Nanoparticles for Bioimaging: Analytical Characterization and Measurements - Nanotechnology for Biomedical Imaging and Diagnostics - page 163

relaxation, inhomogeneities in the main magnetic field also contribute to the decay
of transverse magnetization. The time constant for the combination of these two
factors is called T 2 * and is given by

11 1

=+ inhomo

TTT *

2 2 2

To generate an image, the MRI scanner plays out an imaging pulse sequence.
This is a train of Rf pulses, magnetic field gradients, and signal acquisitions required
to generate the proton signal, record the spatially encoded signals, and reconstruct a
set of 2D or 3D images. To generate the proton signal, the Rf coil produces a B 1
field that rotates the magnetization in the body from the z -axis into the x-y plane. To
spatially localize the proton signal, the magnetic field gradients are turned on during
the B 1 field, after the B 1 field, or during signal acquisition. Turning the gradients on
and off requires a certain period of time, during which the proton signal decays due
to T 2 * relaxation. To generate a measurable proton signal for the MRI scanner to
record, an echo must be formed using either a spin echo pulse sequence or a gradient
echo sequence.
a spin echo is formed by applying a 180° pulse with the Rf coil after the 90°
pulse. The 180° pulse reverses the proton signals within the transverse plane, refo-
cusing the signal loss due to the inhomogeneities in the B 0 field. as a result, an echo
signal is formed at the echo time (TE), which is two times longer than the time bet-
ween the 90° and 180° pulses. Under certain circumstances, applying a 180° pulse is
not feasible, and a gradient echo pulse sequence is required. The gradient echo is
formed by applying a negative gradient after the 90° pulse to dephase the proton
signal in the x-y plane, followed by a positive gradient to rephase the signal and form
the echo. Typically, a gradient echo can be formed with a shorter TE than is possible
with a spin echo sequence, and they are often employed with fast imaging pulse
sequences. The B 0 field inhomogeneities are not refocused due to the lack of a 180°
pulse, causing gradient echo images to be T 2 * weighted.
In order to collect all the data to reconstruct an image, a single echo is not
sufficient. all pixels in the image need to be spatially encoded in three dimensions,
which requires the pulse sequence to be repeated many times. However, if the pulse
sequence is repeated too fast, the signal will be very low due to inadequate T 1 relax-
ation back along the z -axis. The time between successive applications of 90° pulses
is referred to as the repetition time (TR).
as stated earlier, the relaxation mechanisms are often employed to generate MRI
contrast. T 1 -weighted images are designed to accentuate the differences in T 1 relaxa-
tion times between various tissues. T 1 -weighted images can be produced with a pulse
sequence with a short TR and short TE. The short TE minimizes the effects of varia-
tions in T 2 values from different tissues. However, the short TR alters the signal inten-
sity of tissues with different T 1 values. Tissues with short T 1 times can fully relax
back to equilibrium within the short TR and appear bright on the final image. Tissues
with long T 1 values cannot fully relax before the next 90° pulse and appear dark.