Biomedical Engineering Reference
In-Depth Information
Figure 1.2
A series of schematics explaining
nuclear magnetic resonance detection
according to classical physics. (a) When
nuclear spins are in the presence of a
magnetic fi eld, a bulk magnetization vector is
present from the majority of the spins
aligning with the fi eld direction; (b) This
magnetization can be detected by subjecting
the sample to a radiofrequency pulse that tips
the bulk magnetization vector into the x-y
plane; (c) As this vector oscillates in
transverse plan it decays according to
T
2
*
relaxation. This decay is called “spin
dephasing”, which is depicted by the vector
fanning out; (d) Detection of the oscillating
magnetic vector yields an oscillating signal
that decays with a time constant of
T
2
*
.
lation which is detected by the RF detection coil, which is typically the same coil
as that used to generate the RF pulse.
Once generated, the magnitude of the oscillating signal decays according to
spin-spin relaxation, which occurs when a given ensemble of oscillating spins lose
coherence, or synchronicity. This can be depicted by an oscillating vector “fanning
out” over time (Figure 1.2c). A loss of spin coherence leads to a decay in the oscil-
lating signal. A measure of the magnitude of this decay is the time constant
T
2
*
(Figure 1.2 d).
A loss of spin coherence occurs when the spins within an ensemble experience
variations in their Lamour frequencies,
o
, during oscillation in the transverse
plane. Such variations are caused not only by macroscopic inhomogeneities but
also by microscopic fl uctuations in the local
B
o
fi eld. The contribution of macro-
scopic and microscopic
B
o
variations to
T
2
*
relaxation can be differentiated by
specifi c detection sequences, as will be discussed below.
In both USPIO and SSPIO samples, microscopic variations in
B
o
are dominated
by the agglomeration state of the particles. Measuring this contribution to
T
2
*
is
critical for detecting the agglomeration state of particles. Because of their magnetic
properties, superparamagnetic particles create local magnetic fi elds when in the
presence of a
B
o
fi eld; this in turn creates a local fi eld “ gradient ” , or a spatially
ω
