Psy 8960, Fall ‘06 Gradients and Fields1 Gradients and fields Static magnetic field Gradients...

Gradients and Fields 1Psy 8960, Fall ‘06

S = 10=-10Hz

S = 10=-15Hz

S = 0S = 0

S = 10=-10Hz

S = 10=-10Hz

S = 10=-10Hz

S = 10=-5Hz

S = 10=0Hz

S = 10=5Hz

S = 10=10Hz

S = 10=10Hz

S = 10=15Hz

S = 10=15Hz

S = 10=20Hz

S = 10=-5Hz

S = 10=0Hz

S = 10=5Hz

S = 10=-15Hz

S = 10=-20HzS = 10

=-20HzS = 0

S = 10=10Hz

S = 10=-5Hz

S = 10=0Hz

S = 10=5Hz

S = 10=15Hz

S = 10=20Hz

S = 10=10Hz

S = 10=-5Hz

S = 10=0Hz

S = 10=5Hz

S = 10=-15Hz

S = 10=-20Hz

S = 10=-10Hz

S = 10=15Hz

S = 10=20Hz

S = 10=10Hz

S = 10=-5Hz

S = 10=0Hz

S = 10=5Hz

S = 10=-15Hz

S = 10=-20Hz

S = 10=-10Hz

S = 10=15Hz

S = 10=20Hz

S = 10=10Hz

S = 10=-5Hz

S = 10=0Hz

S = 10=5Hz

S = 10=-15Hz

S = 10=-20Hz

S = 10=-10Hz

S = 10=15Hz

S = 10=10Hz

S = 10=-5Hz

S = 10=0Hz

S = 10=5Hz

S = 10=-15Hz

S = 10=-10Hz

S = 10=10Hz

S = 10=-5Hz

S = 10=0Hz

S = 10=5Hz

S = 0

S = 0

S = 0

S = 0

S = 0S = 0 S = 0

S = 0S = 0 S = 0 S = 0S = 0 S = 0 S = 0S = 0 S = 0 S = 0 S = 0

S = 0 S = 0 S = 0

S = 0 S = 0

S = 0

S = 0

S = 0

S = 0

S = 0 S = 0

S = 0 S = 0 S = 0

S = 0

S = 0 S = 0 S = 0

S = 0S = 0 S = 0 S = 0S = 0 S = 0 S = 0S = 0 S = 0 S = 0 S = 0

Gradients and fields

• Static magnetic field• Gradients

– Effect of simultaneous gradients

• Excitation and slice selection

Gradients and Fields 2Psy 8960, Fall ‘06

Imaging

B

Gradients and Fields 3Psy 8960, Fall ‘06

One field, one resonant frequency

B

RF in RF out

Gradients and Fields 4Psy 8960, Fall ‘06

Field gradient: frequency = localization

RF in RF out

Gz = 3 G/cm

Bz = (3 + 0.0003z) T

Gradients and Fields 5Psy 8960, Fall ‘06

Gradients

• Typical strength – 10 to 40 mT/m (1 to 4 G/cm)

• Always produce fields aligned with B0

• Typical rise time: 300 microseconds• Risks to subjects:

– Peripheral nerve stimulation – Cardiac stimulation

http://www.fmrib.ox.ac.uk/~peterj/lectures/hbm_1/sld010.htm

Gradients and Fields 6Psy 8960, Fall ‘06

Slice select gradients• Z gradient during imaging pulse• X gradient during excitation pulse• Y gradient during excitation pulse

• Axial slice• Sagittal slice• Coronal slice

• Y & Z gradients during imaging pulse

• Oblique slice

z

y x

Gradients and Fields 7Psy 8960, Fall ‘06

Pulse bandwidth and slice thickness

Frequency (Hz)

Position (cm)

Gradient, e.g.10 G/cm = 4,260 Hz/cm

Broadband pulse excites fat slice

Gradients and Fields 8Psy 8960, Fall ‘06

Parts of an RF pulse

Carrier (center) frequency

BW

Frequency (Hz)

Gradients and Fields 9Psy 8960, Fall ‘06

Slice selection - edges aren’t exactly hardG = 5.1kHz/cmf

x

Frequency profile of pulse = spatial profile of slice

BW thk = BW/GSS

Gradients and Fields 10Psy 8960, Fall ‘06

Pulse bandwidth and slice thickness

Frequency (Hz)

Position (cm)

Gradient, e.g.10 G/cm = 4,260 Hz/cm

Narrow pulse excites fat slice

Gradients and Fields 11Psy 8960, Fall ‘06

Pulse bandwidth and slice thickness

Frequency (Hz)

Position (cm)

Gradient, e.g.10 G/cm = 4,260 Hz/cm

Strong gradient decreases slice thickness

Gradients and Fields 12Psy 8960, Fall ‘06

Pulse bandwidth and slice thickness

Frequency (Hz)

Position (cm)

Gradient, e.g.10 G/cm = 4,260 Hz/cm

Center frequency determines slice position

Gradients and Fields 13Psy 8960, Fall ‘06

Slice selection: gradient during pulse

Bz = 3T

RF in RF out

Gz = 3 G/cm

Gradients and Fields 14Psy 8960, Fall ‘06

Pulse sequence diagrams

Nrep

RF

GSS

GPE

GRO

DAC

Magnetization preparation

Excitation Read-out