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