From Body Magnetic Resonance Group

The radiofrequency (RF) coil is a resonant circuit that receives and/or transmits magnetic signal. An RF coil is tuned to be resonant at the Larmor frequency of the nuclei of interest by adjusting the values of the capacitors. There are three main RF coil projects in our lab: a dual-tuned (¹H and ²³Na) birdcage knee coil, a dual-tuned (¹H and ²³Na) breast coil, and a phased array ¹H breast coil.

Phased Array ¹H Breast Coil

The physical size and geometry of an RF coil determine its sensitivity profile. For example, let us consider a simple RF coil consisting of a circular copper loop with tuning capacitors. A large RF coil would have greater spatial coverage, but will also receive more noise. A small RF coil has high sensitivity but only in a localized region. A phased array coil – a chain of decoupled small coils – features the high sensitivity of a small coil in addition to the volumetric coverage of a large coil. We have built a phased array breast coil that utilizes a unique coil layout to provide 4-5 times higher SNR than a common industrial coil. This coil also enables high reduction factors (6x or 8 x) using parallel imaging. We have demonstrated the ability to acquire high spatial and temporal resolution clinical breast images using this coil.

Custom-built 18-channel breast coil from the inside (left) and outside (right) views.

Dual-Tuned (¹H and ²³Na) Birdcage Knee Coil

A birdcage coil has two circular ends that are connected by either lines of copper tape or rods of copper tubing. The birdcage coil exhibits multiple resonant modes, and the number of modes corresponds to the number of rungs. The advantage of the birdcage coil is that one of the modes has a very uniform quadrature B1 field. The B1 uniformity makes the birdcage coil ideal for quantification studies. Due to its coil geometry and human anatomy, the birdcage design is applied to head coils and knee coils. We have used the birdcage design to build a dual-tuned (¹H and ²³Na) knee coil.

Dual-Tuned (¹H and ²³Na) Breast Coil

Additional imaging modalities such as sodium (Na) MR have the potential to increase the sensitivity and specificity of breast cancer detection because ²³Na MR can reflect the disruption of the membrane Na-K pump associated with cancer. Results from a study from another lab suggest that ²³Na MR can be used to monitor patient response to chemotherapy. Moreover, recent advances in gradient hardware and pulse sequences have made ²³Na MR feasible for quantitative measurements of sodium concentration. We have built three coils for multinuclear breast imaging: a dual-tuned surface coil, a dual-tuned Helmholtz coil, and a ²³Na coil insert that fits into an industrial breast coil.

Dual-tuned (sodium and proton) Helmholtz breast coil