Magnetic resonance imaging

My discussion: MRI procedure

• generate tissue contrast

• generate spatial localization

larmor frequency and RF pulse

RF pulse(classical view)

0B

• Larmor frequency

y’

z

x’

x

y

B0 M

Tissue contrast

T2 decay constant

20

T

t

xy eMM

intrinsic spin-spin interactions

y

xz Mxy

y

xz

Mxy 90o RF(B1)

pause removed

Classical viewpoint

B’

T1 relaxation constant

)1( 10

T

t

z eMM

Spin-lattice relaxation

90o RF(B1) pause added

energy

Spin-echo

T2 T2*

TR

TE

90o 180o

90o 180o

RF

signal

T2=spin-spin relaxationT2*=T2+static field inhomogeneities

Different contrast images

T2 contrast : TR/TE = 5500/105 ms, 2 Nex, 512x256 matrix

T1 contrast : TR/TE= 450/14 ms, 1 Nex, 256x192 matrix

*Nex: number of acquisitions

TE(14)

TR(450) TR(5500)

TE(105)

WhiteGrey CSF

Spatial localization

• Slice select gradient• Frequency encode gradient• Phase encode gradient

Magnetic field gradient

Slice select gradient: slice thickness

B0

RF bandwidth

ZD1

D2

D3

B-

B+’

Slice select gradient: slice location

B0

RF bandwidth

Z Z1Z2Z3

B-

B+

Frequency encode gradient

digitization

-Bx

+Bx

-f max +f max0

Fourier transform

Phase encode gradient

-

+

#1

#60

#256

#188

#128

-

-

-

+

+

+

Time

Imaging: spin echo

x

y

f(x,y)

kx

ky

k-space

F(kx,ky)

MR image

RF pause

Slice selection gradient

Frequency encode gradient

Phase encode gradient

MR signal

Data acquisition

90o 180o repeat

x

y

f(x,y)

kx

ky

k-space

F(kx,ky)

MR image

RF pause

Slice selection gradient

Frequency encode gradient

Phase encode gradient

MR signal

Data acquisition

90o 180o repeat

Imaging: spin echo planar-FMRI technique

Within T2*

Functional magnetic resonance imaging

BOLD(blood-oxygen-level-dependent) fMRI is

currently the most common fMRI technique

Noninvasive-no injection of radioactive

isotopes

Spin echo planar imaging-extremely fast-50 ms

T2-T2* weighted images

T2-T2* weighted image:T2* is dependent on the presence of blood deoxygenation and that deoxygenated hemoglobin is a "BOLD" effect that can be observed by FMRI at high magnetic fields

shorter T2* longer T2*

more paramagnetic

FMRI actual measurement

FMRI images

1. a high resolution scan is taken as background

2. a series of low resolution scans are taken over time during the experiment

3. the analyzed low resolution images are shown as colored blobs on top of the original high resolution scan, and also in 3D after proper analysis

FMRI head coil

The posterior lateral prefrontal cortex is a key neural substrate underlying the central bottleneck of information processing.

Central bottleneck model:

Experimental result:

Task 1 Task 2

FMRI can precisely locate the brain active spot with 1.5 x 1.5 mm in-plane resolution or even less than 1 mm.

FMRI can catch the blood flow in seconds with total scan time on the order of 1.5 to 2.0 min per run.

Advantage:

Thanks!