Master's Thesis - Studierstube Augmented Reality Project - Graz ...

3.4 Image Contrast
(a) T1-contrast is in a manner of speaking based one the amount of
available - already relaxated - spins parallel to B0. A new RF-pulse will
rotate the whole magnetization vector, whereas for some tissue it will
be higher than for others. In other words, if at the time of a pulse,
spins are still excited, they will be rotated anti-parallel to B0. Only
these which have already relaxated will be turned back perpendicular
to the measurement coil and will be noticed afterwards. For example,
these coherences explain the secularly known behavior of water with T1
weighted images. Nearly all hydrogen nuclei of H2O are still excited at
a short T R. A new RF-pulse makes them ”invisible” or out-of-phase, so
water will appear almost black.
(b) The T2-contrast or T 2 ∗ -contrast is defined by the transversal relaxation.
Because of the different relaxation times the loss of signal will
vary for different tissues depending on the time T E an echo is enforced.
To refer to the showcase water, H2O will produce a bright signal for a
longer echo time T E as a result of many spins remaining in phase and
”visible”.
Figure 3.8: The basic contrast mechanisms for T1 or T2 respectively T 2 ∗ weighted images. For T 1 weighting one has to
consider mainly the time between the repetitions of the sequence T R and a short echo time. For T2 weighting the time
between excitation and the generated echo T E represents the major factors.
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