Compensation of gradient-induced magnetic field perturbations

Abstract

Pulsed magnetic field gradients are essential for MR imaging and localized spectroscopy applications. However, besides the desired linear field gradients, pulsed currents in a strong external magnetic field also generate unwanted effects like eddy currents, gradient coil vibrations and acoustic noise. While the temporal magnetic field perturbations associated with eddy currents lead to spectral line shape distortions and signal loss, the vibration-related modulations lead to anti-symmetrical sidebands of any large signal (i.e. water), thereby obliterating the signals from smaller signals (i.e. metabolites). Here the measurement, characterization and compensation of vibrations-related magnetic field perturbations is presented. Following a quantitative evaluation of the various temporal components of the main magnetic field, a digital B 0 magnetic field waveform is generated which reduces all temporal variations of the main magnetic field to within the spectral noise level.

Proton MRS and MRSI of the brain without water suppression

“…The detailed mechanisms underlying the correlations between the sideband artifacts and the dynamic magnetic fields may be complex and are still not fully understood [41,74]. However, the following may shed some light on the understanding and explanation: (1) The interaction of the magnetic field B 0 and the time-varying electrical current in the gradient coils will exert a magnetic force, called the Lorentz force, on the gradient coil windings, thus causing mechanical vibrations of coils as manifested by the acoustic noise during the MR scans.…”

“…where s m0 is the ideal signal without the influence of the timevarying field induced by the exciting gradient and u(t) can be decomposed into the following terms [74], The ratio of the water peak to the 1st order sidebands is 25:1. When this ratio is larger, the 2nd order sidebands are even smaller relative to the 1st order sidebands.…”

Spectroscopic Imaging of the Mouse Brain

“…The four orders in magnitude difference in concentration of water versus the metabolites of interest not only stresses a strong demand on the dynamic range of the MR receiver but also on the stability of the setup. Any coherence in vibration, for instance, caused by the application of gradients, by breathing of the mouse, or by the cardiac cycle, can result in sidebands of the water signal that contaminates the spectral region of interest (5). Suppression of the signals from water can therefore exclude these potential artifacts.…”

“…Secondly, good B 0 shim needs to be obtained to minimize linewidths of the peaks in order to reduce signal overlap, thereby enabling distinct detection of metabolites (4). Thirdly, the signals from the highly concentrated water content have to be accurately suppressed not only to overcome dynamic range limitations of the MR receiver but also to reduce spectral contamination caused by potential vibrations, eddy currents, and (physiologic) motion (5). Fourthly, contaminating signals originating from areas outside the volume of interest need to be suppressed to below noise level as these may leak into the voxels of interest due to the point spread function in spatial Fourier transform.…”

Image correction during large and rapid B0 variations in an open MRI system with permanent magnets using navigator echoes and phase compensation

“…Their image effects can be corrected in real time by updating the frequency [13] or phase [14] of the radiofrequency (RF) transmitter and receiver or retrospectively with corresponding phase factors [2,8,10,11]. Previous works on compensating B 0 field variation have largely focused on image-to-image artifacts in functional MRI (fMRI) [2,3,10,11,13,[15][16][17][18], spectra deterioration of in vivo MR spectroscopy (MRS) [4,9,12,[19][20][21][22][23][24][25][26] and high-resolution nuclear magnetic resonance (NMR) spectroscopy [27][28][29], which were mainly caused by low frequency B 0 variation due to respiration or external perturbations.…”