35 research outputs found
Free-Breathing Myocardial T1 Mapping using Inversion-Recovery Radial FLASH and Motion-Resolved Model-Based Reconstruction
Purpose: To develop a free-breathing myocardial T1 mapping technique using
inversion-recovery (IR) radial fast low-angle shot (FLASH) and calibrationless
motion-resolved model-based reconstruction. Methods: Free-running
(free-breathing, retrospective cardiac gating) IR radial FLASH is used for data
acquisition at 3T. First, to reduce the waiting time between inversions, an
analytical formula is derived that takes the incomplete T1 recovery into
account for an accurate T1 calculation. Second, the respiratory motion signal
is estimated from the k-space center of the contrast varying acquisition using
an adapted singular spectrum analysis (SSA-FARY) technique. Third, a
motion-resolved model-based reconstruction is used to estimate both parameter
and coil sensitivity maps directly from the sorted k-space data. Thus,
spatio-temporal total variation, in addition to the spatial sparsity
constraints, can be directly applied to the parameter maps. Validations are
performed on an experimental phantom, eleven human subjects, and a young
landrace pig with myocardial infarction. Results: In comparison to an IR
spin-echo reference, phantom results confirm good T1 accuracy, when reducing
the waiting time from five seconds to one second using the new correction. The
motion-resolved model-based reconstruction further improves T1 precision
compared to the spatial regularization-only reconstruction. Aside from showing
that a reliable respiratory motion signal can be estimated using modified
SSA-FARY, in vivo studies demonstrate that dynamic myocardial T1 maps can be
obtained within two minutes with good precision and repeatability. Conclusion:
Motion-resolved myocardial T1 mapping during free-breathing with good accuracy,
precision and repeatability can be achieved by combining inversion-recovery
radial FLASH, self-gating and a calibrationless motion-resolved model-based
reconstruction.Comment: Part of this work has been presented at the ISMRM Annual Conference
2021 (Virtual), submitted to Magnetic Resonance in Medicin
High-frequency climate variability in the Holocene from a coastal-dome ice core in east-central Greenland
An ice core drilled on the Renland ice cap in east-central Greenland contains a continuous climate record dating through the last glacial period. The Renland record is valuable because the coastal environment is more likely to reflect regional sea surface conditions compared to inland Greenland ice cores that capture synoptic variability. Here we present the δ¹⁸O water isotope record for the Holocene, in which decadal-scale climate information is retained for the last 8 kyr, while the annual water isotope signal is preserved throughout the last 2.6 kyr. To investigate regional climate information preserved in the water isotope record, we apply spectral analysis techniques to a 300-year moving window to determine the mean strength of varying frequency bands through time. We find that the strength of 15–20-year δ¹⁸O variability exhibits a millennial-scale signal in line with the well-known Bond events. Comparison to other North Atlantic proxy records suggests that the 15–20-year variability may reflect fluctuating sea surface conditions throughout the Holocene, driven by changes in the strength of the Atlantic Meridional Overturning Circulation. Additional analysis of the seasonal signal over the last 2.6 kyr reveals that the winter δ¹⁸O signal has experienced a decreasing trend, while the summer signal has predominantly remained stable. The winter trend may correspond to an increase in Arctic sea ice cover, which is driven by a decrease in total annual insolation, and is also likely influenced by regional climate variables such as atmospheric and oceanic circulation. In the context of anthropogenic climate change, the winter trend may have important implications for feedback processes as sea ice retreats in the Arctic