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Statistical Parametric Mapping Lecture 6 - Chapter 4 Ultra-Fast fMRI Textbook: Functional MRI an introduction to methods, Peter Jezzard, Paul Matthews, and Stephen Smith Many thanks to those that share their MRI slides online

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Fig. 4.1 BOLD response as a function of TE for different values of T2* r. Note that TE opt ~ T2* and that BOLD response increases with increasing T2* r TE, ms Signal, arb T2* r =80ms 70ms 60ms 50ms 40ms 30ms 20ms 10ms TE opt = optimal TE for BOLD contrast lies between T2* a and T2* r R2* a = R2* relaxation rate during activation (1/T2* a ) R2* r = R2* relaxation rate during rest (1/T2* r ) Echo Time Optimization

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Effects of Field Homogeneity R2* = R2 + R2 mi +R2 ma R2 = transverse relaxation rate due to spin-spin interactions and diffusion through microscopic gradients R2 mi = transverse relaxation rate due to microscopic changes, i.e. deoxyhemoglobin R2 ma = transverse relaxation rate due to macroscopic field inhomogeneity R2* a is relaxation rate during activation R2* r is relaxation rate at rest

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Fig. 4.2 Change in histogram of T2* for thick slab through brain with changing slice thickness. Note broadening of distribution with increasing thickness with shift toward shorter T2* T2*, ms number of voxels 1.9mm 3.8mm 5.9mm Effects of Field Homogeneity

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Fig. 4.3 EPI obtained with TE= 60 and TR=3000 msec and 63 and 95 ky lines. Note recovery of signal loss in d vs c and ghosting in c. Spin Echo 4x4x4 mm 3 Gradient Echo EPI 2x2x2 mm 3

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Fig. 4.4 Phase fluctuations at center of k-space over 42 seconds. Spikes are due to cardiac cycles and slower periodic signal due to respiratory cycles navigator index navigator phase, degrees 0.2 Intra-scan Motion Signal Why would phase advance and retard?

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Fig. 4.5 Gradient echo (GE) echo forms at center of readout window where area under rephasing gradient = area of dephasing gradient. Unlike spin echo dephasing is due to spatial difference in Larmor frequencies during application of gradients. First half of readout window is rephasing and second half is dephasing again. This process repeats at the center of readout window for each ky line in k-space for EPI. gradient echo readout window r.f. read gradient TE dephase rephasedephase For EPI where is the readout signal largest?

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White matter Grey matter Signal (fraction of M 0 ) Flip angle (degrees) Fig. 4.6 Graphical determination of optimal TE for GM and WM signals for multishot GE pulse sequence such as FLASH. Useful for 3D high-resolution images.

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Fig. 4.7 GE EPI pulse sequence and k-space organization of samples. RF Slice Read Phase a) Read Phase b) 1 2 n n n What flip angle is used for EPI?

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Fig. 4.8 Half Fourier (k-space) images. Central 20 percent of ky portion of k-space used for estimating phase correction during conjugation (replacing missing + ky data with acquired -ky data). Note ghosting in B in phase encode direction. A.SE B.GE EPI C.GE (FLASH) D.SE EPI TE=60 msec TE=120 msec

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Effect of system parameters on EPI images for fixed field of view. ParameterEcho Spacing ResolutionSNRGeometric distortion Increase gradient slew rateReduced--- Reduced Increase sampling bandwidth (kx) Reduced---Reduced Increase number of shots (interleaving ky) Reduced---IncreasedReduced Use of ramp sampling (similar to slew rate effect) Reduced--- Reduced Increase read matrix (kx)Increased ReducedIncreased Increase phase matrix (ky)---Increased * Reduced--- Increase field strength--- Increased Table 4.1 from text. * actual resolution increase less than expected due to smoothing effect of signal decay.

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fMRI methods for reduced k-space coverage Keyhole –acquire full k-space as reference –acquire reduced low-frequency k-space fMRI study –fill in missing k-space from reference Half-Fourier –acquire 50-60% of k-space starting at highest ky –theoretical symmetry used to fill in missing ky

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fMRI methods for reduced k-space coverage Sensitivity encoding (SENSE) –Multiple RF coils with independent signal for each (parallel imaging) –Calibration maps from full k-space –each coil part of k-space –2X improvement EPI, 4X for GE UNFOLD –Acquire k-space in sequential time segments time 1 acquire lines 1, 5, 9, time 2 acquire lines 2, 6, 10, time 3 acquire lines 3, 7, 11, time 4 acquire lines 4, 8, 12, reorder into k-space 4x faster per segment reduces inter echo distortions

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