A programmable pre‐emphasis system

Gach, H. Michael; Lowe, I. J.; Madio, David P.; Caprihan, Arvind; Altobelli, Stephen A.; Kuethe, Dean O.; Fukushima, Eiichi

doi:10.1002/mrm.1910400313

Cited by 37 publications

(29 citation statements)

References 6 publications

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“…[4] and [5], and the exponential eddy current model, the integral terms on the right side of Eq. [3] can be shown to be:…”

Section: Theorymentioning

confidence: 99%

“…An apparent system delay model of Eq. [3] could not specifically account for eddy current effects other than those captured by the apparent system delay:…”

Section: Determination Of System Constants and Apparent System Delaymentioning

confidence: 99%

“…System delays and eddy currents can cause the actual trajectory to deviate substantially from the ideal trajectory (1,2). Modern MRI systems include preemphasis hardware for eddy current compensation (3)(4)(5) that correct linear (first-order) eddy currents by altering the digital gradient waveform (1) or high-pass filtering the gradient waveform (6) and correct B 0 (zero-order) eddy currents by modulating the receiver phase (5) or using a current-controlled B 0 coil. Because preemphasis systems are limited to a small number of time-constants, it is not uncommon for short-time constant eddy currents to remain even when they are optimally configured.…”

mentioning

confidence: 99%

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Characterization and correction of system delays and eddy currents for MR imaging with ultrashort echo‐time and time‐varying gradients

Atkinson

Thulborn

2009

Magnetic Resonance in Med

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Reconstruction of high-quality MR images requires precise knowledge of the dynamic gradient magnetic fields used to perform spatial encoding. System delays and eddy currents can perturb the gradient fields in both time and space and significantly degrade the image quality for acquisitions with an ultrashort echo time or with rapidly varying readout gradient waveforms. A technique for simultaneously characterizing and correcting the system delay and linear-and zero-order eddy currents of an MR system is proposed. A single set of calibration scans were used to compute a set of system constants that describe the effects of system delays and eddy currents to enable accurate reconstruction of data collected before uncorrected eddy currents have decayed. Accurate MR image formation requires precise control of the gradient magnetic fields to traverse a defined k-space trajectory during data acquisition. System delays and eddy currents can cause the actual trajectory to deviate substantially from the ideal trajectory (1,2). Modern MRI systems include preemphasis hardware for eddy current compensation (3-5) that correct linear (first-order) eddy currents by altering the digital gradient waveform (1) or high-pass filtering the gradient waveform (6) and correct B 0 (zero-order) eddy currents by modulating the receiver phase (5) or using a current-controlled B 0 coil. Because preemphasis systems are limited to a small number of time-constants, it is not uncommon for short-time constant eddy currents to remain even when they are optimally configured. System delays (e.g., amplifier group delay) are typically determined during manufacturing or installation using calibration scans and manual tuning. However, because eddy currents with time-constants shorter than, or comparable to, the echo-time cannot be distinguished from a true delay (1), the measured delay value is more properly termed the apparent system delay.Traditional Cartesian acquisitions are inherently robust to uncorrected delays and short time-constant eddy currents, particularly when the echo-time is long and the data are primarily acquired under a constant gradient amplitude after all uncorrected eddy currents have decayed. These conditions are often met when preemphasis is used to compensate long time-constant eddy currents, enabling high-quality images to be obtained with minor retrospective data corrections. In contrast, acquisitions that use an ultrashort TE (UTE) or traverse complex k-space trajectories using high slew-rate, time-varying readout gradient waveforms (e.g., Twisted Projection Imaging [TPI], 3D Cones) sample data while uncorrected eddy currents are present, distorting the actual k-space path, and may have an undefined center of k-space due to the unknown system delay, leading to significant image degradation (2).A potential solution is to measure the actual k-space trajectories (7-12) and eddy currents (12) and to use them to accurately reconstruct the acquired data. While such measurement approaches work well in principle, all unique gradient w...

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“…[4] and [5], and the exponential eddy current model, the integral terms on the right side of Eq. [3] can be shown to be:…”

Section: Theorymentioning

confidence: 99%

“…An apparent system delay model of Eq. [3] could not specifically account for eddy current effects other than those captured by the apparent system delay:…”

Section: Determination Of System Constants and Apparent System Delaymentioning

confidence: 99%

mentioning

confidence: 99%

See 1 more Smart Citation

Characterization and correction of system delays and eddy currents for MR imaging with ultrashort echo‐time and time‐varying gradients

Atkinson

Thulborn

2009

Magnetic Resonance in Med

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show abstract

“…Furthermore, they can induce considerable torque on metallic instruments and implants, which can exceed the current limits of MR safety [46]. Besides dedicated hardware designs including magnetic field gradient and shim pulse-shaping networks [47], the effects of eddy currents can be minimized by correcting algorithms using the water signal as a reference [45]. Schulte et al [48] have proposed an efficient 2D eddy current correction procedure, based on a 1D phase deconvolution method.…”

Section: Spectral Resolution and Metabolite Quantificationmentioning

confidence: 99%

Proton MR spectroscopy of the brain at 3 T: an update

et al. 2007

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“…Unfortunately, the use of active shielding layer(s) occupies vital space inside the bore of the magnet, increases system cost, and reduces gradient efficiency. Furthermore, residual eddy currents are never completely removed through active shielding, and experimental compensation methods are also required for optimal results (9,10).…”

mentioning

confidence: 99%

Longitudinal gradient coil optimization in the presence of transient eddy currents

Trakic

Lopez

et al. 2007

Magnetic Resonance in Med

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The switching of magnetic field gradient coils in magnetic resonance imaging (MRI) inevitably induces transient eddy currents in conducting system components, such as the cryostat vessel. These secondary currents degrade the spatial and temporal performance of the gradient coils, and compensation methods are commonly employed to correct for these distortions. This theoretical study shows that by incorporating the eddy currents into the coil optimization process, it is possible to modify a gradient coil design so that the fields created by the coil and the eddy currents combine together to generate a spatially homogeneous gradient that follows the input pulse. During the pulsing of magnetic field gradients in MRI, multiexponentially decaying eddy currents are always induced within the conducting materials of the MR imager. Eddy currents in cold, highly conductive radiation shields of the superconducting magnet produce particularly longacting effects relative to the image acquisition period (1-2). These secondary magnetic fields are known to cause spatial and temporal degradation of the gradient uniformity within the imaging volume, which often results in undesired misregistration and intensity-phase variations in both images and spectra.With the recent push of MRI towards high signal-tonoise ratios (SNRs) and improved image resolution, tremendous efforts have been made to prevent and minimize the eddy-current fields. For instance, active screening is often engaged to minimize leakage fields and hence spatially and temporally complex residual eddy currents induced in the cryostat vessel (3-8). Unfortunately, the use of active shielding layer(s) occupies vital space inside the bore of the magnet, increases system cost, and reduces gradient efficiency. Furthermore, residual eddy currents are never completely removed through active shielding, and experimental compensation methods are also required for optimal results (9,10).During the design of conventional gradient coils (cylindrical, planar, etc.), the current distributions in one or more gradient layers are commonly optimized to obtain a target gradient uniformity in the imaging volume while satisfying other design constraints such as minimum inductance, resistance, leakage fields, force, torque, and maximum gradient efficiency (1). Traditionally, these design approaches do not take eddy currents into direct consideration when optimizing the gradient coil. Consequently, during the pulsing of the gradient current, eddy currents are induced in the cryostat vessel and other conducting materials that lead to degradation of the target field uniformity and the field stability.In 1986, Turner and Bowley (13) were among the first to introduce an analytical technique for passive magnetic screening. Their study considered spatial eddy-current variations in a thick, highly conductive, infinitely long aluminum shield as the secondary source contributing to target gradient fields. By varying the single-layer gradient coil positions to accommodate for the spatial presence of ed...

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A programmable pre‐emphasis system

Cited by 37 publications

References 6 publications

Characterization and correction of system delays and eddy currents for MR imaging with ultrashort echo‐time and time‐varying gradients

Characterization and correction of system delays and eddy currents for MR imaging with ultrashort echo‐time and time‐varying gradients

Proton MR spectroscopy of the brain at 3 T: an update

Longitudinal gradient coil optimization in the presence of transient eddy currents

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