} B 1 } Coil } Gradients } FFT

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1 Introduction to MRI Daniel B. Ennis, Ph.D. Requirements for MRI UCLA DCVI Requirements for MRI Dipoles to Images MR Active uclei e.g. 1 H in H20 Cryogen Liquid He and 2 Magnetic Field (B0) Polarizer ystem (B1) Exciter Coil Receiver Gradients (GX, GY, GZ) patial Encoding Y-grad X-grad Cryostat Z-grad Body Coil (B1) Main Coil (B0) µ Magnetic Moment M z M xy (t) k I ( x) Bulk Magnetization Transverse Magnetization Received ignal k-space signal Image } B 0 } B 1 } Coil } Gradients } FFT Image Adapted From:

2 Dipoles to Images Main Field B0 µ Magnetic Moment M z M xy (t) k I ( x) Bulk Magnetization Transverse Magnetization Received ignal k-space signal Image } B 0 } B 1 } Coil } Gradients } FFT Main Field (B0) - Principles Magnetic Dipoles & Larmor B0 is a strong magnetic field 1.5T, 3.0T, 7.0T, etc. Z-oriented B 0 = B 0 k B0 forces M to precess Larmor Equation B0 generates M More B0, more M = M = B X total n=1 µ n Movie from Don Plewes

3 Bulk Magnetization Zeeman plitting M = X total n=1 µ n E = 1 2 B 0 E =+ 1 2 B 0 } B 0 is o B 0 is on } total=0.24x10 23 spins in a 2x2x10mm voxel = pin-up tate, Low Energy = pin-down tate, High Energy Zeeman plitting hb 0 total 2KT = Hz/T h = J s [Planck Constant] T = 300K (room temperature) K = J/K [Boltzmann Constant] B 0 = 1.5T Pulses B total T 09

4 Dipoles to Images B1 Field - Pulse µ Magnetic Moment M z M xy (t) k I ( x) Bulk Magnetization Transverse Magnetization Received ignal k-space signal Image } B 0 } B 1 } Coil } Gradients } FFT B1 is a radiofrequency () 42.58MHz/T (63MHz at 1.5T) short duration pulse (~0.1 to 5ms) small amplitude <30 µt circularly polarized rotates at Larmor frequency magnetic field perpendicular to B0 B1 Field Lab vs. Rotating Frame Hard Pulse oft inc Pulse Z Z=Z t t =0 t = p t t =0 t = p B 1 =2B e 1(t) cos( t + ) i X Laboratory Frame Y X Y Rotating Frame Envelope Function Carrier Frequency A lot of the math can be done more easily in the rotating frame.

5 Dipoles to Images Coils µ Magnetic Moment M z M xy (t) k I ( x) Bulk Magnetization Transverse Magnetization Received ignal k-space signal Image } B 0 } B 1 } Coil } Gradients } FFT 13 Coils Faraday s Law of Induction The induced electromotive force or EMF in any closed circuit is equal to the time rate of change of the magnetic flux through the circuit. -- Time-varying Magnetic Field Loop of Wire Voltage

6 MR ignal Detection 8-Channel Head Coil Each coil element has a unique sensitivity profile. Coil only detects Mxy Coil does not detect Mz Coil must be properly oriented Faraday s Law of Induction V (t) / sin M xy Dipoles to Images Gradients Gx, Gy, & Gz µ Magnetic Moment M z M xy (t) k I ( x) Bulk Magnetization Transverse Magnetization Received ignal k-space signal Image } B 0 } B 1 } Coil } Gradients } FFT 17

7 Gradients MRI Instrumentation Gradients are a: mall <5G/cm edge of 30cm FOV) patially varying Linear gradients Adds to B0 only in Z-direction Time varying lewrate Max. ~ mT/m/ms Magnetic fields Adds/ubtracts to the B0 field Parallel to B0 Y-Gradient Transceiver Patient Z-Gradient X-Gradient Z Gradients B 0 + B 0 Z-Gradients B 0 + B 0 Maxwell Pair Coil B 0 I B 0 I Z B 0 B 0 B 0 B 0 X

8 X-Gradients X+Z-Gradients Z Z Z X B 0 B 0 B 0 B 0 + B 0 X X X+Z-Gradients Possible lice pin Isochromat Group of spins with the same resonance frequency. k-space Z X 24

9 What is k-space? 1D k-space patial Frequency Mapping Each echo measures some of the spatial frequencies that comprise the object k-space has units of cm -1 or mm -1 Audio signals have units of Hertz (s -1 ) A line of k-space is filled by an echo 2D FT of k-space produces the image time -orspace Any signal/image can be decomposed into a summation of sine waves of appropriate amplitude. 1D k-space 1D k-space time -orspace time -orspace Any signal/image can be decomposed into a summation of sine waves of appropriate amplitude. Any signal/image can be decomposed into a summation of sine waves of appropriate amplitude.

10 1D k-space 1D k-space time -orspace time -orspace Any signal/image can be decomposed into a summation of sine waves of appropriate amplitude. Any signal/image can be decomposed into a summation of sine waves of appropriate amplitude. Fourier Representation What is k-space? k-space image space time -orspace FFT FFT low frequency high k-space is the raw data collected by the scanner.

11 Center What is k-space? Contrast What is k-space? FFT Edges Edges Contrast Information FFT Points in k-space represent different patterns in an image. k-space k-space spikes image space k-space and Field of View ky kx FFT FFT ky FOV = 1 k kx FFT A k-space spike creates a banding artifact. Uniformly skipping lines in k-space causes aliasing.

12 k-space and Resolution ky kx FFT ky Image Contrast kx FFT Acquiring fewer phase encodes decreases resolution. 34 Why Image Contrast? Why Image Contrast? Visual Area of the Thalamus Optic nerve Optic chiasm Optic tract Retina Visual Cortex The human visual system is more sensitive to contrast than absolute luminance.

13 1952 obel Prize in Physics for their development of new methods for nuclear magnetic precision measurements and discoveries in connection therewith Bloch Equations with Relaxation Felix Bloch b. 23 Oct 1905 d. 10 ep 1983 Edward Purcell b. 30 ep 1912 d. 07 Mar 1997 DCVI Bloch Equations d M ~ dt = M ~ B ~ Mxî+M yĵ (M z M 0 ) ˆk + Dr 2 M ~ T 2 T 1 { Precession Precession { Transverse Relaxation { Magnitude of ~M unchanged ~M Longitudinal Relaxation Phase (rotation) of changes due to Relaxation T1 changes are slow O(100ms) T2 changes are fast O(10ms) Magnitude of M can be ZERO Diffusion pins are thermodynamically driven to exchange positions. { ~B Diffusion DCVI Longitudinal & Transverse Relaxation M z (t) =M 0 z e { Initial Condition t T 1 + M 0 1 e M xy (t) =M 0 xye t/t 2 { Initial Condition Return to Equilibrium General solutions to the Bloch equations with relaxation in the rotating frame during free precession. t T 1 { Return to Equilibrium

14 T1 & T2 Relaxation T1 and T2 1.5T M 0 xy M 0 Tissue T1 [ms] T2 [ms] gray matter white matter A.U. Mz Mxy muscle fat kidney liver M 0 z CF Time [ms] T1 Relaxation T1 Relaxation Longitudinal or spin-lattice relaxation Typically, (10s ms)<t1< (100s ms) T1 is long for mall molecules (water) Large molecules (proteins) T1 is short for Fats and intermediate-sized molecules T1 increases with increasing B0 T1 decreases with contrast agents hort T1s are bright on T1-weighted image Fraction of M Fat 260ms Liver 500ms CF 2400ms Decay Time [ms]

15 T2 Relaxation Transverse or spin-spin relaxation Molecular interaction causes spin dephasing Typically, T2<(10s ms) T2 increases with Decreasing molecular size Large molecules have a short T2 Fat has a short T2 Increasing molecular mobility Liquids have long T2s CF, edema Decreasing molecular interactions olids have short T2s Percent ignal [a.u.] T2 Relaxation Liver 43ms Fat 85ms CF 180ms T2 relatively independent of B0 T2 always < T1 Long T2 is bright on T2 weighted image Decay Time [ms] T2 * Relaxation T2 * Relaxation The observed transverse relaxation time constant pin-spin (T2) dephasing combined with... Irreversible Intravoxel field inhomogeneity B0 Typically a few PPM over DV (40-50cm) 1PPM = 640Hz = 1.5µT usceptibility differences (macro and micro) Induce small field perturbations, therefore dephasing Reversible Can be rephased with a spin echo ot with a gradient echo! Diffusion Irreversible

16 T2 * Relaxation T2 * Relaxation 1 T 2 = 1 T 2 + B 0 1 T 2 = 1 T T 0 2 Irreversible Losses Reversible Losses Irreversible Losses Reversible Losses 1 T 2 T2 * Relaxation = 1 T T 0 2 Irreversible Losses Reversible Losses + 1 T D 2 Irreversible Losses + Percent ignal [a.u.] T2 * vs T2 T2 125ms T2 * 90ms T2*<T2 (always!) Decay Time [ms]

17 What are echoes? What are echoes? Two-sided MR signals First half from re-focusing econd half from de-phasing pin Echoes Arise from multiple -pulses Gradient Echoes Arise from magnetic field gradient reversal Line of k-space 48 Why echoes? Pulse equences Free Induction Decay MR signal immediate after an pulse ignal decays rapidly T2 * (<T2)+pectral distribution Imaging requires certain delays lice-selective re-phasing Phase encoding Readout pre-phasing Echoes let us buy some time Free Induction Decay (FID) Contrast Module aturation Recovery Inversion Recovery T2-preparation Imaging Module (Fast) pin Echo (poiled) Gradient Echo aka Host equence

18 Pulse equence Definitions TR - Repetition Time Duration of basic pulse sequence repeating block At least one echo acquired per TR TE - Echo Time Time from excitation to the maximum of the echo pin Echo Imaging 51 pin Echo pin Echo Advantages All spins within voxel rephased Insensitive to off-resonance B0 inhomogeneity Intravoxel Chemical shift signal loss usceptibility Great for T1, T2, ρ contrast ot T2* High R Disadvantages TR can be long AR can be high ignal 90 ome T2* signal losses are reversible.

19 pin Echo pin Echo TE ignal ignal pin Echo pin Echo - Contrast 90 TR 90 TR ignal TE ignal e t T 2 TE e t T 2

20 pin Echo pin Echo - Refocusing 90 TR TE ignal How do you adjust the TR? How do you adjust the TE? pin Echo Contrast pin Echo Parameters pin Density hort Long T1-Weighted hort Intermediate T2-Weighted Intermediate Long A Echo / 1 e TR/T 1 e TE/T 2 pin Echo Contrast pin Echo Parameters pin Density 10-30ms >2000ms T1-Weighted 10-30ms ms T2-Weighted >60ms >2000ms ρ T2 Long TR hort T1 X hort TE Long Images Courtesy of Mark Cohen

21 pin Echo - Contrast pin Echo - Variable TE T2 Contrast TE=13ms TE=26ms TE=53ms TE=106ms TE=145ms TE=172ms Fast pin Echo Fast pin Echo 90 Advantages Glice Turbo factor accelerates imaging Can be used with 2D slice interleaving Allows T2 weighted imaging in a breath hold Disadvantages GPhase GReadout ignal Echo-1 T2-decay Echo-2 Echo-3 High turbo factors (ETL>4): Blur images Alter image contrast Fat & Water are both bright on T2-weighted Water/CF T2 is long Repeated 180s reduce spin-spin interaction This lengthens the moderate T2 of fat AR can be high

22 Inversion Recovery Inversion Recovery Key Features ignal Preparation Block Inversion Pulse TI Inversion Time [ms] ignal Measurement Block pin Echo or Gradient Echo ignal during imaging is dependent on T1 and TI TR is typically long (>2000ms) Better for 2D sequences Can null a single T1 species if TI=ln(2)T1=0.69T1 Can be used for quantitative T1 mapping 62 Inversion Pulses Inversion Recovery

23 Inversion Recovery Contrast Contrast Relax Imaging TI TR Inversion Recovery TE 90 Contrast TR TI Relax Contrast Inversion Recovery 90 TR TI Mz Contrast Relax TE Contrast Inversion Recovery 90 TR TI Mz Contrast Relax TE Contrast

24 Basic Gradient Echo equence e t T 2 Gradient Echo Imaging lice elect Phase Encode Free Induction Decay (FID) Freq. Encode 68 Basic Gradient Echo equence Basic Gradient Echo equence e t T 2 lice elect Free Induction Decay (FID) lice elect Gradient Echo! Phase Encode Phase Encode Freq. Encode Freq. Encode

25 Basic Gradient Echo equence TR TE Basic Gradient Echo equence TR TE lice elect Phase Encode lice elect Phase Encode Wasted Time Freq. Encode Freq. Encode Gradient Echo + poiling Phase Cycling lice elect poiler Gradient poiler Gradient Gradient Echoes & Contrast Phase Encode Freq. Encode

26 poiled Gradient Echo Contrast T2*-weighted Gradient Echo Imaging Axial houlder Axial houlder Gradient Echo Parameters Type of Contrast TE TR Flip Angle pin Density hort Long mall T1-Weighted hort Intermediate Large T2 * -Weighted Intermediate Long mall A echo / 1 e TR/T 1 1 cos e TR/T 1 sin e TE/T 2 Contrast adjusted by changing TR, flip angle, and TE. TE=9ms TE=30ms usceptibility Weighting (darker with longer TE) Bright fluid signal (long T2* is brighter with longer TE) poiled GRE & Ernst Angle Ernst = arccos e TR T 1 Produces the largest MRI signal for a given TR and T1. Tissue T1 [ms] T2 [ms] muscle fat MRI ignal [A.U.] poiled GRE & Ernst Angle Fat Muscle Contrast Flip Angle

27 poiled GRE & Ernst Angle 90 pin Echo EPI High Muscle ignal High Fat ignal Glice GPhase TE TR Highest Contrast GReadout ignal T2*-decay Off Resonance Effects Accumulate pin Echo EPI Advantages Can acquire data in a single shot Can be used with 2D slice interleaving Allows fast T2 * weighted imaging Disadvantages ingle hot EPI Ghosting Blur images Image distortion Alter image contrast Multi-shot EPI lower than single shot Faster than E Applications DWI, Perfusion, fmri µ Magnetic Moment M z M xy (t) k I ( x) Dipoles to Images Bulk Magnetization Transverse Magnetization Received ignal k-space signal Image } B 0 } B 1 } Coil } Gradients } FFT

28 Thanks Daniel B. Ennis, Ph.D (Office) Peter V. Ueberroth Bldg. uite 1417, Room C Le Conte Avenue UCLA DCVI