L 36 Modern Physics [2] How lasers work Medical applications of lasers Applications of high power lasers Medical imaging techniques CAT scans MRI’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 E i (larger energy) E f (smaller energy) Spontaneous emission Stimulated emission Incoming 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 to Cut metals

Laser Fusion Multiple beams of a powerful laser are focused on 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 of different 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 angles passing through the patient. Some of the cuts overlap. A full three dimensional image can be reconstructed using computers.  this procedure is called tomography.

C omputerized T omography 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 N S 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 magnetic field 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.