Determination of pre-emphasis constants for eddy current reduction

Abstract

Magnetic field gradients play a fundamental role in fast magnetic resonance imaging (MRI) methods and spectroscopy imaging. Precise information on gradient waveform shape and rise times is often very useful in experimental MRI. We present a simple method for the measurement of the gradient time course and static magnetic field changes. The method does not require any specialized hardware and can be used with a standard volume coil and a special phantom filled with deionized or doped water. The method is based on an analysis of the instantaneous frequency variation of an MR signal in the time domain, acquired from a mechanically selected thin slice of phantom located at the gradient isocentre. The measurement and approximation of the course of time gradients and of the changes in the static magnetic field result in pre-emphasis constants that substitute the constants determined manually in a time-consuming manner. The described method facilitates the determination of pre-emphasis constants for the compensation of eddy current effects in MR systems. Our results show that residual gradients in the region of interest within 0.5 ms after the gradient is switched off are smaller than 5 µT m−1 (for an applied test gradient of 72 mT m−1) and the shift of the static field is smaller than 0.2 µT. This accuracy is very important for the development of MR spectroscopic imaging technologies using samples with very short relaxation times, and for EPI pulse sequences.

Magnetic Resonance Imaging of Hard Tissues and Hard Tissue Engineered Bio-substitutes

“…Also, a high-performance RF receiver system is demanded to quickly receive the signal without losing any data [18]. Nevertheless, short receiver sampling period and high-demanding gradients required by UTE sequences induce eddy currents into the conducting structures of the scanner (e.g., gradient coils, shim coil, and other parts of the cryostat), which lead to imaging artifacts, localization errors, and signal distortions [19,20]. Imaging imperfections due to eddy currents are common also for MRI sequences based on non-Cartesian sampling acquisitions (e.g., radial and spatial imaging), and have been attenuated by using many strategies, such as pre-emphasis corrections, imaging-based gradient measurements (IGM), and gradient impulse response function (GIRF) [21,22].…”

Modeling and Experimental Verification of Plasma Jet Electromagnetic Signals

“…A set of selected plasma parameters that are significant in terms of the electromagnetic field model created and discussed herein is presented in Table 1. The functionality of the proposed methodology and the actual models of the field [22][23][24][25][26][27][28][29][30] was verified by measuring the electromagnetic field irradiated by the plasma jet, with the plasma located close to the jet orifice; the actual measurement was performed using a suitably designed dipole antenna [5][6][7][8] linked to a frequency analyzer. Table 1.…”

“…This task was centered on the equivalent electromagnetic model of the plasma [31]. The high-frequency electromagnetic field [14,18] was analyzed correspondingly to the pre-specified configuration and the material characterization of the modeled problem [21][22][23][24][25][26], assuming the desired excitation frequency. The geometrical arrangement and the analysis are graphically represented in Figure 2.…”

Detection of seepages in flood embankments using the ElasticNET method

“…Electric tomography is based on the transformation of data taken from the surface of the tested object into the image of its cross-section. There are many methods to optimize the obtained image by solving the appropriate objective function [1][2][3][4][5]13,15,16,[20][21][22][23][24][25]32]. The algorithm based on the ElasticNET presented in this article is a new proposal in tomography.…”