Combination of surface and ‘vertical’ loop elements improves receive performance of a human head transceiver array at 9.4 T

• Published in: NMR in biomedicine Vol. 31; no. 2
• Main Authors: Avdievich, N.I., Giapitzakis, I.A., Pfrommer, A., Borbath, T., Henning, A.
• Format: Journal Article
• Language: English
• Published: England 01-02-2018
• Subjects: Head - anatomy & histology; Humans; Magnetic Resonance Imaging - instrumentation; parallel imaging; Phantoms, Imaging; RF human head coil; Signal-To-Noise Ratio; SNR improvement; transceiver arrays; ultra‐high‐field MRI; parallel imaging; SNR improvement; transceiver arrays; RF human head coil; ultra-high-field MRI
• ISSN: 0952-3480; 1099-1492
• DOI: 10.1002/nbm.3878

Ultra‐high‐field (UHF, ≥7 T) human magnetic resonance imaging (MRI) provides undisputed advantages over low‐field MRI (≤3 T), but its development remains challenging because of numerous technical issues, including the low efficiency of transmit (Tx) radiofrequency (RF) coils caused by the increase in tissue power deposition with frequency. Tight‐fit human head transceiver (TxRx) arrays improve Tx efficiency in comparison with Tx‐only arrays, which are larger in order to fit multi‐channel receive (Rx)‐only arrays inside. A drawback of the TxRx design is that the number of elements in an array is limited by the number of available high‐power RF Tx channels (commonly 8 or 16), which is not sufficient for optimal Rx performance. In this work, as a proof of concept, we developed a method for increasing the number of Rx elements in a human head TxRx surface loop array without the need to move the loops away from a sample, which compromises the array Tx performance. We designed and constructed a prototype 16‐channel tight‐fit array, which consists of eight TxRx surface loops placed on a cylindrical holder circumscribing a head, and eight Rx‐only vertical loops positioned along the central axis (parallel to the magnetic field B0) of each TxRx loop, perpendicular to its surface. We demonstrated both experimentally and numerically that the addition of the vertical loops has no measurable effect on the Tx efficiency of the array. An increase in the maximum local specific absorption rate (SAR), evaluated using two human head voxel models (Duke and Ella), measured 3.4% or less. At the same time, the 16‐element array provided 30% improvement of central signal‐to‐noise ratio (SNR) in vivo relative to a surface loop eight‐element array. The novel array design also demonstrated an improvement in the parallel Rx performance in the transversal plane. Thus, using this method, both the Rx and Tx performance of the human head array can be optimized simultaneously.
To improve the receive (Rx) performance of a tight‐fit head surface loop transceiver (TxRx) array without compromising the transmit efficiency (because of the need to move the loops away from the sample), a novel 16‐channel array was constructed. The array consists of eight TxRx surface loops circumscribing the head and eight Rx‐only ‘vertical’ loops positioned along the central axis of each TxRx loop perpendicular to its surface. Substantial improvement in the signal‐to‐noise ratio (SNR) (up to approximately 30% in the center) and parallel Rx performance relative to the surface loop eight‐channel array were shown.