Modular transmit/receive arrays using very‐high permittivity dielectric resonator antennas

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Ultrahigh field (UHF) MRI suffers from B1 inhomogeneities due to radiofrequency (RF) interferences that arise when the RF wavelength is of the same order as the imaging region of interest 1. It has been shown that JOURNAL/mrim/04.02/01445475-201803000-00058/math_58MM1/v/2018-01-24T161827Z/r/image-png inhomogeneities can be reduced through use of multi‐element transmit array systems 3. Although receive‐only arrays universally are used in UHF, MRI transmit arrays predominantly are used for body imaging using decoupled surface coils 4; microstrip or dipole antenna 5; and to a lesser extent, neuroimaging using decoupled surface coils7 or an array of decoupled transmission line antenna 8. Musculoskeletal (MSK) imaging is less sensitive to JOURNAL/mrim/04.02/01445475-201803000-00058/math_58MM2/v/2018-01-24T161827Z/r/image-png inhomogeneities due to the relatively small dimensions of the typical region of interest; as a result, the birdcage remains the dominant design for RF transmission 10. Nevertheless, recent studies have begun to show the utility of transmit arrays for MSK imaging at 7T 13. Array designs utilizing overlapping surface coils—with the surface coils either fixed on a cylindrical housing into which the region of interest (ROI) is inserted 16, or with two separate surface coil arrays that are placed around the ROI 13—have shown promise. An innovative U‐shaped eight‐channel microstrip array using capacitive decoupling has been used for imaging the ankle joint at 7T 15.
One of the main issues in designing large multi‐element arrays is RF coupling between proximal array elements 17. Aside from causing changes in the impedance of individual array elements, RF coupling also reduces the signal‐to‐noise ratio (SNR) in parallel imaging techniques 18. Many system designs have been proposed to reduce inter‐element coupling, including the overlapping of surface coils, preamplifier decoupling 18, resonant inductive decoupling 20, capacitive decoupling 21, inductive decoupling 22, decoupling annexes 23, and induced current elimination 24. The implementation of these systems typically increases the complexity of the antenna arrays. Decoupling methods that introduce additional decoupling structures to the array, such as the aforementioned resonant inductive decoupling and induced current elimination methods, are highly sensitive to geometric changes to the decoupling structures and subsequently to array deformation. Changes in coil loading also can impact the effectivity of decoupling systems 20 and change the inductance/capacitance value needed for optimal decoupling of array elements 25. Furthermore, the introduction of additional decoupling elements can result in significant alterations to the JOURNAL/mrim/04.02/01445475-201803000-00058/math_58MM3/v/2018-01-24T161827Z/r/image-png distribution compared to independent antenna 26.
High permittivity materials (also referred to as dielectric materials in other literature) have seen increased usage as the trend toward higher magnetic fields continues. High‐permittivity pads placed between the patient and the transmit coil have been used to tailor JOURNAL/mrim/04.02/01445475-201803000-00058/math_58MM4/v/2018-01-24T161827Z/r/image-png fields, with the aim to improving JOURNAL/mrim/04.02/01445475-201803000-00058/math_58MM5/v/2018-01-24T161827Z/r/image-png homogeneity in body‐ and neuroimaging at high field 27 and neuroimaging in ultrahigh field MRI 31, or for strong local focusing of JOURNAL/mrim/04.02/01445475-201803000-00058/math_58MM6/v/2018-01-24T161827Z/r/image-png to improve signal intensity in the cervical spine at 3T 30 and in the inner ear at 7T 33. The interesting electromagnetic (EM) properties of high‐permittivity materials (HPMs) have seen them integrated into several (UHF) antenna designs. The shortened RF wavelength in HPMs has been exploited to reduce the dimensions of bow‐tie antenna by submerging them in water 34 and to construct dielectric waveguide antenna 35, again using water as the HPM. HPMs also have been used as substrates for dipole antenna to improve JOURNAL/mrim/04.02/01445475-201803000-00058/math_58MM7/v/2018-01-24T161827Z/r/image-png penetration and reduce specific absorption rate (SAR) 36.
Recent work has shown that dielectric resonator antenna (DRAs) operating in the transverse electric (TE)01δ mode 38 and hybrid electromagnetic (HEM)11δ40 can be used as transceive antenna in ultrahigh field MRI.

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