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Chapter 2. Imaging Experiments
gradient echo imaging techniques. The T2* effects of solid materials are very short (<100
µs), so special experimental techniques need to be used to obtain images from such
samples.
ti, Inversion Time
ti is an important contrast parameter in imaging. The inversion time contrast between
different components in the sample can be enhanced by performing a ti inversion recovery
sequence (ti ir).
The ti ir imaging sequence involves applying a 180° inversion pulse and a delay, ti, at
the beginning of a conventional imaging sequence. The inversion pulse flips the
magnetization to the z direction. The spins return to equilibrium, exponentially, with a time
constant inversion time, as illustrated in Figure 16.
180°
Exponential recovery
of the longitudinal
magnetization
t
0
tnull=0.69T1
Figure 16. Equilibrium Magnetization
Sampling the magnetization after the inversion time delay results in varying signal
intensities, depending on the ti of the spins. Notice that the signal is negative at short
inversion times and gradually becomes positive at longer inversion times. When
ti=(0.69.T1), the signal is zero, as shown in Figure 16. If the sign of the intensities is
to be retained in the images, it is necessary to phase correct the images instead of taking the
absolute value mode. ti ir imaging experiments can be inefficient because of the long
recycle time required to allow the spins to return to equilibrium.
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VNMR 6.1C User Guide: Imaging
01-999163-00 A0800