Comparison of Prospective and Retrospective Gated 4D Flow Cardiac MR Image Acquisitions in the Carotid Bifurcation.

PURPOSE: To evaluate the agreement of 4D flow cMRI-derived bulk flow features and fluid (blood) velocities in the carotid bifurcation using prospective and retrospective gating techniques. METHODS: Prospective and retrospective ECG-gated three-dimensional (3D) cine phase-contrast cardiac MRI with three-direction velocity encoding (i.e., 4D flow cMRI) data were acquired in ten carotid bifurcations from men (n = 3) and women (n = 2) that were cardiovascular disease-free. MRI sequence parameters were held constant across all scans except temporal resolution values differed. Velocity data were extracted from the fluid domain and evaluated across the entire volume or at defined anatomic planes (common, internal, external carotid arteries). Qualitative agreement between gating techniques was performed by visualizing flow streamlines and topographical images, and statistical comparisons between gating techniques were performed across the fluid volume and defined anatomic regions. RESULTS: Agreement in the kinematic data (e.g., bulk flow features and velocity data) were observed in the prospectively and retrospectively gated acquisitions. Voxel differences in time-averaged, peak systolic, and diastolic-averaged velocity magnitudes between gating techniques across all volunteers were 2.7%, 1.2%, and 6.4%, respectively. No significant differences in velocity magnitudes or components ([Formula: see text], [Formula: see text], [Formula: see text]) were observed. Importantly, retrospective acquisitions captured increased retrograde flow in the internal carotid artery (i.e., carotid sinus) compared to prospective acquisitions (10.4 ± 6.3% vs. 4.6 ± 5.3%; [Formula: see text] < 0.05). CONCLUSION: Prospective and retrospective ECG-gated 4D flow cMRI acquisitions provide comparable evaluations of fluid velocities, including velocity vector components, in the carotid bifurcation. However, the increased temporal coverage of retrospective acquisitions depicts increased retrograde flow patterns (i.e., disturbed flow) not captured by the prospective gating technique.

Accelerated cardiac T1 mapping with recurrent networks and cyclic, model-based loss.

BACKGROUND: Using the spin-lattice relaxation time (T1) as a biomarker, the myocardium can be quantitatively characterized using cardiac T1 mapping. The modified Look-Locker inversion (MOLLI) recovery sequences have become the standard clinical method for cardiac T1 mapping. However, the MOLLI sequences require an 11-heartbeat breath-hold that can be difficult for subjects, particularly during exercise or pharmacologically induced stress. Although shorter cardiac T1 mapping sequences have been proposed, these methods suffer from reduced precision. As such, there is an unmet need for accelerated cardiac T1 mapping. PURPOSE: To accelerate cardiac T1 mapping MOLLI sequences by using neural networks to estimate T1 maps using a reduced number of T1-weighted images and their corresponding inversion times. MATERIALS AND METHODS: In this retrospective study, 911 pre-contrast T1 mapping datasets from 202 subjects (128 males, 56 ± 15 years; 74 females, 54 ± 17 years) and 574 T1 mapping post-contrast datasets from 193 subjects (122 males, 57 ± 15 years; 71 females, 54 ± 17 years) were acquired using the MOLLI-5(3)3 sequence and the MOLLI-4(1)3(1)2 sequence, respectively. All acquisition protocols used similar scan parameters: T R = 2.2 ms $TR\; = \;2.2\;{\rm{ms}}$ , T E = 1.12 ms $TE\; = \;1.12\;{\rm{ms}}$ , and F A = 35 ∘ $FA\; = \;35^\circ $ , gadoteridol (ProHance, Bracco Diagnostics) dose ∼ 0.075 mmol / kg $\sim 0.075\;\;{\rm{mmol/kg}}$ . A bidirectional multilayered long short-term memory (LSTM) network with fully connected output and cyclic model-based loss was used to estimate T1 maps from the first three T1-weighted images and their corresponding inversion times for pre- and post-contrast T1 mapping. The performance of the proposed architecture was compared to the three-parameter T1 recovery model using the same reduction of the number of T1-weighted images and inversion times. Reference T1 maps were generated from the scanner using the full MOLLI sequences and the three-parameter T1 recovery model. Correlation and Bland-Altman plots were used to evaluate network performance in which each point represents averaged regions of interest in the myocardium corresponding to the standard American Heart Association 16-segment model. The precision of the network was examined using consecutively repeated scans. Stress and rest pre-contrast MOLLI studies as well as various disease test cases, including amyloidosis, hypertrophic cardiomyopathy, and sarcoidosis were also examined. Paired t-tests were used to determine statistical significance with p < 0.05 $p < 0.05$ . RESULTS: Our proposed network demonstrated similar T1 estimations to the standard MOLLI sequences (pre-contrast: 1260 ± 94 ms $1260 \pm 94\;{\rm{ms}}$ vs. 1254 ± 91 ms $1254 \pm 91\;{\rm{ms}}$ with p = 0.13 $p\; = \;0.13$ ; post-contrast: 484 ± 92 ms $484 \pm 92\;{\rm{ms}}$ vs. 493 ± 91 ms $493 \pm 91\;{\rm{ms}}$ with p = 0.07 $p\; = \;0.07$ ). The precision of standard MOLLI sequences was well preserved with the proposed network architecture ( 24 ± 28 ms $24 \pm 28\;\;{\rm{ms}}$ vs. 18 ± 13 ms $18 \pm 13\;{\rm{ms}}$ ). Network-generated T1 reactivities are similar to stress and rest pre-contrast MOLLI studies ( 5.1 ± 4.0 % $5.1 \pm 4.0\;\% $ vs. 4.9 ± 4.4 % $4.9 \pm 4.4\;\% $ with p = 0.84 $p\; = \;0.84$ ). Amyloidosis T1 maps generated using the proposed network are also similar to the reference T1 maps (pre-contrast: 1243 ± 140 ms $1243 \pm 140\;\;{\rm{ms}}$ vs. 1231 ± 137 ms $1231 \pm 137\;{\rm{ms}}$ with p = 0.60 $p\; = \;0.60$ ; post-contrast: 348 ± 26 ms $348 \pm 26\;{\rm{ms}}$ vs. 346 ± 27 ms $346 \pm 27\;{\rm{ms}}$ with p = 0.89 $p\; = \;0.89$ ). CONCLUSIONS: A bidirectional multilayered LSTM network with fully connected output and cyclic model-based loss was used to generate high-quality pre- and post-contrast T1 maps using the first three T1-weighted images and their corresponding inversion times. This work demonstrates that combining deep learning with cardiac T1 mapping can potentially accelerate standard MOLLI sequences from 11 to 3 heartbeats.

Highly accelerated cardiac cine phase-contrast MRI using an undersampled radial acquisition and temporally constrained reconstruction.

PURPOSE: To evaluate a method to enable single-slice or multiple-slice cine phase contrast (cine-PC) acquisition during a single breath-hold using a highly sparsified radial acquisition ordering and temporally constrained image reconstruction with a spatially varying temporal constraint. MATERIALS AND METHODS: Simulated and in vivo cine-PC datasets of the proximal ascending aorta were obtained at different acceleration factors using a view projection acquisition order optimized for temporally constrained reconstruction (TCR). Reconstruction of the sparse cine-PC data performed with TCR was compared to reconstructions using zero-filled regridding and temporal interpolation. RESULTS: TCR resulted in more accurate velocity measurements than regridding or temporal interpolation. In one dataset, TCR of undersampled in vivo data (16 views per cardiac phase) resulted in a peak systolic velocity within 3.3% of the value measured by Doppler ultrasound while shortening the scan time to 13 seconds. High temporal-resolution undersampled TCR was also compared lower temporal-resolution, more highly sampled, regridding in three normal volunteers. CONCLUSION: TCR proved to be an effective method for reconstructing undersampled radial PC data. Although TCR utilizes a temporal constraint, temporal blurring was minimized by using appropriate constraint weights in addition to a spatially varying temporal constraint. TCR allowed for the acquisition time to be reduced to the duration of a breath-hold, while still resulting in accurate velocity measurements.