ABSTRACT
Surface coils are widely used in magnetic resonance imaging and spectroscopy. While smaller diameter coils produce higher signal to noise ratio (SNR) closer to the coil, imaging larger fields of view or greater distance into the sample requires a larger overall size array or, in the case of a channel count limited system, larger diameter coils. In this work, we consider reconfiguring the geometry of coils and coil arrays such that the same coil or coil array may be used in multiple field of view imaging. A custom designed microelectromechanical systems switch, compatible with magnetic resonance imaging, is used to switch in/out conductive sections and components to reconfigure coils. The switch does not degrade the SNR and can be opened/closed in 10 µs, leading to rapid reconfiguration. Results from a single coil, configurable between small/large configurations, and a two-coil phased array, configurable between spine/torso modes, are presented.
ABSTRACT
As the number of coils increases in multi-channel MRI receiver-coil arrays, RF cables and connectors become increasingly bulky and heavy, degrading patient comfort and slowing workflow. Inductive coupling of signals provides an attractive "wireless" approach, with the potential to reduce coil weight and cost while simplifying patient setup. In this work, multi-channel inductively coupled anterior arrays were developed and characterized for 1.5T imaging. These comprised MR receiver coils inductively (or "wirelessly") linked to secondary or "sniffer" coils whose outputs were transmitted via preamps to the MR system cabinet. The induced currents in the imaging coils were blocked by passive diode circuits during RF transmit. The imaging arrays were totally passive, obviating the need to deliver power to the coils, and providing lightweight, untethered signal reception with easily positioned coils. Single-shot fast spin echo images were acquired from 5 volunteers using a 7-element inductively coupled coil array and a conventionally cabled 7-element coil array of identical geometry, with the inductively-coupled array showing a relative signal-to-noise ratio of 0.86 +/- 0.07. The concept was extended to a larger 9-element coil array to demonstrate the effect of coil element size on signal transfer and RF-transmit blocking.
Subject(s)
Magnetic Resonance Imaging/instrumentation , Magnetic Resonance Imaging/methods , Adult , Computer Simulation , Equipment Design , Healthy Volunteers , Humans , Image Processing, Computer-Assisted , Middle Aged , Phantoms, Imaging , Radio Waves , Signal Processing, Computer-Assisted , Signal-To-Noise Ratio , Wireless TechnologyABSTRACT
Tissue conductivity and permittivity are critical to understanding local radio frequency (RF) power deposition during magnetic resonance imaging (MRI). These electrical properties are also important in treatment planning of RF thermotherapy methods (e.g. RF hyperthermia). The electrical properties may also have diagnostic value as malignant tissues have been reported to have higher conductivity and higher relative permittivity than surrounding healthy tissue. In this study, we consider imaging conductivity and permittivity using MRI transmit field maps (B1+ maps) at 3.0 Tesla. We formulate efficient methods to calculate conductivity and relative permittivity from 2-dimensional B1+ data and validate the methods with simulated B1+ maps, generated at 128 MHz. Next we use the recently introduced Bloch-Siegert shift B1+ mapping method to acquire B1+ maps at 3.0 Tesla and demonstrate conductivity and relative permittivity images that successfully identify contrast in electrical properties.
