Biomedical Engineering Reference
In-Depth Information
a result of the reduced saturation dependency, short TR of 25-28 msec is used
with the minimal saturation of moving spins. It allows the spins to recover from
previous RF pulses. Longitudinal recovery occurs exponentially with a time
constant T1. Normally, repitition time T R five times the value of T1 is needed to
ensure complete relaxation. However, long TR would limit the amount of data
acquired and make 3D imaging difficult. Full relaxation can be achieved with a
short pulse delay if smaller flip angles are used. Partially relaxed steady states
are easily achieved with shorter pulse delays using the limited flip angles, the
so-called gradient-recalled acquisition in the steady state for 2D or 3D volume
acquisitions. Further reductions in saturation effects may also be realized by
using intravenous contrast agents that shorten the T1 of blood. These refer to
the gradual loss of longitudinal magnetization caused by repeated excitation RF
pulses. This leads to loss of signal-to-noise ratio during 2D acquisition in which
flowing blood has to travel within a slice or in a 3D acquisition in which the blood
travels through a thick imaging volume (or slab). In such a situation, saturation
effects may prevent the imaging of the distal portion of a vessel.
Contrast mechanism for vessel imaging is based on the differences in satu-
ration between blood and stationary tissue, rather than flow itself. This contrast
mechanism is usually dominated by “in flow” effects. TOF techniques differenti-
ate blood only when its magnetization differs from that of surrounding stationary
tissue. Longer blood stay in the imaging volume makes it more difficult to detect
the vessels. Signal loss therefore occurs whenever slowly moving blood enters
the volume of interest and reaches a new saturated steady state. Phase contrast
angiography is less susceptible to this problem of signal loss. This may be due
to saturation effects arising out of decreased TR and increased α factor.
Let us describe these factors.
3.2.6.2.1 Short TR. Short repetition times (TR) cause less recovery of lon-
gitudinal magnetization from one cycle to the next, causing gradual loss of the
M z component. This effect is less pronounced with longer repetition times.
3.2.6.2.2 Larger Flip Angle ( α ). A large flip angle causes more signal loss
due to loss of longitudinal magnetization. Therefore, for a given TR, there is
greater gradual loss of M z with a larger flip angle ( α ) than with a smaller flip
angle (see Fig. 3.24). In GRE, very short TR is selected, as a result saturation
effects pose a problem. The uses of small flip angles counteract this effect. These
saturation effects become especially important in 2D and 3D-plane flow or in
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