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…