PULSE SEQUENCES
FREQUENCY SELECTIVE EXCITATION RF Grad 0 Sir Peter Mansfield
A 1D IMAGE Field Strength / Frequency Position
FOURIER PROJECTIONS MR Image Raw Data FFT of Raw Data
BACK PROJECTION Image Domain Gradient Encoding 2D Fourier Transform real imaginary Fourier Domain Paul Lauterbur
EQUIVALENT STRATEGIES IN K-SPACE* Gradient Samples Time Gradient Gradient Samples Gradient Time *Ignoring effects of signal decay and sample motion Samples
GRADIENT PRE-ENCODING Signal Gradient Samples Signal Gradient Samples Time Signal Gradient Samples
INTERLEAVED SPATIAL ENCODING Gradient 1 Gradient 2 A2 A1 Samples Points indicated in black are affected by Gradient 1 but NOT by Gradient 2 Points indicated in Blue are affected by both gradients
EPI K-SPACE TRAJECTORY k-space plots the integral of the gradient encoding. k-phase k(x,y,t) = γ T 0 G(x,y,t)dt Its Fourier transform is the image. k-frequency
CONVENTIONAL SPATIAL ENCODING tr RF Grad 0 Grad 1 te Grad 2 A2 A2 A2 A1 A1 A1 Samples
CONVENTIONAL K-SPACE TRAJECTORY +Kphase tr -K frequency
SPIRAL ky Gx kx Gy
3D K-SPACE k y kz k x
3D K-SPACE Gz tr Gy Gx Imaging time = tr * Nz * Ny
MULTI-SLICE MRI tr RF Slice 1 Slice 3 Slice 2 Slice 4 Slice 1 te Gz Gy Gx t slice N = slices tr / tslice
SPATIAL ENCODING tr RF Grad 0 Grad 1 te Grad 2 A2 A2 A2 A1 A1 A1 Samples
CONTRAST ENCODING tr tr RF Grad 0 Grad 1 te te Grad 2 A2 A2 A2 A1 A1 A1 Samples
B0 SPIN DEPHASING
T2 AND TE 1 Signal 0.5 S(t) = M xy (t) = M 0 e te T 2 CSF Brain Fat 0 0 60 Time (milliseconds) 120
PARTIAL SATURATION tr tr Sequence of 90 Pulses NMR Signal
EFFECTS OF TE AT LONG TR
EFFECTS OF TR AT SHORT TE te=17 nex=1 thick=3mm Matrix=256x256 BW=16kHz
CONTRAST, TR AND TE Long Proton Density T2-Weighted tr Short T1-Weighted Short Long te
CONTRAST, TR AND TE Density T2 Long tr T1 Short Short te Long
T2-WEIGHTED EPI SCAN Metastatic (cancer) lesions, and many others, typically appear bright on T2- weighted MR images
OBSERVED RELAXATION RATE The Observed Transverse Relaxation Rate, T2*, is the sum of several components: 1 1 1 1 = + + T2* T2 T2 T2 D Molecular Field Inhomogeneity Diffusion
HAHN SPIN ECHO 1. 2. 3. 4. 180 pulse 90 pulse 5. rephasing dephasing 6. spin echo
B0 SUMMARY ANIMATION
90 180 MULTI-ECHO 180 T2* T2* T2 TE1 TE2
INVERSION RECOVERY 180 90 1 Mz 0.5 0 0.5 CSF Brain Fat 1 Mxy 0.5 1 500 0 500 1000 1500 2000 2500 3000 3500 4000 Time (ms) 0 0.5 1 500 0 500 1000 1500 2000 2500 3000 3500 4000 Time (ms) TI=700ms
3D T1 Images TE = 3.2 TR = 14.4 124 slices 1.25 mm thick 1 NEX Flip Angle 20 TI = 500
Sample Data Set (normal) Fast Spin Echo 3 mm Slices 3D IR-SPGR TE = 3.2, TI = 700 SAMPLE DATA SET (NORMAL)
CONTRAST TO NOISE RATIO (GRAY-WHITE) Gray White 0.2 te 0.1 0 3 tr 6 tr, te in seconds -5% 0% +3% -5% 0% 3% Contrast = [(1 e tr /1.2 )e te /.08 ], gray matter [(1 e tr /1.0 )e te /.07 ], white matter
REDUCED FLIP ANGLE IMAGING Outline Determinants of Imaging Time TR, Saturation and Image Quality Reduced Flip Angle Techniques FLASH (=SPGR) FISP (=GRASS) Gradient Echoes Applications of Shallow Flip Imaging Ultra-Fast Imaging
DETERMINANTS OF IMAGING TIME Scan Time = Repetition Time (TR) x Number of Phase Encodes x NEX (Averages) x Number of 3D Steps
TR AND IMAGE QUALITY Reduced TR Yields: Decreased Scan Time Increased T1 Contrast Reduced (Useable) T2 Contrast Reduced Signal to Noise Ratio Increased Power Deposition Reduced Slice Coverage 36
SIGNAL AND FLIP ANGLE Small Flip Angle Large Flip Angle α α 37
SMALL AND LARGE FLIP ANGLE Loss of Longitudinal Magnetization Small Flip Angle Large Flip Angle
FLIP ANGLE AND TR/T1 1 0.8 0.6 90 45 0.4 0.2 0 0 1 2 3 4 20 10 5 0.25 0.2 0.15 0.1 0.05 20 45 10 90 5 0 0 0.02 0.04 0.06 0.08 0.1
Contrast and Flip Angle Large Flip Angles Short Long Long Short Proton Density T1 Weighted T2* Weighted Small Flip Angles Short Long Long Short Proton Density Proton Density T2* Weighted T2* Weighted
A 180 PULSE IS NOT USED IN FLASH IMAGING z x z After 180 Pulse z x y Initial Magnetization y After Small RF Pulse y x
T2 AND T2* T2: Transverse Magnetization Decay from Spin-Spin Interactions T2*: Transverse Magnetization Decay from Local Magnetic Field Variations
SIGNAL AND TE GRADIENT ECHO TE=20 TE=40 TE=60 TE=80 TE=100
MAGNETIC SUSCEPTIBILITY The Extent to Which a Substance Becomes MAGNETIZED when Placed Within a Magnetic Field
MAGNETIC SUSCEPTIBILITY Objects with Susceptibility Different than Air Distort the Magnetic Field Applied Magnetic Field
T2* 100ms 0ms
SIGNAL LOSSES FROM SPIN DEPHASING B Inhomogeneous Magnetic Fields Within Voxels Result in Spin Dephasing and Signal Loss in Gradient Echo Sequences Capillary Gradients of several Gauss/cm may exist near deoxy-hb-filled capillaries.
CONTRAST OPTIMIZATION Contrast 43 ms TE >> T2a=40; T2b=45; te=0:150; >> contrast = exp(-te/t2b) - exp(-te/t2a); >> plot(te,contrast,'linewidth',3); >> find(contrast==max(contrast)) ans = 43
SPIN ECHO RF 90 180 Gz te Gy Gx 10 msec
FLASH RF te Gz Gy Gx 10 msec
FLASH RF te Gz Gy Gx 10 msec
FLASH MAGNETIZATION CYCLE 1. 2. α Longitudinal Recovery 3. α RF pulse followed by data collection Spoiling of transverse magnetization
FISP (GRASS) RF te Gz Gy Gx 10 msec
SSFP MAGNETIZATION CYCLE 1. 2. α Longitudinal Recovery and T2* relaxation α degree RF pulse and data collection 4. 3. α α degree RF pulse and data collection Longitudinal Recovery and T2* relaxation 54
SSFP RF te Gz Gy Gx 55
RF RF RF te te te Gz Gz Gz Gy Gy Gy Gx Gx Gx FLASH GRASS SSFP
3D MP-RAGE RF Gz 180 Repeat Ny times ti tr tr tr Gy Gx
PHASE MAPS RF te slice select readout Time shift in data collection amounts to a phase offset Spins precessing at different rates (different magnetic fields) will acquire different phase shifts
TRADEOFFS Volume Coverage tr slice thickness te
TRADEOFFS SNR tr te flip angle voxel volume contrast imaging time (e.g., averaging)
TRADEOFFS Imaging Time tr resolution total slices (due to vendor optimization)
TRADEOFFS SAR flip angle echo train length number of slices / tr total scan duration