A smart switching system to enable automatic tuning and detuning of metamaterial resonators in MRI scans.

We present a radio-frequency-activated switching system that can automatically detune a metamaterial resonator to enhance magnetic resonance imaging (MRI) performance. Local sensitivity-enhancing metamaterials typically consist of resonant components, which means that the transmitted radio frequency field is spatially inhomogeneous. The switching system shows for the first time that a metamaterial resonator can be detuned during transmission and tuned during reception using a digital circuit. This allows a resonating system to maintain homogeneous transmit field while maintaining an increased receive sensitivity. As a result, sensitivity can be enhanced without changing the system-provided specific absorption rate (SAR) models. The developed digital circuit consists of inductors sensitive to the transmit radio-frequency pulses, along with diodes acting as switches to control the resonance frequency of the resonator. We first test the automatic resonator detuning on-the-bench, and subsequently evaluate it in a 1.5 T MRI scanner using tissue-mimicking phantoms. The scan results demonstrate that the switching mechanism automatically detunes the resonator in transmit mode, while retaining its sensitivity-enhancing properties (tuned to the Larmor frequency) in receive mode. Since it does not require any connection to the MRI console, the switching system can have broad applications and could be adapted for use with other types of MRI scanners and field-enhancing resonators.

Dielectric permittivity of human blood of different lactate levels measured at millimeter waves.

The investigation of variations in dielectric properties of blood based on its biochemical profile is important for determining the feasibility of developing electromagnetic non-invasive sensing systems for monitoring the levels of various metabolites in blood. In this paper, the real and imaginary parts of dielectric permittivity are measured as a function of lactate concentration in the 30-60 GHz frequency range using two different measurement techniques. The blood samples are collected from a healthy subject undergoing three different exercise modes and the dielectric properties are measured with an open-ended coaxial probe technique and a custom-made millimeter wave transmission system. Good correlation is observed in measurements from the two methods, suggesting that an increase in lactate concentration lowers the imaginary part of permittivity and thus causing higher attenuation.