1. Class average = 22.99 Papers-graded “soon”. Generally good, main problem is Description not related to physics principles.

2. Medical Imaging II. (ultrasound and MRI) “ionizing radiation”- EM wave or energetic particle with enough energy to kick electron off atom or molecule (“ionize”). Molecular damage. e 1. X-ray. 2. PET scan (inject radioactive stuff that decays in body producing particles come flying out.) Harmless Methods (if low power, just slight warming) 1. Ultrasound- sound waves don’t effect atoms and molecules at all except shake a bit. S. M. 2. MRI- uses radio waves, very low frequency/energy. High doses- enough dead cells for radiation sickness. Low dose damage DNA  cancer Balance of risks Special homework problems this week- construct your own!

3. Ultrasound: uses sound waves to see inside the body Where am I? HELLO If I hear an echo what do I know? a.That something a head of me reflects sound b.how far away it is c.what its made of d.a and b e.a, b, and c If I hear an echo 1 second after I shout “Hello”, I can conclude that this surface is a.about 331 m away b.about 3x10 8 m away c.about 115 m away d. I don’t remember the speed of sound in air. Answer is d. But do know something of c. Answer is c. Total Dist = velocity * time. Distance to surface = Total Dist/2

4. ULTRASOUND PROBE Transmits: Short pulse of sound wave Frequency of sound (1 MHz – 5 MHz) MHz= megahertz (10 6 Hz) Timing and Amplitude (Distance & something about Material) Listens: Echos ~1% of time transmitting ~99% of time listening

5. Sound waves reflect and refract at a surface interface Acoustic equivalent to “index of refraction of light” depends on: Speed of Sound in substance (Also on density of substance) The bigger the difference in the speed of sound at the interface, the more sound is reflected and the greater the refraction. Which is true? a.Speed of sound in B is greater than A b.Speed of sound in A is greater than B c.I don’t know how to tell Substance ASubstance B TRAVELING SOUND WAVES

6. Ultrasound: Depth measurement SubstanceSpeed of Sound Air 331 m/s Fat 1450 m/s Water 1450 m/s Soft Tissue 1450 m/s Brain 1451 m/s Liver 1549 m/s Kidney 1561 m/s Blood 1570 m/s Muscle1585 m/s Bone 4080 m/s Reflecting layers 1 2 3 4 5 Why do they put the oil between you and the probe? a.To protect the probe b.To protect your skin from ultraviolet rays generated by the probe c.To decrease the reflection of the sound wave off your outer skin d.To allow smooth movement of the probe. Oil/Gel Answer is c. If no gel, some air between probe and your skin. Reflects most of sound… Large difference in speed of sound between air and skin tissue!

7. Ultrasound: Depth measurement Reflecting layers 1 2 3 4 5 Oil/Gel Time Amplitude Time Amplitude (Assume incident soundwave strong enough to reach layer 5) A B C D

8. Ultrasound: Depth measurement Reflecting layers 1 2 3 4 5 Oil/Gel Amplitude (Assume incident soundwave strong enough to reach layer 5) B Features: 1)All reflection peaks ~ same width… all echos of same sound pulse 2)Timing determined by distance from probe: Layer 3 is twice as far as layer 2… takes 2 times as long 3)Peak 1 received almost instantaneously (oil/skin) 4)Peak 5 is larger … skin/air surface gives large reflection 1 2 3 4 5

9. Ultrasound: 2-D Image Use Focused Sound Wave: Measure Layer Depths at different angles --- 2D picture. Speaker Probe

10. Why Ultrasound? Human Hearing: Up to frequencies of 20 kHz Frequency in Ultrasound Measurement: 1 MHz – 5 MHz MHz= megahertz (10 6 Hz) speed of sound = wavelength x frequency ultrahigh frequency means wavelength very short. In tissue, speed = 1540 m/s. = speed/. Wavelength = (1540 m/s )/(5 x 10 6 Hz) = 0.3 mm! Shorter wavelength = higher resolution. Bats use sonar to “see”. Cross section baby torso: Fetus small, need high resolution!

12. 2. Magnetic resonance imaging. (MRI) I. basic idea- detects where hydrogen atoms are. Different tissue has different distribution of H atoms. II. Detect hydrogen atoms by how they absorb radio waves. Do at each little spot in body. We cover basic idea. Lots of more complicated stuff to get better signals, detect environment of H atom (what kind of molecule H is in). H atoms--tiny magnets How detect H atoms with spatial selectivity? “Magnetic resonance.” Wonder of modern physics, but NOT simple. Uses many ideas of quantum physics already have seen.

13. Nucleus of each atom is tiny magnet. Each type of atom nucleus has different size. magnet moment µ H- “1”, N = 1/14, Na ¼, etc. What happens if put magnet in a magnetic field? a. tries to line up with field, b. nothing, c. tries to point perpendicular to mag. field, d. tries to line up opposite to field. d. Less energy if pointing opposite to field “spin down”. takes energy to force it to line up with field “spin up” B B

14. H atom nucleus, internal magnet magnetic field, B magnetic energy = µB down up atom magnet-quantum physics says, only up or down. Two possible energy levels. Like electrons in atom. Differences: 1) gap between levels depends on magnetic field. E = µB. 2) Energy gap is billion times smaller. Detect H atoms with physics similar to atoms and light. Big magnet-- can point any direction. Magnetic of hydrogen nucleus. Only up or down! Like energy levels of electron- only certain energies.

15. e e Using light to tell what kind of atoms you have. method 1: whack atoms with electrons, see what color light comes out. Yellow-sodium, red- neon, etc. method 2?: send in light of all colors. Detect amount of each color that comes through. can we figure out what kind of atoms there are by what comes through? a. no, all atoms absorb blue, so only red light will come through. b. no, all atoms absorb all colors, so fraction of light absorbed only tells number of atoms. c. yes, but color not important, the fraction of total light absorbed depends on atom type. d. yes, color(s) of light absorbed depends on type of atom

16. magnetic energy down up ans. D Send in different color light, only certain colors make electron jump up. That light absorbed, does not come through. E =h Use same method to tell how many H nucleus. How much radio wave is absorbed= # H atoms. E =h, But E about billion times less than light. Radio waves not visible light. Difference in energy. Depends on size of magnet and size of magnetic field E= 2µB = energy of photon = h. If measure that nucleus flipped spin and know energy took to do it matched µ of hydrogen, know if was H atom. Why better than method #1? particular yellow color absorbed-- sodium! certain red light absorbed-- neon

17. radio wave detector magnet on bar demo- little pushes at right frequency. Push with magnet. Detector will see the least radio waves when? a. radio wave is higher freq. than nucleus flip frequency. b. radio wave matches nucleus flip frequency. c. radio wave is lower freq. than spin flip. d. will always be the same independent of radio wave frequency. b. if frequency exactly matches, will flip nuclei, this uses energy, comes out of radio waves. Detector will see LESS.

18. Even simpler, send in only radio waves with exactly energy to flip H. See how much absorbed, tells how many H’s. Analogy-- Barrel of different tuning forks, how many 440 Hz? speaker putting out 440 Hz. 22 detector measures how much of 440 Hz sound absorbed (by exciting 440 tuning forks)

19. Human body- big blob of H atoms (in molecules), more some places than others. detector 1 detector 2 how much power absorbed? = number of atoms in path x B x mag. mom. 1) To make absorbed power large enough to see easily make B BIG! (adjust ν). Magnetic Resonance imaging measures amount of H atoms in different places. radio waves. around 1 MHz. (really puny, not nearly enough energy to break apart molecules so no damage) Which detector will detect MORE radio waves? a. det. 1, b. det. 2 ans. b. detector 2. Less power absorbed in body = more in detector.

20. Make magnetic field different across body. Use magnetic field dependence of resonance. Do as slices, then slices of slices. B x Compare energy needed to flip flip H atom nucleus at left ear (LE) right ear (RE), and nose (N). a. same at all three places. b. RE most, nose second, LE least c. LE most, nose second, RE least d. nose least, RE and LE same and higher. e. nose most energy, RE and LE less. Good for detecting amount of H through whole body, but how to look at details in particular location, like part of brain?? 900 G 1000 G B LE B RE ans. c, E = µB

21. B x E =µB x Power absorbed tells you how many H atoms only in slice of head where µB = hν. Always send in same exact frequency Rf matches only at one B = one slice. Tells how many H in that slice! B x E =hν = m.m. x B x change B, now energy match at different slice. Power absorbed tells you how many H atoms only in new slice of head.

22. Change B variation over time. Get number of H atoms at each different slice. Change B by changing currents through wires. Move a little, makes lots of noise! To get measure of each spot (not just slope) make B vary in 3 D. Absorbed energy all has to come from H atoms at the spot where µx B = hν Have B varying in x,y, z. Measure power absorbed. Change B's and repeat over and over. Map out H atom distribution in entire head/body. Takes a while.

23. Getting even fancier!! If measure frequency really really carefully, can tell what type of molecule the H atom is in. Other atoms change the B field a little. H C C C H C C C O Hemoglobin without oxygen. Hemoglobin with oxygen. Oxygen shifts magnetic field. H atom flips at slightly different frequency! Can tell difference.

24. demo with oscillating magnet in field. energy/ atom = magnetic moment x B = hν Chose different values of ν, find different types of atoms. Nuclear magnetic resonance chemical analysis. magnetic moment of atomic nucleus. Depends on how protons and neutrons arranged. Each type of atom different. hydrogen sodium hν 1 hν 2