Hyperpolarized Nuclei for NMR Imaging and Spectroscopy - Lunds ...

educe the influence of B 0 inhomogeneities, which substantially can degrade
the quality of EPI images.
The trueFISP sequence (Oppelt et al. 1986) used in Paper II, and the fast
low-angle shot (FLASH) sequences (Haase et al. 1986) used in Papers III
and IV, had to be written from the start, mainly because of the nonstandard
triggering behavior imposed by the animal ventilator for hyperpolarized
gas (see section 3.2). The FLASH sequence offered various options
for segmented image acquisition (k-space data of one image could optionally
be acquired during several consecutive inhalations of hyperpolarized gas), as
well as online calculation of the gradient b-value ( s e e se c t i o n 5 .3 .2 , Eq . [ 28 ] ) .
The trueFISP sequence included the options of using a RF preexcitation
before the image acquisition, to rapidly reach a steady state of the magnetization,
and a flip-back RF pulse after the image acquisition, which returned
transverse magnetization to the longitudinal direction, thereby allowing the
generation of several consecutive images with arbitrary time spacing (see
Figure 7). This concept has been described by Scheffler et al. 2001.
28
trueFISP
RF
Read
Slice
Phase
q
2 +X
q ±X
TR loop
Figure 7. Sketch of the trueFISP sequence used in Paper II. The gradient areas indicated
by light and dark gray, respectively, have zero integrals.
The sequence parameters of the imaging sequences used in Papers I–IV
are summarized in Table 5.
q –X
q
2 +X