Biomedical Engineering Reference
In-Depth Information
Figure 1.6
Two different MR pulse sequences
for measuring T
2
. (a) A spin echo sequence
consists of two radiofrequency (RF) pulses: a
90 °
x
phase, and a 180 °
y
phase, separated by
a delay
τ
. The echo signal appears at time 2
τ
.
T
2
is measured by obtaining the echo signal
from successive cycles using incremental
values of
τ
. The recycle delay, d
1
, is typically
1-3 s; (b) A CPMG sequence allows for much
faster T
2
measurements because multiple
echos are acquired in rapid succession by a
series of 180 °
y
phase RF pulses and signal
acquisitions. T
2
measurements acquired with
a CPMG sequence avoids diffusion artifacts
because of the short time over which the
measurement occurs.
those found in high- fi eld superconducting magnets or large nonportable perma-
nent magnets.
More sophisticated pulse sequences allow for the measurement of T
2
relaxation
in the presence of a relatively inhomogeneous magnetic fi eld. To do this, the
contribution of magnetic fi eld inhomogeneities to signal decay must be removed,
which can be achieved using “spin echoes”. The phenomenon and use of spin
echos were discovered early during the development of magnetic resonance by
Erwin Hahn, and later refi ned for rapid T
2
measurements by H.Y. Carr, E.M.
P urcell, S. M eiboom and D. G ill to yield the so - called CPMG sequence [50 - 52] . A
spin echo sequence is composed of two pulses; the fi rst pulse rotates the spins
90 ° , and the second 180 ° . After the fi rst pulse, the spins oscillate in the transverse
plane and begin to dephase, as shown in Figure 1.5. After time
, the fastest oscil-
lating spins are on the leading edge of the “fan”, while the slowest oscillating spins
are at the lagging edge of the fan. A 180 ° pulse fl ips the spins in the transverse
plane to the opposite side of the
z
-axis, switching the relative positions of the
fastest and slowest spins. After time 2
τ
, the fastest spins catch up with the slowest,
thus refocusing the magnetization vectors and the observed signal (Figure 1.6a).
In order to obtain a measure of the decay constant that results from microscopic
magnetic fi eld fl uctuations, or T
2
, a series of spin echo sequences are run with
incremented delay times,
τ
τ
(typically milliseconds). Sequential scans must be
