Healthcare and Medicine Reference
Timing diagram for gradient echo pulse sequence. (From Bernas, L., Preclinical cellular MRI
of glioma using iron oxide contrast agents, PhD thesis, Department of Medical Biophysics,
University of Western Ontario, 2009. University of Western Ontario.)
used rather than 180° RF pulses to refocus the magnetization and generate
echoes. A simple pulse sequence diagram of the GRE sequence is shown in
The signal acquired depends on PD, T1, and T2* (the last because the static
inhomogeneity effects are not eliminated as they are by SE techniques). In
GRE imaging, lip angles smaller than 90° are used, which allows for much
shorter TRs, and therefore shorter scan times than are possible using the SE
sequence, as full T1 recovery is not necessary before the next RF excitation.
After a few excitation pulses, a steady state is reached in which the decrease
in the longitudinal magnetization is exactly balanced by T1 recovery (Reiser
2008). The remaining transverse magnetization may either be destroyed
through the use of a dephasing gradient, as is the case for the spoiled gradi-
ent echo (SPGR) sequence, or be driven to steady state, as in steady-state free
precession (SSFP) sequences.
10.8.3 Balanced Steady-State Free Precession
Cellular MRI is most commonly performed using T2*-weighted GRE
sequences as they are sensitive to the presence of magnetic field-perturbing
contrast agents such as superparamagnetic iron oxide (SPIO) nanoparticles.
These sequences, however, are limited by relatively poor tissue contrast and
a potentially overwhelming “blooming artifact” caused by the SPIO, which
may prevent adequate visualization of the surrounding anatomy. T2-weighted