f MRI Biological Basis and Experiment Design Lecture
f. MRI: Biological Basis and Experiment Design Lecture 9: Pulse sequences, Take 3 • Slice selection homework review • Pulse sequence/kspace trajectory matching • Spin echo http: //www-mrsrl. stanford. edu/~brian/mri-movies/
Protons on water molcules: Ernst's "spies" = B = 267 x 106 (rad/s)/T = 42. 6 MHz/T
Slice selection - edges aren’t exactly hard f G = 12 m. T/m = 1. 2 G/cm = *x*G x BW thk = BW/GSS Frequency profile of pulse = spatial profile of slice
Pulse sequence diagram: Nrep = 256 Flip angle = 7 deg. RF TR = 10 ms Sequence type: FLASH ______ Contrast: ______ T 1 GSS Image resolution: ______ 256 x 256 Total acquisition time: ______ 256 x 10 ms = 2. 5 s GPE TE ~ 5 ms GRO DAC 256 points
K-space trajectory and scaled T 2* decay in gray matter: 256 … M 256 lines time (ms) 4 3 2 1 256 (complex-valued) points Tacq = 2. 5 ms TE = 5 ms
Image:
Pulse sequence diagram: TR = 1. 5 s Flip angle = 75 degrees RF EPI Sequence type: ______ T 1 and T 2* Contrast: ______ GSS Image resolution: ______ 64 x 64 64 x 0. 57 ms = 36. 48 ms Image acq. time: ______ … GPE TE = 30 ms (32 nd line) … GRO (64 times) Echo spacing = 0. 57 ms DAC 64 64 64
K-space trajectory and scaled T 2* decay in gray matter: M 64 lines … 64 (complex-valued) points time (ms) Tacq = 35 ms TE = 30 ms
Image: (resolution) (contrast)
Pulse sequence diagram: TR = 5 s Flip angle = 75 degrees RF Sequence type: ______ GE EPI Contrast: ______ T 2* GSS Image resolution: ______ 64 x 64 Image acq. time: ______ 64 x 0. 57 ms = 36. 48 ms … GPE TE = 30 ms (32 nd line) … GRO (64 times) Echo spacing = 0. 57 ms DAC 64 64 64
K-space trajectory and scaled T 2* decay in gray matter: M 64 lines … 64 (complex-valued) points time (ms) Tacq = 35 ms TE = 30 ms
Image: (resolution) (contrast)
Pulse sequence diagram: TR = 5 s Flip angle = 90 degrees RF Sequencetype: EPI ______ Sequence Contrast: T ______ Contrast: 2* GSS Imageresolution: 128 ______ Image x 128 Imageacq. time: 128 ______ Image x 0. 75 ms = 96 ms … GPE TE = 55 ms (65 th line) … GRO (128 times) Echo spacing = 0. 75 ms DAC 128 128 128
K-space trajectory and scaled T 2* decay in gray matter: M 128 lines … 128 (complex-valued) points time (ms) Tacq = 96 ms TE = 55 ms
Image:
Pulse sequence diagram: TR = 5 s Flip angle = 90 degrees RF Sequence type: ______ EPI Contrast: ______ T 2* GSS Image resolution: ______ 128 x 128 Image acq. time: ______ 96 x 0. 75 ms = 72 ms … GPE TE = 30 ms (32 nd line) … GRO (96 times) Echo spacing = 0. 75 ms DAC 128 128 128
K-space trajectory and scaled T 2* decay in gray matter: M 96 lines … time (ms) Tacq = 72 ms TE = 30 ms 128 (complex-valued) points
Image:
Spin Echo 180 deg. 90 deg. M T 2* time (ms)
Pulse sequence diagram: 90 deg. 180 deg. TR = 5 s RF Sequencetype: SE ______ Sequence EPI Contrast: T ______ Contrast: 2 GSS Imageresolution: 128 ______ Image x 128 Imageacq. time: 96 ______ Image x 0. 75 ms = 72 ms … GPE TE = 60 ms (32 nd line) … GRO (96 times) Echo spacing = 0. 75 ms DAC 128 128
K-space trajectory and scaled T 2 & T 2* decay in gray matter: M T 2 96 lines T 2* … 90 time (ms) 180 Tacq = 72 ms TE = 60 ms 128 (complex-valued) points
Image:
Pulse sequence diagram: Nrep = 4 Flip angle = 40 degrees TR = 0. 25 s RF Sequence type: segmented EPI ______ Contrast: ______ T 1 and T 2* GSS Image resolution: ______ 128 x 128 Segment acq. time: 32 x 0. 75 ms = 24 ms … GPE Image acq. time: ______ 1 sec TE = 20 ms (16 th line) … GRO (32 times) Echo spacing = 0. 75 ms DAC 128 128 128
K-space trajectory and scaled T 2* decay in gray matter: … … M 128 lines time (ms) Tacq, seg = 24 ms TE = 20 ms 128 (complex-valued) points
Image: But not really. . . With the short read-out, the distortion would be much less noticeable. This acquisition had a short TE (using partial Fourier acquisition) but was not a segmented acquisition, so has more distortion than advertised.
Pulse sequence diagram: 90 deg. 180 RF Nrep = 4 180 TR = 2. 5 s TI = 0. 9 s Sequence type: IR ______ SE EPI Contrast: ___________ T 1 and (small amount of) T 2 GSS Image resolution: ______ 128 x 128 … GPE Image acq. time: ______ 4 x 2. 5 s = 10 s TE = 25 ms (16 th line) GRO DAC … … Echo spacing = 0. 75 ms …
K-space trajectory and scaled T 2* decay in gray matter: … … M 128 lines time (ms) Tacq, seg = 24 ms TE = 40 ms 128 (complex-valued) points
Image:
Pulse sequence diagram: Nrep = 176 x 224 TR = 10 ms RF Sequence type: 3 D ______ FLASH Contrast: ___________ T 1 GSS Image resolution: ______ 256 x 224 x 176 Image acq. time: ______ 176 x 224 x. 01 s = 6 min 34 s GPE GRO DAC 256 points
K-space trajectory and scaled T 2* decay in gray matter: M 224 lines time (ms) Tacq = 2. 5 ms TE = 5 ms 176 partitions 256 (complex-valued) points
Image:
- Slides: 31
