Fast Imaging Fast - clinicauniversitariaradiologia.pt

FastImagingS. I. Gonçalves, PhD

Radiology DepartmentUniversity Hospital CoimbraAutumn Semester, 2011

Fast Im

aging

1

“MRI: Principles and Applications”, Friday, 8.30‐9.20 am

Overview

• The necessity for fast imaging

• Segmented k‐Space: Fast Spin Echo and Fast Gradient Echo

• Conventional Echo Planar Imaging

• Fast Imaging in Steady‐State

Fast Im

aging

2

Reducingscantimes

• In traditional scanning methods, only one k‐space line is acquired after each RF pulse;

Fast Im

aging

3

RF

RF

-kmax kmax

0 RF

Spin‐echo

RF

-kmax kmax

0 RF

Gradient‐echo

TR

Reducingscantimes

Fast Im

aging

4

kx

ky

Acquired in one TR

Ny – Number of phase encode linesNz – Number of partition encode lines (3D acquisitions)Nacq – Number of averages

Totalscantime

Fast Im

aging

5

TT = NacqNzNyTR

• Example: Time to acquire one SE image

Acquisition matrix = 256256TR = 2000 ms

TT = 2562000 ms = 8.53 min

Reducingtotalscantime

Fast Im

aging

6

• There are obviously two ways to reduce scan times:

Decrease the number of RF excitations Segmented k‐space and Echo‐Planar Imaging (EPI);

Short TRRapid Imaging in Steady‐State

SSFP (Steady‐State Free Precession) sequences

TT = NacqNzNyTR

Segmentedk‐space

Fast Im

aging

7

• Decrease the number of RF pulses associated with phase encode and partition encoding steps;

• Encode more than one k‐space line per RF pulse – Multi‐Shot acquisition

• FSE and Ultra Fast GE

TSE, FSE, TurboFLASH, Fast SPGR, T1‐TFE

1st k‐line 2nd k‐line 3rd k‐line 4th k‐line

Segmentedk‐space

Fast Im

aging

8

Fast Spin‐EchoNumber of k‐lines per TR is controlled by the Turbo Factor or Echo Train Length (ETL) (2‐32);SAR intensive;Blurring;Robust to susceptibility artifacts;

Q. Chen et al., EJR, 1999

N

• The contrast in FSE is modified in relation to a standard SE sequence; Echoes are received at different echo times;

• The echoes corresponding to the central k‐space lines are the ones that will determine image contrast;

• The moment at which theses echoes are acquired is called effective TE;

• In T1‐weighted FSE sequences, the need to choose a short TR limits ETL;

• Fat has a higher T2 signal in FSE than in standard SE;

• This is due to a lengthening of T2 relaxation time in FSE;k=0

TEeff

FSE

Fast Im

aging

9

FSE‐ Examples

Fast Im

aging

10

A) SE image with TR=650 ms, TE=8 msB) FSE image with TR=650 ms, TEeff=14 ms, ETL=8

T1 weighting

A) FSE image with TR=5.1 s, TEeff=58 msB) FSE image with TR=5.4 s, TEeff=84 ms

T2 weighting

Hendrick, “Breast MRI”, Springer‐Verlag, 2008

• Category of GRE sequences with short TR, small flip‐angle and optimized k‐space filling;

• Due to the small flip‐angle and short TR, these sequences have typically poor T1‐weigthing;

• Therefore it is common to prepare de magnetization with an 180 RF pulse to increase T1‐weigthing;

• The lines in k‐space can be acquired after a single inversion pulse (single‐shot) or can be distributed amongst several inversion pulses (multi‐shot or segmented acquisition);

• Effective inversion time will correspond to the delay between the inversion pulse and acquisition of the central k‐space lines;

• T2‐weigthing can be obtained with a 90‐180‐180‐90 (T2‐Prep);

Ultra‐FastGRE

Fast Im

aging

11

Findingyourwayinbetweenvendoracronyms…

Fast Im

aging

12

Sequence Type Siemens GE Philips

Ultrafast GRE TurboFLASH Fast GRE, Fast SPGR

TFE

Ultrafast GRE 3D MPRAGE 3D Fast GRE, 3D Fast SPGR

3D TFE

Volume Interpolated GRE VIBE FAME, LAVA‐XV THRIVE

SequencediagramforUltrafastGRE

1st k-line

TR

RF

Signal2nd k-line 3rd k-line

Fast Im

aging

13

Ultrafast GRE sequences have a short TR, TE, a low flip‐angle ;

TR is so short that image acquisition lasts less than 1 sec and typically less than 500 ms;

Typical values: TR= 3‐5 msec, TE= 2 msec, and flip angle 5;

ShotN k‐lines per shot

SequencediagramforUltrafastGRE

TR

RF

1st k-line

SignalK=0 3rd k-line

Fast Im

aging

14

TIeff can be adjusted to null the signal from a given tissue, similar to STIR;

Multi‐shot approach especially useful for cadiac imaging where images have to be acquired consistently within different heart phases and adequate temporal resolution has to be used;

The speed of these GRE sequences is particularly suited to monitoring Gadolinium bolus arrival while imaging at arterial phase and in T1 high‐resolution 3D imaging;

TIeff

Ultra‐FastGRE‐ Examples

Fast Im

aging

15

VIBE post‐contrastTR=3 ms, TE=1.1 msVoxel size = 1.561.56 1.70 mm104 imagesTotal scan time: 21 s

3D FLASHTR=1.9 ms, TE=0.8 msVoxel size = 1.561.56 5.00 mm8 imagesTotal scan time: 5.01 s

ConventionalEchoPlanarImaging(EPI)

Fast Im

aging

16

• Fastest imaging modality in MRI (100 ms/slice), but with limited resolution (i.e. 128128 ); Extensively used for perfusion, diffusion and functional MRI;

• Collection of all data necessary to build an image after a single RF pulse (single‐shot);

• Continuous signal acquisition of all k‐space in the form of an gradient‐echo train;

• In EPI, there is water‐fat shift (WFS) in the phase encoding direction due to phase errors and because of the low BW in the PE direction. Fat‐suppression should be selected;


Fast Im

aging

17

• GE‐EPI: single RF excitation pulse, with no preparation, followed by an echo train; T2* weighting

• SE‐EPI: 90‐180 pulses followed by an echo train; T2 weighting

• IR‐EPI: 180 inversion pulse to prepare magnetization followed by an RF excitation pulse and echo train; T1 weighting

• DW‐EPI: preparatory pattern for diffusion weighting


Fast Im

aging

18

GE‐EPI

T2*


Fast Im

aging

19

SE‐EPI

T2 T2


Fast Im

aging

20

DW SE‐EPI

T2T2


Fast Im

aging

21

k‐space trajectory

AlternativeEchoPlanarImaging(EPI)

Fast Im

aging

22

Segment of k‐lines per excitation

Segmented EPI – shorter echo trains


Fast Im

aging

23

Spiral EPI – non‐uniform sampling


Fast Im

aging

24

Angled k‐Space EPI– constant phase encoding gradient

EPI– Examples

Fast Im

aging

25

BOLD response

TR= 3 s6464

fMRI‐BOLD with GRE EPI

EPI– Examples

Fast Im

aging

26

DWI with SE EPI

DW SE‐EPI for b=0

TR=3271 ms, TE=62 msPixel size = 2.572.57 mmSlice Thickness = 10 mmPACE

Fast Im

aging

27

DWI with SE EPI

DW SE‐EPI for b=0

TR=5584 ms, TE=96 msPixel size = 3.13.1 mmSlice Thickness = 10 mmPACE

What went wrong here?

FastImaginginSteady‐State

Fast Im

aging

28

• When TRT2, magnetization does not completly relax until the next RF pulse;

• Transverse and longitudinal co‐exist in one TR;

• The type of steady state depends on the flip angle , TR and gradient pattern that is applied;

• In order for steady state to be established, time and amplitude patterns of gradients must be exactly reproduced between RF pulses;

• TR<10 ms, <60


Fast Im

aging

29

• In steady‐ state gradient echo sequences, residual transverse magnetization will participate in the signal and the contrast will vary according to the type of sequence;

• By maintaining residual transverse magnetization, excitation pulses will produce new echoes (Hahn echos, stimulated echos) in addition to the gradient‐echo;

• There are several variants in the family of steady‐state gradient echo sequences, according to the type of echo recorded (which determines contrast) and how the gradients are adjusted:

FLASH, SPGR, FFE, FISP, PSIF, TrueFisp, N-FFE, T2-FFE, b-FFE, SSFP, FIESTA, GRASS


Fast Im

aging

30

• Breath‐hold scans;

• Fast 3D scans;

• Less SAR deposition when compared to SE;

• High‐resolution imaging;

Steady‐statefreeprecession(SSFP)mechanism

Fast Im

aging

31

TR

+ + +

Rotation DephasingMT

T2 relaxation MZ

T1 relaxation

nth RF pulse (n+1)th RF pulse

SSFPmechanism

Fast Im

aging

32

…… ……

TR(n‐1)th RF nth RF

1n M

1n M

nM

nM

nn MM

1

Different types of SSFP sequences

RF

Slice

PhaseEncode

Read

Signal

Sampling

SSFP-FID(N-FFE, FAST, GRASS, FISP, MPGR)

Fast Im

aging

33


RF

Slice

PhaseEncode

Read

SSFP-ECHO(T2-FFE, CE-FAST,PSIF, SSFP)

Signal

Sampling

Fast Im

aging

34

Low SNR

35


RF

Slice

PhaseEncode

Read

Sampling

DESS – Double echoe steady-state

Signal

FID ECHO

t0 tp

(tp-t0)

RF

Read

Signal

TR

Balanced (b)-SSFP, signal contrast

1

20

21

21

21

11cos

TTMSignal

TTTT

opt

β=

Fast Im

aging

36

(TrueFISP, FIESTA, b-FFE)

SSFPsequences– Examples

Fast Im

aging

37

1.5 T

2D TrueFisp

SSFPsequences– Examples

Fast Im

aging

38

FLASH FISP DESS

Documents

Fast Imaging Fast - clinicauniversitariaradiologia.pt