Multi-echo Spin Echo Pulse Sequence Research Articles

Selective excitation localized by the Bloch-Siegert shift and a gradient.

To perform -selective excitation using the Bloch-Siegert shift for spatial localization. A -selective excitation is produced by an radiofrequency (RF) pulse consisting of two summed component pulses: an off-resonant pulse that induces a -dependent Bloch-Siegert frequency shift and a frequency-selective excitation pulse. The passband of the pulse can be tailored by adjusting the frequency content of the frequency-selective pulse, as in conventional gradient-localized excitation. Fine magnetization profile control is achieved by using the Shinnar-Le Roux algorithm to design the frequency-selective excitation pulse. Simulations analyzed the pulses' robustness to off-resonance, their suitability for multi-echo spin echo pulse sequences, and how their performance compares to that of rotating-frame selective excitation pulses. The pulses were evaluated experimentally on a 47.5 mT MRI scanner using an RF gradient transmit coil. Multiphoton resonances produced by the pulses were characterized and their distribution across predicted. With correction for varying across the desired profile, the proposed pulses produced selective excitation with the specified profile characteristics. The pulses were robust against off-resonance and RF amplifier distortion, and suitable for multi-echo pulse sequences. Experimental profiles closely matched simulated patterns. The Bloch-Siegert shift can be used to perform -gradient-free selective excitation, enabling the excitation of slices or slabs in RF gradient-encoded MRI.

Evaluation of electrical conductivity and anisotropy in muscle tissues using conductivity tensor imaging (CTI)

Low-frequency conductivity of an anisotropic tissue is associated with its cellular structure. Imaging of the conductivity tensor inside the human body could prove invaluable to analyses of interactions between electromagnetic fields and biological systems, such as predictions of current pathways during electrical stimulation. Since the muscle is the most abundant anisotropic tissue in the human body, in vitro and in vivo muscle conductivity tensor assessment has been attempted. In this study, we conducted phantom imaging of biological tissues using conductivity tensor imaging (CTI) to validate its in vivo usefulness. We constructed phantoms using bovine and/or porcine muscles and performed a CTI experiment using MRI. High-frequency conductivity was first obtained using B1 mapping with a multi-echo spin-echo pulse sequence. Information about the cellular space was obtained with a multi-b diffusion tensor imaging sequence. We combined the data from these separate scans to reconstruct the conductivity tensor images of the phantoms. The low-frequency and high-frequency conductivities of the muscle tissues in the phantoms were compared with the corresponding values measured by an impedance analyzer. The anisotropy of each muscle tissue was quantified as an anisotropy ratio (AR), defined as the ratio of the eigenvalues of a conductivity tensor along the longitudinal direction to those along the transversal directions. The isotropic conductivity and conductivity tensor in bovine muscles were less than those of porcine muscles. However, the anisotropy was stronger in bovine muscles based on the AR values by fiber directions. Current CTI is a promising noninvasive tool for evaluation of the muscle microstructure.

Analysis of the multi‐echo spin‐echo pulse sequence

AbstractThe multi‐echo spin‐echo sequence is a series of operators, referred to as periodic operators. Each periodic operator consists of a free rotation (no RF), a refocusing RF pulse, and another free rotation, identical to the first one. A preparation operator that precedes the periodic operators converts the equilibrium magnetization Mz into an initial magnetization Mi. It is shown that a multi‐echo sequence is equivalent to a simple rotation of the magnetization about a tilted axis. The component of Mi along the rotation axis is stationary and provides a stable signal, denoted pseudo steady‐state. The perpendicular component rotates and eventually de‐phases. Using this model, we derive analytic expressions to the signal for different preparation operators, and show how to align Mi with the rotation axis such that the signal is maximized. A simple and efficient algorithm is presented to calculate the Fourier coefficients of the magnetization during the sequence using the discrete Fourier transform. Finally, formulas of the echo signal when unavoidable phase errors are generated are derived. We show how to eliminate artifacts caused by these errors and restore the original image.

Magnetic resonance scanning and image segmentation procedure at 3 T for volumetry of human hippocampal subfields

BACKGROUND: Recent evidence suggests that dementia affects hippocampal substructures differentially and thus identifying anatomical details of this structure is potentially clinically important. Visualising details of human hippocampal substructures in vivo is challenging by imaging due to the small size of the medial temporal lobe structure. METHODS: MRI data were acquired with a 3 T MR scanner using a 2D multi-echo spin echo pulse sequence at such a spatial resolution to reveal hippocampal subfield boundaries. These images were used to develop a manual segmentation procedure for the hippocampal subfields based on image contrast within the structures and/or geometric constraints by anatomical landmarks. RESULTS: The T2-images were used to devise a segmentation protocol for Cornu Ammonis (CA) CA1, CA2, CA3, dentate gyrus, subiculum and lumped Stratum Lacunosum + Stratum Moleculare + Stratum Radiatum. The segmentation protocol was applied to MRI data from healthy young and aged controls as well as a small cohort of mild cognitive impairment (MCI) subjects. The reported subfield volumes showed high levels of inter-rater reliability arguing for potential as a tool in documenting subfield volumetry in clinical research. CONCLUSIONS: A comprehensive and robust MRI protocol which allows the labelling of six separate hippocampal subfields from images acquired with a 3 T clinical scanner is presented to promote hippocampal subfield volumetry of clinical cohorts.

Comparison of conventional single echo and multi‐echo sequences with a fast spin‐echo sequence for quantitative T2 mapping: Application to the prostate

The accuracy of water T2 maps generated from a fast spin-echo (FSE) sequence was compared with data obtained by conventional single and multi-echo spin-echo pulse sequences using a commercial gel phantom. Spatially localized stimulated echo acquisition mode (STEAM) proton spectroscopy was also used to confirm the reported water T2 values of the gels contained in the phantom. The FSE sequence was shown to be superior in accuracy to both the single and multi-echo spin echo sequences and comparable to STEAM, producing results that were within 10% of known values. The effectiveness of the FSE sequence was further demonstrated by generating T2 maps of the normal and diseased prostate in clinically acceptable imaging times, resulting in comparable T2 values to those obtained using STEAM. Accurate quantitative T2 maps can be produced with the FSE sequence.