EPI, Part 1 1Psy 8960, Fall ‘06

EPI, Part 1

• Pulse sequences• Nyquist ghost• Chemical shift

– FLASH– EPI

EPI, Part 1 2Psy 8960, Fall ‘06

Overview: last week• Steady-state magnetization

– when TR is shorter than or comparable to T1, longitudinal magnetization cannot recover between repetitions

– Images have T1-weighting in addition to intended weighting

• Ernst angle: angle that maximizes transverse magnetization at steady-state.

M|| = M0 M|| = Mss

1)cos( TTR

Ernst e

EPI, Part 1 3Psy 8960, Fall ‘06

This week

• FLASH and EPI pulse sequences (review k-space trajectory)• Nyqiust ghost in EPI (and other issues …)

• Relationship between total read-out time (TRO) and voxel displacement

EPI, Part 1 4Psy 8960, Fall ‘06

K-space trajectories: FLASH

FLASH (TE ~ 5 ms)

excitationRead and phase pre-encode (refocusing)

Read-out

NRO

RF

GSS

GPE

GRO

DAC

PE table increments each repetition

Flip angle ~ 7 deg.

TE ~ 5ms

NPE

EPI, Part 1 5Psy 8960, Fall ‘06

EPINrep = 32

Flip angle ~ 60 deg.

TE ~ 30ms

64 pts 64 pts

EPI (TE ~ 30 ms)

Read-out

Read and phase pre-encode (refocusing)

excitation

Phase blip

EPI, Part 1 6Psy 8960, Fall ‘06

Nyquist (half FOV) ghost

• Fourier relationships– Delta k determines FOV– Every-other-line modulation in k-space = ½ FOV modulation in image

EPI, Part 1 7Psy 8960, Fall ‘06

Chemical shift

EPI, Part 1 8Psy 8960, Fall ‘06

Chemical shift: fat

http://www.lipidlibrary.co.uk/nmr/1NMRsat/index.htm

water Subcutaneous fat

3.5 ppm = 3.5 x 128Hz @ 3T

EPI, Part 1 9Psy 8960, Fall ‘06

Chemical shift during gradient encoding

Saturated fat,127,680,447Hz

Water,127,680,000Hz

Relative phase, kx = ky = 1.4 cm-1Resonant frequency, no gradients

EPI, Part 1 10Psy 8960, Fall ‘06

Voxel displacementFLASH: BW = 1/(dwell time) = 1 / (10s) = 100,000 Hz

RO

127.73MHz

127.63MHz127.68MHz

Combination of gradient, sampling rate determine FOV, resolution

PE

EPI, Part 1 11Psy 8960, Fall ‘06

Voxel displacementFLASH: BW = 1/(dwell time) = 1 / (10s) = 100,000 Hz

FLASH: frequency resolution = 100,000 Hz / 64 pixels = 1563 Hz/pixel

identical calculation: TRO = 64*10s = 640s; Hz/pixel = 1/TRO

RO

127.73MHz

127.63MHz127.68MHz

Combination of gradient, sampling rate determine FOV, resolution

FLASH: frequency resolution in phase encode direction is effectively infinite since TPE = 0;

PE

EPI, Part 1 12Psy 8960, Fall ‘06

FLASH & EPI: Tev

FLASHTev,RO = NRO x dwellTime

Tev,PE = 0

excitationRead and phase pre-encode (refocusing)

Read-out

Tev: time between measurement of first and last data point

Read-out

Read and phase pre-encode (refocusing)

excitation

Phase blip

EPITev,RO = NRO x dwellTime

Tev,PE = NPE x NRO x dwellTime