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

Abstract

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.

Continuous Magnetic Field Monitoring Using Rapid Re-Excitation of NMR Probe Sets

“…Field probes based on 19 F NMR permit continuous gradient monitoring concurrently with 1 H imaging procedures and/or with equally continuous 1 H transmit RF recording as demonstrated in this work. The use of 19 F samples also simplifies susceptibility matching of probeheads, since it removes the need to avoid materials that contain 1 H. On the other hand, 19 F field probes are inherently less sensitive than 1 H probes due to lower gyromagnetic ratio and lower spin density than, e.g., water.…”

“…The use of 19 F samples also simplifies susceptibility matching of probeheads, since it removes the need to avoid materials that contain 1 H. On the other hand, 19 F field probes are inherently less sensitive than 1 H probes due to lower gyromagnetic ratio and lower spin density than, e.g., water. Furthermore, suitable liquids of high 19 F content, particularly fluorocarbons, do not or hardly dissolve paramagnetic salts. Therefore it remains challenging to reduce their relaxation times to the order of 100 µs.…”

“…Studying essentially any magnetic field dynamics is no longer limited by probe dephasing and relaxation or restricted to time-consuming snapshot measurements. This capability holds promise for a variety of uses ranging from sequence development, image reconstruction, field control [25], and motion correction [26,27] to system characterization [17,19] and calibration, hardware diagnostics, and quality assurance. Additionally, continuous field monitoring opens up the novel capability of routinely recording entire scans.…”

“…Due to superior sensitivity, 1 H probes are preferable for system diagnostics and calibration, gradient and shim characterization [17,19], and sequence debugging. 19 F probes are less sensitive but offer spectral decoupling for measurement in the presence of 1 H RF transmission or image acquisition.…”

“…Two sets of 4 shortlived 1 H probes (T 2 ≈ 70 µs) were combined with four longlived 19 F probes (T 2 ≈ 36 ms, 1.3 mm droplet diameter in a setup of 23 cm diameter. The use of different nuclei ensured spectral decoupling for simultaneous operation.…”