16.2 Medical Imaging16.2 Medical Imaging and Radiation and Radiation

Ideas for todayIdeas 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 Radiation can be

biologically harmful…biologically harmful…

But radiation can alsoBut radiation can also

be medically helpful!be medically helpful!

•X-ray taken during arthroscopic surgery

Electromagnetic spectrumElectromagnetic spectrum

X-rays and gamma rays are electromagnetic radiation!

Wilhelm RoentgenWilhelm Roentgen

•Discovered x-rays in 1895•1896, being used in medicine!

Bertha Roentgen

X-raysX-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?What are photons?

Single photon source

Two “slits”

Light / electromagnetic waves

Water waves

Camera

2 slit interference2 slit interference

Unlike water, the light makesUnlike water, the light makes

individual spots (“clicks”) on the cameraindividual 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 raysBremsstrahlung 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 orbitalsAtomic 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”Radiative Transitions”

Spectral Spectral lineslines

CharacteristiCharacteristic c X-raysX-rays

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

Producing X-raysProducing 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 tubeX-ray tube

X-ray SpectrumX-ray SpectrumNote the characteristic X-rays are sharp lines on the broad Bremsstrahlung background

X-ray interaction with MatterX-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

SunseSunsett

Photoelectric EffectPhotoelectric 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 me

Photoelectric Photoelectric

X-ray ImagingX-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* ScanningCT* 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 Shadow CTCT

Click me

•31-Mar-2011 •Phys 150 Lecture 19 •20

i>clicker question

X rays (choose the INCORRECT answer)

A. are preferentially absorbed by light elements

B. can be reflectedC. can cause cancerD. are used in CT scans

•31-Mar-2011 •Phys 150 Lecture 19 •21

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 backscatterC. can cause cancer – depends on doseD. are used in CT scans – CAT scans

•31-Mar-2011 •Phys 150 Lecture 19 •22

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 backscatterC. can cause cancer – depends on doseD. are used in CT scans – CAT scans

X ray backscatter at O’Hare

Gamma RaysGamma Rays• Higher photon energy than X rays• Sources:

– Radiative transitions in nuclei– Particle accelerators

(high-energy bremsstrahlung)

Gamma-rays and MatterGamma-rays and Matter

Compton scattering Pair production

Gamma Radiation TherapyGamma 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 Medicine2003 Nobel Prize in Medicine

Lauterbur, UIUC

Mansfield

Magnetic Resonance ImagingMagnetic Resonance Imaging

•31-Mar-2011 •Phys 150 Lecture 19 •28

i>clicker question

MRI uses

A. microwavesB. radio wavesC. X raysD. infrared radiation

•31-Mar-2011 •Phys 150 Lecture 19 •29

i>clicker question

MRI (Magnetic Resonance Imaging) uses

A. microwavesB. radio waves – and a strong magnetic fieldC. X raysD. infrared radiation

MMagnetic agnetic RResonance esonance IImagingmaging

Hydrogen nuclei Hydrogen nuclei in your body not in your body not normally normally polarizedpolarized

MRI uses huge MRI uses huge magnetic field to magnetic field to polarize H nucleipolarize 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 MRIFunctional MRIMeasures 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 The Visible Human Project – CT and MRI data combinedand MRI data combined

MRI using “polarized” gasesMRI using “polarized” gases

Like precessing topsLike 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