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Presented by: Mahyar Nirouei PowerPoint Presentation
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Presented by: Mahyar Nirouei

Presented by: Mahyar Nirouei

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Presented by: Mahyar Nirouei

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  1. Magnetic Resonance Imaging Presented by: Mahyar Nirouei

  2. Synopsis of MRI 1) Put subject in big magnetic field 2) Transmit radio waves into subject [2~10 ms] 3) Turn off radio wave transmitter 4) Receive radio waves re-transmitted by subject0 5) Convert measured RF data to image

  3. Siemens Allegra 3T Scanner B0 = 3 T = 30,000 gauss Earth’s magnetic field = 0.5 gauss

  4. MRI Hardware

  5. RF Coil • RF Coils are used to excite the spins and to receive the signal • A typical coil is a tuned LC circuit and may be considered a near field antenna

  6. Many factors contribute to MR imaging • Quantum properties of nuclear spins • Radio frequency (RF) excitation properties • Tissue relaxation properties • Magnetic field strength and gradients • Timing of gradients, RF pulses, and signal detection

  7. What kinds of nuclei can be used for NMR? • Nucleus needs to have 2 properties: • Spin • charge • Nuclei are made of protons and neutrons • Both have spin ½ • Protons have charge • Pairs of spins tend to cancel, so only atoms with an odd number of protons or neutrons have spin • Good MR nuclei are 1H, 13C, 19F, 23Na, 31P

  8. Hydrogen atoms are best for MRI • Biological tissues are predominantly 12C, 16O, 1H, and 14N • Hydrogen atom is the only major species that is MR sensitive • Hydrogen is the most abundant atom in the body • The majority of hydrogen is in water (H2O) • Essentially all MRI is hydrogen (proton) imaging

  9. A Single Proton m There is electric charge on the surface of the proton, thus creating a small current loop and generating magnetic moment m. The proton also has mass which generates an angular momentum J when it is spinning. J + + + Thus proton “magnet” differs from the magnetic bar in that it also possesses angular momentum caused by spinning.

  10. Magnetic Moment B B I L m= tmax / B = IA W L F t = mB = m B sinq F = IBL t = IBLW = IBA Force Torque

  11. J m r v Angular Momentum J = mw=mvr

  12. The magnetic moment and angular momentum are vectors lying along the spin axis m = gJ g is the gyromagnetic ratio g is a constant for a given nucleus

  13. How do protons interact with a magnetic field? • Moving (spinning) charged particle generates its own little magnetic field • Such particles will tend to line up with external magnetic field lines (think of iron filings around a magnet) • Spinning particles with mass have angular momentum • Angular momentum resists attempts to change the spin orientation (think of a gyroscope)

  14. Ref: www.simplyphysics.com

  15. The energy difference between the two alignment states depends on the nucleus • D E = 2 mz Bo D E = hn • n = g/2p Bo known as Larmor frequency g/2p = 42.57 MHz / Tesla for proton

  16. Resonance frequencies of common nuclei Note: Resonance at 1.5T = Larmor frequency X 1.5

  17. Electromagnetic Radiation Energy X-Ray, CT MRI

  18. MRI uses a combination of Magnetic and Electromagnetic Fields • NMR measures the net magnetization of atomic nuclei in the presence of magnetic fields • Magnetization can be manipulated by changing the magnetic field environment (static, gradient, and RF fields) • Static magnetic fields don’t change (< 0.1 ppm / hr): The main field is static and (nearly) homogeneous • RF (radio frequency) fields are electromagnetic fields that oscillate at radio frequencies (tens of millions of times per second) • Gradient magnetic fields change gradually over space and can change quickly over time (thousands of times per second)

  19. Radio Frequency Fields • RF electromagnetic fields are used to manipulate the magnetization of specific types of atoms • This is because some atomic nuclei are sensitive to magnetic fields and their magnetic properties are tuned to particular RF frequencies • Externally applied RF waves can be transmitted into a subject to perturb those nuclei • Perturbed nuclei will generate RF signals at the same frequency – these can be detected coming out of the subject

  20. Basic Quantum Mechanics Theory of MR The Effect of Irradiation to the Spin System Lower Higher

  21. Basic Quantum Mechanics Theory of MR Spin System After Irradiation

  22. Net magnetization is the macroscopic measure of many spins Bo M

  23. Net magnetization • Small B0 produces small net magnetization M • Larger B0 produces larger net magnetization M, lined up with B0 • Thermal motionstry to randomize alignment of proton magnets • At room temperature, the population ratio of anti-parallel versus parallel protons is roughly 100,000 to 100,006 per Tesla of B0

  24. Quantum vs Classical Physics One can consider the quantum mechanical properties of individual nuclei, but to consider the bulk properties of a whole object it is more useful to use classical physics to consider net magnetization effects.

  25. To measure magnetization we must perturb it • We can only measure magnetization perpendicular to the B0 field • Need to apply energy to tip protons out of alignment • Amount of energy needed depends on nucleus and applied field strength (Larmor frequency) • The amount of energy added (duration of the RF pulse at the resonant frequency) determines how far the net magnetization will be tipped away from the B0 axis

  26. Precession • Precession refers to a change in the direction of • the axis of a rotating object. • It occurs when spinning objects experience a moment outside the plane of rotation

  27. wL Slope = gyromagnetic ratio B0 • Larmor freqency, and is dictated by • The speed of precession of a spinning body in a field is called the Larmor frequency, and we know a few things about it • Zero when B0 = 0 • Increases as magnetic field increases • We could do an experiment and plot the relationship between B0 and precessional frequency • For protons wL is approximately 42 MHz/Tesla B0

  28. Derivation of precession frequency • = m× Bo • = dJ / dt J = m/g dm/dt = g (m× Bo) m(t) = (mxocos gBot + myosin gBot) x + (myocos gBot - mxosin gBot) y + mzoz This says that the precession frequency is the SAME as the larmor frequency

  29. Recording the MR signal • Need a receive coil tuned to the same RF frequency as the exciter coil. • Measure “free induction decay” of net magnetization • Signal oscillates at resonance frequency as net magnetization vector precesses in space • Signal amplitude decays as net magnetization gradually realigns with the magnetic field • Signal also decays as precessing spins lose coherence, thus reducing net magnetization

  30. NMR signal decays in time • T1 relaxation – Flipped nuclei realign with the magnetic field • T2 relaxation – Flipped nuclei start off all spinning together, but quickly become incoherent (out of phase) • T2* relaxation – Disturbances in magnetic field (magnetic susceptibility) increase the rate of spin coherence T2 relaxation • The total NMR signal is a combination of the total number of nuclei (proton density), reduced by the T1, T2, and T2* relaxation components

  31. T2* decay • Spin coherence is also sensitive to the fact that the magnetic field is not completely uniform • Inhomogeneities in the field cause some protons to spin at slightly different frequencies so they lose coherence faster • Factors that change local magnetic field (susceptibility) can change T2* decay

  32. Different tissues have different relaxation times. These relaxation time differences can be used to generate image contrast. • T1 - Gray/White matter • T2 - Tissue/CSF • T2* - Susceptibility (functional MRI)

  33. Spatial Localization • Slice selection • In a homogeneous magnetic field (B0), an magnetic field (B1) oscillating at or very near the resonant frequency, w, will excite nuclei in the bore of the imaging magnet (Larmor equation): w = g Bo 38

  34. Spatial Localization • Slice selection(continued) • We can selectively excite nuclei in one slice of tissue by incorporating a third magnetic field: the “gradient” magnetic field. A gradient magnetic field is a small magnetic field superimposed on the static magnetic field. The gradient magnetic field produces a linear change in the total magnetic field. • Here, “gradient” means “change in field strength as a function of location in the MRI bore”. 39

  35. Spatial Localization • Slice selection(continued) • Since the gradient field changes in strength as a function of position, we use the term “gradient amplitude” to describe the field: • Gradient amplitude = D (field strength) / D (distance) • Example units: gauss / cm 40

  36. Spatial Localization • Slice selection(continued) • To recap, we use these magnetic fields in MRI: • B0 – large, homogeneous field of superconducting magnet. • B1 – temporally-oscillating, RF magnetic field that excites nuclei at resonance. • Bg – spatially-varying, small field responsible for the linear variation in the total field. 41

  37. Spatial Localization • Slice selection(continued) • The linear change of the gradient field can be along the Z axis (inferior to superior), the X axis (left to right), the Y axis (anterior to posterior), or any combination (an “oblique” scanning prescription). • Switching the gradient magnetic fields on/off produces the MRI acoustic noise. 42

  38. Z Y X Superconducting Magnet In a typical superconducting magnet having a a horizontal bore, the Z axis is down the center of the bore, the X axis is horizontal and the Y axis is vertical. 43

  39. Spatial Localization • Slice selection example: • Q: How does the gradient field affect the resonant frequency? • A: The resonant frequency will be different at different locations. • Consider a gradient magnetic field of 0.5 Gauss / cm, using a Z gradient superimposed on a 1.5 T static magnetic field (1.5T = 15,000 Gauss). 44

  40. Spatial Localization • Here’s a picture of the total magnetic field as a function of position: 45

  41. Spatial Localization • Recall the Larmor equation: • w = g Bo • For hydrogen: • = 42.58 MHz/Tesla = 42.58 x 106 Hz/Tesla • Calculating the center frequency at 1.5 Tesla: • w = g Bo • w = (42.58 MHz / Tesla)(1.5 Tesla) • w = 63.87 MHz 46

  42. Spatial Localization What are the frequencies at Inferior 20cm and Superior 20cm? At I 20cm, Btot = 1.499 T: • wI = g Btot • wI = (42.58 MHz / Tesla)(1.499 Tesla) • wI = 63.827 MHz At S 20cm, Btot = 1.501 T: • wS = g Btot • wS = (42.58 MHz / Tesla)(1.501 Tesla) • wS = 63.913 MHz Difference in frequencies: .086 MHz 47

  43. RF Bandwidth • The RF frequency of the oscillating B1 magnetic field has an associated bandwidth. Rather than oscillating at a single frequency of 63.870 MHz, a range or “bandwidth” of frequencies is present. A typical bandwidth for the oscillating B1 magnetic field is + 1 kHz, thus RFfrequencies from 63.869 MHz to 63.871 MHz are present. The bandwidth of RF frequenciespresent in the oscillating B1 magnetic field is inversely proportional to the duration of the RF pulse. 48

  44. RF Bandwidth(continued) • There are actually two RF bandwidths are associated with MRI: • Transmit and Receive • RF Transmit bandwidth ~ +1 kHz affects • slice thickness (we are discussing this) • RF Receive bandwidth ~ +16 kHz is • sometimes adjusted to optimize • signal-to-noise in images (more in a later lecture) 49

  45. RF Bandwidth(continued) 50