Chemistry Reference
In-Depth Information
Tissue magnetization
Transverse
Longitudinal
Produces
T
1
weighted image
T
2
weighted image
T
1
T
1
T
2
T
2
Time
Time
Bright tissue
Dark tissue
Dark tissue or uid
Bright tissue or uid
FIgure 1.16
comparison of T
1
and T
2
properties.
tABle 1.2
Properties of t
1
and t
2
relaxation.
T
1
is called the “spin-lattice” relaxation time
T
2
is called the “spin-spin” relaxation time
recovery of magnetisation along the longitudinal axis:
longitudinal relaxation time
Decay of magnetisation along the transverse axis: transverse
relaxation time
Measurement of the “lattice” around the spin
Measurement of the “spins” around the spin
rotational correlation time “ τr” ~ 1/T
1
Translational correlation “ τc” ~ 1/T
2
Time to rotate one radian (10-12 sec)
= (4
π) η
a
3
/ 3
kT, where a is radius, η is viscosity
Time to diffuse one diameter (10-11 sec)
= (6
π
)
η D / kT, where D is diffusion, η is viscosity
Free protons rotate rapidly and have short τ
r
This is observed as a long T
1
.
Free protons move rapidly and have short τc
.
This is observed as a
long T
2
.
Bound protons hit barriers and have long τ
r.
This translates into
a short T
1
.
Hindered protons move slowly and have long τ
c.
This translates
into a short T
2
.
T
1
and T
2
may be shortened considerably in the presence of a paramagnetic contrast agent. However, a compromise is
necessary because the shortening of T
1
would lead to an increase in signal intensity, while the shortening of T
2
would produce
broader lines with decreased intensity. This reflects a nonlinear relationship between signal intensity and concentration of the
contrast agent. At low concentrations, an increase in the concentration of the contrast agent would cause an increase in signal
intensity until the optimal concentration is reached due to effects on T
1
. Further increase in the concentration would reduce the
intensity of the signal because of the effects on T
2
. The relationship between T
1
and T
2
dictates the design of contrast agents,
which must have a relatively greater effect on T
1
than on T
2
, as well as the use of pulse sequences that emphasise changes in T
1
.
Gadolinium-based contrast agents are usually used in MrI because of their excellent paramagnetic properties and
biological tolerance. However, there are some concerns regarding the problem of toxicity. Problems are related to trans-
metallation, which is the exchange of the metal in the contrast agent with a metal ion in solution (Eq. 1.3) [28].
ML
+
M'
M'L
+
M
(1.3)
Toxicity, specificity, and relaxivity are three important critieria in the design of contrast agents. Many of these paramag-
netic metals are toxic as free ions. Thus, the design and study of contrast agents is very important in MrI because these
