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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 Ei (larger energy) photon Ef (smaller energy) Spontaneous emission Incoming photon Stimulated emission 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. 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 magnetic field N N S 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 magnetic field spin-down spin-up 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