12.1

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  • Nuclear magnetic resonance (NMR) is the spectroscopic study of the magnetic properties of the nucleus of the atom.

    • Protons and neutrons of the nucleus have a magnetic field associated with their nuclear spin and charge distribution.

  • Resonance causes individual nuclei, when placed in a strong external magnetic field, to selectively absorb and release energy unique to those nuclei and their surrounding environment.

  • NMR is not an imaging technique but rather a method to provide spectroscopic data concerning a sample placed in a small volume, high field strength magnetic device.

  • Magnetic field gradients can be used to localize the NMR signal and generate images that display magnetic properties of the proton.

  • Magnetic resonance imaging (MRI) has supplanted many CT and projection radiography methods due to its high contrast sensitivity to soft tissue differences and nonionizing radiation.

  • Drawbacks of MRI include high equipment and siting costs, scan acquisition complexity, long imaging times, image artifacts, patient claustrophobia, and MR safety concerns.

  • This chapter reviews the basic properties of magnetism, concepts of resonance, tissue magnetization and relaxation events, generation of image contrast, and basic methods of acquiring image data.

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12.1 Magnetism, Magnetic Fields, and Magnets

  • Magnetism is a fundamental property of matter generated by moving charges, usually electrons.

  • Magnetic properties of materials result from the organization and motion of electrons in magnetic "domains."

  • Magnetic fields exist as dipoles, with a north pole and a south pole.

  • Magnetic field strength is measured in Tesla (T), with the earth's magnetic field being about 0.00005 T.

  • Magnetic fields can be induced by a moving charge in a wire, and the direction of the magnetic field depends on the sign and direction of the charge.

  • Magnets in MRI systems have different designs, including air core magnets and solid core magnets.

  • Air core magnets have a horizontal magnetic field, while solid core magnets have a vertical magnetic field.

  • Fringe fields, which extend beyond the volume of the magnet, are a potential hazard.

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  • Achieving a high magnetic field strength in MRI requires the electromagnet core wires to be superconductive.

  • Superconductivity is a characteristic of certain metals that exhibit no resistance to electric current at extremely low temperatures.

  • Fringe fields for air core magnets are extensive and increase with larger bore diameters and higher field strengths.

  • Fringe fields for solid core magnets are confined within the design.

  • The main magnetic field in both types of magnets is parallel to the z-axis of the Cartesian coordinate system.

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  • Superconductivity allows the closed-circuit electromagnet to be energized and ramped up to the desired current and magnetic field strength

    • Replenishment of liquid helium must occur continuously to maintain superconductivity

  • Superconductive magnets with field strengths of 1.5 to 3 T are common for clinical systems

  • 4 to 7 T clinical large bore magnets are currently used for research applications

  • Cross section of internal superconducting magnet components

    • Wire coils and cryogenic liquid containment vessel

    • Other necessary components include shim coils, RF coils, and gradient coils

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  • Gradients are produced inside the main magnet with coil pairs

    • Magnetic field strength decreases with distance from the center of each coil

    • When combined, the magnetic field variations form a linear change between the coils, producing a linear magnetic field gradient

  • The MR system is comprised of several components

    • Orchestrated by many processors and control subsystems

    • Components include data storage, digitizer & image processor, host computer, operating console, pulse program, measurement & control, RF transmitter & receiver, shim power supply, gradient power supply, patient table, magnet, clock, and gradient pulse program

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  • Magnetic susceptibility describes the extent to which a material becomes magnetized when placed in a magnetic field

  • Three categories of susceptibility: diamagnetic, paramagnetic, and ferromagnetic

  • Diamagnetic materials have slightly negative susceptibility and oppose the applied magnetic field

  • Paramagnetic materials have slightly positive susceptibility and enhance the local magnetic field

  • Ferromagnetic materials augment the external magnetic field substantially and can distort acquired signals

  • The nucleus exhibits magnetic characteristics on a much smaller scale than atoms/molecules

  • Magnetic properties are influenced by spin and charge distributions intrinsic to the proton and neutron

  • Magnetic characteristics of the nucleus are described by the nuclear magnetic moment

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  • Nuclear magnetic moment is determined by the pairing of protons and neutrons in a nucleus

    • If the sum of protons and neutrons is even, the nuclear magnetic moment is essentially zero

    • If either protons or neutrons are odd, the resultant noninteger nuclear spin generates a nuclear magnetic moment

  • Large numbers of nuclei arranged in a nonrandom orientation generate an observable nuclear magnetic moment

  • Hydrogen is the best element for generating MR signals due to its large magnetic moment and abundance

  • Other elements are orders of magnitude less sensitive, but 23Na and 31P have been used for imaging in limited situations

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  • Protons are considered tiny bar magnets with north and south poles

  • Unbound hydrogen atoms in water and fat have randomly oriented protons due to thermal energy

  • When placed in a strong static magnetic field, protons align with the applied field in parallel and antiparallel directions at two discrete energy levels

  • A stronger magnetic field increases the energy separation and the number of excess protons in the low-energy state

  • The number of excess protons in the low-energy state is approximately 3 protons per million at physiologic temperatures

  • This number of excess protons produces an observable nuclear magnetic moment

  • Protons also experience precession, similar to a spinning top wobbling, due to a torque from the applied magnetic field

  • The precession occurs at an angular frequency proportional to the magnetic field strength

  • The gyromagnetic ratio unique to each element determines the relationship between the magnetic field and the angular precessional frequency

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  • Gyromagnetic ratio is the ratio between precessional frequency and magnetic field strength

    • Gyromagnetic ratio is expressed in MHz/T

    • Each element has a unique gyromagnetic ratio

  • Protons precessing in parallel and antiparallel directions result in a quantized distribution

    • Net magnetic moment of the sample at equilibrium is the vector sum of individual magnetic moments in the direction of B0

    • Magnetic field vector components in the perpendicular direction sum to zero

  • Briefly irradiating the sample with an RF energy pulse tuned to the Larmor frequency promotes protons from low-energy to high-energy direction

    • Magnetization along the direction of the applied magnetic field shrinks

    • Energetic sample returns to equilibrium conditions when protons revert to parallel direction and release RF energy

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  • Magnetic field strengths for MR systems range from 0.3 to 4.0 T

  • Precessional frequency for protons is 42.58 MHz/T

  • Accuracy of precessional frequency is necessary for RF energy absorption by protons

  • Precision of precessional frequency is on the order of Hz to identify location and spatial position of signals

  • Differences in gyromagnetic ratios and precessional frequencies allow selective excitation of elements in the same magnetic field strength

  • Two frames of reference are used: laboratory frame and rotating frame

    • Laboratory frame is stationary and the applied magnetic field is parallel to the z-axis

    • Rotating frame is a spinning axis system where x-y axes rotate at the Larmor frequency

  • Net magnetization vector of the sample is described by three components: longitudinal magnetization (Mz), transverse magnetization (Mxy), and equilibrium magnetization (M0)

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  • Application of RF energy synchronized to the precessional frequency of protons causes absorption of energy and displacement of the sample magnetic moment from equilibrium conditions

  • Return to equilibrium results in emission of energy proportional to the number of excited protons in the volume

  • Excitation, detection, and acquisition of signals are necessary for MRI