Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine
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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.
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Water/fat separation is a classical problem for in vivo proton MRI. Although many methods have been proposed to address this problem, robust water/fat separation remains a challenge, especially in the presence of large amplitude of static field inhomogeneities. This problem is challenging because of the nonuniqueness of the solution for an isolated voxel. ⋯ Simulations and in vivo results are shown to highlight the properties of the proposed method and compare it to previous approaches. Twenty-five cardiac datasets acquired on a short, wide-bore scanner with different slice orientations were used to test the proposed method, which produced robust water/fat separation for these challenging datasets. This paper also shows example applications of the proposed method, such as the characterization of intramyocardial fat.
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Chemical exchange-dependent saturation transfer and paramagnetic chemical exchange-dependent saturation transfer are agent-mediated contrast mechanisms that depend on saturating spins at the resonant frequency of the exchangeable protons on the agent, thereby indirectly saturating the bulk water. In general, longer saturating pulses produce stronger chemical and paramagnetic exchange-dependent saturation transfer effects, with returns diminishing for pulses longer than T1. This could make imaging slow, so one approach to chemical exchange-dependent saturation transfer imaging has been to follow a long, frequency-selective saturation period by a fast imaging method. ⋯ Interleaved, multislice imaging is straightforward. Observation pulses directed at one slice did not produce observable, unintended chemical exchange-dependent saturation transfer effects in another slice. Pulse repetition time and signal-to noise ratio increase in the normal way as more slices are imaged simultaneously.
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Conventionally, MR images are formed by applying gradients to the main static magnetic field (B0). However, the B0 gradient equipment is expensive, power-hungry, complex, and noisy and can induce eddy currents in nearby conducting structures, including the patient. Here, we describe a new silent, B0 gradient-free MRI principle, Transmit Array Spatial Encoding (TRASE), based on phase gradients of the radio-frequency (RF) field. ⋯ Experimental results demonstrate one-dimensional and two-dimensional RF MRI and slice selection using a single-channel, transmit/receive, 0.2 T, permanent magnet, human MR system. The experimentally demonstrated spatial resolution is much higher than that provided by RF receive coil array sensitivity encoding alone but lower than generally achievable with B0 gradients. Potential applications are those in which one or more of the features of simplified equipment, lower costs, silent MRI, or the different physics of the image formation process are particularly advantageous.