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Lecture #2 Review of Classical MR

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Lecture #2 Review of Classical MR Lecture #2! Review of Classical MR • Topics – Nuclear magnetic moments – Bloch Equations – Imaging Equation – Extensions • Handouts and Reading assignments – van de Ven: Chapters 1.1-1.9 – de Graaf, Chapters 1, 4, 5, 10 (optional). – Bloch, “Nuclear Induction”, Phys Rev, 70:460-474, 1946 – Historical Notes • Lauterbur, “Image Formation by Induced Local Interactions: Examples Employing Nuclear Magnetic Resonance”, Nature 242:190-191, 1973. • Mansfield and Grannell, “NMR ‘Diffraction’ in solids?”, J. Phys. C: Solid State Phys., 6:L422-L426, 1973. 1 Spin • Protons (as well as electrons and neutrons) possess intrinsic angular momentum called “spin” • Spin gives rise to a magnetic dipole moment • Useful (though not entirely accurate) to think of a proton as a spinning or rotating charge generating a current, which, in turn, produces a magnetic moment. µ 2 Nuclear Magnetic Moment • Consider a point charge in circular motion: radius r velocity v charge e • From EM theory: in the far field a current loop looks just like a magnetic dipole with magnetic moment µ µ = current • loop area ev 2 e µ = ⋅πr = mvr 2πr 2m angular momentum L gyromagnetic ratio γ • Thus µ⇥ = γL⇥ 3 Gyromagnetic Ratio • γ often expressed as gµb e γ = where µb = and = spin g factor 2m g g ≅ 2 (electrons) Planck’s constant/2 Bohr magneton π (m = electron mass) • For protons gµn e γ = where µn = and g≅ 5.6 2mp γ nuclear magneton = 42.58 MHz/T 2π γ e Important for ESR, NMR Note, for electron spin: = 658 contrast agents, etc γ K. Zavoisky 4 Nuclear Spin in a Magnetic Field • In a uniform magnetic field, a magnetic dipole will experience a torque τ Example: compass ⇥ = µ⇥ B⇥ × • Potential energy given by: angle between E = µ⇥ B⇥ = µB cos θ µ and B − · − Classically, energy can take on any value between ± µB 5 Equation of Motion • Newton’s Law: dL⇥ = ⇥ dt • Combining previous equations: dµ⇥ = ⇥µ B⇥ dt × 6 Physical Picture: ! Single Spin in a Uniform Magnetic Field dµ⇥ = ⇥µ B⇥ Note: µ = constant dt × | | B = B0zˆ z Precession frequency µ θ ω 0 ≡ γB0 Note: Some texts use ω0 = -γB0. y The Larmor Equation x A compass needle has a magnetic € moment and sits in the earth’s field. Why doesn’t it precess? Sir Joseph Larmor 7 Net Magnetization • In tissue, we are always dealing with a large number of nuclei. ! ! • Net magnetization: M = ∑µ volume • Hence, ! dM ! ! = γM × B Equation is valid for € dt non-interacting spins 8 € Bloch Equations • In order to account for inter- and intra-molecular interactions, we can introduce exponential transverse (T2) and longitudinal (T1) relaxation time constants. ! ˆ • For B = B0z ! dM ! M xˆ + M yˆ M − M zˆ € x y ( z 0 ) = γM × B0zˆ − − dt T2 T1 9 € MRI: The Signal Equation ! • Let B = Bzˆ • Following RF excitation (a topic we’ll revisit) and using: ! dM ! € = γM × Bzˆ (ignores relaxation) dt each small tissue volume looks like a tiny oscillating magnetic dipole. € z r −iφ(x,y,z,t ) θ M Mxy (x,y,z)e y r x M M iM xy = x + y 10 € MRI: The Signal Equation • Assuming a uniformly sensitive RF coil, the received signal is given by: −iφ(x,y,z,t ) s(t) = ∫ ∫ ∫ Mxy (x,y,z)e dxdydz x y z • Instantaneous frequency: ω = dφ/dt t t φ(x,y,z,t) = ω(x,y,z,t $) dt $= γ B(x,y,z,t #) dt # € ∫0 ∫0 P. Lauterbur P. Mansfield • In the presence of linear gradients: B(x,y,z,t) = B0 + Gx (t)x + Gy (t)y + Gz (t)z € € demodulate at γB0 t −iγB t −iγ ∫ (Gx (t $ )x +Gy(t $ )y+Gz(t $ )z)dt $ s(t) = e 0 M (x,y,z)e 0 dxdydz e ∫ ∫ ∫ xy € x y z 11 € k-space Comparing... t −iγ (Gx (t $ )x+Gy(t $ )y +Gz(t $ )z)dt $ ∫0 se (t) = ∫ ∫ ∫ Mxy (x,y,z)e dxdydz received signal x y z −i2π (kx x+ky y+kz z) M( kx,ky,kz ) = ∫ ∫ ∫ Mxy (x,y,z)e dxdydz FT of Mxy x y z € k-space interpretation of MRI. & γ t γ t γ t ) s (t) = M ( G (t $) dt $, G (t $) dt $, G (t $) dt $+ €e ' 2π ∫0 x 2π ∫0 y 2π ∫0 z * kx ky kz kz G € Gradients trace a trajectory x through k-space Gy ky Gz t kx 12 Sensitivity • What is M0? 2 2 (Later, we’ll do a more complete γ ! B0 M0 = ρ derivation) 4kT where ρ = spins/unit volume (careful with the term “spin density”), ! = Planck’s constant/2π = 1.05 x 10-34 Joule·s, k ’ -23 € = Boltzmann s constant = 1.38 x 10 J/K, T = absolute temperature. € € • Signal (voltage induced in coil) dM γ 3!2B2 signal ∝ ∝ωM ∝ ρ 0 dt 4kT 13 € Sensitivity (cont.) • What about the noise? 2 Rcoil vc MR Receiver L C Coil low noise preamp 2 Rbody vb Rcoil ∝ ω (due to RF skin depth effects) 2 Rbody ∝ω (from inductive losses, ignoring dialectric losses) 2 • Using Johnson noise spectral density: vn = 4kTR € 2 2 € noise ∝ γ B0 + λ γB0 14 € € Sensitivity (cont.) • Combining with the signal equation: γ 3!2B2 SNR ∝ ρ 0 4kT γ 2B2 + λ γB 0 0 • For high frequencies where body noise dominates coil noise (e.g. γB0 >> 1 Mz) € γ 2!2B SNR ∝ ρ 0 4kT 15 € Summary • Bloch equation ! dM ! M xˆ + M yˆ M − M zˆ = γM × B zˆ − x y − ( z 0 ) dt 0 T T 2 1 • Signal equation t ! ! −iγ ∫ G ⋅r dt % s (t) = M (x,y,z)e 0 ( ) dxdydz € e ∫ ∫ ∫ xy x y z • What’s missing? € 16 T1 and T2 Can be included as k-space weightings: t −iγ G⋅rdt % −t /T2 j ∫0 se (t) = ∑ ∫ Mxy (x,y,z,T1 j ,T2 j )e e dr j r but ... What are the underlying mechanisms? € Why do different tissues have different T1s and T2s? How do contrast agents work? 17 Chemical Shift • Interaction between electron cloud and B0 ω = γBeff = γB0 (1−σ) shielding constant Depends on: electron density • Looks like a new k-space axis molecular geometry, etc t −iγ G⋅rdt & ∫0 −iωt €s e (t) = ∫ ∫ Mxy (x,y,z,ω)e e drdω ω r & γ t γ t γ t t ) s (t) = M ( G (t $) dt $, G (t $) dt $, G (t $) dt $, + e ' 2π ∫0 x 2π ∫0 y 2π ∫0 z 2π * € kx ky kz kω € Note, same equation also holds for B0 inhomogeneity (due to magnetic susceptibility, etc) 18 But what about … • Coupling between spins • Chemical exchange • Nuclei with spin ≠ 1/2 • etc. 19 Next Lecture: Introduction to Quantum Mechanics 20 Biography: Sir Joseph Larmor Joseph Larmor (1857-1942) was educated at the Royal Belfast Academical Institution and the Queens College Belfast. He then took another degree at St. Johns College Cambridge, as was common for promising young students from provincial universities. He won top prize at the final mathematical examination in Cambridge. This was the second year in a row that a student from Belfast had been crowned "senior wrangler". Larmor then returned to Ireland as Professor of Natural Philosophy at Queens College Galway. He held this position for five years but then returned to Cambridge to take up a new Mathematics position and he was later appointed to the prestigious Lucasian Chair of Mathematics. Larmor is well known for his contributions to the theory of electromagnetism, in particular the electron theory of matter. Larmor published his collected papers on electromagnetism in 1900 in a famous book entitled "Aether and Matter". Larmor's work, though rooted in the classical physics in which he had been trained, eventually led to the breakdown of classical physics and the rise of relativity theory and quantum mechanics. He was described as 'one who rekindled the dying embers of the old physics to prepare the advent of the new'. Larmor saw himself as part of an Irish scientific tradition and was involved in editing the collected works of a number of Irish scientists. Larmor spent most of his career in Great Britain but returned to Ireland most summers and moved back permanently after his retirement from the Lucasian chair. He was committed to the Union of Ireland with Great Britain and this led him to serve in Parliament as a member for Cambridge University from 1911 to 1922. 21 Biography: Sir Peter Mansfield (born October 9, 1933, London, England) English physicist who, with American chemist Paul Lauterbur, won the 2003 Nobel Prize for Physiology or Medicine for the development of magnetic resonance imaging (MRI), a computerized scanning technology that produces images of internal body structures, especially those comprising soft tissues. Mansfield received a Ph.D. in physics from the University of London in 1962. Following two years as a research associate in the United States, he joined the faculty of the University of Nottingham, where he became professor in 1979. Mansfield was knighted in 1993. Mansfield's prize-winning work expanded upon nuclear magnetic resonance (NMR), which is the selective absorption of very high-frequency radio waves by certain atomic nuclei subjected to a strong stationary magnetic field. A key tool in chemical analysis, it uses the absorption measurements to provide information about the molecular structure of various solids and liquids. In the early 1970s Lauterbur laid the foundations for MRI after realizing that if the magnetic field was deliberately made nonuniform, information contained in the signal distortions could be used to create two-dimensional images of a sample's internal structure.
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