Biomedical Engineering Reference
In-Depth Information
isochromats can be explained as follows:
δ/δ t = ω 0 = γ ( B 0 + xG x + yG y + zG z )
(3.1)
where γ is gyromagnetic ratio, B 0 is magnetic field strength, x , y , z are position
vectors of a spin isochromat, and G is the applied gradient field vector. This
vector has components viz. G x , G y , and G z along the x , y , and z directions, re-
spectively. Inside the vessels, slight variations in magnetic field make the spin
isochromats precess at different speeds. The spin isochromat precessing in dif-
ferent directions can be represented as different points on a precession circle.
Simultaneously, they lose phase coherence in this process that results in loss of
MR signal. However, two methods are commonly used to recover MR signal loss
viz. refocusing 180 RF pulse and gradient recalled echo (GRE). Spin isochro-
mat magnetization is inverted by applying excitation time less than TE i.e. T =
TE/2. Refocusing 180 RF pulse in spin echo (SE) sequence sent after time T =
TE/2 inverts isochromat magnetization. The refocusing 180 RF pulse creates
a head start. So, it refocuses the slow moving spins to reach the x axis as
shown in Fig. 3.1. This whole process is known as dephasing or defocusing.
Figure 3.1: RF pulse is shown to flip the magnetization out of its orientation
along the z -axis by a variable flip angle θ , magnetetization vector starts to pre-
cess, describing a isochromat circle in the x,y plane (Figure A) for spin-echo
imaging at flip angle 90 . After 90 pulse, the isochromats precess with differ-
ent Larmor frequencies due to experience of different magnetic fields (shown
with arrows). A typical spin-echo pulse is shown with RF pulse flipping mag-
netization 180 and back to create an echo (middle row). In GRE sequence,
inverted readout gradient is used to invert precession and result refocusing
pulse.
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