TEM Body Coils (2012)

A radiofrequency body coil is included with virtually all clinical MRI systems and many research systems, and is most often built into the bore of the MRI magnet immediately behind the patient bore tube. The body coil is commonly used to excite a uniform RF field required for stimulating the nuclear magnetic resonance signal response from a region of interest within the coil's volume. The most ubiquitous body coil design is the birdcage, a monolithic resonator composed of a ladder network of rung elements subtending a cylindrical form and connected by rails or endrings providing manifold feed and return paths for the rungs. The design is ideally suited for imparting a highly homogeneous, circularly polarized RF field centered within its volume, at lower field strengths where longer Larmor wavelengths are relatively uniform even within a body in the coil. As field strengths climb to 3T and above however, circuit lengths increase in wavelength proportion. Large circuits such as the birdcage become less efficient because of increased radiation resistance. Field-induced eddy current losses to the tissue conductor and displacement current losses to the tissue dielectric increase as well, resulting in increased heat and noise in the body load. Interference patterns in the excitation (and reception) fields result in highly non-uniform fields and inhomogeneous images. The TEM body coil has proved to be a good candidate for compensating these high field problems. It preserves some of the qualities of the birdcage, such as the rung structure for higher homogeneity. The TEM coil however replaces the cage coil's end rings with a slotted cavity for the rung elements' return path, rendering the TEM coil as an array of independent transmission line element resonators. These independent elements are electrically short and therefore more efficient at higher frequencies. And these TEM elements can be independently driven or controlled, over five degrees of freedom to manipulate the coil's field in magnitude, phase, space, time, and frequency. By means collectively referred to as parallel transmit and B1 shimming, the “multichannel” TEM coil can be used to mitigate high field problems of image inhomogeneity and SAR incurred by conventional imaging. Additionally, the TEM body coil makes entirely new approaches to imaging possible, such as RF localization of ROIs and optimization of RF-dependent parameters therein. Alternatively, the RF field can be dynamically swept through time, space, frequency, phase, and/or magnitude to “scan” a body that could otherwise not be uniformly excited. Military terms for these approaches are target acquisition, frequency hopping, and beam steering. As with military arrays, the TEM body coil lends itself to significant gains in transmission efficiency and cost reduction by incorporating distributed power amp modules into the TEM elements individually. A sixty-four-channel, 32 kW TEM body coil, for example, would have 64 TEM elements each mated with a 500 W power transistor, distributed in three dimensions over the outer surface of the cylindrical bore liner. These future foundations together with present design considerations, applications examples, and RF safety notes are the bases for the article on TEM Body Coils.