Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine
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This work describes a novel method for highly undersampled projection imaging using constrained reconstruction by Tikhonov-Phillips regularization and its application for high temporal resolution functional MRI (fMRI) at a repetition time of 80 ms. The high-resolution reference image used as in vivo coil sensitivity is acquired in a separate acquisition using otherwise identical parameters. Activation studies using a standard checkerboard activation paradigm demonstrate the inherent high sensitivity afforded by the possibility to separate activation-related effects from "physiological noise.". ⋯ The COBRA method can be extended to three-dimensional (3D) encoding without severe penalty in temporal performance. Analysis of the global signal change demonstrates the excellent reproducibility of COBRA compared to standard EPI. Activation analysis is considerably improved by the possibility to remove electrocardiogram (ECG)-related and breathing-related signal fluctuations by physiological correction of each individual breathing and ECG cycle, respectively.
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An accurate noninvasive method to measure the hemoglobin oxygen saturation (%HbO(2)) of deep-lying vessels without catheterization would have many clinical applications. Quantitative MRI may be the only imaging modality that can address this difficult and important problem. MR susceptometry-based oximetry for measuring blood oxygen saturation in large vessels models the vessel as a long paramagnetic cylinder immersed in an external field. ⋯ The average standard error and coefficient of variation of measurements in phantoms were <2% with tilt correction alone, in agreement with theory, suggesting that high accuracy and reproducibility can be achieved while ignoring noncircularity for tilt angles up to about 30 degrees. In vivo, repeated measurements of %HbO(2) in the femoral vessels yielded a coefficient of variation of less than 5%. In conclusion, the data suggest that %HbO(2) can be measured reproducibly in vivo in large vessels of the peripheral circulation on the basis of the paramagnetic cylinder approximation of the incremental field.
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Single-shot echo-planar imaging (EPI) is well established as the method of choice for clinical, diffusion-weighted imaging with MRI because of its low sensitivity to the motion-induced phase errors that occur during diffusion sensitization of the MR signal. However, the method is prone to artifacts due to susceptibility changes at tissue interfaces and has a limited spatial resolution. ⋯ The technique uses data from a 2D navigator acquisition to perform a nonlinear phase correction and to control the real-time reacquisition of unusable data that cannot be corrected. Measurements on healthy volunteers demonstrate that this approach provides a robust correction for motion-induced phase artifact and allows scan times that are suitable for routine clinical application.
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Reconstruction of high-quality MR images requires precise knowledge of the dynamic gradient magnetic fields used to perform spatial encoding. System delays and eddy currents can perturb the gradient fields in both time and space and significantly degrade the image quality for acquisitions with an ultrashort echo time or with rapidly varying readout gradient waveforms. ⋯ A single set of calibration scans were used to compute a set of system constants that describe the effects of system delays and eddy currents to enable accurate reconstruction of data collected before uncorrected eddy currents have decayed. The ability of the proposed technique to reproducibly characterize small fixed delays (<50 micros) and short-time constant (<1 ms) eddy currents is demonstrated.