Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine
-
With the increasing number of transgenic mouse models of human brain diseases, there is a need for a sensitive method that allows assessing quantitative whole brain perfusion within a reasonable scan time. Arterial spin labeling (ASL), an MRI technique that permits the noninvasive quantification of cerebral blood flow, has been used to assess rodents brain perfusion. ⋯ Pseudo-continuous ASL was experimentally optimized and compared with a standard flow-sensitive alternating inversion recovery sequence for sensitivity, robustness, absolute quantification, and multislice imaging capability. A sensitivity gain up to 40% and clear advantages for multislice imaging are obtained with pseudo-continuous ASL.
-
Susceptometry-based MR oximetry has previously been shown suitable for quantifying hemoglobin oxygen saturation in large vessels for studying vascular reactivity and quantification of global cerebral metabolic rate of oxygen utilization. A key assumption underlying this method is that large vessels can be modeled as long paramagnetic cylinders. ⋯ At a typical venous oxygen saturation of 65%, the absolute error in hemoglobin oxygen saturation estimated via a closed-form cylinder approximation was 2.6% hemoglobin oxygen saturation averaged over three locations in the three veins studied and did not exceed 5% for vessel tilt angles <30° at any one location. In conclusion, the simulation results provide a significant level of confidence for the validity of the cylinder approximation underlying MR susceptometry-based oximetry of large vessels.
-
Comparative Study
The influence of radial undersampling schemes on compressed sensing reconstruction in breast MRI.
Fast imaging applications in magnetic resonance imaging (MRI) frequently involve undersampling of k-space data to achieve the desired temporal resolution. However, high temporal resolution images generated from undersampled data suffer from aliasing artifacts. In radial k-space sampling, this manifests as undesirable streaks that obscure image detail. ⋯ Results from point spread function studies, simulations, phantom and in vivo experiments show that the choice of radial sampling pattern influences the quality of the final image reconstructed by the compressed sensing algorithm. While evenly undersampled radial trajectories are best for specific temporal resolutions, golden-angle radial sampling results in the least overall error when various temporal resolutions are considered. Reduced temporal fluctuations from aliasing artifacts in golden-angle sampling translates to improved compressed sensing reconstructions overall.
-
Comparative Study
On the performance of T2* correction methods for quantification of hepatic fat content.
Nonalcoholic fatty liver disease is the most prevalent chronic liver disease in Western societies. MRI can quantify liver fat, the hallmark feature of nonalcoholic fatty liver disease, so long as multiple confounding factors including T(2)* decay are addressed. Recently developed MRI methods that correct for T(2)* to improve the accuracy of fat quantification either assume a common T(2)* (single-T(2)*) for better stability and noise performance or independently estimate the T(2)* for water and fat (dual-T(2)*) for reduced bias, but with noise performance penalty. ⋯ Optimization was performed for six echoes and typical T(2)* values. This analysis showed that all methods have better noise performance with very short first echo times and echo spacing of ∼π/2 for single-T(2)* correction, and ∼2π/3 for dual-T(2)* correction. Interestingly, when an echo spacing and first echo shift of ∼π/2 are used, methods without T(2)* correction have less than 5% bias in the estimates of fat fraction.
-
Comparative Study
Selective excitation of two-dimensional arbitrarily shaped voxels with parallel excitation in spectroscopy.
Parallel excitation is being studied intensively for applications in MR imaging and in particular for selecting arbitrary shapes as regions of interest. In this work, parallel excitation was applied to arbitrarily shaped voxel selection in spectroscopy and investigated for different excitation k-space trajectories (radial, rectilinear, and spiral) and acceleration factors. Each trajectory was segmented into multiple excitations to increase the overall bandwidth during target selection. ⋯ The selective excitation experiments demonstrated excellent spatial localization and a broad frequency response, although PRESS was superior in direct comparisons with respect to signal-to-noise ratio (SNR) and outer volume suppression. Extensive SNR variation was observed dependent on trajectory (8%-90%), with the preferred radial case producing approximately 40%-60% SNR of the PRESS case. Accelerated trajectories at R = 4 provided comparable artifact signal and target excitation accuracy compared with their nonaccelerated counterparts; however, further acceleration (R = 8) resulted in increased artifact (33% increase at R = 8).