Samantha Mikaiel1,2, Thomas Boyd Martin1,2, Kyung Sung1,2, and Holden H Wu1,2
1Radiological Sciences, University of California, Los Angeles, Los Angeles, CA, United States, 2Biomedical Physics, University of California, Los Angeles, Los Angeles, CA, United States
Synopsis
Real-time visualization is crucial to the
success of MRI-guided minimally invasive cancer interventions. In this work we combine iSSFP with a golden-angle(GA) ordered
radial trajectory and non-Cartesian parallel imaging to create a new real-time MRI
sequence with good tissue contrast while suppressing bSSFP banding artifacts. Phantom and volunteer data were acquired and reconstructed using a combined
sliding-window SPIRiT algorithm, at different frame rates, showing the capability of the sequence to achieve real-time imaging.
These advantages of GA Radial iSSFP show its potential for improving real-time MRI-guided interventions.Introduction
Real-time visualization is crucial to the success of MRI-guided
minimally invasive cancer interventions, especially in the abdomen. The
real-time MRI sequence must be ultrafast, while maintaining high soft-tissue
contrast. RF-spoiled gradient echo (spGRE)
is able to achieve real-time imaging speeds, but the SNR and tissue contrast is
diminished. Balanced-SSFP (bSSFP) has
better T2/T1 contrast and SNR, but suffers from banding artifacts
(1). Half-Fourier
acquisition single-shot turbo spin-echo (HASTE) can provide excellent T1 or T2
contrast, but only reaches low frame rates and is challenged by SAR restrictions.
Compared to these sequences, integrated-SSFP (iSSFP)
(2) is a special case of
steady-state MRI that can suppress banding artifacts associated with bSSFP
while maintaining similar T2/T1 contrast. In this work, we combine iSSFP with a
golden-angle-ordered radial trajectory and non-Cartesian parallel imaging to
create a new real-time MRI sequence with good tissue contrast while reducing banding
artifacts.
Methods
[Real-time Golden-Angle
Radial iSSFP Sequence] To create a radial iSSFP sequence, the slice select
gradient of the bSSFP sequence was modified to achieve 2pi/voxel gradient spoiling
(Fig1). The radial readouts were
continuously acquired with golden-angle (GA) ordering(3).
[Reconstruction]
The GA radial iSSFP data was reconstructed using a combined sliding-window
SPIRiT(4) algorithm (Fig2).
The sliding window reconstruction selects N spokes (temporal footprint
N*TR) for each image frame and then continuously “slides” forward by M spokes (temporal
resolution M*TR) until all of the data is reconstructed. For SPIRiT
reconstruction, a calibration kernel is taken from a fully sampled central
region of k-space for each image frame.
[Experiment Setup]
All experiments were performed on a 3.0T MRI scanner (Prisma, Siemens) with
400-mm field-of-view, 192x192 matrix (unless noted), and 5-mm slices. A
contrast phantom with different T1 and T2 compartments was placed in a
waterbath, and imaged using spGRE (TE/TR=1.48/3 ms, FA=20o), bSSFP
and iSSFP (TE/TR=1.9/3.9 ms, FA=70o), and HASTE (TE=68 ms, 256x256
matrix, 50% partial Fourier). This was first done with a Cartesian trajectory to
investigate image contrast. Subsequently, GA radial versions of bSSFP and iSSFP
were acquired. To demonstrate in vivo
real-time imaging performance, a healthy volunteer was scanned during free
breathing (FB) using the GA radial iSSFP and bSSFP sequences (TE/TR=1.9/3.9 ms,
FA=41o). For comparison, FB GA radial GRE and breath-held (BH)
Cartesian HASTE were also acquired in vivo.
[Imaging Analysis] The
phantom images were visually examined to evaluate image quality, contrast and banding artifacts. The in vivo images were inspected for banding artifacts
and tissue contrast differences.
Results
[Phantom Experiments]
The Cartesian phantom images (Fig3)
demonstrate the lack of T2 contrast in the spGRE image, as well as the banding
artifacts in the bSSFP image. iSSFP has T2/T1 contrast similar to bSSFP,
without any banding artifacts. For the GA radial bSSFP and iSSFP scans, it can
be seen that there is similarly high T2/T1 contrast in both images, with iSSFP again
suppressing the bSSFP banding artifacts.
[Volunteer
Experiments] For the dynamic free-breathing scans, the N and M values for
the sliding-window SPIRiT reconstructions were 144 and 20 respectively, which
achieves a 576-ms temporal footprint (true frame rate 1.7 frames per second, fps)
and a 80-ms temporal resolution (12-fps display frame rate). The data was also reconstructed with N = 55
and M = 20, achieving a 215-ms temporal footprint (true frame rate 4.5 fps) and
12-fps display frame rate. It was observed that iSSFP has contrast similar to the
bSSFP acquisition without banding artifacts (Fig 4). By leveraging the GA radial acquisition pattern, volunteer
motion was reconstructed and resolved with different frame rates (Fig5).
Discussion and Conclusion
Our GA radial iSSFP sequence combines the benefits of the
iSSFP sequence with flexible dynamic GA radial acquisition. In phantom and volunteer scans at 3.0T, it is
seen that GA radial iSSFP achieves good tissue contrast similar to bSSFP while
suppressing banding artifacts. Sliding-window SPIRiT reconstruction allows dynamic
visualization of subject motion with true frame rates from 1.7 to up to 4.5fps.
Higher frame rates may be possible with additional constrained reconstruction. These
advantages of GA Radial iSSFP show its potential for improving real-time MRI-guided
interventions.
Acknowledgements
NSF Graduate Research Fellowship Program
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