Biomedical Engineering Reference
In-Depth Information
Figure 1.5
NMR signal acquisition from two samples that
have (a) a long T
2
value; and (b) a short T
2
value. These two
samples could be a sample of (a) disperse and
(b) agglomerated MRSW particles. When the spin dephasing
is more pronounced (b), the NMR signal decays more
rapidly, corresponding to a lower T
2
value.
The simple 90 ° pulse-detect measurement can be used to measure
T
2
*
. As
discussed above, magnetic fi eld nonuniformities lead to nuclear spin dephasing.
The amplitude of spin dephasing can be measured by determining the decay
constant of the oscillating signal in the transverse plane,
T
2
*
, as shown in Figure
1.2d. For magnets used in most bench-top and portable relaxometers,
T
2
*
is domi-
nated by variations in the applied magnetic fi eld across the sample, which are also
known as magnetic fi eld inhomogeneities. The inhomogeneities of a magnetic
fi eld can be reported in terms of ppm, as calculated by the equation:
1
(
) =
Δω
ppm
(
)
(1.2)
0
*
ω
T
02
where
o
(
ppm
) is a measure of the inhomogeneity of the magnetic fi eld, or rela-
tive change in homogeneity across a specifi c volume,
Δ
ω
o
is the Lamour frequency
(in MHz), and
T
2
*
is the exponential decay rate in seconds of the magnetic reso-
nance signal after a single 90 ° pulse-detect sequence. If one wishes to use
T
2
*
as
a means to measure systems with long effective T
2
values, such as MRSw solu-
tions, then magnets with high homogeneities (
ω
<
0.1 ppm) must be used, such as
