1. 3/13/2009IB Physics HL 21 Ultrasound Medical Imaging Physics – IB Objectives I.2.7Describe the principles of the generation and the detection of ultrasound using piezoelectric crystals. I.2.8Define acoustic impedance as the product of the density of a substance and the speed of sound in that substance. I.2.9Solve problems involving acoustic impedance. I.2.10Outline the differences between A-scans and B-scans. I.2.11Identify factors that affect the choice of diagnostic frequency.

2. 3/13/2009IB Physics HL 22 Ultrasound Production and Detection Based on piezoelectric effect From http://en.wikipedia.org/wiki/Medical_ultrasonography

3. 3/13/2009IB Physics HL 23 Piezoelectric Effect in Crystals Applied electric field produces mechanical vibration Also, mechanical vibration produces electrical signal Single crystal can be both ultrasound source and detector Not at exactly same time Mechanical vibration moves at same frequency as electrical vibration (1 MHz to 20 MHz) + + + - - - - - - - Piezoelectric crystal (e.g., quartz) Electrodes

4. 3/13/2009IB Physics HL 24 Wave Motion in Solids After piezoelectric crystal starts sound wave (ultrasound wave?), wave travels through tissue Wave may reflect, refract, or be transmitted between two different materials (organs, tissue types, etc.) Acoustic impedance (~index of refraction) Acoustic impedance (Z) is product of Density of medium and Speed of wave: Z =  v [units of kg m -2 s -1 ] [Rayl] Ex: water Speed is 1,480 m/s;  = 998 kg/m 3 Z water = 1.48 x 10 6 kg m -2 s -1

5. 3/13/2009IB Physics HL 25 Reflection and Transmission of Waves with Ultrasound Reflection and transmission: When wave goes from medium with impedance Z 1 to a medium with impedance Z 2 Reflection fraction: (Z 2 – Z 1 ) 2 /(Z 2 +Z 1 ) 2 Transmission fraction: (2Z 2 ) 2 / (Z 2 + Z 1 ) 2 Limiting cases: If Z 1 = Z 2, no reflection, and transmission = 1 Reflection fraction + transmission fraction = 1 Note: acoustic impedance is frequency-dependent

6. 3/13/2009IB Physics HL 26 Reflection and Transmission of Waves with Ultrasound - Examples What is fraction of sound reflected and transmitted when Sound travels from water to muscle (Z muscle = 1.7 x 10 6 kg m -2 s -1 ) Sound travels from water to air (Z air = 400 kg m -2 s -1 ) Note: acoustic impedance is frequency-dependent

7. 3/13/2009IB Physics HL 27 Scan Modes with Ultrasound A Mode (Amplitude modulation) B Mode (Brightness mode) M Mode (Moving mode) Doppler (Doppler imaging)

8. 3/13/2009IB Physics HL 28 Scan Modes with Ultrasound A Mode: Amplitude modulation Single transducer generates ultrasound, receives ultrasound Information is available in only one-dimensional scan Height of returning pulse proportional to strength of returning pulse

9. 3/13/2009IB Physics HL 29 Scan Modes with Ultrasound A Mode: Amplitude modulation Assuming speed of sound in muscle / soft tissue is 1,540 m/s How far under the skin does the organ start? How long is the organ? 0.18 ms 35  s

10. 3/13/2009IB Physics HL 210 Scan Modes with Ultrasound B Mode (Brightness mode) Several transducers on handgrip record travel time simultaneously Can build up 2-D picture of reflections Brightness of image on screen is proportional to strength of reflection Transducers

11. 3/13/2009IB Physics HL 211 Scan Modes with Ultrasound B Mode (Brightness mode) Several transducers on handgrip record travel time simultaneously Can build up 2-D picture of reflections

12. 3/13/2009IB Physics HL 212 Ultrasound Frequency Choice High frequency - high resolution Get more detail with a higher frequency scan than a lower frequency scan High frequency – high attenuation Higher frequencies are attenuated faster than lower frequencies Get more penetrating images using lower frequencies

13. 3/13/2009IB Physics HL 213 Ultrasound Imaging - Cautions Difficult to get imaging from brain High-intensity scans can transfer energy to object being scanned Potential warming / damage to imaged object Fetus

14. 3/13/2009IB Physics HL 214 Ultrasound Imaging – Other uses Doppler shift scans Determine blood flow speed High speed – indication of blockage Moving ultrasound (M Scan) Real-time image of moving objects E.g., heart beating Fetus

15. 3/13/2009IB Physics HL 215 Ultrasound - Key Ideas

16. 3/13/2009IB Physics HL 216 Magnetic Resonance Imaging (MRI) Also called Nuclear Magnetic Resonance (NMR) scan

17. 3/13/2009IB Physics HL 217 NMR Scans – IB Objectives I.2.12Outline the basic principles of nuclear magnetic resonance (NMR) imaging I.2.13Describe examples of the use of lasers in clinical diagnosis and therapy.

18. 3/13/2009IB Physics HL 218 Fundamental Concept - Magnetic Energy Atoms in imaged object, especially hydrogen atoms, have a magnetic moment (~a compass) Magnetic moment is a consequence of the spin of the proton No classical analog Charge in motion produces magnetic field Like a compass, the magnetic moments of the hydrogen atoms want to line up in the direction of the applied magnetic field The stronger the field, the more the atoms line up with it

19. 3/13/2009IB Physics HL 219 Fundamental Concept - Magnetic Energy N N N N N Applied magnetic field Most of the atoms in the sample are oriented in the direction of the magnetic field

20. 3/13/2009IB Physics HL 220 Fundamental Concept - Magnetic Energy N N N N N Applied magnetic field When atoms shift their magnetic fields to be opposite the external field, they gain energy (photon). When atoms shift their magnetic fields to be along the external field, they give up energy (photon).

21. 3/13/2009IB Physics HL 221 MRI / NMR Scanner NMR scanners send in a radio signal in to the sample, with just the right amount of energy to flip the nuclear magnetic moments back an forth, from opposite to along the magnetic field. Resonance effect Frequency is called the Larmor frequency Able to localize the resonant area with slightly deformed magnetic fields Gradient fields

22. 3/13/2009IB Physics HL 222 MRI Scanner - Operation Scanner detects where large numbers of hydrogen atoms are ~Water Builds up 2-D image of object / body, which can be converted into a 3-D image Resonance of hydrogen nuclei also sensitive to nearby atoms (electrons) Distinguish compounds that hydrogen is in

23. 3/13/2009IB Physics HL 223 MRI Scanner - Details Useful for imaging skull and brain Whole-body diagnosis

24. 3/13/2009IB Physics HL 224 MRI Simulation - Questions What is the relationship between the applied external magnetic field, and the frequency of the radio-wave energy that flips the spins? Direct, inverse, or no relation How do the fringe fields help localize the RF signal from the body? TUMOR HUNT: Uncheck “Show atomic nuclei” Click “Add tumor” Look for evidence of tumor in RF signal

25. 3/13/2009IB Physics HL 225 MRI Scanner - Cautions Non-ionizing radiation Intense magnetic field No magnetized objects or metal

26. 3/13/2009IB Physics HL 226 MRI - Key Ideas

27. 3/13/2009IB Physics HL 227 MRI - Homework Write a 1-2 paragraph summary of NMR scans. Include: Hydrogen magnetic moment External magnetic field Energy of 50 MHz radio photon Gradient fields (optional)

28. 3/13/2009IB Physics HL 228 Scanning Techniques Excellent table and discussion, p. 502

29. 3/13/2009IB Physics HL 229 Diagnostic and Therapeutic Lasers Excellent table and discussion, p. 501