The design of a double-tuned two-port surface resonator and its application to in vivo Hydrogen- and Sodium-MRI

Abstract

The design and construction of a two-port surface transceiver resonator for both 1 H-and 23 Na-MRI in the rodent brain at 7T is described. Double-tuned resonators are required for accurately co-registering multi-nuclei data sets, especially when the time courses of 1 H and 23 Na signals are of interest as, for instance, when investigating the pathological progression of ischaemic stroke tissue in vivo. In the current study, a single-element two-port surface resonator was developed wherein both frequency components were measured with the same detector element but with each frequency signal routed along different output channels.This was achieved by using the null spot technique, allowing for optimal variable tuning and matching of each channel in situ within the MRI scanner. The 23 Na signal to noise ratio, measured in the ventricles of the rat brain, was increased by a factor of four compared to recent state-of-the-art rat brain studies reported in the literature. The resonator's performance was demonstrated in an in vivo rodent stroke model, where regional variations in 1 H apparent diffusion coefficient maps and the 23 Na signal were recorded in an interleaved fashion as a function of time in the acute phase of the stroke without having to exchange, re-adjust, or re-connect resonators between scans. Using the practical construction steps described in this paper, this coil design.

Sodium-23 Magnetic Resonance Imaging Has Potential for Improving Penumbra Detection but Not for Estimating Stroke Onset Time

“…10 Recent studies involving human sodium MRI in the acute stroke period showed that sodium signal levels measured in the core tissue, defined by visibly increased DWI signal, remained normal up to 7 hours after arterial occlusion before eventually increasing. 8,9 This is consistent with the current and previous 13,17 rat stroke studies, where variable delay times were measured before TSC increased, coupled with early small decreases in sodium levels in the penumbra, suggesting that any increase above normal levels (we propose this as a viability threshold level) would indicate irreversible damage. In this case, permanent tissue damage would not be directly correlated with SOT, that is, the time of arterial occlusion, but rather the time at which sodium signal begins to increase above normal levels.…”

“…For quantitative measurements of the sodium signal, the mean signal in the core and penumbra was normalized to the mean sodium signal values measured in ROIs contralateral to the core and penumbra, respectively. Assuming a linear increase in the sodium signal in ischemic core tissue (as reported in previous studies 12,13,17 ), the signal in core ROI was fitted to a linear function according to Signal Na ðtÞ ¼ c þ s t where the line to which the sodium signal was fitted was characterized by the intercept c (the estimated sodium signal at SOT) and the rate of sodium signal change as a function of time s (the slope).…”

“…13,17 Furthermore, an initial decrease in sodium concentration of~4 mmol/L was observed in the presumed penumbra. 13 Bartha et al 11 also reported a decrease of~8% in stroke tissue (no discrimination made between the core and penumbra) during the acute phase in a rabbit stroke model, whereas Tyson et al 18 measured a 5% sodium reduction, acquired spectroscopically, in normoglycemic tissue during 10 minutes of transient forebrain ischemia.…”

Dedicated receiver array coil for 1H lung imaging with same‐breath acquisition of hyperpolarized 3He and 129Xe gas

“…Of the alternative ways to receive different RF (radio frequency) signals from multiple‐nuclei from the same sample, the most widely explored are: multi‐tuned, switchable and individual isolated coils . Multi (dual)‐tuned coils use a common‐resonant conducting structure that resonates at multiple resonant frequencies . The common‐resonant structure used in these designs intrinsically suffers from moderate isolation between the receivers, which increases noise in the respective receivers.…”

Scan time reduction in 23Na-Magnetic Resonance Imaging using the chemical shift imaging sequence: Evaluation of an iterative reconstruction method

“…A variety of different approaches, including optimized coil designs [11,12], anatomic specific coil-designs [13], development of ultra short echo time sequences [14][15][16] and optimized radial sequence design [17], were suggested for enhancing the detectable 23 Na-MRI signal.…”