Transmission Line Coil Research Articles

Twisted pair transmission line coil – a flexible, self-decoupled and robust element for 7 T MRI

ObjectiveThis study evaluates the performance of a twisted pair transmission line coil as a transceive element for 7 T MRI in terms of physical flexibility, robustness to shape deformations, and interelement decoupling. MethodsEach coil element was created by shaping a twisted pair of wires into a circle. One wire was interrupted at the top, while the other was interrupted at the bottom, and connected to the matching circuit. Electromagnetic simulations were conducted to determine the optimal number of twists per length (in terms of B₁+ field efficiency, SAR efficiency, sensitivity to elongation, and interelement decoupling properties) and for investigating the fundamental operational principle of the coil through fields streamline visualisation. A comparison between the twisted pair coil and a conventional loop coil in terms of B₁+ fields, maxSAR₁₀g, and stability of S₁₁ when the coil was deformed was performed. Experimentally measured interelement coupling between individual elements of multichannel arrays was also investigated. ResultsIncreasing the number of twists per length resulted in a more physically robust coil. Poynting vector streamline visualisation showed that the twisted pair coil concentrated most of the energy in the near field. The twisted pair coil exhibited comparable B₁+ fields and improved maxSAR₁₀g to the conventional coil but demonstrated exceptional stability with respect to coil deformation and a strong self-decoupling nature when placed in an array configuration. DiscussionThe findings highlight the robustness of the twisted pair coil, showcasing its stability under shape variations. This coil holds great potential as a flexible RF coil for various imaging applications using multiple-element arrays, benefiting from its inherent decoupling.

Microstrip Transmission Line RF Coil for a PET/MRI Insert

Purpose:We proposed and developed a new microstrip transmission line radiofrequency (RF) coil for a positron emission tomography (PET) insert for MRI, which has low electrical interactions with PET shield boxes. We performed imaging experiments using a single-channel and a four-channel proposed RF coils for proof-of-concept.Methods:A conventional microstrip coil consists of a microstrip conductor, a ground conductor, and a dielectric between the two conductors. We proposed a microstrip coil for the PET insert that replaced the conventional single-layer ground conductor with the RF shield of the PET insert. A dielectric material, which could otherwise attenuate gamma photons radiated from the PET imaging tracer, was not used. As proof-of-concept, we compared conventional and the proposed single-channel coils. To study multichannel performance, we further developed a four-channel proposed RF coil. Since the MRI system had a single-channel transmission port, an interfacing four-way RF power division circuit was designed. The coils were implemented as both RF transmitters and receivers in a cylindrical frame of diameter 150 mm. Coil bench performances were tested with a network analyzer (Rohde & Schwarz, Germany), and a homogeneous phantom study was conducted for gradient echo imaging and RF field (B1) mapping in a 3T clinical MRI system (Verio, Siemens, Erlangen, Germany).Results:For all coils, the power reflection coefficient was below −30 dB, and the transmission coefficients in the four-channel configuration were near or below −20 dB. The comparative single-channel coil study showed good similarity between the conventional and proposed coils. The gradient echo image of the four-channel coil showed expected flashing image intensity near the coils and no phase distortion was visible. Transmit B1 field map resembled the image performance.Conclusion:The proposed PET-microstrip coil performed similarly to the conventional microstrip transmission line coil and is promising for the development of a compact coil-PET system capable of simultaneous PET/MRI analysis with an existing MRI system.

B 1 field comparison for RF coils in ultra‐high‐field MRI

This reported work demonstrates the use of convex optimisation to localise the transverse magnetic B + 1 field in regions of interest for recently proposed multi-sectioned alternating impedance coils and the traditional transmission line coil. An approach based on different axial slices to identical radio frequency (RF) coils except upper stripline structure is investigated. Electromagnetic simulation results are compared for RF coils and discussed in detail at 7.0 T.

7 T MRI에서의 다양한 RF 코일에 대한 B1+비교

정 자장(<TEX>$B_0$</TEX>)의 세기가 7 T(Tesla) 고자기장 MRI(Magnetic Resonance Imaging) 시스템은 정 자장의 세기가 1.5 T 또는 3 T MRI 시스템에 비하여 인가된 RF(Radio Frequency) 필드의 높은 불균질성을 보여준다. 이러한 문제점를 극복하기 위한 한 방법으로써, 관심 영역에서의 RF 자장이 균일하도록 RF 코일의 각 요소로 들어가는 최적화된 전류의 크기와 위상 값을 얻기 위해서 convex 최적화 방법이 사용된다. 이러한 방법을 7 T MRI 시스템에 다양한 RF 코일에 적용하여 각각의 <TEX>$B_1^+$</TEX> 필드 값을 비교하여 그 성능을 파악하였다. This paper demonstrates the use of the convex optimization to localize the transverse magnetic <TEX>$B_1^+$</TEX> field in regions of interest for recently proposed multi-sectioned alternating impedance coils and the traditional transmission line coil. An approach based on different axial slices to identical RF coils except upper stripline structure is investigated. Electromagnetic simulation results are compared for RF coils and discussed in detail at 7.0 T.

고자장 MRI에서의 영상 영역에 대한 B1+균질성

In high static field magnetic resonance imaging(MRI) systems, fields of 7 T and 9.4 T, the impressed RF field shows larger inhomogeneity than in clinical MRI systems with B0 fields of 1.5 T and 3.0 T. In multi-channel RF coils, the magnitude and phase of the input to each coil element can be controlled independently to reduce the non-uniformity of the impressed RF field. The convex optimization technique has been used to obtain the optimum excitation parameters with iterative solutions for homogeneity in a selected ROI(Region of Interest). To demonstrate the technique, the multichannel transmission line coil was modeled together with a human head phantom at 400 MHz for the 9.4 T MRI system and fields are obtained. In this paper, all the optimized in each isolated ROIs are combined to achieve significantly improved homogeneity over the entire field of view. The simulation results for 9.4 T MRI systems are discussed in detail.

RF Magnetic Signal Localization at Very High Magnetic Field Systems

The impetus of this work originated from the advent of high magnetic field magnetic resonance imaging scanners with B0 fields of 4T, 7T, and 9.4T. These ultrahigh magnetic field systems generally improve the signal to noise ratios. However, B1 field non-uniformity also occurs due to the increased RF field frequencies when wavelengths in the head become shorter than its size. As interest in multiple channel transmission line coils increases, the control of the amplitude and phase of individual coil elements is required in order to develop desired B1 field. The choice of the excitation of the coil elements may be determined by convex optimization. Convex optimization is used provides results very fast, when the problem is formulated globally. In addition, convex optimization provides better signal to noise (SNR) ratio when anatomic specific regions are investigated. In this paper, simulation and experimental results are discussed at 9.4T systems based on the number of elements. The primary objective of this study is to increase the signal in a specific target region and decrease the signal and noise in the outside region termed the suppression region. The convex formulations are minimizing the maximum field point in the suppression region while keeping the center of target maximum. Based on this min-max optimization criterion, an iteration method which modifies the selection of suppression fields is also performed to produce better results. The results of the localization on FDTD human data at 9.4T are shown in Fig. 1. In these figures, the axial slices of the center of human head model provided by XFDTD are used after manipulating with MATLAB and the 16 channel head coil is excited. Figure 1 shows an improvement of the homogeneity in the suppression region when the target region is at center. In Fig. 2, received signal localizations are obtained for three different regions of interest (ROI) after using the convex optimization. Note that the selection of ROI is limited by the geometric setting of phantom in the 8-channel TEM head coil. Convex optimization with an iterative method was performed on both the human head and phantom models with operating frequency 400 MHz to design coil channel excitation parameters. By applying the iterative method to the convex optimization, more homogeneous B1 fields are obtained in the suppression region for 9.4T system.

TEM transmission line coil with double nuclear capability

MR imaging and spectroscopy requires RF fields of high homogeneity. Quadrature volume coils meeting this requirement include the birdcage coil and the TEM resonator. We describe here a quadrature volume coil designed around a modified coaxial transmission line in which multiple inner conductors are arrayed on a circular perimeter and surrounded by a common shield. Current antinodes are established at appropriate points on the inner conductors by series transmission line stubs, either open circuit or short circuit, which terminate the line. Quadrature phasing is generated by a novel circuit constructed from a pair of high-performance current baluns and a commercial quadrature hybrid. The coil is a "pure" TEM coil as it is simply a resonant transmission line. There is no mode structure to consider. The construction of a prototype quadrature four-element coil is described and preliminary test results from this coil in a 4.7T horizontal bore magnet are reported. A related double nuclear coil design for (3)He and (1)H in which two linear transmission line coils are arrayed on a circular perimeter and simultaneously tuned and matched to their respective frequencies is also described. Preliminary tests from this coil in a 1.5T clinical scanner are reported.

Sci‐Fri PM Imaging‐08: Development of Micro‐Strip Coils and “Direct” Method for Bilateral Breast Imaging

In this work, we present a position‐independent decoupling method using micro‐strip transmission line coils. Micro‐strip transmission line coils consist of a grounded back plane together with the conductive strip bonded to either side of a low‐loss, high permittivity material. This two‐part structure exhibits the shielding decoupling properties and excellent decoupling over a range of geometries. Its asymmetrical signal response operates so that signals from each individual breast originate primarily from the coil facing the breast. In this work we also present a parallel imaging method similar to Partially Parallel Imaging with Localized Sensitivities (PILS) method for bilateral breast MRI by taking advantage of the isolation properties of micro‐strip coils to allow independent imaging of both breasts in the time usually required to scan a single breast without resorting to traditional SENSE techniques.

Ultra-broadband NMR probe: numerical and experimental study of transmission line NMR probe

We have reinvestigated a transmission line NMR probe first published by Lowe and co-workers in 1970s [Rev. Sci. Instrum. 45 (1974) 631; 48 (1977) 268] numerically and experimentally. The probe is expected to be ultra-broadband, thus might enable new types of solid-state NMR experiments. The NMR probe consists of a coil and capacitors which are connected to the coil at regular intervals. The circuit is the same as a cascaded LC low-pass filter, except there are nonzero mutual inductances between different coil sections. We evaluated the mutual inductances by Neumann’s formula and calculated the electrical characteristics of the probe as a function of a carrier frequency. We found that they were almost the same as those of a cascaded LC low-pass filter, when the inductance L of a section was estimated from the inductance of the whole coil divided by the number of the sections, and if C was set to the capacitance in a section. For example, the characteristic impedance of a transmission line coil is given by Z=( L/ C) 1/2. We also calculated the magnitude and the distribution of RF magnetic field inside the probe. The magnitude of RF field decreases when the carrier frequency is increased because the phase delay between neighboring sections is proportional to the carrier frequency. For cylindrical coils, the RF field is proportional to (π ν/2 ν d) 1/2exp(− ν/ ν d), where the decay frequency ν d is determined by the dimensions of the coil. The observed carrier frequency thus must be much smaller than the decay frequency. This condition restricts the size of transmission line coils. We made a cylindrical coil for a 1 H NMR probe operating below 400 MHz. It had a diameter 2.3 mm and a pitch 1.2 mm. Five capacitors of 6 pF were connected at every three turns. The RF field strength was 40 and 60 kHz at the input RF power 100 W by a calculation and by experiments, respectively. The calculations showed that the RF field inhomogeneity along the coil axis was caused by a standing wave of current, which arose from the reflections at the coil ends. The calculation showed that the homogeneity could be improved by decreasing the pitch near the both ends and making their impedance close to that at the center.

Artificial Transmission Lines With Distributed Constants

Artificial transmission lines have been very valuable in developing the theory of transmission of power. This theory has been developed in great detail for steady state conditions, and it is now important to extend it to cover transient conditions. This can best be done upon artificial transmission lines but the usual lumpy type of line introduces parasitic reflections caused by the discrete units of inductance, and therefore are of very limited use for this purpose. Attempts have been made from time to time to produce artificial lines with distributed constants, but the results were either unsatisfactory or expensive and bulky. A method, originally proposed by Pupin, has been developed by which small compact and cheap artificial transmission line coils can be designed and built to represent any desired configuration of circuit. These involve tinfoil wound in with the wire between the different layers, insulated from the winding by the usual form of condenser dielectric paper. One of the most difficult parts is to arrive at a coil with the proper constants. Formulas have been derived for doing this which will calculate the result given by the coil within the limits of manufacturing error, and which can be used in a straightforward manner similar to the design of other electrical apparatus. Either single or polyphase lines may be represented, and a single-phase line representing the average configuration of a 110,000-volt line approximately 1000 miles (1600 km.