gradients and fields
DESCRIPTION
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 = 10 =-10Hz. S = 10 =-5Hz. S = 10 =0Hz. S = 10 =5Hz. S = 10 =10Hz. S = 0. S = 0. S = 0. S = 0. S = 0. S = 10 =-15Hz. S = 10 =-10Hz. S = 10 =-5Hz. S = 10 - PowerPoint PPT PresentationTRANSCRIPT
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