wagshul - Pulse Sequences Pulse Sequences Mark Wagshul, PhD...

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Unformatted text preview: Pulse Sequences Pulse Sequences Mark Wagshul, PhD Director, MR Research Center Department of Radiology Stony Brook University MRI Pulse Sequences MRI Pulse Sequences Spin echo and gradient echo sequences basic methods of MRI contrast 3D techniques Preparation techniques secondary contrast methods Fast imaging Gating and k­space segmentation Important basic concepts Images are not acquired in real space, but in inverse­space k­space Image contrast is obtained by manipulating magnetic or biological properties of spins (e.g. relaxation, precession frequency, magnetic susceptibility, diffusion) Image contrast is (almost) always obtained at the expense of signal size “Static” fields (e.g. B0,G) are along z, RF fields are to z Basic Spin Echo Sequence TE/2 RF pulses slice select phase encode Position of BOTH spin­ echo and gradient echo readout TE ω ( x) = γ G xt The 3 step encoding process Basic Gradient Echo Sequence RF Pulse RF Pulse Slice Select Phase Encode Readout ­iγ G xτ iγ G xt e x e x Major differences between Major differences between spin­echo and gradient echo SE – 90 degree pulse, uses all Mz per pulse GE – variable pulse angle, partial use of Mz, allows for faster TR’s SE – refocuses T2*, allows longer TE’s GE – T2* weighting, generally requires short TE’s SE – usually used for T2 weighted images GE – usually used for T1 weighted images, or for speed SE ­ T1 weighting adjusted with TR GE ­ T1 weighting adjusted with TR AND flip angle 3D imaging Reduced gradient Reduced gradient for “slab” selection secondary phase encode: secondary phase encode: Φ (z) = γ G zt la b S me olu V 2D vs. 3D imaging 2D vs. 3D imaging Slice selection replaced with large slab selection Slice selection secondary phase encoding Repetition times: 2D – slices excited serially, TR = Nsl * TR’, imaging time = NPE * TR 3D – all slices excited simultaneously, TR = TR’, imaging time = NPE * Nsl * TR SNR: 2D – signal contributions from the slice, but noise from the entire volume 3D – signal and noise contributions from the entire volume Slice fidelity: 2D – imperfect slice profiles, slice crosstalk 3D – perfect slice profiles, no slice crosstalk 3D sequences essentially trade­off speed for SNR secondary phase encode serves as a signal averaging process Magnetization Preparation Magnetization Preparation Preparation schemes applied prior to initial RF excitation, generally every TR Fat saturation Presaturation Spin Tagging Inversion recovery Magnetization transfer Prep. phase Prep. phase general prep. scheme Fat saturation – selective saturation of fat signal based on chemical shift Presaturation, tagging – selective saturation of all signal based on position IR – selective nulling of signal based on T1 Mag. transfer – saturation of water signal based on exchange with nearby macromolecules Water­Fat Chemical Shift Water­Fat Chemical Shift Signal intensity Water Fat -9 -6 -3 0 3 6 9 Chemical Shift (ppm) Also possible to utilize in­phase/out­of­phase techniques to separate water/fat Presaturation Presaturation Magnetization in this slab is completely destroyed prior to imaging RF pulse Applications: motion suppression spin labeling •Applications: motion suppression spin labeling Inversion recovery Inversion recovery 1.5 1 0.5 0 0 -0.5 -1 -1.5 2 4 6 8 10 12 WM GM CSF IR example – IR example – FLAIR (FLuid Attenuated IR) TI – 2.2 s TE –140 ms TR – 10 s Mag. transfer Preparation pulse water macromolecules -3 -3 -2 -1 0 Frequency (kHz) 1 2 3 Intra­sequence contrast Intra­sequence contrast Components added within sequence to obtain additional contrast Diffusion Flow Motion suppression Phase/Diffusion contrast Phase/Diffusion contrast Spin Phase = γ Gτ ∗ z RF Pulse Slice Select Phase Encode Read Encode Spin Phase = γ Gτ ∗( z+∆ z) θ θ Fast Imaging Fast Imaging SE methods Fast spin echo (FSE) Echo planar imaging (EPI) Steady state imaging (PSIF, CE­FAST) Spoiled GE (SPGR) Echo planar imaging (EPI), Spiral Steady state imaging (True­FISP, FIESTA, Balanced FFE) GE methods Parallel imaging (SENSE, SMASH) Fast Spin Echo Fast Spin Echo Collect multiple k­space lines per 90 degree pulse New parameters – interecho spacing and echo train length (ETL), will determine overall slice TR and attainable TE’s Typical ETL – 8, but can collect the entire image in a single shot (e.g. 128 PE’s) TE determined by echo position of center of k­space For large ETL increasing T2 signal loss in later echoes lead to image blurring (loss of edge of k­ space) 4 ETL Fast Spin Echo Inter echo spacing Phase unwrap k1 k2 k3 k4 FSE k­space coverage } k4 } k3 } k2 k1 } } k2 } k3 } k4 Optimum sampling will place more central portions of k­space close to the k=0 echo Echo Planar Imaging (EPI) Echo Planar Imaging (EPI) Multi­echo gradient echo readout Multi­shot – uses standard gradients and multiple acquisition windows Single shot – uses sinusoidal readout gradient and “blipped” phase encode single acquisition window requires “rebinning” to reconstruct Overall T2* weighting across the echo train blurring and distortion Phase encode Readout EPI k­space coverage EPI Alternate strategy – spiral, Alternate strategy – spiral, much more efficient k­space coverage EPI Applications EPI Applications Functional MRI – BOLD imaging Perfusion T2* weighting, detects changes in blood oxygenation T2* weighting, detects rapid passage of paramagnetic contrast agent Diffusion­weighted, detects differences in water diffusion, e.g. due to stroke, EPI used for rapid coverage of whole brain with minimum motion Diffusion SPGR (Spoiled GE) SPGR (Spoiled GE) Short­TR, low flip angle GE Inter­echo coherent affects are spoiled using RF phase cycling Inter­echo gradient pulses Ideally, gradient spoiling uses varying (random) gradient pulses, in practice often just use fixed large gradient pulse with phase unwrapping Gradient spoiling condition – φ = γ G*∆ x*ton > 2π True FISP (Free Induction Steady state Precession) GE w/o spoiling, magnetization remains in transverse plane More efficient because magnetization is “reused” Contrast ~ T1/T2, excellent for blood/myocardium contrast BUT, very sensitive to field inhomogeneity Rephasing period = TR, so require ∆ B < 2 π/γ TR, usually need TR < 5 ms. Main application to date – near real­time cardiac FLASH FLASH True FISP Parallel Imaging Parallel Imaging Uses spatial information available from an array of RF coils to fill in portions of k­space Maximum reduction factor = number of coils SENSE – obtain smaller FOV images/coil and utilize sensitivity maps of coils to correct aliasing SMASH – use spatial harmonic parameters of coilsto fill in parts of k­space Gating and k­space segmentation Gating and Image acquisition is gated to some physiological signal, e.g. ECG or respiration BUT, extremely inefficient if only one k­space line covered per period collect multiple segments of k­space per period k k k k k k ...
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