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Magnetic Resonance Imaging

Magnetic Resonance Imaging. Magneto Resonance Imaging. Magneto Resonance Imaging. Objectives Students learn to: explain how nuclei behave as small magnets due to their nuclear spin describe the way this nuclear spin aligns in a magnetic field

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Magnetic Resonance Imaging

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  1. Magnetic Resonance Imaging

  2. Magneto Resonance Imaging

  3. Magneto Resonance Imaging • Objectives • Students learn to: • explain how nuclei behave as small magnets due to their nuclear spin • describe the way this nuclear spin aligns in a magnetic field • define nuclear precession, and relate the frequency of precession • (Larmor frequency) to the type of nuclei and the magnetic field strength • explain how subjecting the precessing nuclei to a pulsed radio signal • causes some of them to change the plane of precession • recognise that the effect of this change is to produce a radio signal • and that the amplitude of this signal decreases as the nuclei ‘relax’ and • return to their normal orientation

  4. Magneto Resonance Imaging • explain the differences in relaxation times for fatty and proteinaceous tissues • describe the way external magnetic fields and radio frequency • radiation is used to obtain information on internal body structures in magnetic resonance imaging (MRI) • compare different imaging techniques and identify their major uses.

  5. Uses and Characteristics Magnetic resonance imaging (MRI) · Provides both high resolution, high contrast images of soft tissues and information on their physiology. · presents no radiation risk to the patient. MRI utilizes the precession of nuclei in strong magnetic fields to produce radio frequency signals which can be related to the postion of the nucleus.

  6. Magnetic nuclei A nucleus is composed of neutrons and protons. The individual nucleons also possess their own spin. Protons and neutrons can be visualized as spinning like tops. Their spins may be either up or down. The spins produce a magnetic field called a magnetic dipole.

  7. Magnetic nuclei The angular momentum of a particle rotating around an axis with velocity is given by: L = mvr where m is the mass of a particle, v the velocity and r the distance from the axis of rotation. where I is the angular momentum quantum number

  8. Nuclear Spin The nuclear structure is similar to the structure of the atom in several ways, though on a scale 105 times smaller. The nucleons are arranged in a shell structure similar to that found in the electron structure in atoms and occupy allowed energy levels within the nucleus.

  9. Nuclear Spin • The angular momentum of a nucleus is determined by the spin of the unpaired neutrons and protons and by the orbital angular momentum of the neutrons and protons. • Spin Quantum Number I • The nuclear spin can take only three values. • 1 If the mass number (the number of protons and neutrons), is odd, then I must be a • multiple of 1/2 (3/2, 5/2, 7/2 …) • 2 If the mass number is even and the atomic number (the number of protons), is also even then I must be 0. • 3 If the mass number is even but the atomic number is odd, then I must be a whole number (1, 2, 3, 4, 5 …)

  10. Remember: L Lz= l h/2p For an atom the TOTAL AM = L The same quantisation rule applies to a nuclear particle or nucleus

  11. Remember: orbiting electron An orbiting electron can be thought of as current in a loop m = i A L=mvR m= -(e/2m) L

  12. Nuclear spin and magnetic moment mp= g(e/2m) L = 2.7928 Bn Bn= eh/2mp5.05x10-27 JT-1 A proton can be thought of as a charged, spinning, spherical conductor.

  13. Dipole Energy Energy stored in rotating dipole in external field = m. B = m B cos q

  14. Gyromagnetic Ratio Hz T-1 Larmor Precession Frequency = g B/2p Hz rad/ T

  15. Nuclei in external magnetic fields 1/2 1 Quantum mechanics allows only certain alignment angles for the magnetic moment. Possible alignments for nuclei with I = 1/2 and I = 1 are shown above.

  16. Larmor Precession The nuclei cannot come into perfect alignment with the magnetic field. Instead they precess at fixed angles around the magnetic field The up orientation has a slightly lower energy than the down orientation. The energy separation is given by: DE = -m.B = g h I B 2p The Larmor frequency is given by: w= gB f = gB/2p where g is the gyromagnetic ratio that depends on the nucleus and its magnetic moment

  17. Larmor Precession

  18. Population of energy states The ratio of N + / N- is given by the Boltzmann distribution. At room temperature T =293K A population of 2 000 000 nuclei 1 000 001 might be in the up state 999 999 in the down state. In a sample containing 1017 nuclei this means 1011 more nuclei are in the low energy state than are in the high energy state. In NMR and MRI it is these 1011 nuclei which take part in the imaging process.

  19. Tissue or Bulk magnetization • There are more than 1000 hydrogen protons in each cubic centimeter (cm3) of tissue. • These have randomly oriented spins. • The tissue therefore, has no net magnetic moment or magnetization vector.

  20. Larmor Resonance • Electromagnetic radiation applied • at the natural frequency of the rotating nuclei (Larmor frequency ) • perpendicular to the external magnetic field (z-axis) • will pump energy into each nucleus. • As the nuclei absorb energy they flip their spins into the higher energy spin- down alignment. • The bulk magnetization vector slowly rotates out of alignment with the applied field toward the x-y plane. • An RF field at the Larmor frequency • FLIPS the Bulk Magnetisation Vector • Phases up magnetic moments

  21. Change in Bulk Magnetisation Vector • · The component of the bulk magnetization vector parallel to the magnetic field is called the longitudinal magnetization. • · The component perpendicular to the magnetic field is called the transverse or horizontal magnetization. • The longer the time that the RF single is applied • the more nuclei flip from spin-up to spin-down • the larger the angle between the applied magnetic field • ( z axis) and the the bulk magnetization vector.

  22. RF signal as time goes by x-y plane view

  23. Pulse timing An RF field at the Larmor frequency FLIPS the Bulk Magnetisation Vector • Two RF pulses are used in MRI: • A 180-degree RF pulse reorients the magnetization vector in a direction opposite to the external magnetic field. • A 90-degree RF pulse reorients the magnetization vector into the plane perpendicular to the external magnetic field. • A 180-degree RF pulse takes twice as long as a 90-degree • RF pulse.

  24. MRI Signals or Free induction decay • After a 90-degree RF pulse, • 1: each nuclear magnetic moment is in phase • 2: the magnetization vector precesses at the Larmor frequency about the external magnetic field. • A coil mounted in the horizontal x-y plane will detect a rapidly changing magnetic field as the bulk magnetization vector rotates past the coil entrance at the Larmor frequency • This rotation gives rise to the free induction decay (FID) signal.. • By a sequence of changing magnetic fields coupled with 90 and 180 degree RF spulses the FID signals may be detected, digitized, and, through use of reconstruction algorithms, used to generate MR images.

  25. MRI Signals or Free induction decay

  26. Return to Equilibrium Once the RF signal has ceased the nuclei slowly return to their original energy distribution ( Maxwell Boltzmann Distribution). During this process the vertical component of the bulk magnetization vector (z) slowly increases and the horizontal (x-y) component decreases. Two processes produce this change: T1 relaxation and T2 relaxation

  27. T1 relaxation • Protons placed into a strong magnetic field produce a net magnetization (M) parallel to the magnetic field axis. This process is not instantaneous but grows exponentially from the initial value of zero to the equilibrium value of M with a time constant T1. • At a time equal to T1, 63% of the signal has returned and at 3 x T1, 95% has returned. • When the applied field is switched off, the magnetization M decays exponentially with the same time constant

  28. T1 relaxation • T1 relaxation is called longitudinal or spin-lattice relaxation. • Different tissues with differing densities and chemical composition affect the hydrogen nuclei by producing different relaxation times. • For most tissues, T1 times are a few hundred milliseconds. • T1 relaxation time constants are long in small molecules like water and in large molecule like proteins but short in large (fats) and in intermediate-sized molecules. • In some procedures contrast agents such as gadolinium- • DTPA are used to produce shortened T1. T1 also increases • with increasing magnetic field strength

  29. T1 relaxation

  30. T2 relaxation • After a 90-degree pulse, the magnetization vector rotates at the Larmor frequency in a plane perpendicular to the external magnetic field. • The horizontal component of each nuclear magnetic moments point in the same direction and are in phase. • In a perfectly uniform magnetic field, energy is is lost to neighbouring nuclei through the interaction between nuclear spins. • As this energy is lost the bulk magnetic vector returns to its original value and again lies parallel to the applied magnetic field. • For most tissues, T2 times are typically tens of milliseconds • Liquids generally have long T2 times, whereas large molecules and solids generally have short T2 times.

  31. T2 relaxation

  32. T1 and T2 Relaxation Times T1 (ms) T1 (ms) T2 Tissue 0.5 T 1.5 T (ms) Fat (adipose) 200 260 80 Liver 320 490 45 Kidney 500 650 60 Spleen 540 780 90 Muscle 550 870 50 Grey matter 650 920 100 CSF 2000 2400 180 • The observed FID signal falls exponentially with a decay rate constant T2* which is a few milliseconds and is much shorter than T2. • Materials such as paramagnetic and ferromagnetic contrast agents disrupt the local magnetic field homogeneity and shorten T2*. • T2* <T2 <T1 For all tissues

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