Adrienne E Campbell-Washburn1, Robert J Lederman1, Anthony Z Faranesh1, and Michael S Hansen1
1Cardiovascular and Pulmonary Branch, Division of Intramural Research, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, MD, United States
Synopsis
Balanced SSFP Golden Angle
radial imaging uses a rapidly varying gradient scheme and thus is susceptible
to image distortion caused by gradient delays and eddy currents. We propose
that storing a history of the gradient waveforms in each axis can enable us to
better predict our true k-space coordinates during sampling. We use the
gradient system impulse response function to predict k-space coordinates and
demonstrate reduced image distortion (shading and streaking) in a phantom and
in vivo when utilizing the gradient waveform history. This method will be
useful for dynamic and real-time imaging with Golden Angle balanced SSFP
imaging schemes.Purpose
Golden Angle radial
sampling, where each successive radial arm is rotated by 111.246°,
is appealing for dynamic imaging because it has reduced sensitivity to motion
and provides uniform k-space coverage for any arbitrary number of consecutive projections
[1]. A balanced steady state free precession (bSSFP) sequence with Golden Angle
profile ordering often results in image artifacts caused by gradient delays and
varying eddy currents during the rapidly switching gradient scheme [2]. In
recent work, the gradient system impulse response function (GIRF) has been
characterized and used to predict true k-space trajectories during spiral and
EPI imaging to reduce image distortion [3, 4]. Here, we propose to store a
history of the time-varying gradient waveforms during a Golden Angle
acquisition scheme and apply the GIRF calibration to predict true k-space
coordinates during sampling in order to reduce image artifacts.
Methods
Fully sampled Golden Angle radial
bSSFP imaging was performed at 1.5T (Aera, Siemens Healthcare, Erlangen,
Germany). The GIRF was calibrated for each gradient axis (GIRF bandwidth =
100 kHz and resolution = 26 Hz) [4] and convolved with the nominal x, y and z gradient
waveforms in order predict the true gradient waveforms for arbitrary slice
orientation. Images were reconstructed in MATLAB (R2013a, Mathworks, Natick,
MA) with a nonuniform FFT. Three images were compared: 1) assuming nominal gradient waveforms for perfect
radial projections, 2) using the GIRF-predicted k-space trajectories treating
each radial profile independently (including the prephasing and
readout gradients) and 3) using the GIRF-predicted k-space
trajectories informed by the entire gradient waveform history up to and
including the current profile. Phantom imaging and in vivo imaging of a swine
heart were performed using the following parameters: TE/TR = 1.47/2.94 ms, 201 radial
projections, FOV = 300mm, flip angle = 45°, matrix = 128 x 128, slice thickness = 5 mm,
bandwidth = 1000 Hz/Px. Animal experiments were approved by the institutional
animal care and use committee according to contemporary NIH guidelines.
Results
The improvement in image
artifact was visible in both an axial phantom image (Figure 1) and an oblique short
axis image of the swine heart (Figure 2). Images reconstructed using the
nominal radial trajectories displayed significant image streaking and shading (Figure
1A, 2A). Images were improved by applying GIRF prediction to calculate k-space
trajectories from each radial profile independently, but some residual image artifact remained
(Figure 1B, 2B). Using the entire gradient waveform history to inform k-space
trajectory prediction produced the best quality images, with negligible
remaining artifact (Figure 1C, 2C). The images shown here were in transition to
steady state, creating worst-case scenario artifacts. Figure 3 shows the
trajectories through the center of k-space for the final 6 radial projections,
where the nominal trajectories assume perfect central alignment of the radial
profiles and the GIRF-predicted trajectories depict a mismatch at the k-space
center caused by both gradient delays and eddy currents.
Discussion
Using the
GIRF calibration for a retrospective trajectory prediction can simultaneously correct distortions from both
eddy currents and gradient delays, and can be applied as a standard step during
real-time image reconstruction [4], which could improve the clinical
applicability of radial imaging. In the future, this method will be implemented
in real-time using a buffer of the gradient history and we will apply this
distortion correction during MRI-guided interventions where multiple slice
orientations are interleaved, thus amplifying the eddy current artifacts. Further
studies will assess the duration of waveform history required for sufficient
artifact correction.
Our preliminary data suggests that the ability to store a gradient waveform
history allows a better prediction of the sampled k-space locations with the
gradient system impulse response function.
Acknowledgements
This work was
supported by the NHLBI
Division of Intramural Research (Z01-HL006039, Z01-HL005062)References
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al, An optimal radial profile order based on the Golden Ratio for time-resolved
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Surrogate for the Golden Angle in Time-Resolved Radial MRI Based on Generalized
Fibonacci Sequences. IEEE Trans Med
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System Characterization by Impulse Response Measurements with a Dynamic Field
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al, Real-Time distortion correction of spiral and echo planar images using the
gradient system impulse response function. MRM (2015) doi:
10.1002/mrm.25788