Imaging

skillfulbuyerUrban and Civil

Nov 16, 2013 (3 years and 8 months ago)

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seminar


October, 2008




j. brnjas
-
kraljević

Imaging (MRI)


tomography

technique



the

volume

image

is

built

up

by

images

of

thin

slices

from

which

data

are

taken


two
-
dimensional

distribution

of

certain

physical

parameter

is

image

of

one

tom


measurement of
space distribution

of
same

resonating
nuclei is enabled by introduction of controlled
inhomogeneity of B
0
field
-

gradient of the field

in desired
direction


we measure
resonance/relaxation

of
hydrogen

nuclei

in
water and in fat




dz
dB
G
dy
dB
G
dx
dB
G
z
y
x



in

perfectly

homogen
eous

field

all

protons

have

the

same




--

only

one

signal

is

measured

Signal

is

measured

in

the

presence

of

field

gradient
.

T
he

result

is

distribution

of

nuclei

in

desired

direction
.

Gradients

in

different

direction

built

up

space

distribution

of

nuclei
.

Mathematical

algorithm

transcribes

values

of

measured

voxels

signals

into

gray

scale
.

gradient in direction X
-
ax
is


distinguishes the Larmor

frequency of nuclei depending

on the place in the field




=



0
+x G
x
)


Image

construction



by projection of reordered spectra each volume part, voxel, is give the
value of measured parameters



parameters are displayed in gray scale



specters have to be measured in thin slices
-

the 3D
-
image is built up
from many slices


How is it recorded

?



90
-
FID method recording


pulls simultaneously with gradient in the field
direction


selects the desired tom


changing of the angle of gradient, G
f
, for
frequency differentiation is realized by
combination of two linear gradients in Y i X
direction:


G
y

= G
f

sin
q

and G
x

= G
f

cos
q



the recorded FID is treated by FT
-

gives the
signal distribution by frequencies and phases

G
y

G
x

Imaging


change

of

gradient

angle

is

realized

by

combination

of

two

linear

gradients

and

mathematical

processing

of

signal



analyses

by

Fourier

transform


the

time

of

applying

and

the

with

of

gradients

pulses

in

Y
-

and

X
-

axes

the

voxels

are

differentiated

by

frequency

and

by

phase



third

gradient

in

Z
-

axis

defines

tom

FT

signal

recorded tom

phase differentiation

frequency diff
.

Successive recording of slices in
big volume



frequency

content of excitation RF
-

pulls is changed


to successively
excite single tom along Z
-

axes



gradient pulses in X
-

and Y
-
direction follow the frequencies



after TR interval the first slice is excited again



it is very important not to overlap the frequencies


toms are not
exactly defined

Determination of single volume
parameters

chosen Larmor
frequency excites only
one tom


changes

L

in Y
-

ax; after
that gradient pulls all
moments have again the
same frequency but differ
in phase

distinguishes
frequencies along X
-
ax

gradient is on during
signal detection

gradient
in

Z
ax

gradient u Y
ax

gradient u X
ax

Parameters of a single volume






































ph
a
se


fre
qu
enc
y



FID detected with X
-

gradient on contains frequencies and phases of


precession of protons depending on the space distribution



two
-
dimensional FT method determines the value of frequency and
phase

for each single voxel in XY plane



another FT procedure is used to calculate intensities from each voxel
and to display

it
in gray scale

Detection


artifacts



-

because of spin mobility between different voxels during detection


-

because of diffusion


-

because of covering the small signals by higher ones from undesired
structures


-

because of to weak signal or undistinguishable signal in the whole
volume of interest

help
:


suppression of signals from structures not desired (water or fat)


addition of paramagnetic ions


signal detection in intervals of periodic flow or by special pulls
sequences


Contrast by saturation


IR

method



-

time TI is T
1
ln 2 for T
1
hydrogen
in fat or water



detected are only nuclei in another
tissue



SE method



selective saturation pulls has
frequency spectra
in

resonance
with

longitudinal magnetization of fat



applied before standard pulls
sequence courses the disappearance
of fat magnetization



phase gradient rules out fat
transversal magnetization



imaging sequence does not see fat


MRI angiography



angiography


imaging of blood flow



MRI detects flow
-

intensity


proportional to flow speed



1.
excitation pulls

and
detection
pulls

have different frequencies


two
different slices along Z
-
ax


with
correct TE sees the same blood volume



2.
bipolar gradients



do not detect
static protons


enhances signal from
the ones that flow in direction of
gradient



3.
contrast agents



decreases T
1


in blood


the signal from
surrounding tissue, can be saturated

Parts of imaging system


B
0
field is oriented along the
patients bed


main axis


B
1

field is in transversal plane


RF field coil for excitation is
also the detection coil


it emits and detects certain
white interval of frequencies


detector coils have different
shapes


field shape


three systems of coils build up
the gradients of magnetic field
B
0

in direction X,Y and Z axis

vacuum

liquid helium

liquid nitrogen

housing

superconducted coil
s


Three main gradients

Meaning of magnetic field gradient


gradient in Z
-
axis

-

on while the initial RF
-

pulls is applied;
determines tom in which spins are excited


toms width is determined by steepness of gradient and by frequency
content of RF
-
pulls



gradient in X
-
axis

-

on during the time of detection of relaxation
signal; therefore relaxation frequency is function of x coordinate


gradient in Y
-
axis

-

regularly on and off between two RF
-
pulses;
it determines phase distribution and resolution in XY
-
plane; 128, 256,
512; meaning 360/256 = 1,4
o

phase shift


typical voxel is 2 mm thick, and by matrices of 512
2

has the area of
1mm
2


for B
0

of 1 T and Y
-

gradient of 0,15 mT/cm frequency resolution is
190 Hz

Characteristics and advantages


image



distribution of hydrogen nuclei density



contrast



enhanced by differences in T
1

or in T
2



resolution



determined by magnetic field gradient


bones are “transparent”


the structures inside are easily
seen


dynamics of processes can be investigated


fMRI


follow the activation of certain centers in the brain
during different activities



Risk factors


alternating magnetic fields

induce electric currents
of ions in tissue


t
o weak to course the damage or
local heating


static magnetic field

has so far coursed no damage


method is noninvasive


method must not be applied on patients with metal
implanters (pacemaker,
artificial limb
)



Spin
-
Echo


S =
k

r

(1
-
exp(
-
TR/T
1
)) exp(
-
TE/T
2
)

Inversion Recovery (180
-
90)

S =
k

r

(1
-
2exp(
-
TI/T
1
)+exp(
-
TR/T
1
))

Inversion Recovery (180
-
90
-
180)

S =
k

r

(1
-
2exp(
-
TI/T
1
)+exp(
-
TR/T
1
)) exp(
-
TE/T
2
)

Gradient Recalled Echo


S =
k

r

(1
-
exp(
-
TR/T
1
)) Sin
q

exp(
-
TE/T
2
*) / (1
-
Cos
q

exp(
-
TR/T
1
))


Spin eho imaging

























Inversion recovery


Gradient Recalled Echo Imaging



Contrast agents


Paramagnetic ions that can not
diffuse through membrane


a) increase the local magnetic
field


b) are inert to the biological
tissues