MRI in Review: Simple Steps to Cutting Edge Part I

Transcription

1 MRI in Review: Simple Steps to Cutting Edge Part I DWI is now 2 years old... Mike Moseley Radiology Stanford DWI, b = 1413 T2wt, 28/16 ASN 21 San Francisco + Disclosures: Funding NINDS, NCRR, NCI 45 minutes post MCA- Magn Reson Med. 14, 199 B: igh field at 7T MR Essentials Coils in MRI ΔB: db/dt < 3T/s ethe main magnet B. Protons precess 42.6 Mz / T. B B The Gradient Coils alter B spatial localization or ΔB. Every electric field has a magnetic field... Any moving magnetic field generates an electrical current... Faraday s Law ydrogen nuclei are protons Protons have a positive net charge. This charge spins around the mass. 2. A moving magnetic field creates an electric field... S + Coil Spinning dipoles have local magnetic fields which will align with external magnetic fields. O A moving charge is an electrical current 1. Every electric field has a magnetic field The radiofrequency coil: 1. Creates B1 field with FM energy 2. Detects proton coherence in B - signal # protons aligned ~ Magnetic field strength (B). At 1.5Tesla, this is only 5 of every 1,,. 1 Tesla = 1, gauss. S N B + N

2 The proton spinning charge precesses or wobbles around the B axis due to Angular Momentum The tip of the axis is related to the magnetic gyric ratio (γ) The speed of the precession = Larmor Equation... f = (γ / 2π)B 42.6 Mz@1T Know B = know frequency... Know frequency= know B... B S + N B S + N electric field creates a B1 magnetic field which flips protons away from B... the "flip" is a perturbation... B So, ow Do We Get an MR Signal? e - B1 direction of applied electric field...b1 1. As protons relax, they re-align along B by. Voltage So, ow Do We Get an MR Signal? 4 sec 2. They also lose phase coherence by T2*. B 4 msec B Relaxation is the Process of Realignment 1. As protons relax, they re-align along B by. Voltage 4 sec 2. They also lose phase coherence by T2*. is the recovery of magnetization along the longitudinal axis. Each proton has a unique and 1 Tesla White matter Gray matter Muscle Fat Liver Renal Med. Renal Cor. Blood Tumor** T Signal Signal T2 4 sec 4 msec T2 is the decay of magnetization along the transverse axis. 4 msec

3 Recovery Brain T2 Decay The Spin-" Pulse Sequence: The loss of phase is reversed by Pulses Brain 9 The FID 18 The Spin- Repeat Time = Echo Time = is governed largely by protons interacting with membranes, lipids, cell walls, myelin...spin-lattice relaxation. More macromolecules = large # of stationary protons = shorter and shorter T2. T2 of protons is dominated by interactions with other water protons, etc. T2 is affected by diffusion, perfusion, water content... spin-spin relaxation. More water = large # mobile protons = less interaction = longer and longer T2. The spin- signal strength is related to T2 only... We collect this signal in SE imaging. 9 The Spin- forms at phase 18 The Determines the T2-weighting Longer = more T2 phase decay and less signal. Shorter = less phase loss and more signal. Longer = more T2-weighting. T2-weighted image will have a long to show long T2 tissues 2ms 9 phase 18 Brain 8ms Echo Time = 9 View of First and Second Echo phase Proton relaxation is a process of re-aligning with B. 1. As protons relax, they re-align along B by. Signal 2ms 4 sec is the recovery of magnetization along the longitudinal axis. Short 8ms Long 2. They also lose phase coherence by T2. The signal decay is the FID. Signal 4o msec T2 is the decay of magnetization along the transverse axis.

4 1 M z Longitudinal Magnetization Recovers by over Time Dictates ow Often is Applied WM The Determines the -weighting White matter Gray matter Muscle Fat Liver Renal Med. Renal Cor. Blood x 5 x T2 6 ms 24 ms Short Long The and Indicates the and T2-Weighting ms ms IR for Morphology TI 75 TI 55 Long Short proton density-weighting Short Short -weighting Long Long T2-weighting Short Long Not used!! 2D IR kz FOV2cm 4mm/2 256x192 NSA.5 4:5 cecilcoil TI 35 TI 15 Inversion Recovery Spin Echo Fluid Attenuated Inversion Recovery (FLAIR) 18 9 phase phase 18 TI ms TI ms Mz Mz FLAIR -Mz recovery curve -Mz recovery curve

5 Image Formation Magnitude Phase The MR Pulse Sequence. MR signals are encoded with frequency and phase Ny... -select 9 phaseencode -select encode : k-space --> image k-space time image Gy F Readout gradient encodes protons according to precessional frequency along X... Phase encoding gradient changes phase along Y at each frequency along X... Y X Gradient coils are built to create a magnetic field gradient (G/cm) Along the x, y, and z axes to correspond to 3-D space... The image orientation depends on which gradient is used for slice selection... Y Current passing thro gradient coils creates magnetic fields that add and subtract from B with no effect at isocenter. Z Y X SAGITTAL Z AXIAL X Y CORONAL Z X Current in Current out back isocenter front Subtracts from B No change Adds to B T 1.5 T 1.51 T 63.7 MZ 63.9 MZ 64.1 MZ f = γ / 2π (B + z )

6 To excite protons in the head, we would need an excitation frequency of 64.1 Mz. Other protons at other postions and frequencies, would not be excited (wrong frequency). -select selection uses both a pulse applied during a gradient pulse -select 18 slice position magnet Z gradient along Z 63.91Mhz 4 msec 63.91Mhz 4 msec frequency - z 63.7Mz T 63.9Mz 64.1Mz 1.5 T 1.51 T + z slice position corresponding to Mz.. The signal arises only from the slice phase Gy The MR Pulse Sequence. MR signals are encoded with frequency and phase Ny... -select 9 -select encode : k-space --> image k-space time image F Lower coil Time MR signal from coil Field Frequencies igher 1.499T 1.51T Lower frequencies igher frequencies coil The MR Pulse Sequence MR signals are encoded with frequency and phase Ny... Changing Phase is Another Way of Measuring! -select 9 phaseencode phaseencode -select encode : k-space --> image k-space time image Gy F Gy 9 18 k-space image F

7 Protons alter phase with gradient pulses! Gradient pulse strength Variable strength Mxy Phase change coil 3 3 coil 3 Time Time Time Sequencing Issues and k-space Resolution, Ny Bandwidth FOVx, FOVy 9 rf 18 rf Read A double spin- "pulse sequence": The phase loss can be repeatedly reversed... (as long as there is T2 signal left...) 9 pulse 18 pulse 18 pulse A Multiple Echo "pulse sequence": T2 loss over time The FID The Spin- Second The signal strength is related to T2 only... In Fast Spin Echo we may collect up to 128 es...

8 One week post-onset Single-Shot FSE b= average X, Y, Z FOV24 7.5/ mm no gating The New igh Field Physics 32 Ch. 7T head coil and transmit Coil 3D FSE sequences with long FSE readout (>2 es). Minimum spacing fast acquisition - no artifacts. Flip angle modulation during the readout keeps signal thru long train. Provides best SNR, low SAR at effective. Like CT, 3D allows for efficienct reformats. Sequence can be easily modified for contrast. The Gradient Echo Pulse Sequence. MR signals are encoded with frequency and phase Ny... Foundations of the Gradient Echo Apply a gradient for 1 ms... Time Reverse the gradient for 1 ms... -select 9 Time -encode : k-space --> image phaseencode Ny k-space the Gradient Echo image Ny Gy time 9 F

9 x128 igh-speed MRI -Today s Options GRE EPI What is Magnetic Susceptibility? The magnetic susceptibility is the difference of the magnetic field across a sample. Each substance in a magnetic field alters that field. Iron has a larger MS effect than water, e.g... SE EPI x128 Fe B eff = B (1-²X) Bone - water Air - water Iron - water small gradient across sample Cell large gradient across sample Iron - T2* shortening... Sequence Review Fe SE GRE Observed T2* Observed T2* Time Time Single- SE Multi- FSE Single- GRE Multi- EPI Single-Shot ssfse Multi-Shot FSE Single-Shot EPI Multi-Shot IEPI REWIND or REFOCUS 2D spine MPGR α 4 msec dephase sampling time Long T2* remains, but is in phase with next shot.

10 SPOIL 2D spine SPGR Contrast in GRE α 6 ms For long in GRE: 4 msec dephase sampling time Increase in adds T2-wting... and MS artifacts crush Long T2* crushed, by large gradient or by random. 3 ms 2 Flip 3 MPGR Contrast in GRE 5 o 45 o For long : Increase in flip adds -wting o 2 15 MPGR