MRI basics lecture_9_26

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Presentation Transcript

Magnetic Resonance Imaging:Physical Principles : 

Magnetic Resonance Imaging:Physical Principles , Lewis Center for NeuroImaging

Physics of MRI, An Overview : 

10/22/2008 2 Physics of MRI, An Overview Nuclear Magnetic Resonance Nuclear spins Spin precession and the Larmor equation Static B0 RF excitation RF detection Spatial Encoding Slice selective excitation Frequency encoding Phase encoding Image reconstruction Fourier Transforms Continuous Fourier Transform Discrete Fourier Transform Fourier properties k-space representation in MRI

Physics of MRI : 

10/22/2008 3 Physics of MRI Echo formation Vector summation Phase dispersion Phase refocus 2D Pulse Sequences Spin echo Gradient echo Echo-Planar Imaging Medical Applications Contrast in MRI Bloch equation Tissue properties T1 weighted imaging T2 weighted imaging Spin density imaging Examples 3D Imaging Spectroscopy

Many spins in a voxel: vector summation : 

10/22/2008 4 Many spins in a voxel: vector summation spins in step spins not in step Rotating frame Lamor precession

Phase dispersion due to perturbing B fields : 

10/22/2008 5 Phase dispersion due to perturbing B fields Spin Phase f  gBt B = B0 + dB0 + dBcs + dBpp sometime after RF excitation Immediately after RF excitation sampling

Refocus spin phase – echo formation : 

10/22/2008 6 Refocus spin phase – echo formation time Invert perturbing field: dB -dB Invert spin state: f -f Phase 0 dBt f-dB(t-TE/2) 0 Phase 0 dBt -f+dB(t-TE/2) 0 Echo Time (TE) (gradient echo, k-space sampling) (spin echo)

Spin Echo : 

10/22/2008 7 Spin Echo Spins dephase with time Rephase spins with a 180° pulse Echo time, TE Repeat time, TR (Running analogy)

Frequency encoding - 1D imaging : 

10/22/2008 8 Frequency encoding - 1D imaging m(x) Spatial-varying resonance frequency during RF detection S(t) = m(x)eikxxdx = S(kx), m(x) = FT{S(kx)} S(t) ~ eigBt S(t) ~ m(x)eigGxxtdx kx = gGxt x B = B0 + Gxx

Slice selection : 

10/22/2008 9 Slice selection Spatial-varying resonance frequency during RF excitation z B1 freq band w = w0 + gGzz m+ = mx+imy ~ g b1(t)e-igGzztdt = B1(gGzz) Excited location w Slice profile

Gradient Echo FT imaging : 

10/22/2008 10 Gradient Echo FT imaging kx ky Readout Repeat with different phase-encoding amplitudes to fill k-space

Pulse sequence design : 

10/22/2008 11 Pulse sequence design prewinder spoiler rephasor rewinder spoiler

EPI (echo planar imaging) : 

10/22/2008 12 EPI (echo planar imaging) X Y Z time kx ky Quick, but very susceptible to artifacts, particularly B0 field inhomogeneity. Can acquire a whole image with one RF pulse – single shot EPI RF

Spin Echo FT imaging : 

10/22/2008 13 Spin Echo FT imaging kx ky Readout Repeat with different phase-encoding amplitudes to fill k-space

Spin Relaxation : 

10/22/2008 14 Spin Relaxation Spins do not continue to precess forever Longitudinal magnetization returns to equilibrium due to spin-lattice interactions – T1 decay Transverse magnetization is reduced due to both spin-lattice energy loss and local, random, spin dephasing – T2 decay Additional dephasing is introduced by magnetic field inhomogeneities within a voxel – T2' decay. This can be reversible, unlike T2 decay

Bloch Equation : 

10/22/2008 15 Bloch Equation The equation of MR physics Summarizes the interaction of a nuclear spin with the external magnetic field B and its local environment (relaxation effects)

Contrast - T1 Decay : 

10/22/2008 16 Contrast - T1 Decay Longitudinal relaxation due to spin-lattice interaction Mz grows back towards its equilibrium value, M0 For short TR, equilibrium moment is reduced

Contrast - T2 Decay : 

10/22/2008 17 Contrast - T2 Decay Transverse relaxation due to spin dephasing T2 irreversible dephasing T2/ reversible dephasing Combined effect

Free Induction Decay – Gradient echo (GRE) : 

10/22/2008 18 Free Induction Decay – Gradient echo (GRE) Excite spins, then measure decay Problems: Rapid signal decay Acquisition must be disabled during RF Don’t get central “echo” data time e-t/T2* 90 RF 0 MR signal

Spin echo (SE) : 

10/22/2008 19 Spin echo (SE) time e-t/T2* MR signal e-t/T2

MR Parameters: TE and TR : 

10/22/2008 20 MR Parameters: TE and TR Echo time, TE is the time from the RF excitation to the center of the echo being received. Shorter echo times allow less T2 signal decay Repetition time, TR is the time between one acquisition and the next. Short TR values do not allow the spins to recover their longitudinal magnetization, so the net magnetization available is reduced, depending on the value of T1 Short TE and long TR give strong signals

Contrast, Imaging Parameters : 

10/22/2008 21 Contrast, Imaging Parameters

Properties of Body Tissues : 

10/22/2008 22 Properties of Body Tissues MRI has high contrast for different tissue types!

MRI of the Brain - Sagittal : 

10/22/2008 23 MRI of the Brain - Sagittal T1 Contrast TE = 14 ms TR = 400 ms T2 Contrast TE = 100 ms TR = 1500 ms Proton Density TE = 14 ms TR = 1500 ms

MRI of the Brain - Axial : 

10/22/2008 24 MRI of the Brain - Axial T1 Contrast TE = 14 ms TR = 400 ms T2 Contrast TE = 100 ms TR = 1500 ms Proton Density TE = 14 ms TR = 1500 ms

Brain - Sagittal Multislice T1 : 

10/22/2008 25 Brain - Sagittal Multislice T1

Brain - Axial Multislice T1 : 

10/22/2008 26 Brain - Axial Multislice T1

Brain Tumor : 

10/22/2008 27 Brain Tumor Post-Gd T1 T1 T2

3D Imaging : 

10/22/2008 28 3D Imaging Instead of exciting a thin slice, excite a thick slab and phase encode along both ky and kz Greater signal because more spins contribute to each acquisition Easier to excite a uniform, thick slab than very thin slices No gaps between slices Motion during acquisition can be a problem

2D Sequence (Gradient Echo) : 

10/22/2008 29 2D Sequence (Gradient Echo) Gx Gy Gz b1 acq ky kx TR TE Scan time = NyTR

3D Sequence (Gradient Echo) : 

10/22/2008 30 3D Sequence (Gradient Echo) Gx Gy Gz b1 acq kx ky kz Scan time = NyNzTR

3D Imaging - example : 

10/22/2008 31 3D Imaging - example Contrast-enhanced MRA of the carotid arteries. Acquisition time ~25s. 160x128x32 acquisition (kxkykz). 3D volume may be reformatted in post-processing. Volume-of-interest rendering allows a feature to be isolated. More on contrast-enhanced MRA later

Spectroscopy : 

10/22/2008 32 Spectroscopy Precession frequency depends on the chemical environment (dBcs) e.g. Hydrogen in water and hydrogen in fat have a f = fwater – ffat = 220 Hz Single voxel spectroscopy excites a small (~cm3) volume and measures signal as f(t). Different frequencies (chemicals) can be separated using Fourier transforms Concentrations of chemicals other than water and fat tend to be very low, so signal strength is a problem Creatine, lactate and NAA are useful indicators of tumor types

Spectroscopy - Example : 

10/22/2008 33 Spectroscopy - Example Intensity Frequency

Future lectures : 

10/22/2008 34 Future lectures Magnetization preparation (phase and magnitude, pelc) Fast imaging (fast sequences, epi, spiral…) Motion (artifacts, compensation, correction, navigator…) MR angiography (TOF, PC, CE) Perfusion and diffusion Functional imaging (fMRI) Cardiac imaging (coronary MRA)

3rd dimension – phase encoding : 

10/22/2008 35 3rd dimension – phase encoding Before frequency encoding and after slice selection, apply y-gradient pulse that makes spin phase varying linearly in y. Repeat RF excitation and detection with different gradient area. S(ky, t) =   ( m+(x,y,z)dz)eikyyeigGxxtdxdy