Antennas as Surface Array Elements for Body Imaging at Ultrahigh Field Strengths

Abstract

MR imaging at ultrahigh field strengths requires frequencies in the ultrahigh frequency range. At these frequencies, coils start to behave as antennas with a near field and a far field. In the near field, reactive fields dominate. In the far field, the field turns into an electromagnetic wave, propagating away from the coil. If the imaging target is located in the far field, then the design criteria for the coil change. Rather than creating a resonant coil with a high reactive field, an electromagnetic wave has to be emitted efficiently. At 7 T, this is the case when imaging deeply located body parts such as the prostate. It requires generating E-and B-fields such that the resulting Poynting vector is oriented toward the target region. The antenna impedance should be adjusted to the equilibrium wave impedance within the patient. This will avoid large Eand B-fields at the surface that cause high SAR levels. On the basis of these criteria, we have presented a new type of surface array element: the single-side adapted dipole antenna and demonstrated its superior performance by comparison to a 10 cm diameter loop coil and two microstrip element designs. Volunteer prostate images are shown using an array of eight of these elements.

RF coils: A practical guide for nonphysicists

“…Various types of dipoles used in MRI already exist: monopole antenna , the folded dipole antenna , circular dipole antenna , fractionated dipole , and combinations of loop coils and dipole antennas . Compared to standard loop surface coils, electric dipoles are more likely used in ultrahigh‐field systems, where wavelengths are much shorter than the dimensions of the tissue …”

The fractionated dipole antenna: A new antenna for body imaging at 7 Tesla

“…Various studies show that dipole antennas may have advantages over more conventional coil array elements when imaging deeply located body targets at ultrahigh fields . The use of dipoles as coil array elements was first introduced by Raaijmakers et al in 2009 .…”

Dipole antennas for ultrahigh‐field body imaging: a comparison with loop coils

“…The extent of the near‐field is unclear, but it is relative to the wavelength at that frequency. Raaijmakers and van den Berg have shown that, for 7‐T body imaging (298 MHz), a deeply located imaging target, such as the prostate, is probably located outside of this near‐field region, which imposes different design criteria for the surface array elements.…”