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Abstract

Introduction

Spatial and temporal resolution are limited by the SNR and the time required for spatial encoding. To maximize sensitivity and localization, we implanted a miniaturized coil into the brains of healthy rats, consisting of a fully-integrated CMOS1 NMR transceiver, including RF-amplifier, preamplifiers, signal conditioning electronics and microcoil on a needle-shaped PCB, thus maximizing sensitivity and reducing signal loss. This system samples the MR signal within a very confined spatial region (~10 nl) at a temporal resolution of microseconds, without the use of gradients for spatial encoding.

Methods

The fully-integrated NMR transceiver1,2 includes an on-chip broadband MR-microcoil (300 µm-diameter double-spiral coil) on a silicon substrate with a width of 450 µm, a length of 3000 µm, and a triangular tip for easier insertion into the brain tissue, with a sensitive volume of around 10 nl, directly bonded to a small supporting PCB (Fig 1). The acquired signals are amplified on a signal-conditioning PCB close to the microcoil and further processed outside the scanner room3.

The microcoil was implanted 1.5 mm to 2 mm into the somatosensory cortex of anesthetized rats and used to observe localized FIDs during rest and forepaw stimulation with repetition times between 5 ms and 1 s in a 14.1 T/26 cm horizontal magnet. For control, standard EPI measurements were performed using a surface coil.

Results/Discussion

Fig. 2 shows a comparison of BOLD responses acquired with the conventional surface coil and the microcoil at 1000 ms and 5 ms temporal resolution, respectively. The microcoil response is obtained by averaging over all stimulation epochs, extracting the area under the magnitude of the resulting FIDs, yielding a single value per FID with temporal resolutions of up to 200Hz (TR=5ms). Additionally, the functional signals were low-pass filtered with a 3Hz Gaussian filter to reduce the noise, since no visible stimulation-related features beyond that frequency were observed. Signal changes of around 2% were observed during forepaw stimulation. The contralateral responses for stimulation of the right paw showed no response in any measurement, indicating that the signals are indeed the hemodynamic response to the stimulation of the left paw. By fitting an exponential decay to the FIDs, contributions from inflow and BOLD can be distinguished as changes in amplitude and T2*-decay of the signals.

Conclusion

Exceptional spatial and temporal resolution was possible with the microcoil, detecting BOLD and flow-related signal changes during electrical stimulation within 10 nl of volume and 5 ms of temporal resolution. This new technology offers the potential to detect novel effects or MR-fingerprints of neuronal activation and can be combined with other local and fast methods for neuronal recording such electrophysiology orcalcium recording,