Interventional loopless antenna at 7 T

The loopless antenna magnetic resonance imaging detector is comprised of a tuned coaxial cable with an extended central conductor that can be fabricated at submillimeter diameters for interventional use in guidewires, catheters, or needles. Prior work up to 4.7 T suggests a near‐quadratic gain in signal‐to‐noise ratio with field strength and safe operation at 3 T. Here, for the first time, the signal‐to‐noise ratio performance and radiofrequency safety of the loopless antenna are investigated both theoretically, using the electromagnetic method‐of‐moments, and experimentally in a standard 7 T human scanner. The results are compared with equivalent 3 T devices. An absolute signal‐to‐noise ratio gain of 5.7 ± 1.5‐fold was realized at 7 T vs. 3 T: more than 20‐fold higher than at 1.5 T. The effective field‐of‐view area also increased approximately 10‐fold compared with 3 T. Testing in a saline gel phantom suggested that safe operation is possible with maximum local 1‐g average specific absorption rates of <12 W kg −1 and temperature increases of <1.9°C, normalized to a 4 W kg −1 radiofrequency field exposure at 7 T. The antenna did not affect the power applied to the scanner's transmit coil. The signal‐to‐noise ratio gain enabled magnetic resonance imaging microscopy at 40–50 μm resolution in diseased human arterial specimens, offering the potential of high‐resolution large‐field‐of‐view or endoscopic magnetic resonance imaging for targeted intervention in focal disease. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.