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