Single point measurements of magnetic field gradient waveform

Goodyear, David J.; Shea, Maurice; Beyea, Steven; Shah, N. Jon; Balcom, Bruce J.

doi:10.1016/s1090-7807(03)00152-6

Cited by 13 publications

(24 citation statements)

References 15 publications

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“…Large Amplitude Gradient Measurement Using the Gel Phantom Method (23) Large amplitude gradient measurements are shown for illustrating how the inverse Fourier transform is employed for gel phantom gradient waveform measurements (23). The phase y ¼ gGt p b, in a specific measurement with t p and b fixed, is proportional to the gradient amplitude.…”

Section: Resultsmentioning

confidence: 99%

“…Traditional MR based gradient calibration techniques (15)(16)(17)(18)(19)(20)(21), whether based on an entire FID (17)(18)(19)(20)(21) or a gradient echo (15,16), measure the integral of the gradient waveforms. We have however previously proposed a different category of methods (22)(23)(24), which employ pure phase encode single FID point acquisition and have proven advantageous in measuring gradient waveforms. In this article, we review the basis of these pure phase encode methods.…”

Section: Introductionmentioning

confidence: 99%

“…We have previously proposed a class of fundamentally different strategies (22)(23)(24), single point imaging (SPI), to directly measure magnetic field gradient waveforms. By employing a train of pure phase encode FIDs, only one single point for each FID is acquired and thus it largely avoids signal dephasing caused by large gradient area.…”

Section: Introductionmentioning

confidence: 99%

“…The ideas outlined in this article are a development of the gel phantom approach (23) and also form the basis of the MFGM method (24). The Fourier transform symmetry and shift theorems, permit ready understanding of these pure phase encode methods.…”

Section: Introductionmentioning

confidence: 99%

“…For SPRITE imaging of such short T 2 * species, a short encoding time t p is required and in consequence a high gradient strength is required to maintain an appropriate FOV (28). High gradient strengths are also necessary for nonproton SPRITE imaging (e.g., 23 Na, 7 Li) to compensate for the low gyromagnetic ratio. In these cases a high efficiency gradient coil may be employed for generating high magnetic field gradient strengths (28).…”

Section: Introductionmentioning

confidence: 99%

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Direct measurement of magnetic field gradient waveforms

Han¹,

Ouriadov²,

Fordham³

et al. 2010

Concepts Magnetic Resonance

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As eddy currents increase with gradient amplitude and faster slew rate, they have become a greater problem with the advent of higher-performance gradients in modern MRI scanners. Success in eddy current reduction techniques such as active gradient shielding and waveform pre-emphasis, however, require that the residual eddy currents must be measured with high accuracy for image improvement. Traditional MR based gradient calibration techniques, whether based on an entire FID or a gradient echo, measure the integral of gradient waveforms. We have however previously proposed a different category of methods, which employ pure phase encode FIDs and have proven advantageous in directly measuring the gradient waveforms. In this article, we review the basis of these pure phase encode methods. In keeping with the instructional nature of CMRA, we undertake this review by describing specific experiments and the line of the thought behind the experiments. The pure phase encode approach is sensitive to low amplitude gradients (0.001-1 G/cm), and also permits measurement of high amplitude gradients (10-300 G/cm). The inverse Fourier transform permits ready understanding of these pure phase encode methods. The accuracy of pure phase encode gradient measurement is significantly improved by a multiple FID point acquisition, which permits high temporal resolution of the gradient waveform. The accuracy of gradient measurements is also analyzed and improved through elimination of potential artifacts. As one example of the capability of these methods, waveform measurements were undertaken to reduce the repetition time TR for centric scan SPRITE experiments.

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Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

Direct measurement of magnetic field gradient waveforms

Han¹,

Ouriadov²,

Fordham³

et al. 2010

Concepts Magnetic Resonance

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Accuracy of the Magnetic Field Gradient Waveform Monitor Technique and Consequent Accuracy of Pre‐Equalized Gradient Waveform

Goora

Balcom

2016

Concepts Magn Reson

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The magnetic field gradient waveform monitor (MFGM) technique permits characterization of the temporal evolution of magnetic field gradients in magnetic resonance (MR) instruments (MRIs). Knowledge of the gradient waveform performance permits the development of further techniques, such as gradient waveform pre‐equalization, that correct and optimize gradient waveform distortions due to eddy currents induced during the application of switched magnetic fields and other system limitations. The accuracy of the MFGM technique is important since the overall uncertainty of the gradient waveform measurement will propagate into an uncertainty in corrected gradient waveforms impacting the precision of the resulting MR/MRI measurements. The accuracy of MFGM is investigated through a treatment of the noise present in a MRI. A noisy receiver model provides the basis for characterization of the noise and permits examination of the overall impact of noise on the phase accumulated in a pure‐phase encoded MR signal. Ultimately, a relationship between the signal‐to‐noise ratio of a measurement and the corresponding MFGM uncertainty is developed. The theoretical development is supported through simulation in conjunction with experimental results. The propagation of uncertainties to gradient waveform pre‐equalization is also discussed.

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Gradient system characterization by impulse response measurements with a dynamic field camera

Vannesjo

Haeberlin

Kasper

et al. 2012

Magnetic Resonance in Med

168

321

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This work demonstrates a fast, sensitive method of characterizing the dynamic performance of MR gradient systems. The accuracy of gradient time-courses is often compromised by field imperfections of various causes, including eddy currents and mechanical oscillations. Characterizing these perturbations is instrumental for corrections by pre-emphasis or post hoc signal processing. Herein, a gradient chain is treated as a linear time-invariant system, whose impulse response function is determined by measuring field responses to known gradient inputs. Triangular inputs are used to probe the system and response measurements are performed with a dynamic field camera consisting of NMR probes. In experiments on a whole-body MR system, it is shown that the proposed method yields impulse response functions of high temporal and spectral resolution. Besides basic properties such as bandwidth and delay, it also captures subtle features such as mechanically induced field oscillations. For validation, measured response functions were used to predict gradient field evolutions, which was achieved with an error below 0.2%. The field camera used records responses of various spatial orders simultaneously, rendering the method suitable also for studying cross-responses and dynamic shim systems. It thus holds promise for a range of applications, including pre-emphasis optimization, quality assurance, and image reconstruction.

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Single point measurements of magnetic field gradient waveform

Cited by 13 publications

References 15 publications

Direct measurement of magnetic field gradient waveforms

Direct measurement of magnetic field gradient waveforms

Accuracy of the Magnetic Field Gradient Waveform Monitor Technique and Consequent Accuracy of Pre‐Equalized Gradient Waveform

Gradient system characterization by impulse response measurements with a dynamic field camera

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