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M R I Physics Course

M R I Physics courseChapter 2Basic Physical PropertiesMagnetism

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M R I Physics Course

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    1. M R I Physics Course Jerry Allison Ph.D., Chris Wright B.S., Tom Lavin B.S., Nathan Yanasak Ph.D. Department of Radiology Medical College of Georgia

    4. Magnetic Properties All substances are magnetic (to various degrees). Magnetic susceptibility is the ability of a substance to become magnetized.

    5. Magnetic Properties (continued) External field is applied ? object becomes magnetized, according to susceptibility.

    6. Magnetic Properties (continued) Diamagnetic - develop a small magnetic field in opposition to an applied field (and have a small negative magnetic susceptibility). Non-Magnetic - very weakly diamagnetic

    7. Magnetic Properties (continued)

    8. Magnetic Properties (continued)

    9. Magnetic Properties (continued) Stainless Steel: can be ferromagnetic (exterior surface of the downtown VAMC) or non-magnetic (most surgical steel) depending upon the particular alloy (Fe, Cr, Ni, Mn). Fortunately, most (not all) surgical appliances (staples, clips, etc.) are an alloy that is non-magnetic.

    10. A ferromagnetic substance in an applied magnetic field will develop a magnetic field hundreds of times as strong as the applied field. A spherical iron ball at a distance of 1.6 m from a 1.0 T unshielded magnet will experience equal attractive forces from gravity and the magnet. This phenomenon is the basis for the “projectile hazards” in MRI.

    11. Induced Magnetism (continued) An electrical conductor wrapped around a ferromagnetic iron rod induces a very useful magnetic field when an electric current is flowing: Electromagnets, Transformers, and Motors have iron cores. The induced field is much larger than the magnetic field created by the current flow in the conductor.

    12. Magnetism in Our World Magnetic fields are all around us: Average field in Milky Way: 5x10-6 Gauss Average field in Solar Wind: 5x10-5 Gauss Average field at Moon: 1x10-2 Gauss Average field at Jupiter: 2x104 Gauss 1 Tesla = 10,000 Gauss = 1x104 Gauss 1 T = 10,000 G

    13. The Earth’s magnetic field is 0.5 - 1.0 Gauss at 15° with axis of Earth’s rotation. A 1.5 Tesla (15,000 Gauss) field is 15,000 to 30,000 times greater than the Earth’s magnetic field. Magnetism in Our World (continued)

    14. Origins of Magnetism “Stationary” electrical charges have an electric field (“E-field”). “Moving” electrical charges develop a magnetic field (“B-field”). The basis for ALL MAGNETISM is the motion of electrical charges.

    15. Relationship of E- and B-fields

    16. Because of the inseparability of electric charges and magnetism, we refer to these phenomena in general as “Electromagnetism”. Phenomena of stationary charges ? “Electrostatics”

    19. Origins of Magnetism (continued) The “right hand rule” describes the direction of the magnetic field relative to the direction of movement of electric charges.

    23. Topology of B-field

    24. Magnetic Properties

    27. Magnetic Properties

    28. Magnetic Properties

    30. Magnetic Properties

    35. Radio Frequency Energy (RF) - oscillating magnetic and electric fields (I.e., “electromagnetic” fields) having frequencies between 3 kilohertz (kHz) and 30 Gigahertz (GHz). Examples Radio waves(AM: 535-1605 kHz; FM: 88-108 MHz) MRI (21,43,64,128 MHz?protons in 0.5,1T,1.5T,3T; lower for spectroscopy) Cellphones (824-848 MHz) TV transmission (50-900 MHz; Ch.2-4 ? 54-72 MHz) Microwave Ovens (2.45 GHz) Radar (3-30 GHz)

    36. Radio Frequency Energy In MRI, magnetic fields oscillating at the appropriate resonant frequency are used to stimulate nuclei to either absorb energy or to release energy (spin flip transitions; phase coherence).

    37. Summary All magnetism originates in the movement of electric charge. Magnetic susceptibility describes to what extent a material increases or decreases an applied magnetic field. Resonance: periodic stimulation at the natural frequency can cause energy exchange.

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