Simple eddy current compensation by additional gradient pulses

Abstract

A simple compensation concept for eddy currents generated by additional gradient pulses is presented. Similar to other pre‐emphasis schemes, the gradient pulse shape is overemphasized in the opposite way of the effect of the eddy currents to compensate for the deviations from the anticipated (rectangular) shape. In difference to other schemes no extra hardware is required, but eddy currents have been tried to be cancelled simply by a few short extra gradient pulses. The compensation can be done either by cancelling the amplitude of the eddy currents or their effect on the phase of the NMR‐signal. For certain conditions even analytical solutions can be found. The general treatment is validated by simulations and experiments.

Correction of B0 and linear eddy currents: Impact on morphological and quantitative ultrashort echo time double echo steady state (UTE‐DESS) imaging

“…To simulate linear eddy current, a simple impulse response based on a single exponential model, h e (t), was adopted as follows: where denotes the magnitude of the eddy current and denotes the time constant of the exponential decay in the impulse response. The parameters for this simulated eddy current were based on measurements reported in the literature 29–31 : the B 0 eddy current was simulated with a of 300 μs and of 0.5 x 10 −3 , 29,31 while the linear eddy current was simulated with a of 100 μs and of 5 x 10 −3 30,31 . Although, in reality, impulse response typically results from a combination of eddy currents with different time constants and amplitudes, this experiment simulated only a single component for the purposes of demonstration.…”

“…The parameters for this simulated eddy current were based on measurements reported in the literature [29][30][31] : the B 0 eddy current was simulated with a τ c of 300 μs and m of 0.5 x 10 À3 , 29,31 while the linear eddy current was simulated with a τ c of 100 μs and m of 5 x 10 À3 . 30,31 Although, in reality, impulse response typically results from a combination of eddy currents with different time constants and amplitudes, this experiment simulated only a single component for the purposes of demonstration.…”

Design, Characterisation and Performance of an Improved Portable and Sustainable Low-Field MRI System

“…These interactions are minimized by actively shielding the gradients [33]. The quantification and correction of these effects are well documented in conventional systems [41][42][43][44][45]. In Halbachbased systems eddy currents produced by this interaction might be expected to be much lower, since the magnetic material is discretized into small elements, and the copper RF shield placed between the RF coil and gradient coils is much thinner than the skin depth at low frequencies.…”

Integrator Drift Compensation of Magnetic Flux Transducers by Feed-Forward Correction

“…However, in the more common case of iron-dominated magnets, precise prediction of non-linear and dynamic effects is beyond the current state of the art. While a number of well-established techniques exist to improve reproducibility of the magnets (e.g., by pre-cycling [14]) and to stabilize the field on flat-tops (e.g., by applying current overshoots [15]), these are often too costly in terms of lost beam time. In such cases, feedback control based on magnetic field measurements is necessary.…”

“…The uncertainty of the corrected field can be obtained as a function of time by error propagation in Equation (15). By neglecting the contributions of the NMR measurement, the coil area and the flux integration, as done in the previous subsection, the following expression is obtained:…”