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Pseudo spiral compressed sensing accelerated whole-heart 4D flow MRI: validation against EPI readout
Carmen PS Blanken1, Lukas M Gottwald1, Jos JM Westenberg2, Eva S Peper1, Bram F Coolen1, Gustav J Strijkers1, Aart J Nederveen1, R Nils Planken1, and Pim van Ooij1

1Amsterdam University Medical Centers, location AMC, Amsterdam, Netherlands, 2Leiden University Medical Center, Leiden, Netherlands

Synopsis

4D flow MRI facilitates detailed evaluation of cardiac hemodynamics in patients with cardiovascular disease. In this study, we investigated the performance of pseudo spiral compressed sensing (CS) accelerated whole-heart 4D flow MRI in a comparison with a clinically used EPI readout. CS-accelerated 4D flow MRI yielded similar results to EPI-accelerated 4D flow MRI in terms of velocity vector fields during ventricular ejection and filling and led to consistent blood flow measurements across heart valves. Our data suggest that CS 4D flow MRI has the potential to be accelerated even further for quantitative whole-heart hemodynamic imaging.

Introduction

4D flow MRI facilitates detailed evaluation of cardiac hemodynamics in patients with cardiovascular disease. Whole-heart coverage within clinically feasible scan times requires acceleration. EPI readout is a widely used acceleration technique, but is susceptible to image distortion artifacts for high acceleration factors due to off-resonance effects. In this study, we investigate the performance of compressed sensing (CS) accelerated whole-heart 4D flow MRI using a pseudo spiral Cartesian sampling technique with random undersampling in time (1-3). Comparison of velocity vector fields is conducted against a clinically used EPI readout strategy (4) and blood flow measurements through the heart valves are tested for inter-valve consistency. We hypothesize that CS accelerated 4D flow MRI performs equally well to EPI in measuring cardiothoracic velocities and yields consistent blood flow measurements across all four heart valves.

Methods

9 healthy volunteers (aged 24 ± 4y, 3 female) underwent cardiac MRI including whole-heart CS (n=9) and EPI 4D flow MRI (n=4) at 3T. 4D flow data were acquired in 30 cardiac phases during free-breathing with retrospective ECG-gating. Acquired and reconstructed spatial resolution were 3.0x3.0x3.0 mm3 and 2.8x2.8x3.0mm3 and three-directional VENC was set to 150 cm/s. Scan times ranged from 7 to 10 minutes, depending on FH-dimension of the (transversal oblique) FOV and the volunteer’s heart rate during the EPI scan. EPI was obtained with an EPI factor of 5 and a SENSE factor of 2. The CS undersampling factor ranged from 5.5 to 8.2 to keep the scan times for both scans the same. CS 4D flow scans were reconstructed offline with ReconFrame (Gyrotools, Zurich, Switzerland) and using the Berkeley Advanced Reconstruction Toolbox (BART) (5). A sparsifying total variation transform in time was used with regularization parameters of r = 0.001 and 20 iteration steps.

Time-averaged phase contrast MRA images were created from the EPI and CS datasets by multiplication of the magnitude with the absolute velocity images, and used to segment the entire heart including connected large arteries and veins (Mimics, Materialize, Leuven, Belgium). The overlap of both segmentations was maximized using rigid registration. CS velocity vectors were interpolated to the EPI segmentation for ventricular ejection and filling phases, followed by whole-heart voxelwise comparison for velocity magnitude and vector angle.

Semi-automated retrospective tracking of all four heart valves was performed using 2D cine bSSFP by dedicated software (CAAS MR 4D Flow 2v0, Pie Medical Imaging) with through-plane valve motion correction and automatic aliasing and phase offset correction.

Bland-Altman analysis was used to determine the mean difference and 95% limits of agreement (LOAs) between the CS and EPI velocity vectors. Furthermore, orthogonal regression was performed (Pearson correlation coefficient denoted by r).

Results

Figure 1 shows an example of voxelwise comparison between CS and EPI velocity vector fields in a single volunteer at peak-ejection. Averaged over the cohort, a Pearson correlation of 0.87 ± 0.02 m/s was found at peak-ejection and 0.73 ± 0.06 m/s at peak-filling (Table 1). Bland-Altman analysis revealed a mean difference of -0.014 ± 0.028 m/s at peak-ejection and -0.001 ± 0.010 m/s at peak-filling, indicating no significant bias. Limits of agreement were in the order of 1/5 (peak-ejection) and 1/7 (peak-filling) of the VENC. Median angle difference was 16 ± 1 degrees for peak-ejection and 25 ± 2 degrees for peak-filling.

An example of an EPI and CS streamline visualization obtained with semi-automated retrospective valve tracking is shown in Figure 2. In Figure 3, measured flow curves in a single subject are displayed for both EPI and CS. The measurements revealed good consistency between heart valves (Figure 4).

Discussion

In this study, we aimed to validate CS accelerated whole-heart 4D flow MRI in a comparison with a clinically used EPI readout. A good overall agreement was found between velocity vector fields. Pearson correlation and directional agreement were better at peak-ejection than peak-filling. A possible explanation is that due to lower velocities, velocity-to-noise ratios were lower during ventricular filling than during ventricular ejection. Good consistency of measured blood flow over the heart valves indicates robustness in non-diseased hearts, despite slight deviations from direct proportionality. Enlargement of the cohort and application in a cardiac flow phantom or ex-vivo beating heart may strengthen the validation for future clinical use. We expect that CS acceleration of 8.2 is not yet the limit and can be increased.

Conclusion

CS accelerated whole-heart 4D flow MRI yields similar results to EPI-based 4D flow MRI in terms of velocity vector magnitude and direction and leads to consistent blood flow measurements across heart valves.

Acknowledgements

No acknowledgement found.

References

1. Lustig M, et al., Sparse MRI: The application of compressed sensing for rapid MR imaging. Magn. Reson. Med. 2007; 58:1182-1195

2. Gottwald LM, Peper ES, Zhang Q et al. Pseudo Spiral Compressed Sensing for Aortic 4D Flow MRI: a Comparison with k-t Principal Component Analysis. Proc 27rd Annu Meet ISMRM, Paris, France. 2018

3. Peper ES , Gottwald LM, Zhang Q et al. 30 times accelerated 4D flow MRI in the carotids using a Pseudo Spiral Cartesian acquisition and a Total Variation constrained Compressed Sensing reconstruction. Proc 27rd Annu Meet ISMRM, Paris, France. 2018

4. Westenberg JJ, Roes SD, Ajmone Marsan N, et al. Mitral valve and tricuspid valve blood flow: accurate quantification with 3D velocity-encoded MR imaging with retrospective valve tracking. Radiology. 2008;249:792–800.

5. Uecker M, et al. Berkeley advanced reconstruction toolbox. Proc. Intl. Soc. Mag. Reson. Med. 23; 2015

Figures

Figure 1. Voxelwise comparison of CS- and EPI-accelerated whole-heart acquisitions in a single volunteer at peak ventricular ejection. Top: rigid registration leads to maximal overlap between the velocity vector fields. The CS dataset is projected onto the EPI segmentation. Bottom: Orthogonal regression, Bland-Altman and angle distribution analysis reveal good agreement between the two acquisitions.

Table 1. Results of the Bland-Altman, orthogonal regression and angle difference analyses for voxelwise comparison of CS- and EPI-accelerated whole-heart 4D flow MRI during ventricular ejection and filling phases.

Figure 2. EPI (left) and CS (right) accelerated 4D flow MRI streamline visualization of blood flow through the aortic valve (yellow contour), mitral valve (orange), pulmonary valve (blue) and tricuspid valve (green) in a 23-year old healthy volunteer, resulting from semi-automated retrospective valve tracking. Valve tracking was performed on bSSFP cine images, on two orthogonal views for each heart valve. 4-chamber bSSFP view is visible in the background.

Figure 3. An example of valve flow measurements over time in a 28-year old subject, obtained by means of retrospective valve tracking on EPI- and CS-accelerated 4D flow MRI.

Figure 4. Blood flow measurement on CS- and EPI-accelerated 4D flow MRI data by means of retrospective valve tracking revealed good agreement between heart valves (close to identity: dashed line). One volunteer did not undergo 2D CINE bSSFP and is thus not included in the valve tracking analysis.

Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)
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