Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine
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Spatial resolution in single-shot imaging is limited by signal attenuation due to relaxation of transverse magnetization. This effect can be reduced by minimizing acquisition times through the use of short interecho spacings. However, the minimum interecho spacing is constrained by limits on gradient switching rates, radiofrequency (RF) power deposition and RF pulse length. ⋯ In vivo single-shot echoplanar imaging (EPI), BURST, and half-Fourier single-shot turbo spin-echo (HASTE) images are then presented demonstrating the practical implementation of these resolution enhancement strategies. Images acquired with SMASH show increased spatial resolution and improved image quality when compared with images obtained with the conventional acquisitions. The general principles presented for imaging with SMASH can also be applied to other partially parallel imaging techniques.
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Brain activity was studied with functional magnetic resonance imaging (fMRI) following thermal stimulation. Two groups (n = 6/group) of human male volunteers were given up to four noxious (46 degrees C) and four non-noxious (41 degrees C) stimuli. In the 46 degrees C experiment, positive signal changes were found in the frontal gyri, anterior and posterior cingulate gyrus, thalamus, motor cortex, somatosensory cortex (SI and SII), supplementary motor area, insula, and cerebellum. ⋯ A significant attenuation of the signal change was observed by the third stimulus for the 46 degrees C, but not for 41 degrees C experiment. Similar findings were replicated in the second group. These fMRI findings specify differences between somatosensory and pain sensation and suggest a number of rich avenues for future research.
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Comparative Study
High-accuracy MR tracking of interventional devices: the Overhauser marker enhancement (OMEN) technique.
A new technique for visualization of interventional devices in magnetic resonance imaging is presented. Determination of the position of an invasive device is made possible by incorporating into the device a small marker that emits the NMR signal. This signal is enhanced by the use of the Overhauser phenomenon. ⋯ A prototype marker was constructed and inserted into an inductively fed loop-gap resonator that couples saturation energy with the marker. Circuit analogies are presented that model the Overhauser phenomenon and the coupling circuit. In vitro experiments demonstrated that the marker is visible in MR images up to a slice thickness of 50 mm when inserted in excised animal liver and fat tissues.
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An improved MR tracking coil design is described that provides more robust tracking performance. It is shown theoretically and experimentally that a coil equipped with an internal spin source increases the signal-to-noise ratio in comparison to a coil system without internal source. ⋯ The experiments showed no significant effect on the signal-to-noise ratio between the two types. The improved MR-tracking coil design with internal source was successfully used in cholecystostomies and in laparoscopic interventions; both procedures were performed on swine.
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To separate water and lipid resonance signals by phase-sensitive MRI, a two-point Dixon (2PD) reconstruction is presented in which phase-unwrapping is used to obtain an inhomogeneity map based on only in-phase and out-of-phase image data. Two relaxation-weighted images, a "water image" and a "fat image," representing a two-resonance peak model of proton density, are output. The method is designed for T1- or density-weighted spin-echo imaging; a double-echo scheme is more appropriate for T2-weighted spin-echo imaging. The technique is more time-efficient for clinical fat-water imaging than 3PD schemes, while still correcting for field inhomogeneity.