Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine
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Arterial spin labeling offers great potential in clinical applications for noninvasive measurement of cerebral blood flow. Arterial spin labeling tagging methods such as the flow sensitive alternating inversion recovery technique require efficient spatial inversion pulses with high inversion accuracy and sharp transition zones between inverted and noninverted magnetization, i.e., require a high performance inversion pulse. This work presents a comprehensive comparison of the advantages offered by a variable-rate selective excitation variant of the hyperbolic secant pulse against the widely used conventional hyperbolic secant pulse and the frequency offset corrected inversion pulses. ⋯ Both the hyperbolic secant and frequency offset corrected inversion pulses have small variations in inversion profiles that may lead to unwanted subtraction errors in arterial spin labeling at a level where the residual signal is comparable to the desired perfusion contrast. The variable-rate selective excitation pulse is shown to have improved inversion efficiency indicating its potential in perfusion MRI. The variable-rate selective excitation pulse variant also showed greatest tolerance to radiofrequency variation and off-resonance conditions, making it a robust choice for in vivo arterial spin labeling measurement.
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The purpose of this study was to establish a normal range for the arterial arrival time (AAT) in whole-brain pulsed arterial spin labeling (PASL) cerebral perfusion MRI. Healthy volunteers (N = 36, range: 20 to 35 years) provided informed consent to participate in this study. AAT was assessed in multiple brain regions, using three-dimensional gradient and spin echo (GRASE) pulsed arterial spin labeling at 3.0 T, and found to be 641 +/- 95, 804 +/- 91, 802 +/- 126, and 935 +/- 108 ms in the temporal, parietal, frontal, and occipital lobes, respectively. ⋯ Significant AAT sex differences were also found using voxelwise permutation testing. An atlas of AAT values across the healthy brain is presented here and may be useful for future experiments that aim to quantify cerebral blood flow from ASL data, as well as for clinical comparisons where disease pathology may lead to altered AAT. Pulsed arterial spin labeling signals were simulated using an identical sampling scheme as the empiric study and revealed AAT can be estimated robustly when simulated arrival times are well beyond the normal range.
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In vivo radiofrequency (RF) field B(1) mapping represents an essential prerequisite for parallel transmit applications. However, the large dynamic range of the transmit fields of the individual coil elements challenges the accuracy of MR-based B(1) mapping techniques. In the present work, the B(1) mapping error and its impact on the RF performance are studied based on a coil eigenmode analysis. ⋯ In addition, the weighting of the eigenmodes is tailored to potential target applications, e.g., specific absorption rate (SAR) reduced RF shimming or multidimensional RF pulses, resulting in improved RF performance. The basic theoretic principles of the concept are elaborated and validated by corresponding simulations. Furthermore, results on B(1) mapping and RF shimming experiments, performed on phantoms and in vivo using a 3-T scanner equipped with an eight-channel transmit/receive body coil, are presented to prove the feasibility of the approach.
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Contrast-enhanced three-dimensional T(1)-weighted imaging based on magnetization-prepared rapid-gradient recalled echo is widely used for detecting small brain metastases. However, since contrast materials remain in both blood and the tumor parenchyma and thus increase the signal intensity of both regions, it is often challenging to distinguish brain tumors from blood. In this work, we develop a T(1)-weighted, black-blood version of single-slab three-dimensional turbo/fast spin echo whole-brain imaging, in which the signal intensity of the brain tumor is selectively enhanced while that of blood is suppressed. ⋯ To avoid a signal loss resulting from the flow-sensitizing scheme, the first refocusing flip angle is forced to 180 degrees. Composite restore pulses at the end of refocusing pulse train are applied to achieve partial inversion recovery for the T(1)-weighted contrast. Simulations and in vivo volunteer and patient experiments are performed, demonstrating that this approach is highly efficient in detecting small brain metastases.