Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine
-
The inherent distortions in echo-planar imaging that arise due to inhomogeneities in the static magnetic field can lead to difficulties when attempting to obtain structurally accurate diffusion-tensor imaging data. Parallel acceleration techniques can reduce the magnitude of these distortions but do not remove them entirely. Images can be corrected using a measured field map, but this is prone to error. ⋯ This allows benefits of the blip-reversed approach to be exploited, with only a modest increase in scan time and, due to the extra data acquired, no significant loss of signal-to-noise efficiency. A novel approach to recombining blip-reversed data is also presented, which involves refining the measured field map, using an algorithm to minimize the difference between the corrected images. The field map refinement is also applicable to conventionally acquired blip-reversed sequences.
-
We have discovered a simple and highly robust method for removal of chemical shift artifact in spin-echo MR images, which simultaneously decreases the radiofrequency power deposition (specific absorption rate). The method is demonstrated in spin-echo echo-planar imaging brain images acquired at 7 T, with complete suppression of scalp fat signal. When excitation and refocusing pulses are sufficiently different in duration, and thus also different in the amplitude of their slice-select gradients, a spatial mismatch is produced between the fat slices excited and refocused, with no overlap. ⋯ This enables greater volume coverage per unit time, well suited for functional and diffusion studies using spin-echo echo-planar imaging. Moreover, the method can be generally applied to any sequence involving slice-selective excitation and at least one slice-selective refocusing pulse at high magnetic field strengths. The method is more efficient than gradient reversal methods and more robust against inhomogeneities of the static (polarizing) field (B(0)).
-
High spectral and spatial resolution MRI, based on echo-planar spectroscopic imaging, has been applied successfully in diagnostic breast imaging, but acquisition times are long. One way of increasing acquisition speed is to apply the sensitivity encoding algorithm for complex high spectral and spatial resolution data. ⋯ Very low g factors are obtained using the breast coils, and the signal-to-noise ratio (SNR) penalty for water resonance peak height and water resonance asymmetry images is small at acceleration factors of up to 6 and 4, respectively, as evidenced by high Pearson correlation factors between fully sampled and accelerated data. This is the first application of the sensitivity encoding algorithm to characterize the structure of the water resonance at high spatial resolution.
-
This pilot study presents a technique for three-dimensional and quantitative analysis of meniscus shape, position, and signal intensity and compares results in knees with (n = 20) and without (n = 11) radiographic osteoarthritis. 3-T MR images with 2mm section thickness were acquired using a proton density-weighted, fat-suppressed, coronal, fast spin-echo sequence. Segmentation of the tibial, femoral, and external surface of the medial meniscus and the tibial joint surface was performed. ⋯ Key results included a greater size (i.e., volume, surface areas, and thickness), increased medial extrusion (i.e., greater extrusion distance, greater meniscal area uncovered by tibial surface), and elevated signal intensity of the medial meniscus in osteoarthritis than in nonosteoarthritis knees, particularly in the meniscus body. These results need to be confirmed in larger cohorts, preferably under weight-bearing conditions.
-
Conventional T(2)-weighted turbo/fast spin echo imaging is clinically accepted as the most sensitive method to detect brain lesions but generates a high signal intensity of cerebrospinal fluid (CSF), yielding diagnostic ambiguity for lesions close to CSF. Fluid-attenuated inversion recovery can be an alternative, selectively eliminating CSF signals. However, a long time of inversion, which is required for CSF suppression, increases imaging time substantially and thereby limits spatial resolution. ⋯ Dual acquisition in each time of repetition is performed, wherein an in phase between CSF and brain tissues is achieved in the first acquisition, while an opposed phase, which is established by a sequence of a long refocusing pulse train with variable flip angles, a composite flip-down restore pulse train, and a short time of delay, is attained in the second acquisition. A CSF-suppressed image is then reconstructed by weighted averaging the in- and opposed-phase images. Numerical simulations and in vivo experiments are performed, demonstrating that this single pulse sequence may replace both conventional T(2)-weighted imaging and fluid-attenuated inversion recovery.