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Comparative Study
. 2024;26(2):101052.
doi: 10.1016/j.jocmr.2024.101052. Epub 2024 Jun 25.

The effects of field strength on stimulated echo and motion-compensated spin-echo diffusion tensor cardiovascular magnetic resonance sequences

Andrew D Scott  1 , Ke Wen  2 , Yaqing Luo  2 , Jiahao Huang  3 , Simon Gover  4 , Rajkumar Soundarajan  4 , Pedro F Ferreira  5 , Dudley J Pennell  5 , Sonia Nielles-Vallespin  5
Affiliations
Comparative Study

The effects of field strength on stimulated echo and motion-compensated spin-echo diffusion tensor cardiovascular magnetic resonance sequences

Andrew D Scott et al. J Cardiovasc Magn Reson. 2024.

Abstract

Background: In-vivo diffusion tensor cardiovascular magnetic resonance (DT-CMR) is an emerging technique for microstructural tissue characterization in the myocardium. Most studies are performed at 3T, where higher signal-to-noise ratio (SNR) should benefit this signal-starved method. However, a few studies have suggested that DT-CMR is possible at 1.5T, where echo planar imaging artifacts may be less severe and 1.5T hardware is more widely available.

Methods: We recruited 20 healthy volunteers and performed mid-ventricular short-axis DT-CMR at 1.5T and 3T. Acquisitions were performed at peak systole and end-diastole using both stimulated echo acquisition mode (STEAM) and motion-compensated spin-echo (MCSE) sequences at matched spatial resolutions. DT-CMR parameters were averaged over the left ventricle and compared between 1.5T and 3T sequences using both datasets with and without the blow reference data included.

Results: Eleven (1.5T) and 12 (3T) diastolic MCSE acquisitions were rejected as the helix angle (HA) demonstrated <50% normal appearance circumferentially or the acquisition was abandoned due to poor image quality; a maximum of one acquisition was rejected for other datasets. Subjective HA map quality was significantly better at 3T than 1.5T for STEAM (p < 0.05), but not for MCSE and other DT-CMR quality measures were consistent with improvements in STEAM at 3T over 1.5T. When blow data were excluded, no significant differences in mean diffusivity were observed between field strengths, but fractional anisotropy was significantly higher at 1.5T than 3T for STEAM systole (p < 0.05). Absolute second eigenvector orientation (E2A, sheetlet angle) was significantly higher at 1.5T than 3T for MCSE systole and STEAM diastole, but significantly lower for STEAM systole (all p < 0.05). Transmural HA distribution was less steep at 1.5T than 3T for STEAM diastole data (p < 0.05). SNR was higher at 3T than 1.5T for all acquisitions (p < 0.05).

Conclusion: While 3T provides benefits in terms of SNR, both STEAM and MCSE can be performed at 1.5T. However, MCSE is unreliable in diastole at both field strengths and STEAM benefits from the improved SNR at 3T over 1.5T. Future clinical research studies may be able to leverage the wider availability of 1.5T CMR hardware where MCSE acquisitions are desirable.

Keywords: Cardiac microstructure; DTI; Diffusion tensor; Field strength; Healthy volunteers; Stimulated echo.

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Conflict of interest statement

Declaration of competing interests The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: The Cardiovascular Magnetic Resonance Unit at the Royal Brompton Hospital receives research support from Siemens. Ke Wen and Yaqing Luo are partly funded by Siemens.

Figures

ga1
Graphical abstract
Fig. 1
Fig. 1
Diffusion-weighted images showing an apparent loss of SNR at 1.5T using STEAM, but not so noticeably using MCSE. Images are shown from one typical subject, acquired in both cardiac phases at both field strengths and using both sequences. Images are shown for the same diffusion encoding direction (apart from the blow reference data) with the same contrast and brightness for each dataset (sequence, phase, and field strength). The red arrow heads highlight residual fat signal in the MCSE data at 3T which partially covers the LV myocardium. For equivalent full field-of-view images, see Supplementary Figs. 1 and 2. For equivalent images for all encoding directions after registration and averaging, see Supplementary Fig. 3. SNR signal-to-noise ratio, STEAM stimulated echo acquisition mode, MCSE motion-compensated spin-echo, LV left ventricle
Fig. 2
Fig. 2
Example DT-CMR parameter maps acquired in a typical subject at all combinations of field strength, sequence type, and cardiac phase. DT-CMR diffusion tensor cardiovascular magnetic resonance, STEAM stimulated echo acquisition mode, MCSE motion-compensated spin-echo, E2A absolute value of the angle of the second eigenvector.
Fig. 3
Fig. 3
An example of the diffusion tensor represented as an ellipsoidal glyph in the LV for all systolic acquisitions in one volunteer. The glyphs are color-coded by HA. All datasets show good-quality data. Insets show a zoomed-in region from the septum. See Supplementary Fig. 5 for the equivalent data shown in diastole. LV left ventricle, HA helix angle, STEAM stimulated echo acquisition mode, MCSE motion-compensated spin-echo.
Fig. 4
Fig. 4
Histograms of subjective HA map quality score, showing the significant increase in the HA map appearing as expected in the 3T vs 1.5T data for STEAM but not for MCSE. Datasets scored 0 (<50% of the myocardium demonstrating the normal transmural variation in HA) were excluded from further analysis. “Counts” are equivalent to subjects. HA helix angle, STEAM stimulated echo acquisition mode, MCSE motion-compensated spin-echo.
Fig. 5
Fig. 5
Violin plots of MD, FA, transmural HA gradient, and absolute E2A calculated without blow reference data. The equivalent figure with data processed with all b-values (including blow reference data are shown in Supplementary Fig. 7). The median is shown as a white circle in each plot and the mean is shown as a horizontal line. MD mean diffusivity, FA fractional anisotropy, HA helix angle, E2A absolute value of the angle of the second eigenvector, STEAM stimulated echo acquisition mode, MCSE motion-compensated spin-echo.
Fig. 6
Fig. 6
Violin plots comparing eigenvalues calculated without blow reference data between field strengths. Equivalent plots calculated with the blow reference data are shown in Supplementary Fig. 8. STEAM stimulated echo acquisition mode, MCSE motion-compensated spin-echo, sys systole, dias diastole.
Fig. 7
Fig. 7
Quality metrics for data excluding the blow reference apart from the SNR in the blow images. See Fig. 5 for an explanation of the violin plots. Equivalent plots, processed including the blow reference data, are shown in Supplementary Fig. 9 (apart from SNR). SNR values are shown for the b = 450 smm−2 data for MCSE data and for the blow data for STEAM due to the differences in main b-value between the field strengths for STEAM and the high-intensity blood signal in the blow images for MCSE. SNR signal-to-noise ratio, MCSE motion-compensated spin-echo, STEAM stimulated echo acquisition mode, LV left ventricle, TA transverse angle, HA helix angle, sys systole, dias diastole.
Fig. 8
Fig. 8
Variation of the FA and MD over the left ventricular myocardium depending on field strength from data processed without the blow reference data. For an explanation of the violin plots, see Fig. 5. Equivalent plots for data with blow reference data included are provided in Supplementary Fig. 11. FA fractional anisotropy, MD mean diffusivity, MCSE motion-compensated spin-echo, STEAM stimulated echo acquisition mode.
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