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








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 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 ))
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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