Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine
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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.
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In subtractive imaging modalities, the differential longitudinal magnetization decays with time, necessitating signal-efficient scanning methods. Balanced steady-state free precession pulse sequences offer greater signal strength than conventional spoiled gradient echo sequences, even during the transient approach to steady state. ⋯ The validity of the technique is shown using two phantoms, and its potential is demonstrated in vivo with a variable angle schedule to increase the signal-to-noise ratio (SNR) in myocardial tissue. Using variable flip angles, the mean SNR improvement in subtractive imaging of myocardial tissue was 18.2% compared to conventional, constant flip angle, balanced steady-state free precession (P = 0.0078).
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Magnetization transfer effects represent a major source of contrast in multislice turbo spin echo sequences (TSE)/fast spin echo sequences. Generally, low refocusing flip angles have become common in such MRI sequences, especially to mitigate specific absorption rate problems. ⋯ In particular, fewer signal attenuations are observed for TSE with low flip angles such as hyperecho-TSE and smooth transitions between pseudo steady states-TSE, leading to contrast that is less dependent on the number of slices. It is shown that the strength of the magnetization transfer-induced signal attenuations can be understood and described by a physical framework, which is based on the mean square flip angle of a given TSE sequence.
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While most diffusion-weighted imaging (DWI) is acquired using single-shot diffusion-weighted spin-echo echo-planar imaging, steady-state DWI is an alternative method with the potential to achieve higher-resolution images with less distortion. Steady-state DWI is, however, best suited to a segmented three-dimensional acquisition and thus requires three-dimensional navigation to fully correct for motion artifacts. In this paper, a method for three-dimensional motion-corrected steady-state DWI is presented. ⋯ For image reconstruction, batches of cardiac-synchronized readouts are used to form three-dimensional navigators from a fully sampled central k-space cylinder. In vivo steady-state DWI with TURBINE is demonstrated in human brain. Motion artifacts are corrected using refocusing reconstruction and TURBINE images prove less distorted compared to two-dimensional single-shot diffusion-weighted-spin-EPI.