mri, fbp and phase encoding
DESCRIPTION
MRI, FBP and phase encoding. Spins. Precession. RF pulse. T1 and T2. Bloch Equations. Receiver. T1 and T2. Effect of tissue. Slice selection. Slice selection. FBP. Filtered Back Projection. Filtered B a ck Projection. Filtered Back Projection. Filtered backprojection. - PowerPoint PPT PresentationTRANSCRIPT
MRI, FBP and phase MRI, FBP and phase encodingencoding
SpinsSpins
PrecessionPrecession
RF pulseRF pulse
T1 and T2T1 and T2
Bloch EquationsBloch Equations
ReceiverReceiver
T1 and T2T1 and T2
Effect of tissueEffect of tissueT1 and T2 CONSTANTS
T1 Constants at 1.5 T Controlled by TR
T2 Constants at 1.5 T Controlled by TE
Fat85
Muscle86045
White matter78090
Gray matter920100
CSF30001400
Slice selectionSlice selection
Slice selectionSlice selection
FBPFBP
Filtered Back ProjectionFiltered Back Projection
FilteredFiltered B Baack Projectionck Projection
Filtered Back Projection
Filtered backprojection• Filter the measured projection data at different projection angles with a special function.• Backproject the filtered projection data to form the reconstructed image.
Filtering can be implemented in 2 ways, in the spatial domain, the filter operation is equivalent to to convolving the measured projection data using a special convolving function h(t)
p t p t h t, ( , ) ( , ) ( )
More efficient multiplication will be in the spatial frequency domain.• FFT the measured projection data into the frequency domain:p(,)=FT {p(t, )• Multiply the the fourier transform projections with the special function.•Inverse Fourier transform the product p’(,).
Phase EncodingPhase Encoding
Gradient
Slice Plane Slice Phase Frequency
XY Z X or Y Y or X
XZ Y X or Z Z or X
YZ X Y or Z Z or Y
Phase encodingPhase encoding
K spaceK space
K SpaceK Space
Partial K Partial K space space
reconstrucreconstructiontion
Partial K space Partial K space reconstructionreconstruction
Partial K space Partial K space reconstructionreconstruction