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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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**Echo Time Optimization**

0.025 0.020 0.015 0.010 0.005 0.000 50 100 150 TE, ms Signal, arb T2*r=80ms 70ms 60ms 50ms 40ms 30ms 20ms 10ms TEopt = 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) Fig BOLD response as a function of TE for different values of T2*r. Note that TEopt ~ T2* and that BOLD response increases with increasing T2*r.

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**Effects of Field Homogeneity**

R2* = R2 + R2mi +R2ma R2 = transverse relaxation rate due to spin-spin interactions and diffusion through microscopic gradients R2mi = transverse relaxation rate due to microscopic changes, i.e. deoxyhemoglobin R2ma = 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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**Effects of Field Homogeneity**

50 100 150 T2*, ms 4000 3000 2000 1000 number of voxels 1.9mm 3.8mm 5.9mm 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*.

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4x4x4 mm3 2x2x2 mm3 Spin Echo Gradient Echo EPI 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.

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**Intra-scan Motion Signal**

500 1000 1500 navigator index 0.1 0.0 -0.1 -0.2 -0.3 navigator phase, degrees 0.2 Fig Phase fluctuations at center of k-space over 42 seconds. Spikes are due to cardiac cycles and slower periodic signal due to respiratory cycles. Why would phase advance and retard?

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**For EPI where is the readout signal largest?**

gradient echo readout window r.f. read gradient TE dephase rephase 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. For EPI where is the readout signal largest?

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0.00 0.02 0.04 0.06 0.08 0.10 15 30 45 60 75 90 White matter Grey matter Signal (fraction of M0) Flip angle (degrees) Fig 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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RF Slice Read Phase a) Read Phase b) n n-1 1 n-1 2 n 2 1 Fig GE EPI pulse sequence and k-space organization of samples. What flip angle is used for EPI?

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SE GE EPI GE (FLASH) SE EPI TE=60 msec TE=120 msec 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.

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**Effect of system parameters on EPI images for fixed field of view.**

Echo Spacing Resolution SNR Geometric distortion Increase gradient slew rate Reduced --- Increase sampling bandwidth (kx) Increase number of shots (interleaving ky) Increased Use of ramp sampling (similar to slew rate effect) Increase read matrix (kx) Increase phase matrix (ky) Increased* Increase field strength 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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