The Stimulated Echo Acquisition Mode (STEAM) has been the primary method for in vivo cardiac Diffusion Tensor Imaging (DTI)¹. STEAM, however, requires two consecutive heartbeats to encode the signal, hence requiring breath holding or narrow-window respiratory gating for consistent signal formation². Additionally, the inherent signal loss of STEAM relative to Spin Echo (SE) and the impact of cardiac strain during STEAM encoding render STEAM DTI acquisition of the beating heart particularly challenging^3,4. To address these limitations, single shot SE sequences may be used in combination with respiratory navigator gating and motion compensated diffusion gradient waveforms⁵. It is the objective of the present work to compare respiratory gating and SNR efficiency along with diffusion tensor metrics between STEAM DTI and second-order motion compensated SE DTI of the in vivo human heart^6,7. Cardiac-triggered diffusion-weighed STEAM and second-order motion compensated SE sequences (Figure 1) were implemented on a 1.5T Philips Achieva system (Philips Healthcare, Best, The Netherlands) equipped with a 5-channel cardiac receiver array and a gradient system delivering 80mT/m @ 100mT/m/ms. Data were acquired in 7 healthy subjects (5 female, weight 62±9kg, age 24±2years, heart rate 66±10beats/min). Written informed consent according to institutional guidelines was obtained prior to imaging. STEAM was implemented as described previously². For second-order motion compensated SE⁷, fat suppression was performed using a 1-3-3-1 binomial spatial-spectral excitation pulse⁸. A variable rate selective excitation (VERSE)⁹ echo pulse was employed to reduce echo times. Both STEAM and SE used reduced field-of-view excitation. Data acquisition was performed during free-breathing using navigator gating (NAV1 gating window: 5mm). For STEAM, data were accepted if the relative displacement between NAV1 and NAV2 was within ± 0.5mm (Figure 1a). Diffusion encoding was along 3 (b=100s/mm²) and 9 (b=450s/mm²) directions¹⁰. Imaging parameters were: in-plane resolution 2.8×2.8mm², slice thickness 8mm, FOV: 230×98mm², TE/TR(STEAM): 31ms/2-R-R, TE/TR(SE): 70ms/1-R-R, number of signal averages STEAM/SE: 8/16 to obtain identical nominal scan times. Imaging was triggered to the systolic strain “sweet spot” to avoid strain effects in STEAM³. Myocardial SNR of STEAM vs. SE was assessed per single average in a separate scan using b=100s/mm² and 450s/mm² encoded along 6 diffusion directions (16mm slice thickness, 4 signal averages). SNR was normalized to unit time yielding SNR efficiency SNR_t. DTI analysis was performed on mean diffusivity (MD), fractional anisotropy (FA), the local helix elevation (helix angle) and the deviation of the helix from circumferential contour (transverse angle). For all scans, navigator gating efficiency was recorded.Navigator efficiency of STEAM was 29% lower on average compared to SE resulting in effective scan times across all volunteers of 8:28±2:56 min:sec vs. 5:59±2:43 min:sec for STEAM vs. SE. The ratio of SNR efficiency SNR_t(SE/STEAM) = SNR_t(SE)/SNR_t(STEAM) for SE versus STEAM was 2.9±0.7 and 2.4±0.6 for b=100 and 450s/mm², respectively. A comparison of helix and transverse angle maps acquired in the same subject is given in Figure 2. The linear transmural change of helix angles is noted for both techniques in Figure 2b. In the STEAM case, however, endo- and epicardial helix angles were found to be less steep with increased angle variation relative to SE. Transverse angles were close to zero degrees with significantly increased standard deviation in the STEAM case compared to SE (-1.6±20.7° vs. -2.6±16.5°, p<0.01) (Figure 2c). The root-mean-square error (RMSE) of linear regression of transmural helix angles was significantly higher for STEAM vs. SE (19.8±4.2° vs. 15.9±3.7°, p<0.01). While 3.5±1.4% of the diffusion tensors derived from STEAM had negative eigenvalues, no negative eigenvalue was detected for SE. FA values obtained with STEAM were significantly higher relative to SE (0.60±0.03 vs. 0.37±0.03, p<0.001) while MD values were lower for STEAM vs. SE (1.01±0.06x10^-3mm²/s vs. 1.49±0.15x10^-3mm²/s, p<0.001).Free-breathing cardiac diffusion tensor imaging using second-order motion-compensated SE DTI allows for significantly shorter scan times (29%) while yielding a 2.65-fold increase in SNR when compared to STEAM DTI of the in vivo heart. The higher SNR of SE DTI translates into improved diffusion tensor accuracy as evidenced by helix and transverse angle distributions. Of note, systematic differences for MD and FA between STEAM and SE were found. These differences may be explained by the long diffusion time ΔT of STEAM (ΔT_STEAM = 1000ms vs. ΔT_SE = 25ms) and associated motional narrowing effects during STEAM encoding¹¹. Navigated, second-order motion compensated SE DTI presents an attractive alternative to STEAM DTI for free-breathing mapping diffusion tensors of the in vivo heart on modern MR systems with high-performance gradients.