Nuclear Magnetic Resonance-Bushberg Chapter 14 Diagnostic Radiology Imaging Physics Course GRE-Steady-state Precession with GRE Sequence with Long TR(>200 msec) Short TR(<50 msec) ◆For long TR(e200msec)GRE and flip0>45°：contrast Steady-state precession:equilibrium of M,and Mfrom behavior similar to SE pulse to pulse in a repitition sequence Major difference signal dependence on T2 rather than T2 For very short TR(<T2*),persistent M occurs .Mechanisms of T2contrast different than T2,especially for During each pulse aMxyM:and bM2M(a,b<1) contrast agents T1-weighting achieved with short TE Steady-state M,and M components co-exist in dynamic equilibnum ◆For flip0<30°：small M,reduces T1 differences GRASS=Gradient Recalled Acq.in the Steady State differences the major contrast attributes for short TE FISP =Fast Imaging with Steady-state Precession .Longer TE provides T2-weighting FAST=Fourier Acquired STeady state GRE not useful with long TR except for demonstrating Practical only with short and very short TR magnetic susceptibility differences Flip 0 has the major impact on contrast GRE-Steady-state Precession with Short TR(<50 msec)and Contrast Weighting GRE-"SPoiled"GRE Techniques(SPGR) ◆Small flip=5-30°：p-weighted4 ◆Very short TR一poor T1-. contrast; weighting ◆Moderate flip6=30-60 T2*influence reduced by T2/T1-weighted contrast(some "spoiling"the steady-state My T1) 。Large fip0=75-90°：T2and T1-weighted contrast Mostly T1-weighted contrast Typical parameter values for .Short TR,short TE,moderate contrast desired in GRE and to large flip e and spoiled M steady-state acquisitions produces greatest1 contras SPGR (TR =8 mec.TE 1.9 GRASS/FISP TR=35 msec. GRASS sequence(TR=24 msec,TE 4.7 Better gadolinium contrast than TE=3 msec and flip8=20° rkab comparable spin echo,but .Unremarkable contrast but flow increased artifacts and SNR 监“06 9,19and26May2005 16 Nuclear Magnetic Resonance – Bushberg Chapter 14 Diagnostic Radiology Imaging Physics Course 9, 19 and 26 May 2005 16 © UW and Brent K. Stewart, PhD, DABMP 61 GRE Sequence with Long TR (> 200 msec) GRE Sequence with Long TR (> 200 msec) ™ For long TR (> 200 msec) GRE and flip For long TR (> 200 msec) GRE and flip θ > 45°: contrast : contrast behavior similar to SE ™ Major difference signal dependence on T2* rather than T2 ™ Mechanisms of T2* contrast different than T2, especially for Mechanisms of T2* contrast different than T2, especially for contrast agents ™ T1-weighting achieved with short TE ™ For flip For flip θ < 30°: small Mxy reduces T1 differences ™ ρ differences the major contrast attributes for short TE ™ Longer TE provides T2*-weighting ™ GRE not useful with long TR except for demonstrating GRE not useful with long TR except for demonstrating magnetic susceptibility differences © UW and Brent K. Stewart, PhD, DABMP 62 GRE - Steady-state Precession with Short TR (< 50 msec) Short TR (< 50 msec) ™ Steady-state precession: equilibrium of Mz and Mxy from pulse to pulse in a pulse to pulse in a repitition sequence ™ For very short TR (< T2*), persistent Mxy occurs ™ During each pulse aMxy → Mz and bMz → Mxy (a, b <1) (a, b <1) ™ Steady-state Mz and Mxy components co-exist in exist in dynamic equilibrium ™ GRASS = GRASS = Gradient radient Recalled ecalled Acq. in the . in the Steady State ™ FISP = Fast Imaging with maging with Steady-state Precession ™ FAST = Fourier Acquired cquired STeady state ™ Practical only with short and very short TR ™ Flip θ has the major impact on contrast has the major impact on contrast © UW and Brent K. Stewart, PhD, DABMP 63 GRE - Steady-state Precession with Short TR (< 50 msec) and Contrast Weighting ™ Small flip Small flip θ = 5-30°: ρ-weighted weighted contrast; contrast; ™ Moderate flip Moderate flip θ = 30-60°: T2/T1-weighted contrast (some weighted contrast (some T1) ™ Large flip Large flip θ = 75-90°: T2*- and T1-weighted contrast weighted contrast ™ Typical parameter values for Typical parameter values for contrast desired in GRE and contrast desired in GRE and steady-state acquisitions ™ GRASS/FISP TR = 35 msec, GRASS/FISP TR = 35 msec, TE = 3 msec and flip TE = 3 msec and flip θ = 20° ™ Unremarkable contrast but flow c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., pp. 406-407. GRASS sequence (TR = 24 msec, TE = 4.7 GRASS sequence (TR = 24 msec, TE = 4.7 msec, flip θ=50°) volume acquisition. Contrast unremarkable for white/gray matter due to unremarkable for white/gray matter due to T2/T1-weighting dependence. Blood appears bright – MR angiography MR angiography – reduce contrast of anatomy relative to vasculature. © UW and Brent K. Stewart, PhD, DABMP 64 GRE - “SPoiled” GRE Techniques (SPGR) E Techniques (SPGR) ™ Very short TR Very short TR → poor T1- weighting ™ T2* influence reduced by T2* influence reduced by “spoiling” the steady-state Mxy by adding phase shift to by adding phase shift to successive RF pulses ™ Mostly T1-weighted contrast weighted contrast ™ Short TR, short TE, moderate Short TR, short TE, moderate to large flip to large flip θ and spoiled Mxy produces greatest T1 contrast produces greatest T1 contrast ™ Better gadolinium contrast than Better gadolinium contrast than comparable spin echo, but comparable spin echo, but increased artifacts and increased artifacts and ↓ SNR c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 408. SPGR sequence (TR = 8 msec, TE = 1.9 SPGR sequence (TR = 8 msec, TE = 1.9 msec, flip θ = 20°) 3D volume acquisition. T1 contrast evident as well as bright blood and magnetic susceptibility artifact (?). and magnetic susceptibility artifact (?)