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
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