Transcript Document
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 B0 field - gradient of the field in desired
direction
we measure resonance/relaxation of hydrogen nuclei in
water and in fat
dB
Gx
dx
dB
Gy
dy
dB
Gz
dz
in perfectly homogeneous field
all protons have the same -only one signal is measured
gradient in direction X-axis
distinguishes the Larmor
frequency of nuclei depending
on the place in the field
= (B0 +x Gx)
Signal is measured in the presence of field gradient. The 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.
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, Gf, for
frequency differentiation is realized by
combination of two linear gradients in Y i X
direction:
Gy = Gf sin q and Gx = Gf cos q
the recorded FID is treated by FT - gives the
signal distribution by frequencies and phases
G
Gx
y
Imaging
FT
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
recorded tom
phase differentiation
frequency diff.
signal
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
gradient in Z ax
chosen Larmor
frequency excites only
one tom
gradient u Y ax
changes L in Y- ax; after
that gradient pulls all
moments have again the
same frequency but differ
in phase
gradient u X ax
distinguishes
frequencies along X-ax
gradient is on during
signal detection
Parameters of a single volume
phase
frequency
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 T1ln 2 for T1 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 T1
in blood – the signal from
surrounding tissue, can be saturated
Parts of imaging system
vacuum
liquid helium
liquid nitrogen
housing
superconducted coils
B0 field is oriented along the
patients bed – main axis
B1 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
B0 in direction X,Y and Z axis
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 = 1,4o phase shift
typical voxel is 2 mm thick, and by matrices of 5122 has the area of
1mm2
for B0 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 T1 or in T2
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 – to 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 )) exp(-TE/T )
Inversion Recovery (180-90)
S = k r (1-2exp(-TI/T )+exp(-TR/T ))
Inversion Recovery (180-90-180)
S = k r (1-2exp(-TI/T )+exp(-TR/T )) exp(-TE/T )
Gradient Recalled Echo
S = k r (1-exp(-TR/T )) Sinq exp(-TE/T *) / (1 -Cosq exp(-TR/T ))
1
1
2
1
1
1
1
2
2
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