1575
Receiver ring-down attenuation for Ultra-Low field MR1Centre for advanced imaging, University of Queensland, Brisbane, Australia, 2Physikalisch-Technische Bundesanstalt (PTB), Berlin, Germany
Synopsis
Ultra-low field MR detector coils experience long dead-times which reduce acquisition efficiency. We present a simple low insertion loss Q-damping scheme and a post-processing method that, combined, allow earlier signal acquisition. Proposed methods have been empirically verified with a cylindrical detector at 2.5 kHz. This approach can improve imaging efficiency for ULF MR considerably, promoting the use of inexpensive resistive coils for low-cost, portable ULF MR instruments.
Synopsis
Ultra-low field MR detector coils experience long dead-times which reduce acquisition efficiency. We present a simple low insertion loss Q-damping scheme and a post-processing method that, combined, allow earlier signal acquisition. Proposed methods have been empirically verified with a cylindrical detector at 2.5kHz. This approach can improve imaging efficiency for ULF MR considerably, promoting the use of inexpensive and robust detectors for low-cost and portable instruments.Introduction
Air-core magnetometers employed in ULF MR suffer from long dead-times, often of tens of milliseconds, when used in the presence of pre-polarization or radio-frequency pulses1. This delay reduces the SNR and lengthens acquisition time. Several methods have been proposed to reduce the dead-time. These include insertion of carefully calibrated ring-down cancelling Rf-pulses2,3, combination of equivalent acquisitions with opposite or different ring-down phases by software4-6, critically damping the resonant circuit temporarily7,8, using complex feedback electronics2,9, and using backward linear prediction to extrapolate initial data points from the signal10. These methods either increase the complexity of acquisition sequences or suffer from limited signal efficiency.Methods
Frequency increase Q-damping: Optimal coil de-energizing follows an exponential decay with a time constant ( dependent on the resonant frequency of the detector (f0) such that . We propose to accelerate this dissipation by temporarily re-tuning the circuit to a higher frequency, which is achieved by reducing the capacitance of the resonant circuit. We have opted to use the coil’s natural resonant frequency of 85kHz, which offers a 34 fold acceleration compared to the 2.5kHz acquisition frequency. Component switching and pre-amplifier protection are achieved by a set of 3 reed relays (Fig.1).
Software ring-down attenuation: The long ring-down re-induced by switching Q-damping relays is attenuated by subtracting estimated re-ringing from the signal. Three re-ringing estimation algorithms have been tested:
- Mono-exponential fitting: A mono-exponential oscillatory decay has been fitted to the section where the re-ringing is dominant over the signal following Equation(1). Circuit resonant frequency (f0) and decay time (τ) were estimated from an averaged NMR signal free re-ringing. Equation(1) is then used to estimate the phase (Φ) and amplitude (Ard) of the re-ringing overlapped with the MR signal.
$$V_rd=A_{rd}e^{-t\over τ_{rd}}cos(2\pi f_0t+\phi).\space\space\space\space(1)$$
- Rigid recorded ring-down: A recorded NMR signal free re-ringing is averaged and subtracted from the signal.
- Adaptive recorded ring-down: The phase and magnitude of the recorded averaged re-ringing is adjusted and subtracted individually for each acquisition. To estimate the phase and amplitude, a sinusoidal lobe is fitted to one of the first re-ringing lobes where the re-ringing dominates over the MR signal.
NMR experiment: Test coil parameters: 25.6Ω AC resistance at 1kHz, 52.6mH, 29mm inner diameter, 48.1mm outer diameter and 34mm height. Signals from a 20ml water sample and an in-vivo human thumb were acquired (Fig.4).
Results
Experimental acquisitions show that the coil was de-energized from a 180° Rf pulse in less than 2ms. The corresponding estimated decay time is 1.9μs (critically damped with 20kΩ). The switching of the mechanical relays generate a re-ringing which decays to noise floor level in 20 ms, and is highly reproducible regardless of the preceding Rf pulse intensity (0°, 90° or 180°)(Fig.2).
All three software ring-down attenuation methods improved the spectrum(Fig.3-4). Directly subtracting the averaged ring-down had the poorest performance, reducing ring-down effects by about 60%. The exponential fitting and adaptive recorded ring-down subtraction methods attenuated ring-down by 80%, reducing sensor dead-time to 4ms.
Discussion
Switching the resonant frequency to higher frequencies has allowed us to completely dissipate energy within 2ms, which is remarkably fast considering employed relays have a 0.2ms response time. The characteristics of the re-ringing are independent of employed Rf power, which confirms the efficacy of the proposed damping approach.
Software ring-down attenuation reduces acquisition dead-time significantly. The limited attenuation efficiency of the direct subtraction of the averaged recorded ring-down implies that the reproducibility of re-ringing is limited; likely due to the use of electromechanical switches. Hence, adjusting the amplitude and phase of the averaged ring-down improves ring-down attenuation. The exponential fitting method can attenuate the ring-down with minimal noise insertion but is more sensitive to imprecision in estimation of re-ringing.
Conclusion
Faster energy dissipation from the detector is achieved by increasing the resonant frequency of the Q-damping circuit. This large attenuation allows the re-ringing to be independent of the preceding pulse, becoming more reproducible and improving its attenuation through post-processing. By combining both methods, we have been able to reduce the dead-time below 4ms, less than half of recently reported values1. This approach can improve imaging efficiency for ULF MR considerably, promoting the use of inexpensive and robust signal detectors for low-cost and portable instruments.Acknowledgements
No acknowledgement found.References
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