MRI Process MR Image Contrast Apply RF pulse to tip NMV into transverse plane B1 Wm. Faulkner, B.S.,R.T.(R)(MR)(CT), FSMRT H2O CH3 MRI Process Sample transverse magnetization MRI Process Wait while protons (spins) relax back to Thermal Equilibrium x y
Relaxation The process by which the spins and therefore! the net magnetization returns to thermal equilibrium T2-Relaxation Spin-Spin Relaxation! Exchange of energy among the spins! Results in a decay of transverse magnetization T1-Relaxation T2-Relaxation Rate Spin-Lattice Relaxation! Loss of energy from the spins (energy lost to molecular lattice)! Results in a recovery of longitudinal magnetization Transverse! (Signal) Exponential! Decay
T2-Relaxation T2-Relaxation Transverse! (Signal) for 63% of! transverse to decay for 63% of! transverse to decay Affected by Tissue Slightly by field strength Temperature Fat = short T2-relaxation time Water = long T2-relaxation time T1-Relaxation Rate T1-Relaxation Longitudinal! (Signal) Exponential! Recovery Longitudinal! (Signal) for 63% of! longitudinal to recover
T1-Relaxation for 63% of! longitudinal to recover Affected by Tissue Significantly by field strength Temperature Fat = short T1-relaxation time Water = long T1-relaxation time Let s Review Exposure to an RF field at the Larmor Frequency causes the spins to precess in phase! Continuing the RF application causes some spins to gain energy from the RF field and move to the high energy state! As more and more spins move to the high energy state, the NMV tips into the XY plane Let s Review Upon removal of the RF field, the NMV is precessing through a receiver coil oriented in the XY plane! The NMR signal is induced in the coil according to Faraday s Law of Induction! Since the RF field is no longer present, the NMV will relax back to it s alignment with the external magnetic field (B0) Let s Review The NMV relaxation consists of two simultaneous yet independent processes: T2 and T1! T2: characterized by the loss of detectable magnetization in the transverse plane! T1: characterized by the recovery of magnetization along the longitudinal axis
Phase Dispersion RF Tx Receive FID Free Induction Decay Returned RF Signal Loss of phase coherence MR Signal Decay RF! Pulse Decay due to! - T2-relaxation! Signal
MR Signal Decay RF! Pulse Signal FID decay due to T2* Decay due to! - T2-relaxation! - Field and local! inhomogeneities! - Chemical shift Refocusing (correcting) off-resonance effects (T2 ) Spin Echo Pulse Sequence Gradient Echo Pulse Sequence 1/2 TE T2 T2* FID! T2* Readout (freq) Gradient! Magnetic Field
What s the Difference? Spin Echo Gradient Echo Tissue Relaxation s Water! Fat! Long T1! Long T2 (T2*)! Short T1! Short T2 (T2*) Most Pathology has an associated increase in water
MR Image Contrast Control T1-weighted: Image contrast primarily dependent on tissues T1 times! http://nacnud.net/images/nacnud-pics/ws_board.jpg T2-weighted: Image contrast primarily dependent on tissues T2 times! PD-weighted: Image contrast primarily dependent on tissues proton (spin) density http://ww4.hdnux.com/photos/23/03/51/4995707/3/628x471.jpg TR ( of Repetition) TR! ( of Repetition) TR ( of Repetition) The TR determines how much longitudinal! magnetization recovers based on the T1-relaxation time of the tissue RF Measured! Signal
TR ( of Repetition) The TR determines how much longitudinal! magnetization recovers based on the T1-relaxation time of the tissue TR is your T1control knob T1-Weighted Images TE ( to Echo) TE! ( to Echo) RF Measured! Signal
TE ( to Echo) TE ( to Echo) The TE determines how much transverse! magnetization has decayed at the time! of sampling based on the T2-relaxation time of the tissue TE ( to Echo) TE ( to Echo) SI SI
TE ( to Echo) TE ( to Echo) SI SI TE ( to Echo) TE ( to Echo) SI SI TE is your T2control knob
500 1000 TR 2000 4000 8000 12,000 25 50 75 100 TE Dual Echo: Spin Echo Long TR Dual Echo: Gradient Echo A chemical shift effect TE 1 TE 2 Out-of-Phase In-Phase
MR Contrast is Relative Relative to Parameters! Relative to Field Strength! Relative to Tissue s T1 and T2 MR Contrast is Relative Relative to Parameters! Relative to Field Strength! Relative to Tissue s T1 and T2 1.5 T 3.0 T 3 mm 400 TR 800 TR 400 TR 800 TR MR Contrast is Relative Relative to Parameters! Relative to Field Strength! Relative to Tissue s T1 and T2 1.5 T Let s Review Immediately following an RF pulse, the transverse magnetization decays due to 3 main factors! field and local inhomogeneities! chemical shift (fat and water)! T2 (spin-spin interaction)! The decay of the FID is therefore due to T2* 400 TR 800 TR
Let s Review There are two main types of MR pulse sequences! Gradient Echo! Spin Echo! A GRE sequence uses a gradient reversal only to produce the echo. Transverse decay is therefore due to T2*! A SE sequence employs a 180 degree RF pulse prior to sampling the echo Let s Review With a SE sequence, the transverse magnetization decays at rate dependent on the tissue s T2 time! The TR ( of Repetition) controls image contrast contribution from T1! The TE ( to Echo) controls image contrast contribution from T2 (T2* w/gre) Tissue Relaxation s MR Image Contrast: SPIN ECHO T1 T2 TR TE Fat Short Short T1 Short Short T2 Long Long Water Long Long PD Long Short
Parameter Selection: SPIN ECHO TR T1 400-700 TE 25 or less T2 PD 3000 or higher 80-120 3000 or higher 25 or less