Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine
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Three-dimensional segmented echo planar imaging (3D-EPI) is a promising approach for high-resolution functional magnetic resonance imaging, as it provides an increased signal-to-noise ratio (SNR) at similar temporal resolution to traditional multislice 2D-EPI readouts. Recently, the 3D-EPI technique has become more frequently used and it is important to better understand its implications for fMRI. In this study, the temporal SNR characteristics of 3D-EPI with varying numbers of segments are studied. ⋯ When operating in the thermal noise dominated regime, fMRI experiments with a motor task revealed that the 3D variant outperforms the 2D-EPI in terms of temporal SNR and sensitivity to detect activated brain regions. Thus, the theoretical SNR advantage of a segmented 3D-EPI sequence for fMRI only exists in a low SNR situation. However, other advantages of 3D-EPI, such as the application of parallel imaging techniques in two dimensions and the low specific absorption rate requirements, may encourage the use of the 3D-EPI sequence for fMRI in situations with higher SNR.
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Comparative Study
Ultrashort echo time imaging using pointwise encoding time reduction with radial acquisition (PETRA).
Sequences with ultrashort echo times enable new applications of MRI, including bone, tendon, ligament, and dental imaging. In this article, a sequence is presented that achieves the shortest possible encoding time for each k-space point, limited by pulse length, hardware switching times, and gradient performance of the scanner. In pointwise encoding time reduction with radial acquisition (PETRA), outer k-space is filled with radial half-projections, whereas the centre is measured single pointwise on a Cartesian trajectory. ⋯ The signal to noise ratio and Contrast-to-noise ratio (CNR) performance, as well as possible limitations of the approach, are investigated. In-vivo head, knee, ankle, and wrist examples are presented to prove the feasibility of the sequence. In summary, fast imaging with ultrashort echo time is enabled by PETRA and may help to establish new routine clinical applications of ultrashort echo time sequences.
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Three-dimensional black-blood MRI is a promising noninvasive imaging technique for the assessment of atherosclerotic carotid artery disease. However, this technique is inherently susceptible to motion. In particular, swallowing can result in considerable wall motion at the carotid bifurcations, which may induce drastic image degradation or substantial overestimation of wall thickness. ⋯ Self-gating signal readouts along the superior-inferior direction during each repetition time period are used to derive the projection profiles of the imaging volume. Based on cross-correlation analysis between the projection profiles and the corresponding reference profiles, swallowing motion can be detected and the motion-contaminated data will subsequently be discarded and reacquired in the next repetition time. The self-gated SPACE sequence was validated on eight healthy volunteers and two patients and, when compared with the conventional SPACE sequence, proved to be more resistant to swallowing motion and significantly improved image quality as well as the sharpness of carotid artery wall boundaries.
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Patients with highly hypoxic primary tumors show increased frequency of locoregional treatment failure and poor survival rates and may benefit from particularly aggressive treatment. The potential of gadolinium diethylene-triamine penta-acetic acid-based dynamic contrast-enhanced-MRI in assessing tumor hypoxia was investigated in this preclinical study. Xenografted tumors of eight human melanoma lines were subjected to dynamic contrast-enhanced-MRI and measurement of the fraction of radiobiologically hypoxic cells and the fraction of pimonidazole-positive hypoxic cells. ⋯ Tumors showing high K(trans) values and high v(e) values had low fractions of hypoxic cells, whereas tumors showing both low K(trans) values and low v(e) values had high hypoxic fractions. K(trans) differentiated better between tumors with low and high hypoxic fractions than did v(e). This study supports the current attempts to establish dynamic contrast-enhanced-MRI as a method for assessing the extent of hypoxia in human tumors, and it provides guidelines for the clinical development of valid assays.
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Parallel imaging with accelerated acquisition was noted to pronounce Gibbs artifacts which appear as ripples propagated in the phase-encoding (PE) direction near the susceptibility-affected region in echo-planar imaging (EPI). Using the extended EPI sequence, which collected extended readouts outside the regular data sampling time, the pronounced Gibbs artifact was analyzed and found to be caused by an increased echo shift in the pre-echo time (T(E)) of accelerated parallel imaging. This was also confirmed by theoretical derivation of the echo shift caused by the inplane susceptibility gradient in the PE direction (ISG(PE)). ⋯ The nonaccelerated portion in the pre-T(E) used the delay for the optimum blood oxygen level dependent (BOLD) sensitivity at 3 T, maintaining the same slice coverage as the symmetrical acceleration in both pre-T(E) and post-T(E). The increased data sampling points resulted in an increase of the signal-to-noise ratio (SNR). The restored signal and enhanced SNR of the proposed method were confirmed to deliver a better BOLD functional MRI (fMRI) result in the breath holding experiment.