Synopsis
We proposed a cardiac and respiratory self-gated, 4D multi-phase steady-state imaging with contrast (MUSIC) technique for detailed evaluation of cardiovascular anatomies. A rotating cartesian k-space sampling pattern was designed that integrates frequently sampled k-space centerline as self-gating signal and allows retrospective data-binning based on derived motion signal. Phantom and in-vivo results on 7 clinical indicated pediatric CHD patients show that the proposed self-gated MUSIC could potentially eliminates the need of external physiological signal for motion gating, has increased scan efficiency while maintaining or exceeding the image quality of the original MUSIC. Purpose
Conventional
contrast-enhanced MRA is generally performed without cardiac gating due to the
time constraint imposed by breath-hold and the first passage of the gadolinium
based agent and therefore provides limited delineation on blood vessels
that are subject to cardiac motion. To address this issue, we recently proposed
the MUSIC
1 approach that acquires cardiac-phase-resolved 4D images without
breath-holds during the steady-state distribution of ferumoxytol, an intra-vascular
contrast agent. Using ECG and airway pressure signal for motion gating, the MUSIC has been routinely applied in our institution on pediatric patients with congenital heart disease
(CHD) and provides exquisitely detailed information on both intra- and
extra-cardiac vascular anatomy
1. However, the airway pressure signal is only available
when general anesthesia (GA) is performed and the ECG signal is not always
reliable especially in high field strength due to interference from the RF pulses and gradients. To overcome these
limitations, we propose the self-gated MUSIC (sgMUSIC) where a retrospective
motion self-gating strategy is used to eliminate the need for external
physiological signal and make the MUSIC potentially available on general patient population. In this study, we tested the sgMUSIC on pediatric CHD
patients underwent GA so that the derived motion signal can be validated
against the recorded physiological signal and the sgMUSIC images compared with original MUSIC.
Methods
(a)Data Acquisition: A 3D GRE sequence is modified using ROtating
Cartesian K-space (ROCK) where kykz views of
3D Cartesian grid were reordered using quasi-spiral pattern with each arm
starts from the outer k-space and ends at the center(Fig.1a). The arms are rotated using segmented
golden ratio
2 in which the entire k-space is divided into N=7
angular segments, each segment is sampled using golden ratio and different
segments are sampled in a pseudo-random order. The extra degree of randomness
ensures more uniform k-space sampling even after data sorting.
(b)Data Sorting: Cardiac and respiratory self-gating
signal was derived from the repeatedly sampled centerline (kykz=0) using
cross-correlation
3 and further separated by band-pass filter. The
acquired data is then retrospectively soft-gated
4 based on the
derived respiratory self-gating signal and sorted into
different cardiac phase based on the derived cardiac self-gating signal.
(c)Image Reconstruction: 3D images were reconstructed independently for
each cardiac phase using ESPIRiT, a compressed sensing and parallel imaging
combined approach
5. The data sorting and image reconstruction
algorithms were implemented in our GPU-accelerated image reconstruction
framework
5 so that 4D images are available at the scanner console 5-8
minutes after the scan.
(d)Phantom Experiment: Static phantom experiment was
performed to validate the ROCK pattern in different settings: 1) direction of
quasi-spiral arms (inward vs. outward); 2) rotating strategy (segmented golden
ratio vs. golden angle). Data was sorted using physiological signal recorded
from previous patient scans.
(e)In-vivo Experiment: 7 clinically indicated pediatric CHD patients
(aged 1 month to 8 years) underwent GA with controlled mechanical ventilation
were included in this study. 4mg-Fe/kg ferumoxytol was administrated. The sgMUSIC was performed after standard clinical protocol, which
includes the original MUSIC with matching resolution. Sequence parameters
included: 0.9/2.9ms, FA=25°, 500x300x150,
0.8-1.0mm
3 isotropic resolution, 9 phases, TA=5±1.5min. The self-gated
images were reconstructed without using ECG or ventilator pressure signal.
(f)Image Evaluation: Subjective image quality scores were visually
assessed on original MUSIC and sgMUSIC in 3 different anatomical regions by a
cardiologist using criteria listed in Table 1.
Results
The time
series of SG projections from static phantom (Fig.1b) shows that the inward
quasi-spiral arm could significantly reduce the eddy-current interference in
self-gating signal because an adjacent k-space line is acquired in previous TR
instead of a distant one in the case of outward arm. The k-space patterns (Fig.1c)
show the segmented golden ratio provides more uniform k-space sampling even
after data sorting.
Fig.2 shows the in-vivo result from a 3-month-old
female where the derived SG signal corresponds well with the recorded
physiological signal and the sgMUSIC
images using self-gating signal have similar quality with the ones using recorded
physiological signal. When compared with the original MUSIC acquired using with
4X GRAPPA, the sgMUSIC acquisition took less time due to the use of variable
density sampling pattern and non-linear image reconstruction algorithm.
Scores in Table.2 shows that the
sgMUSIC maintains or exceeding the image quality of original MUSIC, with significant
improvement in 1 of 3 regions. (P<0.05)
The volumetric images allows reformatting into
arbitrary planes for visualization of detailed vascular structures (Fig.3).
Conclusion
The proposed
cardiac and respiratory self-gated MUSIC eliminates the need of external
physiological signal for motion gating, has slightly increased scan efficiency,
while maintaining or exceeding the image quality of the original MUSIC.
Acknowledgements
NIH 1R01HL127153References
1. Han, F., Rapacchi, S., Khan,
S., Ayad, I., Salusky, I., Gabriel, S., Plotnik, A., Finn, J. P. and Hu, P.
(2015), Four-dimensional, multiphase, steady-state imaging with contrast
enhancement (MUSIC) in the heart: A feasibility study in children. Magn Reson
Med, 74: 1042–1049. doi: 10.1002/mrm.25491
2. Han, F., Zhou, Z., Rapacchi,
S., Nguyen, K.-L., Finn, J. P. and Hu, P. (2015), Segmented golden ratio radial
reordering with variable temporal resolution for dynamic cardiac MRI. Magn
Reson Med. doi: 10.1002/mrm.25861
3. Hu, P., Hong, S., Moghari, M.
H., Goddu, B., Goepfert, L., Kissinger, K. V., Hauser, T. H., Manning, W. J.
and Nezafat, R. (2011), Motion correction using coil arrays (MOCCA) for
free-breathing cardiac cine MRI. Magn Reson Med, 66: 467–475.
doi: 10.1002/mrm.22854
4. Cheng, J. Y., Zhang, T.,
Ruangwattanapaisarn, N., Alley, M. T., Uecker, M., Pauly, J. M., Lustig, M. and
Vasanawala, S. S. (2015), Free-breathing pediatric MRI with nonrigid motion
correction and acceleration. J. Magn. Reson. Imaging, 42: 407–420.
doi: 10.1002/jmri.24785
5. Uecker, M., Lai, P., Murphy,
M. J., Virtue, P., Elad, M., Pauly, J. M., Vasanawala, S. S. and Lustig, M.
(2014), ESPIRiT—an eigenvalue approach to autocalibrating parallel MRI: Where
SENSE meets GRAPPA. Magn Reson Med, 71: 990–1001.
doi: 10.1002/mrm.24751
6. Han, F., Zhou, Z., Sung, K., Finn, J. P. and Hu, P. A Low-Cost,
Clinical Friendly Non-Linear Parallelized MR Image Reconstruction Framework:
Initial Proof of Concept on Pediatric Contrast Enhanced MRA Application.
Proceedings ISMRM 23rd Scientific Sessions, 2015, Toronto.