Psy 8960, Fall ‘06 Gradients and Fields1 Gradients and fields Static magnetic field Gradients –Effect of simultaneous gradients Excitation and slice selection

Psy 8960, Fall ‘06 Gradients and Fields2 Imaging B

Psy 8960, Fall ‘06 Gradients and Fields3 One field, one resonant frequency B RF inRF out

Psy 8960, Fall ‘06 Gradients and Fields4 Field gradient: frequency = localization RF inRF out G z = 3 G/cm B z = ( z) T

Psy 8960, Fall ‘06 Gradients and Fields5 Gradients Typical strength – 10 to 40 mT/m (1 to 4 G/cm) Always produce fields aligned with B 0 Typical rise time: 300 microseconds Risks to subjects: –Peripheral nerve stimulation –Cardiac stimulation

Psy 8960, Fall ‘06 Gradients and Fields6 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

Psy 8960, Fall ‘06 Gradients and Fields7 Pulse bandwidth and slice thickness Frequency (Hz) Position (cm) Gradient, e.g. 10 G/cm = 4,260 Hz/cm Broadband pulse excites fat slice

Psy 8960, Fall ‘06 Gradients and Fields8 Parts of an RF pulse Carrier (center) frequency BW Frequency (Hz)

Psy 8960, Fall ‘06 Gradients and Fields9 Slice selection - edges aren’t exactly hard G = 5.1kHz/cm f x Frequency profile of pulse = spatial profile of slice BWthk = BW/G SS

Psy 8960, Fall ‘06 Gradients and Fields10 Pulse bandwidth and slice thickness Frequency (Hz) Position (cm) Gradient, e.g. 10 G/cm = 4,260 Hz/cm Narrow pulse excites fat slice

Psy 8960, Fall ‘06 Gradients and Fields11 Pulse bandwidth and slice thickness Frequency (Hz) Position (cm) Gradient, e.g. 10 G/cm = 4,260 Hz/cm Strong gradient decreases slice thickness

Psy 8960, Fall ‘06 Gradients and Fields12 Pulse bandwidth and slice thickness Frequency (Hz) Position (cm) Gradient, e.g. 10 G/cm = 4,260 Hz/cm Center frequency determines slice position

Psy 8960, Fall ‘06 Gradients and Fields13 Slice selection: gradient during pulse B z = 3T RF inRF out G z = 3 G/cm

Psy 8960, Fall ‘06 Gradients and Fields14 Pulse sequence diagrams N rep RF G SS G PE G RO DAC Magnetization preparation ExcitationRead-out