Christine Law1 and Gary Glover1
1Stanford University, Stanford, CA, United States
Synopsis
We optimized
a spiral sequence for dual-echo cardiac gating applied to cervical spine fMRI. Dual-echo obviates dependency on variable
initial longitudinal magnetization that would, otherwise, require T1 correction. Spiral acquisition allows short echo time and
readout duration which maximizes SNR and BOLD contrast. Our technique improves tSNR and fMRI activation
when compared with ungated sequence.
Introduction
Functional
Magnetic Resonance Imaging (fMRI) of the cervical spine is challenging partly
due to cardiac-related physiological noise.
Cardiac influence causes vascular pulsation noise that may obscure
spinal cord BOLD (blood-oxygen-level-dependent) response since main arteries
and veins run along the cord and adjacent to grey matter.1 Also, cardiac pulsation induces pulsatile
movement in cerebrospinal fluid causing non-rigid spinal cord motion and signal
variation.2 It is possible to
minimize cardiac-induced noise by cardiac-gated acquisition; each image volume
being acquired synchronized to heart rate instead of a fixed repetition time TR. The first echo, of a dual-echo acquisition, can
eliminate variable initial longitudinal magnetization in cardiac gating that would,
otherwise, require correction by precise knowledge of pixel-wise longitudinal
relaxation time T1 of imaged anatomy.3,4 Here, we improve this dual-echo technique to
detect activation in cervical spine by optimizing echo times TE and minimizing readout time by means of a
custom spiral sequence5. Using the proposed techniques, we increase temporal-SNR (tSNR) and can detect
more activation in cervical spine than a non-cardiac gating technique.Methods
In
a cardiac-gated dual-echo sequence (Fig.1), amount of initial transverse
magnetization S0 is different for each image volume because
each is acquired synchronized to heart rate (TR is not fixed, Fig.2).
S1(t,i)|t=TE1=S0(ti)e−TE1R∗2(ti)i=1...N(1)
S2(t,i)|t=TE2=S0(ti)e−TE2R∗2(ti)i=1...N(2)
S2(i)/S1(i)=e−△TE1R∗2(ti)i=1...N(3)
Equations
(1) and (2) represent readout magnetization signal intensity at two TE within the same cardiac acquisition period (collected for N cardiac periods). Unknown initial signal intensity S0(ti) can be
eliminated by dividing the second echo readout by the first (Eqn. 3), which results
in a dependence only on R∗2.
Experiment
To
demonstrate, we gathered cervical spinal data using a cardiac-gated dual-echo
spiral-out sequence on a healthy volunteer.
Vertebral levels between C4 and C8 were imaged (Fig. 3). Scanning parameters are: TE1=5ms, TE2=35ms, matrix size=64 (two shots), FOV = 16cm, slice
thickness = 4mm, gap=1mm, #slices=14. A
custom-built MRI-compatible pneumatically driven finger-moving device was engaged
for passive motor task: velcro straps secure volunteer’s hands to the pneumatic
device which moves each finger of each hand individually and randomly during on-blocks. Two 128-volume scans were collected: one with
cardiac gate every two heart beats, and one with fixed TR=2s. Passive task was a block design alternating
between on and off blocks of 15s each. A
pulse oximeter was attached to the volunteer’s toe for gating.Analysis
For
cardiac-gated data, ratio images were first calculated according to Eqn.3 (with
masking to avoid division by zero), then the time-series was resampled to a
fixed interval of 2s. To determine
activation, time-series of each voxel was correlated with sine and cosine functions
at the fundamental task frequency. (Higher-order harmonics were not modeled
because they were strongly attenuated by hemodynamic filtering and because of
relatively short block periods.) tSNR
images were calculated for both cardiac-gated and ungated data for comparison.Results
Figure
4 shows activation in three slices: from ungated and cardiac-gated data, each
overlaid on anatomical images. Activation
was reliably detected using our optimized cardiac gating method but not with
the ungated method. Figure 5a shows eight slices (after dividing second echo by
the first) and their corresponding tSNR images (Fig.5b). Cardiac-gating significantly improves tSNR
over ungated acquisition.Discussion
The basic premise of dual-echo cardiac gating is: division
of two echos in order to eliminate unknown initial magnetization S0; thus, avoidance of imperfect
T1 effect correction. We have optimized this
technique, beyond earlier attempts,3,4 by employing a spiral sequence
instead of EPI. Since the spiral begins traversal
of k-space starting from its center, TE1 can be set to a much shorter value than
that of EPI (5ms vs 21ms). Shorter TE1 and TE2 maximize SNR and minimize
division error, while keeping △TE equal to T∗2 is optimal for activation detection. We set echo times for optimal BOLD
contrast at 3T (TE1=5ms, TE2=35ms, △TE=30ms). Because spinal cord is highly inhomogeneous, spiral's relatively short readout duration (when compared with EPI) minimizes signal
dropout and distortion.5
Cardiac-gating
acquisition is inherently less time-efficient, than acquisition at a fixed TR,
simply because some time is spent waiting for the next heart beat. These relatively longer periods between each
acquisition, on the other hand, allow longitudinal
signal to recover more; hence, increasing SNR.
We also showed that there is further advantage, due to elimination of
motion-related signal loss in these small cervical structures, resulting in
increased tSNR; thus, improving image quality.
Acknowledgements
GE Healthcare and NIH P41-EB0015891References
1.
Cohen-Adad J, Gauthier CJ, Brooks JCW, Slessarev
M, Han J, Fisher JA, Rossignol S, Hoge D. BOLD signal responses to controlled
hypercapnia in human spinal cord. NeuroImage 2010; 50:1074–1084.
2.
O’Connell JEA. The Vascular Factor In
Intracranial Pressure and the Maintenance of the Cerebrospinal Fluid
Circulation. Brain 2010; 66:204–228.
3.
Zhang WT, Mainero C, Kumar A, Wiggins CJ, Benner
T, Purdon PL, Bolar DS, Kwong KK, Sorensen AG. Strategies for improving the
detection of fMRI activation in trigeminal pathways with cardiac gating.
Neuroimage. 2006;31(4):1506-12.
4.
Beissner F, Baudrexel S, Volz S, Deichmann R,
Dual-echo EPI for non-equilibrium fMRI - implications of different echo
combinations and masking procedures. Neuroimage 2010; 52(2):524-31.
5.
Glover GH. Spiral imaging in fMRI. Neuroimage
2012;62(2):706-12.