Nguyên lý tạo ảnh trong MRI (NMR dx rad imaging physics course) phần 2

Nuclear Magnetic Resonance – Bushberg Chapter 14 Diagnostic Radiology Imaging Physics Course Take Aways: Aways: Five Things You should be able to Explain after the NMR Lectures ¬ 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: http://courses.washington.edu/radxphys/PhysicsCourse04 http://courses.washington.edu/radxphys/PhysicsCourse04 05.html 05.html © UW and Brent K Stewart, PhD, DABMP The magnetic characteristics of the nucleus and the magnetic properties of matter How the NMR signal is generated and detected T1 and T2 relaxation: how they arise and how they are measured Pulse sequence methods used and pulse sequence timing (e.g., TR and TE) and inherent NMR parameters (e.g., T1 and T2) give rise to tissue contrast 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 2003 Nobel Prize for Medicine - MRI Soft Tissue Transparency and First NMR Image ¬ ¬ ¬ ¬ Laterbur and Mansfield (2003, medicine): discoveries concerning magnetic resonance imaging (MRI) Rabi (1944, physics): nuclear magnetic resonance (NMR) methodology Bloch and Purcell (1952, physics): NMR precision measurements Ernst (1991, chemistry): highhigh-resolution NMR spectroscopy c.f Mokovski, A Medical Imaging Systems, p © UW and Brent K Stewart, PhD, DABMP 9, 19 and 26 May 2005 © UW and Brent K Stewart, PhD, DABMP Nuclear Magnetic Resonance – Bushberg Chapter 14 Diagnostic Radiology Imaging Physics Course NMR T1 for Tumor and Normal Tissue Nuclear Magnetic Resonance ¬ ¬ ¬ ¬ ¬ ¬ ¬ 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 Magnetic Resonance Imaging – magnetic gradients and mathematical reconstruction algorithms produce the NNdimensional image from NMR freefree-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 © UW and Brent K Stewart, PhD, DABMP Image Contrast – What does it depend on? ¬ ¬ ¬ ¬ ¬ ¬ c.f Mansfield, et al NMR Imaging in Biomedicine, 1982, p 22 ¬ ¬ ¬ Exception: permanent magnet Magnetic susceptibility – extent to which a material becomes magnetized when placed in a magnetic field Three categories of magnetic susceptibility ¬ Diamagnetic – opposing applied field ¬ Paramagnetic – enhancing field, no selfself-magnetism ¬ Ferromagnetic – ‘superparamagnetic’, greatly enhancing field ¬ ¬ ¬ © UW and Brent K Stewart, PhD, DABMP 9, 19 and 26 May 2005 Mag field generated by moving charges (e- or quarks) Most materials not exhibit overt magnetic properties ¬ ¬ intrinsic: ρH,T1, T2, flow, perfusion, diffusion, 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 Magnetism and the Magnetic Properties of Matter Radiation needs to interact with the body’s tissues in some differential manner to provide contrast X-ray/CT: differences in e- density (e-/cm3 = ρ · e-/g) Ultrasound: differences in acoustic impedance (Z = ρ·c) Nuclear Medicine: differences in tracer concentration (ρ (ρ) MRI: many intrinsic and extrinsic factors affect contrast ¬ c.f http://www.gg.caltech.edu/~dhl http://www.gg.caltech.edu/~dhl// Ca, H2O, most organic materials (C and H) O2, deoxyhemoglobin and GdGd-based contrast agents Exhibits selfself-magnetism: Fe, Co and Ni © UW and Brent K Stewart, PhD, DABMP Nuclear Magnetic Resonance – Bushberg Chapter 14 Diagnostic Radiology Imaging Physics Course 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(N-S) and repulsion (N(N-N & SS-S) Magnetic field strength (flux density): B ¬ ¬ ¬ Magnetism and the Magnetic Properties of Matter Measured in tesla (T) and gauss (G): 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 magnitude) © UW and Brent K Stewart, PhD, DABMP Magnetic Characteristics of the Nucleus ¬ ¬ ¬ ¬ ¬ c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 374 and 377 © UW and Brent K Stewart, PhD, DABMP 10 Nuclear Magnetic Characteristics of the Elements Magnetic properties of nuclei determined by the spin and charge distribution (quarks) of the nucleons (p+ and n) Magnetic moment (µ (µ) describes the nuclear B field magnitude Pairing of p+-p+ or nn-n causes µ to cancel out So if P (total p+) and N (total n) is even no/little µ If N even and P odd or P even and N odd resultant µ (NMR eff.) eff.) ¬ ¬ ¬ ¬ ¬ ¬ c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 375 © UW and Brent K Stewart, PhD, DABMP 9, 19 and 26 May 2005 11 Biologically relevant elements that are candidates for NMR/MRI Magnitude of µ Physiologic concentration Isotopic abundance Relative sensitivity 1H (p+) provide 104-106 times the signal from 23Na or 31P c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 376 © UW and Brent K Stewart, PhD, DABMP 12 Nuclear Magnetic Resonance – Bushberg Chapter 14 Diagnostic Radiology Imaging Physics Course Nuclear Magnetic Characteristics of the Elements ¬ ¬ ¬ ¬ ¬ ¬ Spinning p+ considered ‘classically’ as a bar magnet Thermal energy randomizes direction of µ no net magnetization Application of an external magnetic field (B0) two energy states Lower energy µ parallel w/ B0 and higher energy µ antianti-parallel w/ B0 Number of excess µ @ 1.0T and 310 K ppm (very small effect) For typical voxel in MRI: 1021 p+ 3x1015 more µ in lower state Larmor Frequency ¬ ¬ ¬ ¬ ¬ ¬ ‘Classically’ a torque on µ by B0 causes precession Precession occurs at an angular frequency (rotations/sec or radians/sec)* radians/sec)* Larmor equation: ω0(radians/sec)= γ·B0 ; f0(rotations/sec or Hz)= ( )·B0 = gyromagnetic ratio (MHz/T) unique to each element Choice of freq the resonance phen to be ‘tuned’ to a specific element For 1H @ 1.5T = 64 MHz (Channel 3) * Note: 360° = 2π radians, radian = 57.3° c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 377 © UW and Brent K Stewart, PhD, DABMP c.f http://www.hull.ac.uk/mri /lectures/ /lectures/gpl_page.html 13 Larmor Frequency & US VHF Broadcast Spectrum c.f Hendee, et al Medical Imaging Physics, 4th ed., p 357 ¬ ¬ ¬ 3.0 T = 128 MHz ¬ c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p.18 © UW and Brent K Stewart, PhD, DABMP 9, 19 and 26 May 2005 c.f http://www.rentcom.com/wpapers/ telex/telex3.html 15 c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 379 14 Nuclear Magnetic Characteristics of the Elements ¬ 1.5 T = 64 MHz © UW and Brent K Stewart, PhD, DABMP At equilibrium, no B field ⊥ B0 (all along zz-axis) Random distribution of µ in xx-y plane averages out: Bxy = Small µz add up to measurable M0 (equilibrium magnetization) Absorbed radiofrequency EM radiation lowlow-E to highhigh-E HighHigh-E nuclei lose energy to environment: return to equilibrium state and Mz (longitudinal magnetization) M0 c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 378 © UW and Brent K Stewart, PhD, DABMP c.f http://www.hull.ac.uk/mri /lectures/gpl_page.html /lectures/gpl_page.html 16 Nuclear Magnetic Resonance – Bushberg Chapter 14 Diagnostic Radiology Imaging Physics Course Raphex 2000 Diagnostic Questions ¬ ¬ ¬ ¬ ¬ ¬ Raphex 2003 Diagnostic Questions D42 D42 Which of the following elements would not be of interest in an MRI image? Element Z A A Hydrogen 1 B Carbon 13 C Oxygen 16 D Sodium 11 23 E Phosphorus 15 31 © UW and Brent K Stewart, PhD, DABMP ¬ D53 D53 For hydrogen imaging in a 1.0 T MRI unit, the frequency of the RF signal is about: ¬ A 400 kHz B MHz C 40 MHz D 400 MHz E GHz ¬ ¬ ¬ ¬ 17 © UW and Brent K Stewart, PhD, DABMP Geometric Orientation ¬ ¬ ¬ ¬ Resonance and Excitation Two frames of reference used ¬ Laboratory frame – stationary reference from observer’s POV Rotating frame – angular frequency equal to the Larmor precessional frequency ¬ ¬ ¬ Rotating Frame Both frames are useful in explaining various interactions Mxy: transverse magnetization, ¬ ¬ ¬ ⊥ B0 (at equilibrium = 0) ¬ Return to equilibrium results in RF emission from µ with ¬ Lab Frame 18 When RF applied, Mz tipped into the xx-y (transverse) plane ¬ Amplitude proportional the number of excited nuclei (spin ρ) Rate depends on the characteristics of the sample (T1 and T2) 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 Absorption of RF energy promotes lowhighlow-E µ high-E µ Amplitude and duration of RF pulse determines the number of nuclei that undergo the energy transition (θ (θ) Continued RF application induces a return to equilibrium Rotating Frame c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., pp 380380-381 © UW and Brent K Stewart, PhD, DABMP 9, 19 and 26 May 2005 19 © UW and Brent K Stewart, PhD, DABMP 20 Nuclear Magnetic Resonance – Bushberg Chapter 14 Diagnostic Radiology Imaging Physics Course Resonance and Excitation Changing Reference Frames RF Pulse Angle Tip: ¬ ¬ ¬ 0° ¬ ¬ Why is MRI so hard to learn? Changing reference frames Classical versus Quantum Mechanical explanation Lab and rotating frames Changing scales ¬ 90° 90° ¬ ¬ ¬ 180° 180° Higher energy state c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 382 © UW and Brent K Stewart, PhD, DABMP Start with a voxel of mm x mm x 10 mm as a starting point and then split up later into smaller and smaller pieces 21 © UW and Brent K Stewart, PhD, DABMP Resonance and Excitation ¬ ¬ ¬ ¬ ¬ 22 Resonance and Excitation B1 field component rotating at Larmor f0 (offlittle effect) (off-freq Rotating reference frame: B1 stationary in xx-y plane B1 applied torque to Mz rotation: θ = γ · B1· t Flip angle (θ (θ) describes the rotation through which the longitudinal magnetization (Mz) is displaced to generate transverse magnetization (Mxy) Common angles: 90° 90° (π/2 radians: π/2 pulse) and 180° 180° (π radians) ¬ ¬ ¬ Time required 1010-100 µsec 90° largest Mxy 90° pulse (signal) generated For flip angle (θ) (θ) < 90° 90° ¬ ¬ ¬ ¬ Rotating Frame Macroscopic Intermediate (spin isochromats) isochromats) Microscopic/QM smaller Mxy component generated and less signal less time necessary to displace Mz greater amount of Mxy (signal) per excitation time Low flip angle (θ) very important in rapid MRI scanning Lab Frame c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 384 © UW and Brent K Stewart, PhD, DABMP 9, 19 and 26 May 2005 23 c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 384 © UW and Brent K Stewart, PhD, DABMP 24 Nuclear Magnetic Resonance – Bushberg Chapter 14 Diagnostic Radiology Imaging Physics Course Free Induction Decay: T2 and T2* Relaxation Free Induction Decay: T2 and T2* Relaxation ¬ ¬ ¬ 90° 90° pulse produces phase coherence of nuclei As Mxy rotates at f0 the receiver coil (lab frame) through magnetic induction (dB/dt) produces a damped sinusoidal electronic signal: free induction decay (FID) ¬ ¬ ¬ c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 385 25 © UW and Brent K Stewart, PhD, DABMP c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 385 ¬ ¬ ¬ ¬ ¬ ¬ T2 decay mechanisms det by the molecular structure of the sample sample Mobile molecules (e.g., CSF) exhibit a long T2 as rapid molecular molecular motion reduces intrinsic B inhomogeneities Large, stationary structures have short T2 B0 inhomogeneities and susceptibility agents (e.g., contrast materials) cause more rapid dephasing T2* decay c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 386 © UW and Brent K Stewart, PhD, DABMP 9, 19 and 26 May 2005 c.f http://www.hull.ac.uk/mri /lectures/gpl_page.html /lectures/gpl_page.html 26 © UW and Brent K Stewart, PhD, DABMP Return to Equilibrium: T1 Relaxation Free Induction Decay: T2 and T2* Relaxation ¬ Decay of the FID envelope due to loss of phase coherence of the individual spins due to intrinsic micro magnetic field variations in the sample: spinspin-spin interaction T2 decay constant Mxy(t) = M0e-(t/T2): decay of Mxy after 90° 90° pulse T2: time required for Mxy to to 37% (1/e) peak level T2 relaxation relatively unaffected by B0 27 ¬ ¬ ¬ Loss of Mxy phase coherence (T2 & T2* decay) occurs relatively quickly Return of Mz to M0 (equilibrium) takes longer Excited spins release energy to local environment (‘lattice’): spinT1 spin-lattice relaxation decay constant Mz(t) = M0[1[1-e-(t/T1)]: recovery of Mz after 90° 90° pulse T1: time required for Mz to to 63%: (1(1-e-1) M0 After t = T1 Mz(t) ≅ M0 c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 387 © UW and Brent K Stewart, PhD, DABMP c.f http://www.hull.ac.uk/mri /lectures/ /lectures/gpl_page.html 28 Nuclear Magnetic Resonance – Bushberg Chapter 14 Diagnostic Radiology Imaging Physics Course Return to Equilibrium: T1 Relaxation ¬ ¬ ¬ Return to Equilibrium: T1 Relaxation Method to determine T1: use various ∆t between 90° 90° pulses pulses and estimate by curve fitting Dissipation of absorbed energy into the lattice (T1) varies substantially for various tissue structures and pathologies (prev Damadian table) Energy transfer most efficient when the precessional frequency of the excited nuclei 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 ¬ ¬ ¬ ¬ ¬ ¬ 29 Comparison of T1 and T2 ¬ ¬ ¬ ¬ ¬ c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 389 © UW and Brent K Stewart, PhD, DABMP 9, 19 and 26 May 2005 © UW and Brent K Stewart, PhD, DABMP 30 T1 and T2 versus B Field Strength T1 > T2 > T2* (T2 44-10X shorter than T1) Small molecules: long T1 and long T2 (e.g., water, CSF) Intermediate molecules: short T1 and short T2 (most tissues) Large/bound molecules: long T1 and short T2 The differences in T1 and T2, as well as spin density (ρ (ρ) provide much to MRI contrast and exploited for the diagnosis of pathologic conditions c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., pp 390390-391 Large slowslow-moving molecules low vibrational freq (very small overlap with f0: longest T1) Moderately sized molecules (e.g., lipids, proteins and fat) and viscous fluids low & intermed freq (great overlap: short T1) Small molecules low, intermediate and high freq (small overlap with f0: long T1) T1: Soft tissue [0.1,1] and aqueous substances [1,4] T1 relaxation as B0 Contrast agents: spinspin-lattice sink 1.5 T = 64 MHz 3.0 T = 128 MHz 31 c.f Mansfield, et al NMR Imaging in Biomedicine, 1982, p 23 © UW and Brent K Stewart, PhD, DABMP 32 Nuclear Magnetic Resonance – Bushberg Chapter 14 Diagnostic Radiology Imaging Physics Course Raphex 2003 Diagnostic Questions Raphex 2003 Diagnostic Questions ¬ D56 D56 In MRI, pure water will have a T1 and a T2 ¬ D55 D55 In MRI contrast is created by all of the following except: ¬ A long, long B long, short C short, long D short, short ¬ A Administration of a contrast agent B Differences in atomic number C Differences in hydrogen content D Differences in T1 time of tissues E Differences in T2 time of tissues ¬ ¬ ¬ ¬ ¬ ¬ ¬ © UW and Brent K Stewart, PhD, DABMP 33 © UW and Brent K Stewart, PhD, DABMP Raphex 2002 Diagnostic Questions ¬ D52 In biological tissue, relaxation times are ordered: ¬ A T1 < T2 < T2* B T1 < T2* < T2 C T2* < T2 < T1 D T2 < T2* < T1 E T2 < T1 < T2* ¬ ¬ ¬ ¬ Raphex 2000 Diagnostic Questions ¬ D46 D46 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 Tesla MRI system, one might expect the T2 relaxation time to: ¬ A Decrease significantly B Decrease slightly C Increase significantly D Increase slightly E Remain the same ¬ ¬ ¬ ¬ © UW and Brent K Stewart, PhD, DABMP 9, 19 and 26 May 2005 34 35 © UW and Brent K Stewart, PhD, DABMP 36 Nuclear Magnetic Resonance – Bushberg Chapter 14 Diagnostic Radiology Imaging Physics Course Pulse Sequences ¬ ¬ ¬ Spin Echo (SE) - Echo Time (TE) ¬ Tailoring pulse sequence emphasizes the image contrast dependent on ρ, T1 and T2 contrast weighted images Timing, order, polarity, pulse shaping, and repetition 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 37 c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 392 Spin Echo (SE) - Echo Time (TE) ¬ ¬ ¬ ¬ ¬ Initial 90° maximal Mxy and phase coherence 90° pulse (t = 0) FID exponentially decays via T2* relaxation At t = TE/2 a 180° induces spin rephasing 180° pulse is applied Spin inversion: spins rotate in the opposite direction, undoing all the T2* dephasing through ∆t = TE/2 at t = TE (∆ (∆t = 2· 2·TE/2) An FID waveform echo (“ (“spin echo” echo”) produced at t = TE © UW and Brent K Stewart, PhD, DABMP 9, 19 and 26 May 2005 38 SE - Repetition Time (TR) & Partial Saturation 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 events FID echo envelope centered at TE sampled and digitized with ADC Multiple echos generated by successive 180° 180° pulses allow 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 ¬ ¬ ¬ ¬ 39 Standard SE pulse sequences use a series of 90° 90° pulses separated by ∆t = TR (repetition time, msec): [300,3000] This ∆t allows recovery of Mz through T1 relaxation processes After the 2nd 90° 90° pulse, a steadysteady-state Mz produces the same FID amplitude from subsequent 90° 90° 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 © UW and Brent K Stewart, PhD, DABMP 40 10 Nuclear Magnetic Resonance – Bushberg Chapter 14 Diagnostic Radiology Imaging Physics Course Spin Echo Contrast Weighting ¬ ¬ ¬ ¬ ¬ Spin Echo: T1T1-weighting How the NMR signal changes with different tissue types and pulse sequence parameters S ∝ ρ · [1[1-e-(TR/T1)] · e-(TE/T2) ρ, T1 and T2 are tissue properties TR and TE are pulse sequence parameters Each of these values can alter voxel contrast ¬ ¬ ¬ ¬ ¬ Short TR to maximize differences in Mz during return to equilibrium Short TE to minimize differences in T2 dependency of the FID How T1 values modulate the FID When TR ranges 400400-600 msec differences in Mz emphasized Short TE preserves the T1 FID differences with minimum T2 decay (x,y) c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 394 © UW and Brent K Stewart, PhD, DABMP 41 c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 395 Spin Echo: T1T1-weighting © UW and Brent K Stewart, PhD, DABMP 42 Spin Echo: Spin (Proton) Density Weighting (TR=549, TE=11) ¬ ¬ ¬ ¬ ¬ T1T1-weighted (TR=500, TE=8) Fat most intense signal White and gray matter with intermediate signal CSF with lowest signal Typical pulse sequence parameters ¬ ¬ ¬ ¬ ¬ ¬ ¬ Image contrast due to differences in the nuclear spin density (ρ (ρ) Very hydrogenous tissues (e.g., lipids and fats) have high ρ compared with proteinaceous soft tissues Aqueous tissues (e.g., CSF) also have a relatively high ρ Long TR to minimize T1 differences (CSF > fat > GM > WM) Short TE to minimize T2 decay TR: 400400-600 msec TE: 55-30 msec T1 c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 395 © UW and Brent K Stewart, PhD, DABMP 9, 19 and 26 May 2005 43 c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 397 © UW and Brent K Stewart, PhD, DABMP 44 11 Nuclear Magnetic Resonance – Bushberg Chapter 14 Diagnostic Radiology Imaging Physics Course Spin Echo: Spin (Proton) Density Weighting Spin Echo: T2T2-weighting (TR=2400, TE=30) ¬ ¬ ¬ ¬ ρ-weighted (TR=2,400, TE=30) Fat and CSF – relatively bright Slight contrast inversion between WM and GM Typical pulse sequence parameters ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ Reduce T1 effects with long TR, TR, accentuate T2 effects with long TE T2T2-weighted signal usu the second echo of a multimulti-echo sequence Compared with a T1inversion of tissue contrast T1-weighted image Short T1 tissues short T2, long T1 tissues long T2 TR: 1,5001,500-3,500 msec TE: 55-30 msec Highest SNR for SE pulse sequences Image contrast relatively poor ρ c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 397 © UW and Brent K Stewart, PhD, DABMP 45 c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 398 © UW and Brent K Stewart, PhD, DABMP 46 T1 Spin Echo: T2T2-weighting Spin Echo Parameters (TR=2400, TE=90) ¬ ¬ ¬ T2− T2−weighted (TR > 2,000, TE > 80) As TE increased, more T2 contrast is achieved at the expense of reduced Mxy Typical pulse sequence parameters ¬ ¬ TR: 1,5001,500-3,500 msec TE: 6060-150 msec T2 c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 398 © UW and Brent K Stewart, PhD, DABMP 9, 19 and 26 May 2005 T1 ρ 47 c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 399 © UW and Brent K Stewart, PhD, DABMP 48 12 Nuclear Magnetic Resonance – Bushberg Chapter 14 Diagnostic Radiology Imaging Physics Course Raphex 2002 Diagnostic Questions Raphex 2000 Diagnostic Questions ¬ D51 A higher intensity MRI spin echo signal is produced by: ¬ A Long T1, long T2 B Long T1, short T2 C Short T1, long T2 D Short T1, short T2 ¬ ¬ ¬ ¬ D43 A spin echo pulse sequence is used with a TE time of 20 ms and a TR of 3000 ms The MR image obtained by this technique will be weighted ¬ A Atomic number (Z) B Mass number (A) C Proton density (PD) D T1 E T2 ¬ ¬ ¬ ¬ © UW and Brent K Stewart, PhD, DABMP 49 © UW and Brent K Stewart, PhD, DABMP Raphex 2001 Diagnostic Questions ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ Raphex 2000 Diagnostic Questions D47D47-D49 Using a normal spinspin-echo pulse sequence in MRI, match the timing with the type of image: TE (msec) TR (msec) A 20 400 B 100 400 C 100 2000 D 1000 400 E 20 2000 9, 19 and 26 May 2005 ¬ D47 D47 A spinspin-echo MRI pulse sequence in which water is bright and soft tissues are darker would utilize: ¬ A Long TE, long TR B Long TE, short TR C Short T1, long T2 D Short TE, long TR E Short TE, short TR ¬ ¬ ¬ ¬ D47 D47 T1T1-weighted D48 D48 T2T2-weighted D49 D49 Proton density weighted © UW and Brent K Stewart, PhD, DABMP 50 51 © UW and Brent K Stewart, PhD, DABMP 52 13 Nuclear Magnetic Resonance – Bushberg Chapter 14 Diagnostic Radiology Imaging Physics Course Inversion Recovery (IR) Inversion Recovery (IR) ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ Emphasizes T1 by expanding the amplitude of Mz by 2X Initial 180° - Mz 180° pulse inverts Mz After ∆t = TI (inversion (inversion time), time), a 90° 90° pulse rotates Mz into Mxy At ∆t = TI + TE/2, a second 180° 180° pulse induces an FID echo at TE TR = period between initial 180° 180° pulses TR < T1 causes partial saturation ¬ ¬ ¬ Echo amplitude depends on TI, TE, TR and |Mz| S ∝ ρ · [1[1-2e-(TI/T1)+e-(TR/T1)] · e-(TE/T2) TI controls contrast between tissues Can produce negative Mz (out of phase) when short TI used FID amplitude phase (phase sensitive detection – quadrature receiver coil) can be preserved or the magnitude taken (x,y) c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 400 © UW and Brent K Stewart, PhD, DABMP 53 c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 400 IR - T2 Short Tau IR 54 © UW and Brent K Stewart, PhD, DABMP IR - T2 Short Tau IR (TI=150, TR=5520, TE=29) ¬ ¬ ¬ ¬ ¬ Short Tau Inversion Recovery (STIR) Uses very short TI and magnitude signal processing Materials w/ short T1 have lower sig intensity (reverse of std T1T1-weighting) All tissues pass through zero amplitude (Mz = 0) Judicious TI selection suppress a given tissue signal (bounce point) ¬ ¬ ¬ ¬ (TR=750, TE=13) Null point: TI = ln(2) · T1 Example: fat suppression; T1 = 260 msec (B0=1.5T) TI = 180 msec Compared with a T1T1-weighted sequence, STIR ‘fat suppression’ suppression’ reduces distracting fat signal and eliminates chemical shift artifacts Typical STIR: TI = 140140-180 msec; TR = 2,500 msec FLAIR (TI=2400 TR=10K, TE=150) c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 402 © UW and Brent K Stewart, PhD, DABMP 9, 19 and 26 May 2005 55 c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 402 © UW and Brent K Stewart, PhD, DABMP T2 (TR=2400, TE=90) 56 14 Nuclear Magnetic Resonance – Bushberg Chapter 14 Diagnostic Radiology Imaging Physics Course IR - Field Attenuated IR and Contrast Comparison ¬ ¬ ¬ ¬ ¬ Long TI increases the signal levels of CSF & other long T1 tissues tissues FLuid FLuid Attenuated IR (FLAIR): bounce point at CSF T1 (3,500 msec) Nulling CSF requires: TI = ln(2) · T1 = 2,400 msec TR = 7,000 typically employed to allow reasonable Mz recovery Contrast comparison: T1T1-, ρ-, and T2T2-weighted plus FLAIR Raphex 2001 Diagnostic Questions ¬ D44 In MRI: ¬ A For most soft tissues, T2 is longer than T1 B T1 decreases with field strength C T1 of CSF is longer than T1 of soft tissue D T2 increases with field strength E T2 of soft tissue is longer than T2 of CSF ¬ ¬ ¬ ¬ c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 403 © UW and Brent K Stewart, PhD, DABMP 57 © UW and Brent K Stewart, PhD, DABMP Gradient Recalled Echo (GRE) ¬ ¬ ¬ ¬ ¬ Gradient Recalled Echo (GRE) Magnetic field gradient used to induce the formation of an echo Gradient changes local magnetic field (B0+∆B): f = (γ/2π) (γ/2π)··(B0+∆B) FID signal generated under a linear gradient dephases quickly Inverted gradient (opposite polarity) used to produce an FID echo echo Not a spinspin-echo technique; does not cancel T2* effects c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 404 © UW and Brent K Stewart, PhD, DABMP 9, 19 and 26 May 2005 58 ¬ ¬ ¬ ¬ 59 Echo time controlled through gradient magnitude or time offset Flip angle (θ) a major variable determining contrast in GRE seq Less time to excite the spins short TR smaller flip θ For short TR (< 200 msec) more Mz generated w/ small flip θ c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 404 © UW and Brent K Stewart, PhD, DABMP 60 15 Nuclear Magnetic Resonance – Bushberg Chapter 14 Diagnostic Radiology Imaging Physics Course GRE - SteadySteady-state Precession with Short TR (< 50 msec) GRE Sequence with Long TR (> 200 msec) ¬ For long TR (> 200 msec) GRE and flip θ > 45° 45°: contrast behavior similar to SE ¬ Major difference signal dependence on T2* rather than T2 Mechanisms of T2* contrast different than T2, especially for contrast agents T1T1-weighting achieved with short TE ¬ ¬ ¬ ¬ ¬ ¬ For flip θ < 30° 30°: small Mxy reduces T1 differences ¬ ¬ ¬ ¬ ¬ ρ differences the major contrast attributes for short TE Longer TE provides T2*T2*-weighting ¬ ¬ GRE not useful with long TR except for demonstrating magnetic susceptibility differences © UW and Brent K Stewart, PhD, DABMP ¬ ¬ 61 © UW and Brent K Stewart, PhD, DABMP GRE - SteadySteady-state Precession with Short TR (< 50 msec) and Contrast Weighting ¬ ¬ ¬ ¬ ¬ ¬ Small flip θ = 55-30° 30°: ρ-weighted contrast; Moderate flip θ = 3030-60° 60°: T2/T1T2/T1-weighted contrast (some T1) Large flip θ = 7575-90° 90°: T2*T2*- and T1T1-weighted contrast Typical parameter values for contrast desired in GRE and steadysteady-state acquisitions GRASS/FISP TR = 35 msec, TE = msec and flip θ = 20° 20° Unremarkable contrast but flow ¬ ¬ ¬ GRASS sequence (TR = 24 msec, TE = 4.7 msec, flip θ=50° 50°) volume acquisition Contrast unremarkable for white/gray matter due to T2/T1T2/T1-weighting dependence Blood appears bright – MR angiography – reduce contrast of anatomy relative to vasculature 9, 19 and 26 May 2005 62 GRE - “SPoiled SPoiled”” GRE GRE Techniques (SPGR) ¬ c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., pp 406406-407 © UW and Brent K Stewart, PhD, DABMP SteadySteady-state precession: equilibrium of Mz and Mxy from pulse to pulse in a repitition sequence For very short TR (< T2*), persistent Mxy occurs During each pulse aMxy Mz and bMz Mxy (a, b [...]... Physics of Medical Imaging, 2nd ed., p 399 © UW and Brent K Stewart, PhD, DABMP 48 12 Nuclear Magnetic Resonance – Bushberg Chapter 14 Diagnostic Radiology Imaging Physics Course Raphex 20 02 Diagnostic Questions Raphex 20 00 Diagnostic Questions ¬ D51 A higher intensity MRI spin echo signal is produced by: ¬ A Long T1, long T2 B Long T1, short T2 C Short T1, long T2 D Short T1, short T2 ¬ ¬ ¬ ¬ D43 A spin... (TR =24 00, TE=90) ¬ ¬ ¬ T2− T2−weighted (TR > 2, 000, TE > 80) As TE increased, more T2 contrast is achieved at the expense of reduced Mxy Typical pulse sequence parameters ¬ ¬ TR: 1,5001,500-3,500 msec TE: 6060-150 msec T2 c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 398 © UW and Brent K Stewart, PhD, DABMP 9, 19 and 26 May 20 05 T1 ρ 47 c.f Bushberg, et al The Essential Physics. .. minimize T2 decay TR: 400400-600 msec TE: 55-30 msec T1 c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 395 © UW and Brent K Stewart, PhD, DABMP 9, 19 and 26 May 20 05 43 c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 397 © UW and Brent K Stewart, PhD, DABMP 44 11 Nuclear Magnetic Resonance – Bushberg Chapter 14 Diagnostic Radiology Imaging Physics Course... Physics of Medical Imaging, 2nd ed., p 4 02 © UW and Brent K Stewart, PhD, DABMP 9, 19 and 26 May 20 05 55 c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 4 02 © UW and Brent K Stewart, PhD, DABMP T2 (TR =24 00, TE=90) 56 14 Nuclear Magnetic Resonance – Bushberg Chapter 14 Diagnostic Radiology Imaging Physics Course IR - Field Attenuated IR and Contrast Comparison ¬ ¬ ¬ ¬ ¬ Long TI increases... Essential Physics of Medical Imaging, 2nd ed., p 403 © UW and Brent K Stewart, PhD, DABMP 57 © UW and Brent K Stewart, PhD, DABMP Gradient Recalled Echo (GRE) ¬ ¬ ¬ ¬ ¬ Gradient Recalled Echo (GRE) Magnetic field gradient used to induce the formation of an echo Gradient changes local magnetic field (B0+∆B): f = (γ /2 ) (γ /2 )··(B0+∆B) FID signal generated under a linear gradient dephases quickly Inverted gradient... short T2, long T1 tissues long T2 TR: 1,5001,500-3,500 msec TE: 55-30 msec Highest SNR for SE pulse sequences Image contrast relatively poor ρ c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 397 © UW and Brent K Stewart, PhD, DABMP 45 c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 398 © UW and Brent K Stewart, PhD, DABMP 46 T1 Spin Echo: T2T2-weighting... = ln (2) · T1 = 2, 400 msec TR = 7,000 typically employed to allow reasonable Mz recovery Contrast comparison: T1T1-, ρ-, and T2T2-weighted plus FLAIR Raphex 20 01 Diagnostic Questions ¬ D44 In MRI: ¬ A For most soft tissues, T2 is longer than T1 B T1 decreases with field strength C T1 of CSF is longer than T1 of soft tissue D T2 increases with field strength E T2 of soft tissue is longer than T2 of CSF... type of image: TE (msec) TR (msec) A 20 400 B 100 400 C 100 20 00 D 1000 400 E 20 20 00 9, 19 and 26 May 20 05 ¬ D47 D47 A spinspin-echo MRI pulse sequence in which water is bright and soft tissues are darker would utilize: ¬ A Long TE, long TR B Long TE, short TR C Short T1, long T2 D Short TE, long TR E Short TE, short TR ¬ ¬ ¬ ¬ D47 D47 T1T1-weighted D48 D48 T2T2-weighted D49 D49 Proton density weighted... currents c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 410 ¬ T1 ρ © UW and Brent K Stewart, PhD, DABMP T2 FLAIR 69 c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 4 12 Magnetization Transfer Contrast ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ MR arthrograms of shoulder in 32- year-old man with suspected glenohumeral instability Axial 3D gradient-echo MR image obtained using parametric... minimize differences in T2 dependency of the FID How T1 values modulate the FID When TR ranges 400400-600 msec differences in Mz emphasized Short TE preserves the T1 FID differences with minimum T2 decay (x,y) c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 394 © UW and Brent K Stewart, PhD, DABMP 41 c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 395 Spin