Optical Link as an Alternative for MRI Receive Coils: Toward a Passive Approach.

OBJECTIVE: The objective of this work was to propose an alternative solution to NMR signal transmission by replacing the coaxial cables of the receiver radiofrequency (RF) coil in the context of MRI so as to improve safety. Starting from the analysis of previous studies and reports on the topic, the difficulty of supplying power wirelessly to an RF coil was identified. To avoid this difficult task, the development of a passive analog optical link was studied. METHODS: In order to quantify the requirements for achieving an analog conversion, the performance of the link was evaluated based on the input NMR signal amplitude and the optical power and compared with that of a galvanic link. Acquisitions were performed on a 7-T preclinical MRI system with a doped saline solution as phantom. A passive and MRI-compatible polarization-state custom-made modulator was tested as well as a commercial Mach-Zehnder interferometer. RESULTS: The conversion was not sensitive enough to keep similar SNRs, but the main source of noise was identified along with parameters for improvement. Optical power emitted by the laser, insertion loss, and full-phase inversion voltage of the modulators were found to be crucial characteristics for the application. These data indicate that custom application devices are required since the frequency, bandwidth, and amplitude of NMR signals are quite different to usual telecommunication signals. CONCLUSION: An electro-optic modulation and a transmission channel were successfully conceived and tested. Images were reconstructed with some significant SNR drawbacks that are expected to be compensated with an appropriate modulator. SIGNIFICANCE: While technical challenges remain, our approach to a two-decades-long problem could solve a major issue of MRI safety by removing the need for supplying on-coil electrical current.

Sub-wavelength terahertz imaging through optical rectification.

We record a sub-wavelength terahertz image of a caster sugar grain thanks to optical rectification in the sample excited with a femtosecond laser beam. The lateral spatial resolution of this technique is given by the laser spot size at the sample and here its measured value is 50 µm, i.e. ~λ/12. We give an estimation of the ultimate resolution that could be achieved with this method.

Full electro-optic terahertz time-domain spectrometer for polarimetric studies.

We present a terahertz (THz) time-domain spectrometer dedicated to polarimetric studies. THz pulses are generated through optical rectification in a ⟨111⟩-cut cubic crystal. The ⟨111⟩ crystal cut produces a THz polarization state similar to that of the exciting laser beam. Detection of the THz pulses is performed by electro-optic sampling in a similar crystal, and a dual detection scheme allows us to measure simultaneously the two polarization components of the THz beam. We experimentally illustrate that the polarization of the THz beam can be adjusted by adjusting the laser polarization. This technique alleviates the need for a polarization controller at THz frequencies.

Electro-optic probe for real-time assessments of RF electric field produced in an MRI scanner: Feasibility tests at 3 and 4.7 T.

During magnetic resonance imaging (MRI) examinations, the average specific absorption rate (SAR) of the whole body is calculated as an index of global energy deposition in biological tissue without taking into account the presence of metallic implants or conductive materials. However, this global SAR calculation is not sufficient to ensure patient safety and a local SAR measurement should be carried out. Several measurement techniques have already been used to evaluate the local SAR, in particular electric field (E-field) probes, but the accuracy of the measurements and the resolutions (spatial and temporal) depend strongly on the measurement method/probe. This work presents an MR-compatible, subcentimeter probe based on an electro-optic (EO) principle enabling a real-time measurement of the local E-field during MRI scans. The experiments using these probes were performed on two different MR systems (preclinical and clinical) having different static magnetic field strengths and with different volume coil geometries. The E-field was measured with unloaded (in air) and loaded volume coils in order to assess the sensing characteristics of the optical probe. The results show an excellent linearity between the measured E-field and the radiofrequency (RF) magnetic field in both experimental conditions. Moreover, the distribution of the E-field throughout the volume coil was experimentally determined and was in good agreement with numerical simulations. Finally, we demonstrate through our measurements that the E-field depends strongly on the dielectric properties of the medium.

Electric field and temperature measurement using ultra wide bandwidth pigtailed electro-optic probes.

We present pigtailed electro-optic probes that allow a simultaneous measurement of high frequency electric fields and temperature using a unique laser probe beam. This has been achieved by the development of a novel probe design associated with a fully automated servo-controlled optical bench, initially developed to stabilize the electric field sensor response. The developed electro-optic probes present a stable response in outdoors conditions over a time duration exceeding 1 h, a frequency bandwidth from kHz to tens of GHz with a sensitivity of 0.7 Vm(-1)Hz(-(1/2)), and a temperature accuracy of 40 mK.

High-finesse Fabry-Perot electro-optic sensors with enhanced sensitivity and high spatial resolution.

We present a high-finesse optical cavity containing a LiTaO(3) electro-optic crystal, devoted to free-space electric field characterization. Theoretical considerations will show that the modulation depth is directly related to the transversal components of the field to be measured, thus opening the way to vectorial mapping of the electric field using a single electro-optic crystal. Also, a discussion about noise and sensitivity will be given. As the latter increases with the effective cavity length, and bandwidth decreases, a trade-off is realized, allowing us to measure an electric field of 60 mV/m/ sqrt[Hz] in a 110 MHz bandwidth. Cavity dimensions are less than 8 mm(3), giving an inner-crystal transverse spatial resolution of 70 microm and allowing pigtailed systems to integrate.