MRI Signal Calculations (hopefully) Made Easy Brian Hargreaves Jan 11, 2012 Purpose • Overview of Bloch/Matrix simulations – Exact simulation of many cases – Numerical simulations often reasonable • In-depth explanation of EPG method – Extended phase graph – Simulates “dephased states” – Common in many MRI sequences Matrix Simulations • Single Spin • RF rotation • Gradient- or off-resonance-induced rotation • Relaxation Jaynes 1955 “The Matrix Treatment of Nuclear Induction” Rotations RF (x) Rotation: Any rotation is just a matrix multiplication •RF can rotate about any transverse axis •Rotations due to precession are just about z Magnetization Propagation Relaxation: Can represent any propagation in the form (Jaynes – 1955) Magnetization Expressions • Matrix expression for magnetization propagation: • Steady state solution: If we let then
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Matrix Simulations Rotations - Stanford Universitybah/software/epg/EPG-Lecture.pdf · •Overview of Bloch/Matrix simulations –Exact simulation of many cases –Numerical simulations
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MRI Signal Calculations (hopefully)Made Easy
Brian HargreavesJan 11, 2012
Purpose• Overview of Bloch/Matrix simulations
– Exact simulation of many cases– Numerical simulations often reasonable
• In-depth explanation of EPG method– Extended phase graph– Simulates “dephased states”– Common in many MRI sequences
Matrix Simulations• Single Spin• RF rotation• Gradient- or off-resonance-induced rotation• Relaxation
Jaynes 1955 “The Matrix Treatment of Nuclear Induction”
RotationsRF (x) Rotation:
Any rotation is just a matrix multiplication
•RF can rotate about any transverse axis•Rotations due to precession are just about z
Magnetization PropagationRelaxation:
Can represent any propagation in the form
(Jaynes – 1955)
Magnetization Expressions
• Matrix expression for magnetization propagation:
• Steady state solution:
If we let
then
Example: T1 WeightingAssume Longitudinal Magnetization dies out
RF
Sig
nal
90º 90º
1
0
Mz
90ºMz
Mxy
Example: T1-Weighted Imaging
M1 M2 M1 M2 M1 M2
α α α
1
Comments• 3x3 Matrix reduced to 1D• Other examples reduce to 2D• Sometimes analytic solution is reasonable
Zur Y, Stokar S, Bendel P. An analysis of fast imaging sequences with steady-state transverse magnetization refocusing. Magn Reson Med 1988; 6:175–193.
Sekihara K. Steady-state magnetizations in rapid NMR imaging using small flip angles and short repetition intervals. IEEE Trans Med Imaging 1987; 6:157–164.
van der Meulen P, Groen JP, Tinus AMC, Bruntink G. Fast field echo imaging: An overview and contrast calculations. Magn Reson Imaging 1988; 6:355–368.
Buxton RB, Fisel CR, Chien D, Brady TJ. Signal intensity in fast NMR imaging with short repetition times. J Magn Reson 1989; 83:576–585.
Steady-State EPG -- I• Can calculate state propagation repeatedly• Difficult, often unnecessary to do analytically• Fast when number of states can be limited
Steady-State EPG -- II• Recall steady states: M’ = AM + B• Is there an EPG form?
– Assuming a finite number (N) of states, yes!– Write each state as real + imaginary– Expand to vector of length 6N– RF rotations become block-diagonal matrices– Gradient transformation is mostly off-diagonal and
diagonal 1’s, except for F0* to F0 state (conjugate)
Gradient-Spoiled EPG SimulationRF
Gz
RF-Spoiled Gradient Echo
RF
Gz
TR(Quadratic Phase Increment)
Sig
nal
1
0
Mz
X XEliminate transverse magnitization: T1 contrast