This family of
sequences uses a
balanced gradient waveform. This waveform will act on any stationary
spin on resonance between 2 consecutive RF pulses and return it to the same
phase it had before the gradients were applied.
A balanced sequence starts out with a
RF pulse of 90° or less and the spins in the steady state. Prior to the next TR in the
slice encoding, the
phase encoding and the
frequency encoding direction, gradients are balanced so their net value is zero. Now the spins are prepared to accept the next
RF pulse, and their corresponding signal can become part of the new
transverse magnetization. If the balanced gradients maintain the longitudinal and
transverse magnetization, the result is that both
T1 and
T2 contrast
are represented in the image.
This
pulse sequence produces images with increased signal from fluid (like T2 weighted
sequences), along with retaining
T1 weighted tissue
contrast. Balanced
sequences are particularly useful in
cardiac MRI. Because this form of sequence is extremely dependent on
field homogeneity, it is essential to run a
shimming prior the acquisition.
Usually the gray and white matter contrast is poor, making this type of sequence unsuited for
brain MRI. Modifications like
ramping up and down the flip angles can increase
signal to noise ratio and contrast of
brain tissues (suggested under the name COSMIC - Coherent Oscillatory State acquisition for the Manipulation of Image Contrast).
These
sequences include e.g.
Balanced Fast Field Echo (bFFE),
Balanced Turbo Field Echo (
bTFE),
Fast Imaging with Steady Precession (
TrueFISP, sometimes short TRUFI),
Completely Balanced Steady State (CBASS) and
Balanced SARGE (BASG).
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] and 2 definitions [
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