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Highly Accelerated Balanced SSFP Cardiac Cine 3D-DENSE Quantifies Human 3D Myocardial Strain1MRI Research Center, Auburn University, Auburn University, AL, United States
Synopsis
Quantification of 2D and 3D myocardial strain at cine frame-rates has been previously demonstrated with echo-planar and spiral sequence versions of Displacement Encoding with Stimulated Echoes (DENSE). However, these non-conventional acquisition methods, with their sensitivities to off-resonance, has hindered their integration into mainstream cardiac MRI application. Here we present a more conventional, but highly accelerated, balanced SSFP (bSSFP) version of Cardiac Cine 3D-DENSE for application to human subjects. In vivo human scans at 3T demonstrated good agreement of myocardial radial (Err), circumferential (Ecc) and longitudinal Ell) strain values between bSSFP 3D-DENSE to those reported in previous literature.
Introduction
Quantification of human myocardial strain has been previously well demonstrated with echo-planar (for 2D) and spiral (2D & 3D) sequence versions of Displacement Encoding with Stimulated Echoes (DENSE)1-3. However, these non-conventional acquisition methods, with their sensitivities to off-resonance, has hindered their integration into mainstream cardiac MRI (CMR) application. Balanced Steady-State Free Precession (bSSFP) that’s known for high SNR and versatility is well favored in CMR. Past efforts to integrate displacement encoding methods into bSSFP has been incomplete, or had limited success4, 5. Here we present a novel bSSFP version of Cardiac Cine 3D-DENSE that incorporates high x4 GRAPPA acceleration and complementary three-axis encoding for effective T1 artifact suppression. The bSSFP Cine 3D-DENSE is demonstrated in human subjects at 3T.Methods
Fig-1 illustrates the bSSFP Cardiac Cine 3D-DENSE sequence that runs continuously as steady-state with ECG-trigger to acquire data on every heartbeat. The DENSE encode uses a standard 1-1 SPAMM/CSPAMM module selectable for in-plane, or through-plane encoding. This SPAMM/CSPAMM method is well established for the suppression of the undesired T1 echo in the DENSE signal. The cine bSSFP readout is customized with ramped start-up flip angle for fast approach to steady-state and modified gradients to incorporate the DENSE de-encode moments while maintaining gradient moment nulling in all axes. The end of cardiac R-R cycle can incorporate either α/2 magnetization restore method, or just run spoil gradients before the next triggered DENSE encode. For each 3D partition, the 3D-DENSE serially acquires four complimentary image sets including encoded readout (x-axis), encoded phase-encode (y-axis), encoded through-plane (z-axis), and non-encoded phase-reference directions. For each encoded axis, the bSSFP readout module integrates the necessary de-encode moments into its relevant gradients.
Imaging parameters for bSSFP 3D-DENSE: FOV = 256x240 mm, Partitions = 16, Partition thickness = 4 mm, Matrix = 128x120, Pixel size = 2x2x4 mm, Bandwidth = 1116 Hz/Pixel, GRAPPA acceleration = x4, Flip angle = 30°, Cine frames = 20, Averages = 1, DENSE in-plane/through-plane encode = 0.1/0.08 cyc/mm. 3D-DENSE scans placed the 16 partitions at mid-Left Ventricle (LV) with the partition “slices” as cardiac short-axis (SA) direction. All image and strain analysis were performed offline using customized Matlab programs (Mathworks, Natick, MA). Myocardial mid-LV SA, six sector and global average strain values for radial (Err), circumferential (Ecc), and longitudinal (Ell) directions were calculated and presented.
Importantly, similar to navigator gating, our sequence employed a manual push-button gating method. The subject being scanned would press and hold the gating button during exhaled breath-hold (allowing acquisition) then release the button before and during breathing (non-acquisition steady-state). The button process is repeated on the subject’s next breath-hold and repeated as necessary for the duration of the scan. Five healthy human subjects, 22-29yo, 2 female, with informed consent, were scanned in a 3T Verio scanner (Siemens, Erlangen, Germany) with a 32-chan anterior/posterior RF coil array (Invivo, Gainesville, Florida).
Results
From all subjects, bSSFP 3D-DENSE provided displacement encoded and reference image sets suitable for strain analysis. The manual push-button gating method proved very easy and effective for acquiring long (10-15 minute) multi-breath-hold scans. During analysis, 4 partitions from each end of the 3D stack were discarded due to through-plane aliasing – leaving 8 partitions of good 3D data. Figs-2&3 present representative 3D-DENSE image and strain analysis results at mid-LV. Fig-2B shows a good 2D in-plane displacement field pattern and same displacement field from a 3D side-view in Fig-2C. Fig-2E,F,G present strain maps for the three Err, Ecc, and Ell directions. Fig-3 presents AHA-convention, six-sector strain and global mean strain graphs for the three Err, Ecc, and Ell directions for each cine frame. Subjects with slow heart-rates (RR > 1 sec) showed increased DENSE T1 signal fading and subsequent noise in the late-diastole frames – as evident in the Fig-3 strain graphs The bSSFP 3D-DENSE showed good agreement per subject, however, variation was high across subjects with global mean Err = 0.42±0.33, Ecc = 0.17±0.29 and Ell = 0.14±0.26 (Mean±StdDev).Discussion
This highly accelerated bSSFP 3D-DENSE for human 3D cardic strain quantification is a promising start. The x4 acceleration with a long 3D scan gave good SNR (≈>50). The DENSE T1 fading is problematic, so work is needed to optimize the bSSFP flip angle method. The manual push-button gating works well with cooperative subjects, but impractical for otherwise. Although not included in this abstract, the 3D-DENSE also performs cine 2D-DENSE (single slice) with a 24-sec breath-hold per slice acquisition time.Conclusions
We developed and demonstrated a highly accelerated bSSFP Cine 3D-DENSE sequence for human CMR application. Future work will improve on its overall performance.Acknowledgements
Special thank you for programmatic and volunteer support goes to Julie Rodiek, Julio Yanes, and Lily Strassberg.References
1. Aletras AH, Ding S, Balaban RS, Wen H. “DENSE: Displacement Encoding with Stimulated Echoes in Cardiac Functional MRI”, J Magn Reson. 1999; 137(1): 247–252.
2. Kim D, Gilson WD, Kramer CM, Epstein FH, “Myocardial Tissue Tracking with Two-dimensional Cine Displacement-encoded MR Imaging: Development and Initial Evaluation”, Radiology 2004; 230:862-871.
3. Zhong X, Spottiswoode BS, Meyer CH, Kramer CM, Epstein FH, “Imaging Three-Dimensional Myocardial Mechanics Using Navigator-gated Volumetric Spiral Cine DENSE MRI”, MRM 2010; 64(4):1089-1097.
4. Cowart EA, Gilson WD, Kramer CM, Epstein FH, “Imaging 3D Myocardial Motion with SSFP Cine DENSE”, Proc. Intl. Soc. Mag. Reson. Med. 11, 2004.
5. Kim D, Kellman P, “Improved cine displacement-encoded MRI using balanced steady-state free precession and time-adaptive sensitivity encoding parallel imaging at 3T”, NMR Biomed 2007; 20(6):591-601.
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