS

Solenoid for producing a strong magnetic field by passing a large current

through a set of coils

Magnetic Resonance Imaging• The rules of atomic physics (quantum

mechanics) require that the atomic hydrogen bar magnets can only have 2 orientations when placed in a magnetic field either parallel or antiparallel to it, we call this spin-up or spin-down

spin-up spin-down

magneticfield

protons

Magnetic Resonance• Protons has a “spin” that can be either

“up” or “down” relative to the direction of the magnetic field

• If radio waves (FM) hit the protons, it can cause it them to flip from one spin state to the other at a frequency that depends on the strength of the magnetic field

• These spin flips result in the absorption or release of radio wave energy that can be detected electronically

Magnetic Resonance Imaging

• In effect, the magnetic field makes the protons act like tiny radio transmitters that only broadcast their signal when the value of the magnetic field is just right

• By varying the strength of the magnetic field as a function of position in the body, the spin flips can be detected in various parts of the body

• A computer is used to combine the signals from various parts of the body to generate detailed cross-sectional images

MRI DEVICE

Homer

MRI safety considerations• The magnetic field used in MRI are very

strong – 30,000 times the strength of the earth’s magnetic field.

• Because the magnet coils are cooled to liquid helium temperatures, they are usually kept on all the time

• Because the magnetic field is on, all iron and steel objects must not be allowed to enter the room. http://www.mercola.com/2001/aug/15/mri.htm