Transcript fMRI: Biological Basis and Experiment Design

fMRI: Biological Basis and Experiment Design
Lecture 8: Pulse sequences, Take 2
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Gradient echo review
K-space review
Slice selection
K-space navigation
– Bandwidth
– Echo time
– Read-out time
The read-out signal is the 1D FFT of the sample
GRO
-
0

Magnitude of signal in RF coil
Real part of signal in RF coil
Imaginary component of signal in RF coil
Pulse sequence diagram: slow 2D FLASH (64 x 64)
Nrep = 64
Flip angle ~ 56 deg.
RF
GSS
GPE
PE table increments
each repetition
GRO
DAC
64 points
TR ~ 640us
Slice selection
f
G = 5.1kHz/cm
RF Pulse (bandwidth = 1kHz)
x
Excited slice
thickness = BWRF/G = 0.2cm = 2mm
Slice selection - edges aren’t exactly hard
f
G = 5.1kHz/cm
x
BW
thk = BW/GSS
Frequency profile of pulse = spatial profile of slice
Slice thickness is determined by bandwidth of pulse
Slice position is determined by center frequency of pulse
f
x
1st TR
Overlap of slices leads to non-uniform
signal intensity for first few TRs
f
x
30th TR
Pulse sequence diagram: normal 2D FLASH (2562)
Nrep = 256
Flip angle ~ 7 deg.
RF
GSS
GPE
PE table increments
each repetition
TE ~ 5ms
GRO
DAC
256 points
TR ~ 10ms
Pulse sequence diagram: normal 2D FLASH (2562)
Nrep = 256
Flip angle ~ 7 deg.
RF
Read gradient amplitude, GRO: 35mT/m
Dwell time (per data point): 3.5s
Sampling rate (bandwidth): 286kHz
GSS
GPE
FOV = 286kHz/(1.49MHz/m) = 19.2cm
PE table increments
each repetition
TE ~ 5ms
NRO = 256
TRO (line) = 256*3.5us = 0.9ms
Resolution = FOVRO/NRO = 0.75mm
GRO
DAC
TR ~ 10ms
Frequency resolution (Hz/pixel)
RO: 1.1kHz/px
PE: effectively infinite
256 points
TE = 5ms
FLASH sequences read one line per excitation
Relative phase of spins
-
0

EPI sequences zig-zag back and forth across k-space
-
0

Pulse sequence diagram: EPI (64 x 64 image)
Nrep = 32
RF
GSS
GPE
GRO
DAC
64 pts
64 pts
Pulse sequence diagram: EPI (64 x 64 image)
Read gradient amplitude, GRO: 35mT/m
Dwell time (per data point): 3.5s
Sampling rate (bandwidth): 286kHz
RF
FOV = 286kHz/(1.49MHz/m) = 19.2cm
GSS
NRO = 64
TRO (line) = 64*3.5us = 0.224ms
TRO (image) = 64*64*3.5us = 14.3ms
…
GPE
Resolution = FOVRO/NRO = 3.0mm
…
GRO
DAC
64 64 64 64
Frequency resolution (Hz/pixel)
RO: 4.5kHz/px (1/TRO,line)
PE: 70 Hz/px (1/TRO,image)
64 64
TEmin = TRO/2 + fudge = 9ms
Pulse sequence diagram: EPI (more realistic)
Read gradient amplitude, GRO: 15mT/m
Dwell time (per data point): 8.1s
Sampling rate (bandwidth): 122kHz
RF
FOV = 122kHz/(0.64MHz/m) = 19.2cm
GSS
NRO = 64
TRO (line) = 64*8.1us = 0.518ms
TRO (image) = 64*64*3.5us = 33ms
…
GPE
Resolution = FOVRO/NRO = 3.0mm
…
GRO
DAC
64 64 64 64
Frequency resolution (Hz/pixel)
RO: 1.9 kHz/px (1/TRO,line)
PE: 30 Hz/px (1/TRO,image)
64 64
TEmin = TRO/2 + fudge = 18ms
A 3D object has a 3D k-space
Pulse sequence: 3D FLASH (176 x 224 x 256)
RF
GSS
GPE
Slab is selected
with very weak
gradient
Partition Encoding
increments every
224 repetitions
PE table increments
each repetition;
cycles for each
partition
GRO
DAC
256 points
Nrep = 176 x 224
kz
ky
kx