2009. 5. 11. 14:54ㆍJames/Lecture
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Understanding of MRI Pulse Sequences Diagram.
Feburay 15. 2007.
Ho NamKoong.
◈ Contents. 2. Basic terms to understand Pulse Sequences. 3. Spin Echo Family. 4. Fast Spin Echo Family. 5. Inversion Recovery Family. 6. Gradient Echo Family. 7. Clinical Application.
1. Why need to know Pulse Sequences?
1. Why need to know Pulse Sequences?
MR Images should show the location and specific characters of the lesion.
ex)
▷ specific characters : T1-Weighted, T2-Weighted, T2*-Weighted, Flow Character (Angiography), Contrast Media(Dynamic), Fat Saturated, Cine, Diffusion, Perfusion, MRS, fMRI...
▷ location : planes ( axial, sagittal, coronal, oblique).
▷ Signal Intensity (SI)
► SSE ∝ [1-e(-TR/T1)]e(-TE/T2)
► SI ∝ Density of proton × T1 factors × T2 factors
▷ The points of scanning in clinical filed.
Patients (Scan Time) Doctors (Resolution) Technologists (SNR) Best Image What is the best image?
2. Basic terms to understand Pulse Sequences.
▷ Voxel (Volume of Element) / Pixel (Picture of Element)
Matrix = Phase encoding ×Frequency Encoding.
▷ FOV (Field of View): Anatomic region of interest.
FOV = Matrix × Pixel size.
▷ TR (Repetition Time): the time between two 90o RF Pulses.
► amount of magnetization vector.
à Signal Production
▷ TE (Echo Time): the time between initial 90o RF Pulse and Echo.
► the results of T2 dephasing.
▷ FA (Flip Angle): The angle to flip the NMV to the transverse plane.
► amount of transverse magnetization.
► amount of signal.
▷ IR (Invresion Time):
▷ NEX (Number of Acquisition, Number of Excitation), NSA(Number of Signal Averages):
► Produce one image.
▷ Gradient performance: mT/m, G/cm.
B A
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3. Spin Echo Family.
. Try to draw SE Pulse diagram! (Fig1.)
▷ Erwin Hahn, 1949.
① FID (Free Induction Decay)
► too process quickly to turn on and measure it.
► No spatial information.
② Encoding: for spatial information.
► direct impact on the overall scan time.
③ Refocusing.
► use 180o RF Pulse or Gradient.
④ Read-out the Signal.DAS(Data Acquisition System)
► data acquisition.
► frequency encoding of the signal.
▷ Gradient Phase Effects.
► Phase dispersion.(Please draw the phase dispersion according to gradient!)
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Gx |
► Balancing Pulse: refocusing phases at Slice selection and read out.(Fig, 3, 4, 5 and 6)
▷ multi echo (double, triple echo)
► limit number of slice.
► PD and T2 weighted images.
► same matrix (phase encoding)
▷ multi slice
► using dead time.
► slice number = TR / TE + To.
▷ The TR and TE values in conventional SE.
TR: Short (less than 750ms), Long (more than 1500ms)
TE: Short (less than 40ms), Long (more than 75ms)
4. Fast Spin Echo Family.
RARE (Rapid Acquisition with Relaxation Enhancement): Dr. J Henning, 1986.
à FSE (Fast Spin Echo), TSE (Turbo Spin Echo).
RF 180O 90O 180O 180o
▷ ETL (Echo Train Length), TF (Turbo Factor): number of echos.
▷ Scan Time = TR × NP × NEX / ETL
▷ Rewind the phase encoding to undo the dephasing of the spins.
▷ Central segment determines the effective echo time affects the contrast.
▷ The feature of FSE.
► Reduced scan time.
► reduced slice number à use long TR.
► higher RF exposures à increased SAR limits.
► complicated contrast behavior.
► possibly reduced resolution (Ginging or Gibb’s artifacts)
à using high matrix or filtering.
► fat brightening.
subtle contrast changes in the knee.
high level of motion artifacts from the highly motional suborbital fat.
rapid train of refocusing pulse à breaking J-coupling (T2á)
Long IES à GRASE(TGSE): conserve the J-coupling & more Spin-Echo like.
▷ Singl-Shot FSE and HASTE.
► one initial 90O RF pulse.
► very long echo train (all phase-encoding steps).
► scan time = Nslice × (scan time for slice + TD).
► predominately fluid structures (biliary system: MRCP)
5. Inversion Recovery Family.
▷ IR (Inversion time): the time.
RF 180O 90O 180O
► Heavily T1 weighted images.
► TR = 3 × TI (to optimize SNR).
► Null point = 0.69 × T1
► very long measurement time.
► STIR (Short TI Inversion Recovery): Breast.
► FLAIR (Fluid Attenuated Inversion Recovery)
- nulling the water signal (CSF).
- MS (Multiple Sclerosis).
- Periventricular lesion.
▷ IR-FSE.
► Useful for examining the degree of myelination in the immature brain.
▷ interleaved IR-FSE.
► Interleaved TI for long TI (FLAIR).
▷ Real Reconstruction (true) IR.
► Good contrast in neonatal brain.
Magnitude reconstruction Real-valued reconstruction Signal TI/T1
6. Gradient Echo Family.
Conventional SE (long TR, TE) à reduced flip angle, gradient reveral.
110O 0O SNR TR:500ms TR:100ms TR:10ms The Ernst Angle.
▷ Try to draw Fig 7 and 8.
▷ Using a small flip angle à TR can be reduced dramatically without saturation.
▷ TE (less than 10ms) à used early two decay of signal determined by T2* not T2. ( T2* contrast: spin-spin interaction, magnetic field inhomogeneities, magnetic susceptibility.)
▷ TR > T2: FID signal depend largely upon T1.
TR < T2: remnant transverse magnetization remains until next RF pulse is applied. à in addition to the FID.
▷ Reduce RF power deposition (SAR), but more work for the gradients and noisier for the patient.
▷ Increased number of slice.
▷ relatively low SN but high CN.
▷ show unpredictable contrast but increased sensitivity to hemorrhage.
▷ TR á = T1 dependence ↓, SNR á
TE á = T2* dependence á, SNR ↓
FA á = T1 dependence á, SNR ↓
TE: Short (less than 15ms), Long (more than 30ms)
FA: Small (less than 40o), Large (more than 50o).
▷ The branches of in GE.
Transverse magnetization: Spoiling, rewinding, stead-state, modified.
① FID (T1 Weighted): Spoiling, incoherence: spoiled transverse signal on Gs
à successfully relaxed in longitudinal direction à only longitudinal signal contributes to next RF pulse à provide more T1 dependence.(Fig. 9)
► Hahn echo and Stimulated echo (multiple echo).
► insufficient spoiling à flash bands (balanced SSFP that reduce it).
FLASH (Fast Low Angle Shot; Siemens), SPGR (SPoiled Gradient Recalled echo; GE), T1-FFE (T1 Fast Field Echo; Philips), FAST (RF Spoiled Fourier Acquisition Steady-state Technique; Picker).
② FID and Echo (T1/T2 mixed): Rewinding, coherence: rewinding transverse signal on Gp à more signal than spoiled but more complicated weighting.(Fig. 10).
► Ideal contrast depends upon the flip angle and the ratio T1/T2.
► True FISP (Balanced FFE): rewinding gradients in all three directions à excellent SNR and Contrast between blood and myocardium (cardiac imaging).
GRASS (Gradient Recalled Acquisition in the steady-state; GE), GRE (Gradient Recalled Echo; GE), FISP (Fast Imaging with Steady-state Precession; Siemens), FFE (Fast Field Echo; Philips), FAST (Fourier Acquisition Steady-state Technique; Picker).
③ Echo only (T2): Steady-state free precession: echo from prior TR (reverse TE > TR).
► Actually not a gradient echo (Hahn echo origin)
► motion artifacts á and SNR ↓
PSIF (reversed FISP; Siemens), SSFP (Steady-state Free Precession; GE), CE-FAST (Contrast-Enhanced Fourier Acquired Steady-state Technique; Picker), T2-FFE (T2 Fast Field Echo; Philips)
④ Double Echo: Double Echo Steady-state: two echo acquired (FISP echo and PSIF echo): combined high resolution and strong T2-weighted.
► 3D orthopedic study.
► IAC
DESS (Double Echo Steady-state, Siemens), CISS (Constructive Interference in Steady-State, GE), FADE (Fast Acquired Dual Echo)
⑤ Ultra-Fast GE Imaging: extremely short TR and very low flip angle using inversion pulse.
► TR ↓, FA ↓ à T1 very poor. (so, using IR pulse).
► phase encoding order à Contrast.
► changing matrix size in phase encoding à changed the contrast.
▷ MP-RAGE: very high resolution T1-weighted images à showing very good anatomical detail (brain).
TFL (Turbo-FLASH, Siemens), MP-RAGE (Magnetization Prepared Rapid Acquired Gradient Echo, Siemens), FSPGR (Fast SPGR, GE).
▷ GE Echo Planar Imaging.
► fastest pulse sequence.
► real-time cardiac imaging.
► functional MRI of brain.
7. Clinical Application.
Pulse Sequences
Applications
SE
SE
T1, PD, T2 for anatomical areas.
MT-frequency off set 8~16ms MT pulse.
Fermi Pulse (high FA, lower amplitudes)
FSE
FSE
T1, PD, T2-WI images
breathhold abdominal images
joint scan
use blurring cancellation, ESPò
T1-FLAIR(IR-FSE)
achieve better tissue CNR and SNR
in same or shorter scan time than SE T1.
T2-FLAIR(IR-FSE)
the null point of CSF, TI:2000~2300ms
Double or Tripple IR-FSE
cardiac anatomy, myocardial wall masses
valve leaflets, black blood, carotid imaging
FR-FSE
refocused -90o RF.
PD, T2-WI of spine
abdomianal breath hold, head, joint.
MRCP, T2-spine, IAC
SSFSE(IR-SSFSE)
short ESP, 0.5NEX (half-fourier)
to reduce motion artifacts and scan time
to scan uncooperative patients
for BH abdominal and cardiac imaging with long TE (300~1300ms) to GB & biliary tree
IR-FSE
suppress the signal from fat in ABD & extrimity
do not use with contrast studies because enhancing pathology could be suppresed
GE
SPGR
T1-WI,
short TR (40~60ms), moderate flip angle (30o~50o)
GRE
generally create T2* contrast
C-spine, Knee
Sequential Fast GRE/SPGR
T1, T2* BH ABD & pelvis imaging
ultra-fast localizers
BH cardiac/aortic arch imaging.
Multi-phase Fast SPGR
temporally resolved contrast studies
joint motion studies of knee, TMJ, Wrist
3D Fast GRE/SPGR
high resolution T1, T2* joint & musculoskeletal image when fast scan times are desired
reformat into multiple planes
BH ABD & breast w/wo SPECIAL
multi-phase contrast enhanced volume imaging
FIESTA
fully balanced steady-state coherent image
high SNR & T2/T1 contrast at very short TR
cardiac function, valve assessment, wall motion, qantitative analysis, great vessels, valvular morphology
(with fat-sat) biliary tree, urinary tract, great vessels & branches in abdomen / when contrast adminstration is contraindicated.
3D FIESTA
whole body iamging, bile ducts, cochelia, joint, spine for disc herniation, spinal cord block, IACs
3D FIESTA-C
cycling of the RF phase
to reduce banding artifacts and improve image homogeneity. (when TR >4~5ms, dark band appears on the 3D FIESTA)
inter-vertebral discs, hydrocephalus obstructions, biliary tree dilatation, cholangio-pancreato graphy, IAC applications
MERGE
Multi-Echo Recalled Gradient Echo: 2D fast GRE with multiple echos
use larger receive bandwidth (31.24KHz) which reduces the chemical and susceptibility artifacts.
T2* WI C-spine imaging: improve visualization of cord compression and parenchymal disease.
LAVA
3D SPGR with Partial Kz filling techniques
segmented SPECIAL technique
liver imaging.