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December 2, 2009
Durgesh Kumar DwivediDepartment of NMR & MRI
AIIMS, New Delhi, India
ContentsBasic terminology of MR pulse sequencesRARE/ FSESE (CSE) vs FSEContrast in FSEAdvantagesDisadvantagesSSFSE, HASTE
Pulse sequence and timing diagram
Four lines are needed radio-frequency (RF) pulse three gradients - slice, phase, frequency/ readout
SPIN ECHO (SE) SEQUENCES
Manufacturer Single echo SE Multiple echo SE Echo train SE
Siemens Single SE SEDouble echo
Turbo spin echo (TSE)Half Fourier acquisition turbo SE (HASTE)
GE SE Multiecho multiplanar (MEMP)Variable echo multiplanar (VEMP)
Fast SE (FSE)
Single shot FSE (SSFSE)
Philips SE,
Modified SE
Multiple SE (MSE)
Turbo spin echo (TSE)Ultra fast Spin echo (UFSE)
Contrast parameters
Two key parameters : repetition time (TR) and echo time (TE) - are key to the creation of image contrast.
TR (in milliseconds) is the time between the application of an RF excitation pulse and the start of the next RF pulse
TE (in milliseconds) is the time between the application of the RF pulse and the peak of the echo detected
Effect of TR and TE on MR image contrast
Imaging technique
TR TE
T1 weighting
Short Short
T2 weighting
Long Long
PD weighting
Long Short
* Short TR & long TE produces very low SNR and should be avoided
TR (in ms) TE (in ms)
Sequence
Short Long Short Long
SE 250-700 >2000 10-25 >60
GRE <50 >100 1-5 >10
Spin echo
90° pulse flips the net magnetization vector into the transverse plane
A 180° pulse is applied at a time equal to one-half of TE to rephase the spinning nuclei
When the nuclei are again spinning in phase (at total TE), an echo is produced and read
FSE
RARE (Hennig et al 1986) (Rapid Acquisition with Relaxation Enhancement)
FSE (Fast spin echo) (Mulkern et al 1990) TSE (Turbo spin echo)
FSE Fast scan (Based on principle of echo imaging) Long TR (Multiple RF pulse) - T2W 강조
Conventional SE Fast SE
1 NEX 4 NEX
9 min 28 sec 2 min 25 sec
Fourier Transform
k-space
Frequency encoding axis
If 256 개의 phase encoding 을 채울 경우 TR 256 번을 반복P
has
e en
cod
ing
axis
(
Ky)
+ 127
- 127
0
RF signal (Echo)
(Phase) 부호화 경사 ( Gy )
(Frequency) 부호화 (Gx)
RF Pulse
90° 180° 90°
If you fill the 256 phase encoding TR 256 iterations
Fast Spin echo …Echo train length (ETL) : Number of 180° RF pulse : Scan time ∝ (1/ETL)
Echo train spacing (ETS) : Space between 180° RF pulse : Dwell time (in phase encoding direction)
Effective echo time (TEeff) : TE of k-space mid-line
180° 180° 180° 180° 180° 180° 180° 180°90°
ETS
ETL
K-space
Overallcontrast
Detailed Description
+ 127
- 127
0 (Ky)
Fast Spin Echo
RF Pulse
Phase-encodinggradient
Echo
TE5 TE1 TE2 TE3 TE4 TE6 TE7 TE8
ETS90° 180° 180° 180° 180° 180° 180°
TE eff
ETL=8
Ky=0 Centre of k-space
+ 127
- 127
0 (Ky)
Effective TETEeff 40.36 msec 80.72 msec 96.86 msec 137.22 msec
T2 effect ↑, SNR ↓
TE 40 TE 100
TEeff 40 TEeff 100
Echo Train Length
ETL 4 ETL 8 ETL 16 ETL 32
TR 5000msec, NEX = 2ETL governs by: (1)T2 relaxation, (2) ETS
10 min 45 sec 5 min 25 sec 2 min 45 sec 1 min 25 sec
ETL ↑ : Time ↓
Issues : Slice number ↓
Correction: TR ↑, Slice thickness ↑
Scan Time(SE) = (TR)(Ny)(NEX)
Scan Time
Scan Time(FSE) = (TR)(Ny)(NEX) /ETL
NyNEXETL
:::
Phase-encoding stepsNumber of excitationEcho Train Length
Example: TR = 3000 msec, NEX=1, Matrix 256 X 256. ETL of 8.Calculate time for CSE and FSE images?
CSE
3000(TR) * 256(Ny) * 1(NEX)
= 12.8min
FSE
3000(TR) * 256(Ny) * 1(NEX) / 8
= 1.6 min
Contrast in FSE
Images illustrate how the various regions of K space (upper row) can be reconstructed, with the corresponding images (bottom row). Reconstructions are shown for all of the data (left), the center of k space (center), and the outer regions of k space (right).
T1-weighted images obtained with a conventional spin-echo sequence (right) and a fast spin-echo sequence (left) with an echo train length of four and a 500-msec TR. Below image: T2-weighted images obtained with a conventional spin-echo sequence (right) and a fast spin-echo sequence (left) with an echo train length of four and a 2,000-msec TR. TE was 68 msec for the conventional image, and the TE encoding the center of k space in the fast spin-echo image was also 68 msec. Blurring seen in the T1-weighted fast spin-echo image is not apparent in the T2-weighted fast spin-echo image because the earlier echoes are used to sample the higher frequency phase-encoding views.
FSE SE
T1W
T2W
FSE vs CSE
90° 180° 90°
TE80
90° 180° 180° 180° 180° 90°
TE30 TE60 TE80 TE100
CSEFSE
TR
Phase (Phase)Signed slope
RF Pulse
Multiple 180° pulse
TE 80
Time saving
K-space TEeff 80
No time saving
Advantage
Scan time ↓↓: ETL∝
Image quality ↑ : Scan time saving; trade off – ETL and Slice thickness
Based on spin echo and similar contrast
Artifact (motion, susceptibility) ↓ : by 180° refocusing pulse
Disadvantage
BlurringTEeff 50 ms
T2 weightedTEeff 150 ms
Disadvantage Bright fat signal : J-coupling
: Remedy- Fat suppression image
Conventional SE Fast SE
Fat suppression
Fast SE
Disadvantage Specific absorption rate (SAR) : Total RF energy (E) dissipated in a sample over
exposure time (texp) per unit mass(M) (watts per
kilogram)
SAR= E/ (texp*M)
Also,
SAR α Bo2 * θ (Theta)2 *Bandwidth
Use low flip angles
FSE 의 변형 3D FSE (+ 3D)
SSFSE, HASTE
(+ Single shot FSE)
(+ Half fourier acquired sigle shot turbo spin echo)
3D FSE
Thinner imagePhase encoding : z Direction (Nz), addMultiple slices → SlabMore scan timeScan time = (TR*NEX*Ny*Nz)/ETL
2D Image
3D Image
SSFSE , HASTE
Single shot : A very long one echo train (64-128 locations)
Ny = ETL, so, Scan time = TR* NEX
FSE (Single Shot) + Half fourier acq. Partial fourier technique : K-space Fill in the date part of the
Frequency encoding ( Kx)
Pha
se e
ncod
ing
( K
y)
Partial fourier technique
K-space
90° 180° 180° 180° 180° 180° 180° 180° 180° 180° 180° 180° 180° 180° 180° 180° 180°
64-128
Single shot FSE, Half Fourier acquired sigle shot turbo spin echo
HASTE, SSFSE Ultra-fast : 1-2 초 (single breath hold) Abdomen, chest imaging
MRCP Liver MR
Summary Characteristics of spin echo
Fast scan ∝ETL
Advantage : Image quality ↑, Artifact↓
Disadvantage : Fat signal ↑, Slice number ↓
ETL, ETS, TE eff
Advancement due to SSFSE & HASTE- better image quality
THANK YOU…