A resistive Q-switch for low-field NMR systems

Zhen, John; O’Neill, Keelan; Fridjonsson, Einar O.; Stanwix, Paul L.; Johns, Michael L.

doi:10.1016/j.jmr.2017.12.006

Cited by 18 publications

(5 citation statements)

References 15 publications

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“…Nuclear magnetic resonance (NMR) is an example where producing short RF pulses presents a challenge [4]- [11], [18]- [23]. The inset of Fig.…”

Section: Nmr Applicationmentioning

confidence: 99%

“…Here, QL is the loaded Q which, under impedance matched conditions, is ½ the unloaded Q of the isolated tank circuit. This problem has typically been addressed with active Q-suppression circuits [22]- [23] which add complexity and only partially mitigate the problem.…”

Section: Nmr Applicationmentioning

confidence: 99%

“…Since the coil ringdown voltage may start out orders of magnitude larger than the FID, there is considerable dead time, sometimes as much as 20τQ, after the pulse ends before it is possible to begin recording the FID. This dead time introduces phase errors, complicates the transformation of the FID into a spectrum, and significantly reduces the available signal to noise ratio [23]. The problem is typically solved by using spin echoes, which are widely separated from the RF pulse but there are times when an FID is needed.…”

Section: Nmr Applicationmentioning

confidence: 99%

See 2 more Smart Citations

Modal Engineering of Inductive Circuits to Achieve Rapid Settling Times

Javor¹,

Barrett²,

Imboden³

et al. 2021

Preprint

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Inductive circuits and devices are a ubiquitous and important design element in many applications such as magnetic drives, galvanometers, magnetic scanners, applying DC magnetic fields to systems, RF coils in NMR systems and vast array of other applications. They are widely used to generate both DC and AC magnetic fields. Many of these applications require a rapid step and settling time, turning the DC or AC magnetic field on and off quickly. The inductive response normally makes this a challenging thing to do. In this article we discuss open loop control algorithms for achieving rapid step and settling times in four general categories of applications: DC and AC systems where the system is either under or over damped. Each of these four categories requires a different algorithm which we describe here. We show the operation of these drive methods using Simulink and Simscape modeling tools, analytical solutions to the underlying differential equations and in experimental results using an inductive magnetic coil and a Hall sensor. Finally, we demonstrate application of these techniques to significantly reduce ringing in a standard NMR circuit. We intend this article to be practical with useful, easy to apply algorithms and helpful tuning tricks.

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“…Nuclear magnetic resonance (NMR) is an example where producing short RF pulses presents a challenge [4]- [11], [18]- [23]. The inset of Fig.…”

Section: Nmr Applicationmentioning

confidence: 99%

Section: Nmr Applicationmentioning

confidence: 99%

Section: Nmr Applicationmentioning

confidence: 99%

See 1 more Smart Citation

Modal Engineering of Inductive Circuits to Achieve Rapid Settling Times

Javor¹,

Barrett²,

Imboden³

et al. 2021

Preprint

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show abstract

“…For example, in a spin echo sequence this would mean that the 90˚and 180˚pulses could not be applied appropriately for square RF pulses. The ring-down time can be shortened by either using a Q-switch [15][16][17][18], or through the application of a damping pulse which is 180˚out-of-phase with main pulse [13,18]. Similarly, the rise time can be shortened by applying pre-emphasis pulses with greater amplitude [13,18].…”

Section: Introductionmentioning

confidence: 99%

Practical method for RF pulse distortion compensation using multiple square pulses for low-field MRI

Selvaganesan

et al. 2022

PLoS ONE

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Since recovery time of the RF coil is long at low field MRI, the rising and the ring-down times of the square pulse are also long, which means the applied sinc pulse can easily be distorted from the changing amplitude. However, both the rising time and ring-down time can be calculated using Q-factor. Using this information, an RF square pulse were compensated by appending two square pulses before and after the RF pulse. The durations of these RF square pulses were calculated using the Q-factor. Since the amplitude of the sinc pulse changes continuously, a series of square pulses were applied to apply sinc pulse to the coil. The minimum number of square pulses and the amplitude of the square pulses were calculated. It was successfully demonstrated that the sinc pulse can be compensated using a series of square pulses. The more number of square pulses were used, the smoother sinc pulse was applied to the RF coil. The Q-factor was experimentally calculated from the ring-down time of a signal induced in a sniffer loop which was connected to an oscilloscope. The resulting Q-factor was then used to calculate both the duration and amplitude of the square pulses for compensation. Echo trains were also acquired in an inhomogeneous B0 field using the compensated RF pulses. In order to enhance the SNR of the echo trains, a pre-polarization pulse was added to the CPMG spin echo sequence. The SNRs of the echo signal acquired using compensated pulses were compared with those of signal obtained with uncompensated pulses and showed significant improvements of 61.1% and 51.5% for the square and sinc shaped pulses respectively.

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“…We have developed a set of MOSFET-based switches to distribute large rf currents to several transmit coils, and report here on their characterisation and efficiency for the management of rf pulses involved in TRASE imaging. With their sub-µs switching times and their wide bandwidth (down to DC currents), they can be used to switch currents in static field or gradient coils as well, and applications to signal duplexing [11] and low-frequency Q-switching [12] have been reported.…”

Section: Introductionmentioning

confidence: 99%

Doppler-free spectroscopy of the lowest triplet states of helium using double optical resonance

et al. 2021

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TRansmit Array Spatial Encoding (TRASE) MRI uses trains of rf pulses alternatively produced by distinct transmit coils. Commonly used coil switching involving PIN diodes is too slow for low-and ultra-low-field MRI and would introduce wait times between pulses typically as long as each individual pulse in a few mT. A MOSFET-based rf switch is described and characterised. Up to hundreds of kHz, it allows for sub-µs switching of rf currents from a single amplifier to several coils with sufficient isolation ratio and negligible delay between pulses. Additionally, current switching at null current and maximum voltage can be used to abruptly stop or start pulses in series-tuned rf coils, therefore avoiding the rise and fall times associated with the Q-factors. RF energy can be efficiently stored in tuning capacitors for times as long as several seconds. Besides TRASE MRI, this energy storage approach may find applications in fast repeated spin-echo experiments. Here, a three-fold acceleration of TRASE phase-encoding is demonstrated when MOSFET switches are used instead of fast reed relays.

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A resistive Q-switch for low-field NMR systems

Cited by 18 publications

References 15 publications

Modal Engineering of Inductive Circuits to Achieve Rapid Settling Times

Modal Engineering of Inductive Circuits to Achieve Rapid Settling Times

Practical method for RF pulse distortion compensation using multiple square pulses for low-field MRI

Doppler-free spectroscopy of the lowest triplet states of helium using double optical resonance

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