Nuclear Magnetic Resonance-Bushberg Chapter 14 Diagnostic Radiology Imaging Physics Course Nuclear Magnetic Resonance-Chapter 14 othe nucleus and the roperties of matter How the NMR signal is generated and detected .T1 and T2 relaxation:how they arise and how they are Brent K.Stewart.PhD.DABMP ulse se Soft Tissue Transparency and First NMR Image edicine-MR an 参 ce (NM 3 9,19and26May2005
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 NMR T1 for Tumor Nuclear Magnetic Resonance and Normal Tissue NMR the study of the magnetic properties of the nucleus Magnetic field associated with nuclear spin/chg.distr. Not an imaging technique-provides spectroscopic data 润 Magnetic Resonance Imaging-magneticgradients and mathematical reconstruction algorithms produce the N- dimensional image from NMR free-induction decay data High contrast sensitivity to soft tissue differences Does not use ionizing radiation(radio waves) Important to understand the underlying principles of NMR in order to transfer this knowledge to MRI Image Contrast-What does it depend on? Magnetism and the Magnetic Properties of Matte Radiation needs to interact with the body's tissues in some differential manner to provide contrast Mag.field generated by moving charges(e-or quarks) X-ray/CT:differences in e density (e/cm3=pe/g) Most materials do not exhibit overt magnetic properties Ultrasound:differences in acoustic impedance(Z=p-c) ◆Exception:perma hent magnet Nuclear Medicine:differences in tracer concentration(p) Magnetic susceptibility-extent to which a material becomes magnetized when placed in a magnetic field MRI:many intrinsic and extrinsic factors affect contrast Three categories of magnetic susceptibility intrinsic:pT1.T2.flow,perfusion,diffusion,. Diar extrinsic:TR,TE.TI.flip angle, ost organic ma 0gneic aoneanae (Cand) age 4 9,19and26May2005
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 Magnetism and the Magnetic Properties of Matter Magnetism and the Magnetic Properties of Matter Magnetic fields arise from magnetic dipoles(N/S) N-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=10.000 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 magnitude) 8Ts4n Magnetic Characteristics of the Nucleus Nuclear Magnetic Characteristics of the Elements Magnetic moment (u)describes the nuclear B field magnitude Mucena 2持 ◆Pairing of p'-porn-n causesμto cancel out 。So if P(total p")and N (total n)is even-→nonittle u If N even and P odd or P even and N odd-resultantu (NMR eff.) TABLE 14-1.PROPERTIES OF THE NEUTRON AND PROTON Neutron Proton levant clemets hat are canddates for RMR kg 1 股8 Physiologic concentration Isotopic abundance 6所10n 2g*10- Relative sensitvty H (p")provide 104-10 times the signal from 2Na or 3P 0UW and Rn 9.19and26May2005
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 Nuclear Magnetic Characteristics of the Elements Larmor Frequency .Spinning p*considered 'classically'as a bar magnet 'Classically a torque on u by B causes precession 。Thermal energy randomizes direction ofμ→no net magnetization Precession occurs at an angular frequency(rotations/sec or radians/sec) .Application of an extemal magnetic field(B)-two energy states Larmor equation:eo(radians/sec)=TB:fo(rotations/sec or Hz)=(W2x)B ·Lower energy-parallel wB 2=gyromagnetic ratio(MHz/T)unique to each element 。Number ofe cess u 1.0T and 310K-3 pp very small effect) Choice of freg.-the resonance phen.to be 'tuned'to a specific element For typical voxel in MRI:102p →3x105moreμn lower state For 'H @1.5T =64 MHz (Channel 3) EMENT IN MAGNETIC ESOMANCEOR USEFU Y2s (MHz/T) radan =57.3 Larmor Frequency US VHF Broadcast Spectrum Nuclear Magnetic Characteristics of the Elements ◆At equllibrium,no B field⊥B ong z-axis) Random distribution ofin x-y plane averages out B=0 Small H add up to measurable Mo(equilibrium magnetization) .Absorbed radiofrequency EM radiation一→low-E to high-E 424%4% .High-E nuclei lose energy to environment:retumn to equilibrum state and M. (longitudinal magnetization) Bo 18 9,19and26May2005
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 Raphex 2000 Diagnostic Questions Raphex 2003 Diagnostic Questions D42.Which of the following elements would not be of D53.For hydrogen imaging in a 1.0 T MRI unit,the interest in an MRI image? frequency of the RF signal is about: Element 2 A ◆A.Hydrogen 1 1 ◆A.400kHz ◆B.Carbon 6 13 ◆B.4MHz ◆C.Oxygen 16 ◆C.40MHz ◆D.Sodium 11 23 ◆D.400MHz ◆E.Phosphorus 15 31 ◆E.4GHz 17 Geometric Orientation Resonance and Excitation Two frames of reference used Return to equilibrium results in RF emission from u with Amplitude proportional the number of excited nuclei(spin p) Rate depends on the characteristics of the sample(T1 and T2) Rotating frame-angular Excitation,detection analysis the basics for NMR/MRI Resonance occurs when applied RF magnetic field(B) is precisely matched in frequency to that of the nuclei explaining various interactions ◆Absorption of RF energy promotes low-Eμ→high-Eμ M:transverse magnetization Amplitude and duration of RF pulse determines the LB.(at equilibrium=0) number of nuclei that undergo the energy transition(0) Whe Continued RF application induces a retum to equilibrium ap UW and Rr 9.19and26May2005 J
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