The Basics of MRI
Current MRI technology displays images as multiple sets of
gray tone images. Visualization and interpretation of the multiparameter
images may be optimized by assigned color tissue segmentation.
Researcher H. Keith Brown, Ph.D. has developed technology
that creates color composite images that indicate the unique physical and
chemical properties of the human tissues represented by those images.
NMR imaging is a powerful
technique to obtain high resolution
images of comparable quality with
CT scans without the disadvantages
of possible radiation damage.
The important aspects which determine the
resolution and contrast in the final image are the
bandwidth of the rf-signal which causes. The
resonance absorption (excitation of spin-up
protons to spin-down protons), the relaxation
time scale for establishing the equilibrium value,
and the field gradient G in the external magnetic
field Bj.
This loss process has an oscillatory exponential behavior:
The actual T2 relaxation is reduced to a
true relaxation time T2* by field inhomogeneities
and field gradients.
Typically T1 > T2 but the relaxation times typically
depend of the particular kind of body tissue (influence of
difference in proton density due to differences in molecular
structure of body tissues)
MRI techniques use the differences in
relaxation time to highlight different
tissue materials and to obtain optimum
contrast and resolution!
Different techniques are used for pulse sequences for the rf-signals.
The rf-signal typically has a certain bandwidth around the Larmor
frequency for the material to be observed. For medical MRI this is
typically hydrogen.
Two pulse sequence techniques are typically used; the saturation
recovery sequence, SRS and the spin-echo sequence, SES.
The SRS is comprised of a series of 90° pulses separated by a period
of time (time of repetition TD) Each applied 90° rf-pulse rotates the
magnetization from z-direction into the xy-plane, an antenna is used to
pick up the signal (FID) induced by the change of magnetization. The
oscillating (0) FID signal decays following the time constant ti before
the next pulse occurs.
Before complete relaxation has occurred (relaxation time T1 a second
90° pulse follows.
Saturation recovery sequence
The free induction signal
(which contains many frequencies)
is converted into absorption mode
signal by Fourier transformation, it
has a Lorentzian form:
In the SES the 90° pulse is followed by an
additional 180° pulse at a time TE/2 (TE  echo
time) to refocus the Mxy magnetization before the
next 90° pulse occurs. This causes an additional
echo signal.
The signal for the SES image is
described as a function of pulse
repetition time TD and echo time TE:
This equation allows to make the choice of scanning parameters TD
and TE to emphasize the differences for T1 and T2 in different tissue
materials. TD emphasizes the weighting of T1 and TE emphasizes the
weighting of T2.
The right choice of pulse repetition TD and echo-time TE allows to
emphasize the T1 relaxation time characteristics for different body
tissues. Short TD emphasizes tissues with short relaxation times T1
like fat and blood, short TE minimizes T2 decay effects. Long
repetition times TD emphasize tissues with long relaxation times T1
like cerebral tissues.
The relative intensity in the NMR signal for different body tissues
can be calculated as a function of the relaxation times T1 and T2
for different choices of retardation time TD and echo-time TE.
The relative intensity of a signal for body tissue i is:
The contrast is determined by the difference in the
relative signal intensity:
TD
TE
ICSF
Igm
I
500ms
10ms
0.386
0.199
0.209
EXAMPLE:
Compare the intensity of the MRI signal
for a magnetic field strength of B0=1.5 T for cerebrospinal
fluid (CSF) and gray matter (gm) and calculate the contrast
in the MRI image.
For a magnetic field strength of B=1.5 T the relaxation
times for cerebrospinal fluid and for gray matter are:
T1(CSF) = 2400 ms, T2(CSF) = 160 ms
T1(gray matter) = 900 ms, T2(fat) = 100 ms
Nuclear Magnetic Resonance Image Acquisition
Most of the MRI imaging methods are based on the fact that
the resonant frequency is proportional to the field strength.
Thus a small field gradient G = B/z is added along the axis of field
B0, z which causes the resonance frequency 0 to change with position z:
If the resulting FID signal is Fourier transformed to obtain the frequency
distribution, the frequency axis would be equivalent to the z-displacement.
A field gradient Gss (slice-selection gradient) can be used to
localize the MR excitation to a region within the body.
If the rf-pulse has only a small bandwidth (ss  1-2 kHz),
only spins in a thin slice resonating at frequencies within that
bandwidth would be excited (selective excitation).
Each position zi corresponds
to a resonance frequency 0
The choice of field gradient
and band width of the rf-pulse
determines the slice thickness:
A fixed gradient Gss allows to modify the slice thickness by
changing the band width of the rf-signal. Typically, however, the band width
is fixed and the gradient varies.
If an NMR facility has a field of B=2 Tesla and a
gradient of G=0.01xB (T/cm)
If the field is directed along the length axis of the
head
We can calculate the NMR Larmor frequency
as a function of the position along that axis…
0= g.B + g.z.G
where g= 42.58 MHz/T
So the equation becomes…
0= 85.16 + (0.85 x z)
For z = 1.0 cm
0= 86.0 MHz
For z = 5.0 cm
0= 89.4 MHz
For z = 10.0 cm
0= 93.7 MHz
4.7 Tesla/33cm SISCO IMAGING SYSTEM
This Scanner has a magnetic field of 4.7T and 200 MHz resonance
frequency for protons. The shielded gradient coils and Oxford gradient
power supply are able to produce a gradient field of 6.5 G/cm. The diameter
of the space in the magnet bore available to users is 22 cm. The usual size
of the objects for MR imaging is 14 x 14 x 14 cm.
Siemens 3 Tesla Magnetom Allega MR Headscanner
The Siemens 3T Allegra is a state-of-the-art system designed
especially for neurological and cognitive fMRI studies.
The Allegra system provides a gradient strength of 40 mT/m and a
slew rate 400 T/m/sec. It has excellent linearity across a 22 cm FOV. It will
facilitate simultaneous optical and MR or eye-tracking and MR recording.
Varian 600 MHz Wide-Bore Spectrometer
The Varian 600 MHz wide-bore
system will be equipped for both high
resolution
NMR
spectroscopy
and
microimaging. With gradients in place the
clear bore of the magnet is 3.5 cm. Proton
5 mm and 10 mm probes are available for
imaging studies. In addition, there are four
receiver channels for the implementation of
multiple coils, phased arrays and parallel
acquisition schemes. High resolution and
imaging software is available, with the
identical operating system to the 4. 7 Tesla
SISCO system.