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Balanced SSFP Highly Accelerated Cine ‘Watermark’ Quantifies 2D Myocardial Strain with comparison to 2D Spiral Cine DENSE1MRI Research Center, Auburn University, Auburn University, AL, United States
Synopsis
Cardiac MRI (CMR) myocardial tagging enables quantification of myocardial strain. However, tagging remains limited to a research context due to the time-intensive analysis and need to run multiple sequences. CMR sequences must be fast and efficient. We previously developed low-acceleration FLASH and Balanced-SSFP versions of Cine ‘Watermark’ (CWM) that acquire normal cine magnitude images plus ‘hidden’ (via phase) cardiac strain data for calculating myocardial strain. Here we present a highly x4 accelerated bSSFP CWM with improved performance and scan efficiency. The bSSFP CWM is demonstrated in human subjects at 3T and its performance is compared to 2D Spiral Cine DENSE.
Introduction
Cardiac MRI (CMR) myocardial tagging enables quantification of myocardial strain throughout the cardiac cycle1, 2. However conventional tagging remains limited to a research context due to the time-intensive analysis and need to run multiple sequences. CMR sequences must provide maximum diagnostic information with minimum operator effort, time and cost3. Previously we developed low-acceleration FLASH and Balanced SSFP versions of Cine Watermark (CWM) that acquire conventional magnitude cine plus a ‘hidden’ capability to acquire cardiac strain data while requiring no extra operator effort4,5. The CWM acquires normal cine magnitude images plus spatial-modulated tagging added only into the image phase (‘watermark’) allowing quantitative cine myocardial strain calculation. Here we present a highly x4 accelerated bSSFP version of CWM with improved performance and scan efficiency. The bSSFP CWM is demonstrated in human subjects at 3T and its performance is compared to 2D Spiral Cine DENSE.Methods
Fig-1A illustrates the bSSFP CWM sequence that runs continuously as steady-state with ECG-trigger to acquire data on every heartbeat. A classic “1-1” saturation SPAMM was applied with each non-selective rectangular RF pulse flip angle = 45°. The bSSFP CWM readout includes GRAPPA x4 acceleration, customized ramped start-up flip angle for fast approach to steady-state, and at end of cardiac R-R cycle can incorporate either α/2 magnetization restore method, or just apply spoil gradients before the next triggered SPAMM encode. Within one 18-second breath-hold, five cine acquisitions are performed that include SPAMM and CSPAMM in both readout (RO) and phase encode (PE) directions, and a phase-reference scan. Image and strain analyses were performed offline using customized Matlab programs (Mathworks, Natick, MA). Fig-1B illustrates the CWM process that separates non-tagged magnitude from tagged phase images by sum and difference of the SPAMM and CSPAMM complex images. Fig-2 illustrates segmentation lines drawn onto non-tagged magnitude images were used to segment corresponding tagged phase images. Discrete Model Free6 strain analysis was applied to the CWM RO and PE line-tag sets to calculate left ventricular (LV), short-axis (SA), six-sector, circumferential strain (Ecc) graphs. For performance comparison, 2D Spiral Cine DENSE, with its own Matlab analysis program, was used with as similar imaging parameters as possible on the same image slices.
Imaging parameters common for both bSSFP CWM and Spiral DENSE: Slice thickness = 8 mm, Pixel size = 2x2 mm, Cine frames = 20, Averages = 1; bSSFP CWM only: FOV = 256x224 mm, Matrix = 128x112, Bandwidth = 1116 Hz/Pixel, GRAPPA acceleration = x4, Flip angle = 30°, Tag spacing = 8 mm; Spiral DENSE only: FOV = 320 mm, Matrix = 160x160, Interleaves = 10, Flip angle = 12°, Simple 3-point XY encoding, In-plane/Through-plane encode = 0.1/0.08 cyc/mm. Six healthy human subjects, 19-32 yo, 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). Scan slice locations included one long-axis (LA) 4-chamber and 1-3 mid-LV SA views.
Results
Within 18-second breath-holds, all CWM subjects provided cine magnitude and tagged phase image sets suitable for strain analysis. 2D Spiral Cine DENSE required 25-second breath-holds. Fig-2 presents representative CWM cine frame images at end-systole (ES) time-points with subsequent LV segmentation and tagged phase images for the RO, PE and combined RO+PE (grid) directions. All show strong tag contrast with clear displacement and bending of the tag patterns at ES. Fig-3 presents representative CWM non-filtered/non-smoothed Ecc strain compared to 2D Spiral Cine DENSE – where the peak strains differed, but not significantly. Notably, the 2D Spiral Cine DENSE curves appear filtered/smoothed which may affect the comparison. All CWM sectors showed consistent trajectories, although noisy without filtering. Some CWM subjects with slow heart-rates (RR > 1 sec) presented tagging with T1 fading and noise in the late cardiac cycle frames.Discussion
CWM acquires normal cine magnitude and spatially-tagged cine phase images within a single 18-sec breath-hold scan. The bSSFP high SNR and GRAPPA x4 acceleration were essential for CWM to achieve this fast capability. The bSSFP ramped flip angle greatly reduced artifacts. CWM peak strains did not agree with those of 2D Spiral Cine DENSE possibly due to variations with the strain analysis programs. The unfiltered/unsmoothed CWM strain values tended higher than previous literature2 values while the filtered/smoothed 2D Spiral Cine DENSE values tended lower than previously reported.Conclusions
Highly accelerated bSSFP Cine ‘Watermark’ with its integrated hidden capability to acquire tagging in the phase, collects clinically relevant qualitative and quantitative data in one fast scan per slice. Future work is needed to reduce T1 fading effects and improved strain analysis.Acknowledgements
Special thank you for programmatic and volunteer support goes to Julie Rodiek, Julio Yanes, and Lily Strassberg.References
1. Axel L, Dougherty L. “MR imaging of motion with spatial modulation of magnetization”, Radiology 1989;171:841–9.
2. Edvardsen T, et al. “Quantitative Assessment of Intrinsic Regional Myocardial Deformation…”, Circulation 2002;106:50-56.
3. Edelstein WA, et al. “MRI: Time Is Dose—and Money and Versatility”, J Am Coll Radiol 2010;7(8): 650–2.
4. Beyers RJ, et al. “Accelerated Cardiac Cine ‘Watermark’ MRI Provides Cardiac Function via Magnitude Cine and 2D Myocardial Strain Via Spatially Modulated Phase”, Proc.Intl.Soc.Mag.Reson. Med. 25, 2017.
5. Beyers RJ, et al. “Cardiac Single Breath-hold Balanced SSFP Cine ‘Watermark’ provides Cardiac Function via Magnitude and 2D Myocardial Strain via Phase”, Proc.Intl.Soc.Mag.Reson. Med. 26, 2018.
6. Denney TS Jr, McVeigh ER. “Model-free reconstruction of three-dimensional myocardial strain from planar tagged MR images”, J Magn Reson Imaging 1997;7(5):799-810.
Figures
A: bSSFP Cine WaterMark (CWM) is a steady-state ECG-triggered sequence. The CWM encode is a SPAMM/CSPAMM with 45°, 45° RF pulse flip angles for saturation modulation, as opposed to, inversion modulation by conventional 90°, 90° flip angle. The bSSFP readout includes ramp-up flip angle after encoding to rapidly reach steady-state and minimize artifacts. Within one 18-sec breath-hold the full SPAMM and full CSPAMM sets are acquired for readout (RO) and phase encode (PE) directions plus a phase-reference scan.
B: The complex SPAMM + CSPAMM sum returns a normal, non-modulated, T1-weighted image and the complex SPAMM – CSPAMM difference returns a cosine modulation-only image with tagging.
