Gradients and fields

1
Gradients and fields

• Static magnetic field
• Gradients
• Effect of simultaneous gradients
• Excitation and slice selection

2
Imaging
B
3
One field, one resonant frequency
B
RF in
RF out
4
Field gradient frequency localization
Bz (3 0.0003z) T
Gz 3 G/cm
RF in
RF out
5
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/s
ld010.htm
6
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

y
x
z
7
Pulse bandwidth and slice thickness
Frequency (Hz)
Gradient, e.g. 10 G/cm 4,260 Hz/cm
Position (cm)
8
Parts of an RF pulse
Carrier (center) frequency
BW
Frequency (Hz)
9
Slice selection - edges arent exactly hard
BW
thk BW/GSS
Frequency profile of pulse spatial profile of
slice
10
Pulse bandwidth and slice thickness
Frequency (Hz)
Gradient, e.g. 10 G/cm 4,260 Hz/cm
Narrow pulse excites fat slice
Position (cm)
11
Pulse bandwidth and slice thickness
Frequency (Hz)
Gradient, e.g. 10 G/cm 4,260 Hz/cm
Strong gradient decreases slice thickness
Position (cm)
12
Pulse bandwidth and slice thickness
Frequency (Hz)
Gradient, e.g. 10 G/cm 4,260 Hz/cm
Center frequency determines slice position
Position (cm)
13
Slice selection gradient during pulse
Bz 3T
Gz 3 G/cm
RF in
RF out
14
Pulse sequence diagrams
Magnetization preparation
Excitation
Read-out
Nrep
RF
GSS
GPE
GRO
DAC