Dipole antennas for ultrahigh‐field body imaging: a comparison with loop coils. (17th August 2015)

• Record Type: Journal Article
• Authors: Raaijmakers, A. J. E.; Luijten, P. R.; van den Berg, C. A. T.
• Other Names: van den Berg Cornelis guestEditor.; Klomp Dennis guestEditor.; Petridou Natalia guestEditor.
• Is Part Of: NMR in biomedicine. Volume 29:Number 9(2016:Sep.)
• Page Start: 1122
• Page End: 1130
• Publication Date: 2015-08-17
• Subjects: Ultrahigh field -- Dipole antennas -- Engineering -- EM simulations; Nuclear magnetic resonance -- Periodicals; Magnetic Resonance Spectroscopy -- Periodicals; 574
• DOI: 10.1002/nbm.3356
• Languages: English
• ISSNs: 0952-3480

Abstract

Abstract : Although the potential of dipole antennas for ultrahigh‐field (UHF) MRI is largely recognized, they are still relatively unknown to the larger part of the MRI community. This article intends to provide electromagnetic insight into the general operating principles of dipole antennas by numerical simulations. The major part focuses on a comparison study of dipole antennas and loop coils at frequencies of 128, 298 and 400 MHz. This study shows that dipole antennas are only efficient radiofrequency (RF) coils in the presence of a dielectric and/or conducting load. In addition, the conservative electric fields (E‐fields) at the ends of a dipole are negligible in comparison with the induced E‐fields in the center. Like loop coils, long dipole antennas perform better than short dipoles for deeply located imaging targets and vice versa. When the optimal element is chosen for each depth, loop coils have higher B 1 + efficiency for shallow depths, whereas dipole antennas have higher B 1 + efficiency for large depths. The cross‐over point depth decreases with increasing frequency: 11.6, 6.2 and 5.0 cm for 128, 298 and 400 MHz, respectively. For single elements, loop coils demonstrate a better B 1 + /√SARmax ratio for any target depth and any frequency. However, one example study shows that, in an array setup with loop coil overlap for decoupling, this relationship is not straightforward. The overlapping loop coils may generate increased specific absorption rate (SAR) levels under the overlapping parts of the loops, depending on the drive phase settings. Copyright © 2015 John Wiley & Sons, Ltd. Abstract : This article provides electromagnetic insight into the operating principles of dipole antennas by numerical simulations. It focuses on a comparison study of dipole antennas and loop coils at the frequencies of 128, 298 and 400 MHz. When the optimal element is chosen for each depth, loop coils have higher B 1 + efficiency for shallow depths, whereas dipole antennas have higher B 1 + efficiency for large depths. Loop coils demonstrate a better B 1 + /√SARmax ratio, but not in an array setup with overlapping elements.