Presentation on theme: "Statistical Parametric Mapping"— Presentation transcript:
1 Statistical Parametric Mapping Lecture 6 - Chapter 4Ultra-Fast fMRITextbook: Functional MRI an introduction to methods, Peter Jezzard, Paul Matthews, and Stephen SmithMany thanks to those that share their MRI slides online
2 Echo Time Optimization 0.0250.0200.0150.0100.0050.00050100150TE, msSignal, arbT2*r=80ms70ms60ms50ms40ms30ms20ms10msTEopt = optimal TE for BOLD contrast lies between T2*a and T2*rR2*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.
3 Effects of Field Homogeneity R2* = R2 + R2mi +R2maR2 = transverse relaxation rate due to spin-spin interactions and diffusion through microscopic gradientsR2mi = transverse relaxation rate due to microscopic changes, i.e. deoxyhemoglobinR2ma = transverse relaxation rate due to macroscopic field inhomogeneityR2*a is relaxation rate during activationR2*r is relaxation rate at rest
4 Effects of Field Homogeneity 50100150T2*, ms4000300020001000number of voxels1.9mm3.8mm5.9mmFig. 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*.
5 4x4x4 mm32x2x2 mm3Spin EchoGradient Echo EPIFig. 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.
6 Intra-scan Motion Signal 50010001500navigator index0.10.0-0.1-0.2-0.3navigator phase, degrees0.2Fig 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?
7 For EPI where is the readout signal largest? gradientechoreadout windowr.f.read gradientTEdephaserephaseFig. 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?
8 0.000.020.040.060.080.10153045607590White matterGrey matterSignal (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.
9 RFSliceReadPhasea)ReadPhaseb)nn-11n-12n21Fig GE EPI pulse sequence and k-space organization of samples.What flip angle is used for EPI?
10 SEGE EPIGE (FLASH)SE EPITE=60 msecTE=120 msecFig. 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.
11 Effect of system parameters on EPI images for fixed field of view. Echo SpacingResolutionSNRGeometric distortionIncrease gradient slew rateReduced---Increase sampling bandwidth (kx)Increase number of shots (interleaving ky)IncreasedUse of ramp sampling (similar to slew rate effect)Increase read matrix (kx)Increase phase matrix (ky)Increased*Increase field strengthTable 4.1 from text.* actual resolution increase less than expected due to smoothing effect of signal decay.
12 fMRI methods for reduced k-space coverage Keyholeacquire full k-space as referenceacquire reduced low-frequency k-space fMRI studyfill in missing k-space from referenceHalf-Fourieracquire 50-60% of k-space starting at highest kytheoretical symmetry used to fill in missing ky
13 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-spaceeach coil part of k-space2X improvement EPI, 4X for GEUNFOLDAcquire k-space in sequential time segmentstime 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-space4x faster per segment reduces inter echo distortions