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Ultrasound Medical Imaging Physics IB Objectives

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Ultrasound Medical Imaging Physics IB Objectives

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    1. 3/13/2009 IB Physics HL 2 1 Ultrasound Medical Imaging Physics IB Objectives

    2. 3/13/2009 IB Physics HL 2 2 Ultrasound Production and Detection Based on piezoelectric effect

    3. 3/13/2009 IB Physics HL 2 3 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)

    4. 3/13/2009 IB Physics HL 2 4 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/m3 Zwater = 1.48 x 106 kg m-2 s-1

    5. 3/13/2009 IB Physics HL 2 5 Reflection and Transmission of Waves with Ultrasound Reflection and transmission: When wave goes from medium with impedance Z1 to a medium with impedance Z2 Reflection fraction: (Z2 Z1)2/(Z2+Z1)2 Transmission fraction: (2Z2)2 / (Z2 + Z1)2 Limiting cases: If Z1 = Z2, no reflection, and transmission = 1 Reflection fraction + transmission fraction = 1 Note: acoustic impedance is frequency-dependent

    6. 3/13/2009 IB Physics HL 2 6 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 106 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. 3/13/2009 IB Physics HL 2 7 Scan Modes with Ultrasound A Mode (Amplitude modulation) B Mode (Brightness mode) M Mode (Moving mode) Doppler (Doppler imaging)

    8. 3/13/2009 IB Physics HL 2 8 Scan Modes with Ultrasound A Mode: Amplitude modulation Single transducer generates ultrasound, receives ultrasound Information is available in only one-dimensional scan

    9. 3/13/2009 IB Physics HL 2 9 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?

    10. 3/13/2009 IB Physics HL 2 10 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

    11. 3/13/2009 IB Physics HL 2 11 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. 3/13/2009 IB Physics HL 2 12 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. 3/13/2009 IB Physics HL 2 13 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. 3/13/2009 IB Physics HL 2 14 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. 3/13/2009 IB Physics HL 2 15 Ultrasound - Key Ideas

    16. 3/13/2009 IB Physics HL 2 16 Magnetic Resonance Imaging (MRI) Also called Nuclear Magnetic Resonance (NMR) scan

    17. 3/13/2009 IB Physics HL 2 17 NMR Scans IB Objectives

    18. 3/13/2009 IB Physics HL 2 18 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. 3/13/2009 IB Physics HL 2 19 Fundamental Concept - Magnetic Energy

    20. 3/13/2009 IB Physics HL 2 20 Fundamental Concept - Magnetic Energy

    21. 3/13/2009 IB Physics HL 2 21 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. 3/13/2009 IB Physics HL 2 22 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. 3/13/2009 IB Physics HL 2 23 MRI Scanner - Details Useful for imaging skull and brain Whole-body diagnosis

    24. 3/13/2009 IB Physics HL 2 24 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. 3/13/2009 IB Physics HL 2 25 MRI Scanner - Cautions Non-ionizing radiation Intense magnetic field No magnetized objects or metal

    26. 3/13/2009 IB Physics HL 2 26 MRI - Key Ideas

    27. 3/13/2009 IB Physics HL 2 27 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. 3/13/2009 IB Physics HL 2 28 Scanning Techniques Excellent table and discussion, p. 502

    29. 3/13/2009 IB Physics HL 2 29 Diagnostic and Therapeutic Lasers Excellent table and discussion, p. 501

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