16.2 Medical Imaging and Radiation

Ideas for today What are x-rays and gamma rays? How are x-rays used for imaging? How are gamma rays used to kill tumors? Magnetic Resonance Imaging (MRI)

Radiation can be biologically harmful… biologically harmful…

But radiation can also be medically helpful! be medically helpful! X-ray taken during arthroscopic surgery

Electromagnetic spectrum X-rays and gamma rays are electromagnetic radiation!

Wilhelm Roentgen Discovered x-rays in , being used in medicine! Bertha Roentgen

X-rays Short-wavelength electromagnetic waves An x-ray photon has lots of energy –Enough to do chemical damage to molecules –Enough to knock electrons out of atoms Produced by energetic events –Rapid acceleration of a charge –Radiative transition in a highly excited atom

What are photons? Single photon source Two “slits” Light / electromagnetic waves Water waves Camera 2 slit interference

Unlike water, the light makes individual spots (“clicks”) on the camera Photons are the smallest, quantum unit of light Each photon (“click”) has energy h×f (h is Planck’s constant and f is the frequency)

Bremsstrahlung X rays When a fast-moving electron swings around a heavy nucleus, it accelerates rapidly Electron emits much of its energy as X-ray photon

Atomic orbitals Atomic orbitals Electrons have special orbits around nucleus Only orbits with certain energies are allowed Photons (light) can be absorbed or emitted by an atom only at certain values corresponding to energy differences between electron energy levels. “Radiative Transitions” Spectral lines

Characteristic X-rays When a colliding electron excites an atom to high- energy state, that atom can then radiate an x-ray photon

Producing X-rays Accelerate electrons to 10kV - 100kV Electrons hit heavy atoms Some x-rays emitted via bremsstrahlung and some as characteristic X rays X-ray tube filters away lowest energy photons X-ray tube

X-ray Spectrum Note the characteristic X-rays are sharp lines on the broad Bremsstrahlung background

X-ray interaction with Matter Can be absorbed by an atom (to decay later) Rayleigh scattering (that deflects the X-rays and is the same process that makes the sky blue!) Photoelectric effect Sunset

Photoelectric Effect X-ray causes radiative transition in an atom and ejects an electron Effect is most likely when electron energy is low Effect is strongest in many-electron atoms Click mePhotoelectric

X-ray Imaging Any atom that blocks X rays casts a shadow –Many-electron atoms (like Ca) produce shadows –Few-electron atoms cast essentially no shadows All atoms Rayleigh scatter x-rays, causing haze X-ray imaging observes shadows of large atoms Haze can be filtered away

CT* Scanning Many separate X-ray images produce CT database –X rays from different angles mix shadows differently Computer recreates original 3-D geometry from data *Computed (Axial) Tomography Shadow CT

Click me

31-Mar-2011 Phys 150 Lecture 1920 i>clicker question X rays (choose the INCORRECT answer) A.are preferentially absorbed by light elements B.can be reflected C.can cause cancer D.are used in CT scans

31-Mar-2011 Phys 150 Lecture 1921 i>clicker question X rays (choose the INCORRECT answer) A.are preferentially absorbed by light elements - Absorbed by heavy elements B.can be reflected – X ray backscatter C.can cause cancer – depends on dose D.are used in CT scans – CAT scans

31-Mar-2011 Phys 150 Lecture 1922 i>clicker question X rays (choose the INCORRECT answer) A.are preferentially absorbed by light elements - Absorbed by heavy elements B.can be reflected – X ray backscatter C.can cause cancer – depends on dose D.are used in CT scans – CAT scans X ray backscatter at O’Hare

Gamma Rays Higher photon energy than X rays Sources: –Radiative transitions in nuclei –Particle accelerators (high-energy bremsstrahlung)

Gamma-rays and Matter Compton scattering Pair production

Gamma Radiation Therapy Gamma rays are highly penetrating in tissue Most gamma ray events are Compton or Pair Produced Either event damages many molecules Gamma radiation kills cells Approaching tumors from many angles minimizes collateral damage to healthy tissue

2003 Nobel Prize in Medicine Lauterbur, UIUC Mansfield Magnetic Resonance Imaging

31-Mar-2011 Phys 150 Lecture 1928 i>clicker question MRI uses A.microwaves B.radio waves C.X rays D.infrared radiation

31-Mar-2011 Phys 150 Lecture 1929 i>clicker question MRI (Magnetic Resonance Imaging) uses A.microwaves B.radio waves – and a strong magnetic field C.X rays D.infrared radiation

Magnetic Resonance Imaging

Hydrogen nuclei in your body not normally polarized MRI uses huge magnetic field to polarize H nuclei

“New mothers respond uniquely to their own babies. As this movie representing data from an functional magnetic resonance imaging (fMRI) brain scan illustrates, one area of the brain, shown in red, activates when mothers see their own infants; another area, shown in blue, activates when they see someone else's infant. (Movie courtesy Terry Oakes, Lab for Affective Neuroscience, UW-Madison)” Functional MRI Measures brain activity through blood flow

See you next class! For next class: come with questions! We’ll do a review of the entire semester.

The Visible Human Project – CT and MRI data combined

MRI using “polarized” gases

Like precessing tops Analogy: rf is like a torque applied to a gyroscope Rate of precession (spinning) depends on magnetic field AC voltage induced in pickup coil at precession rate