《医学影像物理学》课程教学资源（书籍文献）磁共振NMR NMR Dx Rad Imaging Physics.pdf_大学文库

Nuclear Magnetic Resonance – Bushberg Chapter 14 Diagnostic Radiology Imaging Physics Course 9, 19 and 26 May 2005 1 © UW and Brent K. Stewart, PhD, DABMP 1 Nuclear Magnetic Resonance Nuclear Magnetic Resonance – Chapter 14 Brent K. Stewart, PhD, DABMP Professor, Radiology and Medical Education Director, Diagnostic Physics a copy of this lecture may be found at: a copy of this lecture may be found at: http://courses.washington.edu/radxphys/ ington.edu/radxphys/PhysicsCourse04-05.html © UW and Brent K. Stewart, PhD, DABMP 2 Take Aways: Five Things You should be able : Five Things You should be able to Explain after to Explain after the NMR Lectures the NMR Lectures The magnetic characteristics of the nucleus and the The magnetic characteristics of the nucleus and the magnetic properties of matter magnetic properties of matter How the NMR signal is generated and detected T1 and T2 relaxation: T1 and T2 relaxation: how they arise and how they are how they arise and how they are measured Pulse sequence methods used and pulse sequence Pulse sequence methods used and pulse sequence timing (e.g., TR and TE) and inherent NMR parameters timing (e.g., TR and TE) and inherent NMR parameters (e.g., T1 and T2) give rise to tissue contrast (e.g., T1 and T2) give rise to tissue contrast How a 1D gradient can be used to provide an echo and How a 1D gradient can be used to provide an echo and allow for quick imaging with shallow flip angle sequences © UW and Brent K. Stewart, PhD, DABMP 3 Soft Tissue Transparency and First NMR Image c.f. Mokovski, A. Medical Imaging Systems, p. 3. © UW and Brent K. Stewart, PhD, DABMP 4 2003 Nobel Prize for Medicine for Medicine - MRI Laterbur and Mansfield Laterbur and Mansfield (2003, medicine): (2003, medicine): discoveries concerning discoveries concerning magnetic resonance magnetic resonance imaging (MRI) imaging (MRI) Rabi (1944, physics): Rabi (1944, physics): nuclear magnetic nuclear magnetic resonance (NMR) resonance (NMR) methodology Bloch and Purcell (1952, Bloch and Purcell (1952, physics): NMR precision physics): NMR precision measurements Ernst (1991, chemistry): Ernst (1991, chemistry): high-resolution NMR resolution NMR spectroscopy

Nuclear Magnetic Resonance – Bushberg Chapter 14 Diagnostic Radiology Imaging Physics Course 9, 19 and 26 May 2005 2 © UW and Brent K. Stewart, PhD, DABMP 5 Nuclear Magnetic Resonance NMR the study of the magnetic properties of the nucleus Magnetic field associated with nuclear spin/chg. distr. Magnetic field associated with nuclear spin/chg. distr. Not an imaging technique Not an imaging technique – provides spectroscopic data Magnetic Resonance Imaging Magnetic Resonance Imaging – magnetic gradients and magnetic gradients and mathematical reconstruction algorithms produce the Ndimensional image from NMR free-induction decay data High contrast sensitivity to soft tissue differences Does not use ionizing radiation (radio waves) Does not use ionizing radiation (radio waves) Important to understand the underlying principles of Important to understand the underlying principles of NMR in order to transfer this knowledge to MRI NMR in order to transfer this knowledge to MRI © UW and Brent K. Stewart, PhD, DABMP 6 NMR T1 for Tumor NMR T1 for Tumor and Normal Tissue c.f. Mansfield, et al. NMR Imaging in Biomedicine, 1982, p. 22. c.f. http://www.gg.caltech.edu/~dhl/ © UW and Brent K. Stewart, PhD, DABMP 7 Image Contrast Image Contrast – What does it depend on? Radiation needs to interact with the body’s tissues in Radiation needs to interact with the body’s tissues in some differential manner to provide contrast some differential manner to provide contrast X-ray/CT: differences in e- density (e-/cm3 = ρ · e-/g) Ultrasound: differences in acoustic impedance (Z = Ultrasound: differences in acoustic impedance (Z = ρ·c) Nuclear Medicine: differences in tracer concentration ( Nuclear Medicine: differences in tracer concentration (ρ) MRI: many intrinsic and extrinsic factors affect contrast MRI: many intrinsic and extrinsic factors affect contrast intrinsic: intrinsic: ρH,T1, T2, flow, perfusion, diffusion, . ,T1, T2, flow, perfusion, diffusion, . extrinsic: TR, TE, TI, flip angle, . extrinsic: TR, TE, TI, flip angle, . c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 257. © UW and Brent K. Stewart, PhD, DABMP 8 Magnetism and the Magnetic Properties of Matter Magnetism and the Magnetic Properties of Matter Mag. field generated by moving charges (e- or quarks) or quarks) Most materials do not exhibit overt magnetic properties Exception: permanent magnet Exception: permanent magnet Magnetic susceptibility Magnetic susceptibility – extent to which a material extent to which a material becomes magnetized when placed in a magnetic field Three categories of magnetic susceptibility Diamagnetic Diamagnetic – opposing applied field Ca, H2O, most organic O, most organic materials (C and H) materials (C and H) Paramagnetic Paramagnetic – enhancing field, no self enhancing field, no self-magnetism O2, deoxyhemoglobin and Gd-based contrast agents Ferromagnetic Ferromagnetic – ‘superparamagnetic’, greatly enhancing field Exhibits self Exhibits self-magnetism: Fe, Co and Ni magnetism: Fe, Co and Ni

Nuclear Magnetic Resonance – Bushberg Chapter 14 Diagnostic Radiology Imaging Physics Course 9, 19 and 26 May 2005 3 © UW and Brent K. Stewart, PhD, DABMP 9 Magnetism and the Magnetic Properties of Matter Magnetism and the Magnetic Properties of Matter Magnetic fields arise from magnetic dipoles (N/S) Magnetic fields arise from magnetic dipoles (N/S) N – side the origin of magnetic field lines (arbitrary) side the origin of magnetic field lines (arbitrary) Attraction (N-S) and repulsion (N-N & S-S) Magnetic field strength (flux density): B Measured in tesla (T) and gauss (G): 1 T = 10,000 G Earth magnetic field ~ 1/20,000 T or 0.5 G Magnetic fields arise from Permanent magnets Current through a wire or solenoid (current amplitude sets B Current through a wire or solenoid (current amplitude sets B magnitude) magnitude) © UW and Brent K. Stewart, PhD, DABMP 10 Magnetism and the Magnetic Properties of Matter Magnetism and the Magnetic Properties of Matter c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 374 and 377. © UW and Brent K. Stewart, PhD, DABMP 11 Magnetic Characteristics of the Nucleus Magnetic Characteristics of the Nucleus Magnetic properties of nuclei determined by the spin and charge Magnetic properties of nuclei determined by the spin and charge distribution (quarks) of the nucleons (p+ and n) Magnetic moment ( Magnetic moment (µ) describes the nuclear B field magnitude Pairing of p+-p+ or n-n causes n causes µ to cancel out to cancel out So if P (total p+) and N (total n) is even ) and N (total n) is even → no/little no/little µ If N even and P odd or P even and N odd If N even and P odd or P even and N odd → resultant resultant µ (NMR eff.) c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 375. © UW and Brent K. Stewart, PhD, DABMP 12 Nuclear Magnetic Characteristics of the Elements Nuclear Magnetic Characteristics of the Elements Biologically relevant elements that are candidates for NMR/MRI Biologically relevant elements that are candidates for NMR/MRI Magnitude of Magnitude of µ Physiologic concentration Isotopic abundance Relative sensitivity 1H (p+) provide 104-106 times the signal from times the signal from 23Na or 31P c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 376

Nuclear Magnetic Resonance – Bushberg Chapter 14 Diagnostic Radiology Imaging Physics Course 9, 19 and 26 May 2005 4 © UW and Brent K. Stewart, PhD, DABMP 13 Nuclear Magnetic Characteristics of the Elements Nuclear Magnetic Characteristics of the Elements Spinning p+ considered ‘classically’ as a bar magnet considered ‘classically’ as a bar magnet Thermal energy randomizes direction of Thermal energy randomizes direction of µ → no net magnetization Application of an external magnetic field (B0) → two energy states Lower energy Lower energy µ parallel w/ B0 and higher energy and higher energy µ anti-parallel parallel w/ B0 Number of excess Number of excess µ @ 1.0T and 310 K @ 1.0T and 310 K → 3 ppm (very small effect) 3 ppm (very small effect) For typical voxel in MRI: 1021 p+ → 3x1015 more µ in lower state c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 377. c.f. http://www.hull.ac.uk/mri /lectures/gpl_page.html © UW and Brent K. Stewart, PhD, DABMP 14 Larmor Frequency Larmor Frequency ‘Classically’ a torque on Classically’ a torque on µ by B0 causes precession Precession occurs at an angular frequency (rotations/sec or radi Precession occurs at an angular frequency (rotations/sec or radians/sec)* Larmor equation: Larmor equation: ω0(radians/sec)= (radians/sec)= γ·B0 ; f0(rotations/sec or Hz)= ( (rotations/sec or Hz)= (γ/2π)·B0 γ/2π = gyromagnetic ratio (MHz/T) unique to each element = gyromagnetic ratio (MHz/T) unique to each element Choice of freq. Choice of freq. → the resonance phen. to be ‘tuned’ to a specific element the resonance phen. to be ‘tuned’ to a specific element For 1H @ 1.5T = 64 MHz (Channel 3) H @ 1.5T = 64 MHz (Channel 3) c.f. Bushberg, et al. The Essential Physics of c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 379. * Note: 360° = 2π radians, → 1 radian = 57.3° c.f. Hendee, et al. Medical Imaging Physics, 4th ed., p. 357. © UW and Brent K. Stewart, PhD, DABMP 15 Larmor Frequency & US VHF Broadcast Spectrum c.f. http://www.rentcom.com/wpapers/ telex/telex3.html 1.5 T = 64 MHz c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p.18. 3.0 T = 128 MHz © UW and Brent K. Stewart, PhD, DABMP 16 Nuclear Magnetic Characteristics of the Elements Nuclear Magnetic Characteristics of the Elements At equilibrium, no B field At equilibrium, no B field ⊥ B0 (all along z-axis) axis) Random distribution of Random distribution of µ in x-y plane averages out: Bxy = 0 Small µz add up to measurable add up to measurable M0 (equilibrium magnetization) (equilibrium magnetization) Absorbed radiofrequency EM Absorbed radiofrequency EM radiation radiation → low-E to high-E High-E nuclei lose energy to E nuclei lose energy to environment: return to environment: return to equilibrium state and Mz (longitudinal magnetization) (longitudinal magnetization) → M0 c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 378. c.f. http://www.hull.ac.uk/mri /lectures/gpl_page.html

Nuclear Magnetic Resonance – Bushberg Chapter 14 Diagnostic Radiology Imaging Physics Course 9, 19 and 26 May 2005 5 © UW and Brent K. Stewart, PhD, DABMP 17 Raphex 2000 Diagnostic Questions Raphex 2000 Diagnostic Questions D42. Which of the following elements would not be of . Which of the following elements would not be of interest in an MRI image? Element Element Z A A. Hydrogen 1 1 B. Carbon 6 13 C. Oxygen 8 16 D. Sodium 11 23 E. Phosphorus 15 31 © UW and Brent K. Stewart, PhD, DABMP 18 Raphex 2003 Diagnostic Questions Raphex 2003 Diagnostic Questions D53. For hydrogen imaging in a 1.0 T MRI unit, the . For hydrogen imaging in a 1.0 T MRI unit, the frequency of the RF signal is about: frequency of the RF signal is about: A. 400 kHz B. 4 MHz C. 40 MHz D. 400 MHz E. 4 GHz © UW and Brent K. Stewart, PhD, DABMP 19 Geometric Orientation Two frames of reference used Laboratory frame – stationary stationary reference from observer’s reference from observer’s POV Rotating frame – angular angular frequency equal to the Larmor frequency equal to the Larmor precessional frequency Both frames are useful in Both frames are useful in explaining various interactions Mxy: transverse magnetization, : transverse magnetization, ⊥ B0 (at equilibrium = 0) (at equilibrium = 0) When RF applied, Mz tipped into the x-y (transverse) plane c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., pp. 380-381. Rotating Frame Lab Frame Rotating Frame © UW and Brent K. Stewart, PhD, DABMP 20 Resonance and Excitation Return to equilibrium results in RF emission from Return to equilibrium results in RF emission from µ with Amplitude proportional the number of excited nuclei (spin Amplitude proportional the number of excited nuclei (spin ρ) Rate depends on the characteristics of the sample (T1 and T2) Rate depends on the characteristics of the sample (T1 and T2) Excitation, detection & analysis the basics for NMR/MRI Excitation, detection & analysis the basics for NMR/MRI Resonance occurs when applied RF magnetic field (B1) is precisely matched in frequency to that of the nuclei is precisely matched in frequency to that of the nuclei Absorption of RF energy promotes low-E µ → high-E µ Amplitude and duration of RF pulse determines the Amplitude and duration of RF pulse determines the number of nuclei that undergo the energy transition ( number of nuclei that undergo the energy transition (θ) Continued RF application induces a return to equilibrium

Nuclear Magnetic Resonance – Bushberg Chapter 14 Diagnostic Radiology Imaging Physics Course 9, 19 and 26 May 2005 6 © UW and Brent K. Stewart, PhD, DABMP 21 Resonance and Excitation c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 382. RF Pulse Angle Tip: 0° 90° 180° Higher energy state © UW and Brent K. Stewart, PhD, DABMP 22 Changing Reference Frames Changing Reference Frames Why is MRI so hard to learn? Changing reference frames Classical versus Quantum Classical versus Quantum Mechanical explanation Lab and rotating frames Changing scales Macroscopic Intermediate (spin Intermediate (spin isochromats) Microscopic/QM Start with a voxel of 1 mm x 1 mm Start with a voxel of 1 mm x 1 mm x 10 mm as a starting point and x 10 mm as a starting point and then split up later into smaller and then split up later into smaller and smaller pieces © UW and Brent K. Stewart, PhD, DABMP 23 Resonance and Excitation B1 field component rotating at Larmor f field component rotating at Larmor f0 (off-freq. → little effect) little effect) Rotating reference frame: B1 stationary in x-y plane B1 applied torque to applied torque to Mz → rotation: rotation: θ = γ · B1· t Flip angle ( Flip angle (θ) describes the rotation through which the longitudinal ) describes the rotation through which the longitudinal magnetization (Mz) is displaced to generate transverse ) is displaced to generate transverse magnetization (Mxy) Common angles: 90° (π/2 radians: /2 radians: π/2 pulse) and 180° (π radians) radians) c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 384. Rotating Frame Lab Frame © UW and Brent K. Stewart, PhD, DABMP 24 Resonance and Excitation Time required 10-100 µsec 90° pulse → largest Mxy (signal) generated For flip For flip angle ( angle (θ) < 90° smaller Mxy component component generated and less signal generated and less signal less time necessary to less time necessary to displace Mz greater amount of greater amount of Mxy (signal) (signal) per excitation time Low flip Low flip angle (θ) very important in rapid MRI important in rapid MRI scanning c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 384

Nuclear Magnetic Resonance – Bushberg Chapter 14 Diagnostic Radiology Imaging Physics Course 9, 19 and 26 May 2005 7 © UW and Brent K. Stewart, PhD, DABMP 25 Free Induction Decay: T2 and T2* Relaxation 90° pulse produces phase coherence of nuclei pulse produces phase coherence of nuclei As Mxy rotates at f rotates at f0 the receiver coil (lab frame) through the receiver coil (lab frame) through magnetic induction (dB/dt) produces a damped magnetic induction (dB/dt) produces a damped sinusoidal electronic signal: sinusoidal electronic signal: free induction decay (FID) c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 385. © UW and Brent K. Stewart, PhD, DABMP 26 Free Induction Decay: T2 and T2* Relaxation Decay of the FID envelope due to loss of phase Decay of the FID envelope due to loss of phase coherence of the individual spins due to intrinsic micro coherence of the individual spins due to intrinsic micro magnetic field variations in the sample: magnetic field variations in the sample: spin-spin interaction interaction → T2 decay constant T2 decay constant Mxy(t) = M0e-(t/T2): decay of Mxy after 90° pulse T2: time required for Mxy to ↓ to 37% (1/e) peak level to 37% (1/e) peak level T2 relaxation relatively unaffected by B0 c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 385. © UW and Brent K. Stewart, PhD, DABMP 27 Free Induction Decay: T2 and T2* Relaxation T2 decay mechanisms det. by the molecular structure of the sampl T2 decay mechanisms det. by the molecular structure of the sample Mobile molecules (e.g., CSF) exhibit a long T2 as rapid molecular motion reduces intrinsic B inhomogeneities Large, stationary structures have short T2 B0 inhomogeneities and susceptibility agents (e.g., contrast inhomogeneities and susceptibility agents (e.g., contrast materials) cause more rapid dephasing materials) cause more rapid dephasing → T2* decay c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 386. c.f. http://www.hull.ac.uk/mri /lectures/gpl_page.html © UW and Brent K. Stewart, PhD, DABMP 28 Return to Equilibr Return to Equilibrium: T1 Relaxation Loss of Mxy phase coherence phase coherence (T2 & T2* decay) occurs (T2 & T2* decay) occurs relatively quickly Return of Mz to M0 (equilibrium) takes longer (equilibrium) takes longer Excited spins release energy Excited spins release energy to local environment (‘lattice’): to local environment (‘lattice’): spin-lattice relaxation lattice relaxation → T1 decay constant decay constant Mz(t) = M0[1-e-(t/T1)]: recovery of : recovery of Mz after 90° pulse T1: time required for Mz to ↑ to 63%: (1-e-1) M0 After t = 5 T1 After t = 5 T1 → Mz(t) ≅ M0 c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 387. c.f. http://www.hull.ac.uk/mri /lectures/gpl_page.html

Nuclear Magnetic Resonance – Bushberg Chapter 14 Diagnostic Radiology Imaging Physics Course 9, 19 and 26 May 2005 8 © UW and Brent K. Stewart, PhD, DABMP 29 Return to Equilibr Return to Equilibrium: T1 Relaxation Method to determine T1: use Method to determine T1: use various various ∆t between t between 90° pulses and estimate by curve fitting Dissipation of absorbed energy Dissipation of absorbed energy into the lattice (T1) varies into the lattice (T1) varies substantially for various tissue substantially for various tissue structures and pathologies structures and pathologies (prev. Damadian table) (prev. Damadian table) Energy transfer most efficient Energy transfer most efficient when the precessional when the precessional frequency of the excited nuclei frequency of the excited nuclei overlaps with the vibrational overlaps with the vibrational frequencies of the lattice c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 388. © UW and Brent K. Stewart, PhD, DABMP 30 Return to Equilibr Return to Equilibrium: T1 Relaxation Large slow-moving molecules moving molecules → low vibrational freq. (very small low vibrational freq. (very small overlap with f overlap with f0: longest T1) : longest T1) Moderately sized molecules (e.g., Moderately sized molecules (e.g., lipids, proteins and fat) and lipids, proteins and fat) and viscous fluids viscous fluids → low & intermed. low & intermed. freq. (great overlap: short T1) freq. (great overlap: short T1) Small molecules Small molecules → low, intermediate and high freq. (small intermediate and high freq. (small overlap with f overlap with f0: long T1) : long T1) T1: Soft tissue [0.1,1] and T1: Soft tissue [0.1,1] and aqueous substances [1,4] aqueous substances [1,4] T1 relaxation T1 relaxation ↑ as B0 ↑ Contrast agents: spin-lattice sink c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 389. © UW and Brent K. Stewart, PhD, DABMP 31 Comparison of T1 and T2 T1 > T2 > T2* (T2 4-10X shorter than T1) shorter than T1) Small molecules: long T1 and Small molecules: long T1 and long T2 (e.g., water, CSF) long T2 (e.g., water, CSF) Intermediate molecules: short Intermediate molecules: short T1 and short T2 (most tissues) T1 and short T2 (most tissues) Large/bound molecules: long Large/bound molecules: long T1 and short T2 The differences in T1 and T2, The differences in T1 and T2, as well as spin density ( as well as spin density (ρ) provide much to MRI contrast provide much to MRI contrast and exploited for the diagnosis and exploited for the diagnosis of pathologic conditions c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., pp. 390-391. © UW and Brent K. Stewart, PhD, DABMP 32 T1 and T2 versus B Field Strength T1 and T2 versus B Field Strength c.f. Mansfield, et al. NMR Imaging in Biomedicine, 1982, p. 23 1.5 T = 64 MHz 3.0 T = 128 MHz

Nuclear Magnetic Resonance – Bushberg Chapter 14 Diagnostic Radiology Imaging Physics Course 9, 19 and 26 May 2005 9 © UW and Brent K. Stewart, PhD, DABMP 33 Raphex 2003 Diagnostic Questions Raphex 2003 Diagnostic Questions D56. In MRI, pure water will have a _ T1 and a . In MRI, pure water will have a _ T1 and a _ T2. _ T2. A. long, long B. long, short B. long, short C. short, long D. short, short D. short, short © UW and Brent K. Stewart, PhD, DABMP 34 Raphex 2003 Diagnostic Questions Raphex 2003 Diagnostic Questions D55. In MRI contrast is created by all of the following . In MRI contrast is created by all of the following except: except: A. Administration of a contrast agent. A. Administration of a contrast agent. B. Differences in atomic number. B. Differences in atomic number. C. Differences in hydrogen content. C. Differences in hydrogen content. D. Differences in T1 time of tissues. D. Differences in T1 time of tissues. E. Differences in T2 time of tissues. E. Differences in T2 time of tissues. © UW and Brent K. Stewart, PhD, DABMP 35 Raphex 2002 Diagnostic Questions Raphex 2002 Diagnostic Questions D52. In biological tissue, relaxation times are ordered: In biological tissue, relaxation times are ordered: A. T1 < T2 < T2* A. T1 < T2 < T2* B. T1 < T2* < T2 C. T2* < T2 < T1 D. T2 < T2* < T1 E. T2 < T1 < T2* E. T2 < T1 < T2* © UW and Brent K. Stewart, PhD, DABMP 36 Raphex 2000 Diagnostic Questions Raphex 2000 Diagnostic Questions D46. The T2 relaxation time of a tissue is about 60 msec . The T2 relaxation time of a tissue is about 60 msec on an MRI system with a 0.5 Tesla magnet. On a 1.5 on an MRI system with a 0.5 Tesla magnet. On a 1.5 Tesla MRI system, one might expect the T2 relaxation Tesla MRI system, one might expect the T2 relaxation time to: time to: A. Decrease significantly. A. Decrease significantly. B. Decrease slightly. B. Decrease slightly. C. Increase significantly. C. Increase significantly. D. Increase slightly. D. Increase slightly. E. Remain the same. E. Remain the same

Nuclear Magnetic Resonance – Bushberg Chapter 14 Diagnostic Radiology Imaging Physics Course 9, 19 and 26 May 2005 10 © UW and Brent K. Stewart, PhD, DABMP 37 Pulse Sequences Pulse Sequences Tailoring pulse sequence Tailoring pulse sequence emphasizes the image contrast emphasizes the image contrast dependent on dependent on ρ, T1 and T2 , T1 and T2 → contrast weighted images Timing, order, polarity, pulse Timing, order, polarity, pulse shaping, and repetition shaping, and repetition frequency of RF pulses and frequency of RF pulses and gradient (later) application Three major pulse sequences Spin echo Inversion recovery Gradient recalled echo c.f. http://www.indianembassy.org/dydemo/page3.htm © UW and Brent K. Stewart, PhD, DABMP 38 Spin Echo (SE) Spin Echo (SE) - Echo Time (TE) Echo Time (TE) Initial Initial 90° pulse (t = 0) pulse (t = 0) → maximal Mxy and phase coherence FID exponentially decays via T2* relaxation At t = TE/2 a 180° pulse is applied pulse is applied → induces spin rephasing Spin inversion: spins rotate in the opposite direction, undoing Spin inversion: spins rotate in the opposite direction, undoing all the all the T2* dephasing through through ∆t = TE/2 at t = TE ( = TE/2 at t = TE (∆t = 2·TE/2) An FID waveform echo ( An FID waveform echo (“spin echo”) produced at t = TE c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 392. © UW and Brent K. Stewart, PhD, DABMP 39 Spin Echo (SE) Spin Echo (SE) - Echo Time (TE) Echo Time (TE) Maximum echo amplitude depends on T2 and not T2* FID envelope decay still dependent on T2* SE formation separates RF excitation and signal acquisition events FID echo envelope centered at TE sampled and digitized with ADC Multiple echos generated by successive Multiple echos generated by successive 180° pulses allow pulses allow determination of sample T2 determination of sample T2 - exponential curve fitting: Mxy(t) ∝ e-t/T2 c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 393. © UW and Brent K. Stewart, PhD, DABMP 40 SE - Repetition Time (TR) Repetition Time (TR) & Partial Saturation Standard SE pulse sequences use a series of Standard SE pulse sequences use a series of 90° pulses separated pulses separated by ∆t = TR (repetition time, msec): [300,3000] t = TR (repetition time, msec): [300,3000] This ∆t allows recovery of Mz through T1 relaxation processes After the 2nd 90° pulse, a steady-state Mz produces the same FID produces the same FID amplitude from subsequent 90° pulses: pulses: partial saturation Degree of partial saturation dependent on T1 relaxation and TR c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 394