Biomedical Engineering Reference
In-Depth Information
1
T2
t=T2
T2*
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
100
200
300
400
500
Time (ms)
FIGURE 16.37
Plots of the magnetization vector component in the
x
0
-
y
0
for a decay time
T
2
versus time for a
p
*
2
¼
5 ms.
/2 rotation for gray matter and
T
z'
z'
z'
y'
y'
y'
t
1
> t
0
t
2
> t
1
t
3
> t
2
x'
x'
x'
(a)
(b)
(c)
FIGURE 16.38
Time sequence of dephasing plots for spin-spin decay with time constant
T
2
after p/2 rotation
pulse occurring at
t
¼
t
p
.
T
2
is sensitive to molecular interactions and inhomogeneities in the applied field
B
0
. A more realistic definition of this relaxation constant includes the effects of varia-
tions
D
B
0
:
1
T
T
2
þ
gD
B
1
0
2
¼
ð
16
:
66
Þ
2
Through a comparison of Figures 16.36 and 16.37, it is evident that the
T
2
constants are
*
2
much shorter than
T
1
and
T
is the shortest of all. Typical values for relaxation times can
be found in Table 16.3.
