Birdcage Coil Design Research Articles

Extended curvature birdcage coil design for localized magnetic field in 9.4 T MRI for multiple mouse imaging

Magnetic resonance imaging (MRI) for preclinical analysis is useful for testing medical procedures, therapies, and drug development. Preclinical MRI uses small animals, such as mice, for multiple studies. Preclinical studies are often needed to control target mouse models with different parameters; thus, performing simultaneous MRIs is desired to reduce study time and provide a fair comparison. In this study, we propose a variation of a birdcage coil that is capable of providing the same field intensity for each mouse compared to a traditional birdcage coil configuration. The arrangement of the proposed birdcage coil produces a uniform and strongly localized magnetic |B1|-field in each of the four target mice. The design involves rotating the legs of the birdcage coil so that the current distribution produces a uniform field inside each mouse. Electromagnetic simulations were performed to validate the proposed design. The proposed design exhibited a better field intensity than the reference linear, circularly polarized mode birdcage coils and an eight-channel loop array coil. The proposed customized birdcage coil arrangement promises a better field intensity and uniformity across multiple mouse imaging using 9.4 T preclinical MRI scanners.

A Modified Quadrature Birdcage Coil Incorporated With a Curved Feature for In Ovo MR Imaging.

Goal: This study presents a novel MRI coil design approach explicitly tailored for chick embryo measurements, with the primary objective of improving sensitivity and coverage. Methods: The limitations posed by conventional birdcage coils were addressed by introducing a curvature feature into a standard coil. The performance of the modified coil was assessed using EM simulations and experimental evaluations, which were subsequently validated using a 7 T MRI scanner. A comparative analysis was conducted against a standard quadrature low-pass birdcage coil to evaluate key factors. Results: The proposed coil demonstrated improved SNR and uniformity, particularly in the proximity of the end-rings. These results were consistent with the findings obtained from the simulations. Conclusions: The use of our innovative birdcage coil design holds promise and offers practical potential for in ovo studies.

Wireless amplified NMR detector for improved visibility of image contrast in heterogeneous lesions.

To demonstrate the capability of a wireless amplified NMR detector (WAND) to improve the visibility of lesion heterogeneity without the use of exogenous contrast agents, a cylindrically symmetric WAND was constructed to sensitively detect and simultaneously amplify MR signals emitted from adjacent tissues. Based on a two-leg high-pass birdcage coil design, this WAND could be activated by a pumping field aligned along the main field (B0 ), without perturbing MR signal reception. Compared with an equivalent pair of external detectors, the WAND could achieve more than 10-fold gain for immediately adjacent regions. Even for regions with 3.4 radius distance separation from the detector's cylindrical center, the WAND was at least 1.4 times more sensitive than an equivalent pair of surface arrays or at least twice as sensitive as a single-sided external surface detector. When the WAND was inserted into a rat's rectum to observe adjacent tumors implanted beneath the mucosa, it could enhance the detection sensitivity of lesion regions, and thus enlarge the observable signal difference between heterogeneous tissues and clearly identify lesion boundaries as continuous lines in the intensity gradient profile. Hyperintense regions observable by the WAND existed due to higher levels of blood supply, which was indicated by a similar pattern of signal enhancement after contrast agent administration. By better observing the endogenous signal contrast, the endoluminal WAND could characterize lesions without the use of exogenous contrast agents, and thus reduce contrast-induced toxicity.

Investigation of the B1 field distribution and RF power deposition in a birdcage coil as functions of the number of coil legs at 4.7 T, 7.0 T, and 11.7 T

The proper design of birdcage (BC) coils plays a very important role in the acquisition of highresolution magnetic resonance imaging (MRI) of small animals such as rodents. In this context, we investigate multiple-leg (8-, 16-, 32-, 64-, and 128-leg) BC coils operating at ultra-high fields (UHF) of 7.0 T and 11.7 T and a high-field (HF) of 4.7 T for rodent magnetic resonance imaging (MRI). Primarily, Our study comparatively examines the parameters of the radiofrequency (RF) transmission (|B1 +|)-field, the magnetic flux (|B1|)-field, and RF power deposition (RF-PD) as functions of the number of BC-coil legs via finite-difference time-domain (FDTD) calculations under realistic loading conditions with a biological phantom. In particular, the specific ratio |E/B1 +| is defined for predicting RF-PD values in different coil structures. Our results indicate that the optimal number of legs of the BC coil can be chosen for different resonance frequencies of 200 MHz, 300 MHz, and 500 MHz and that this choice can be lead to superior |B1 +|-field intensity and |B1|-field homogeneity and decreased RF-PD. We believe that our approach to determining the optimal number of legs for a BC coil can contribute to rodent MR imaging.

Numerical methods and software tools for simulation, design, and resonant mode analysis of radio frequency birdcage coils used in MRI

ABSTRACTDesign of magnetic resonance imaging (MRI) radiofrequency (RF) coils using lumped circuit modeling based techniques begins to fail at high frequencies, and therefore more accurate models based on the electromagnetic field calculations must be used. Field calculations are also necessary to understand the interactions between the RF field and the subject inside the coil. Furthermore, observing the resonance behavior of the coil and the fields at the resonance frequencies have importance for design and analysis. In this study, finite element method (FEM) based methods have been proposed for accurate time‐harmonic electromagnetic simulations, estimation of the tuning capacitors on the rungs or end rings, and the resonant mode analysis of the birdcage coils. Capacitance estimation was achieved by maximizing the magnitude of the port impedance at the desired frequency while simultaneously minimizing the variance of RF magnetic field in the region of interest. In order for the proposed methods to be conveniently applicable, two software tools, resonant mode and frequency domain analyzer (RM‐FDA) and Optimum Capacitance Finder (OptiCF), were developed. Simulation results for the validation and verification of the software tools are provided for different cases including human head simulations. Additionally, two handmade birdcage coils (low‐pass and high‐pass) were built and resonance mode measurements were made. Results of the software tools are compared with the measurement results as well as with the results of the lumped circuit modeling based method. It has been shown that the proposed software tools can be used for accurate simulation and design of birdcage coils. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 13–32, 2015

Chronic exertional compartment syndrome of the lower extremities: improved screening using a novel dual birdcage coil and in-scanner exercise protocol

The purpose of this study was to design and evaluate an MRI screening protocol for chronic exertional compartment syndrome (CECS) of the lower legs using an in-scanner exercise protocol and novel dual birdcage coil design for improved imaging. Coil and phantom studies: a custom-made dual birdcage coil designed for this protocol was evaluated for uniformity and signal-to-noise ratio (SNR) compared with a conventional phased-array receive-only torso coil and the body coil. Phantom and normal subject studies were performed to confirm coil performance. In-vivo studies: eight unaffected subjects and 42 patients with lower extremity symptoms suggestive of CECS were imaged with the dual birdcage coil and an in-scanner exercise protocol which included imaging at rest, during isometric resisted dorsi flexion, at rest (recovery), during isometric resisted plantar flexion and, again, at rest. Of 42 patients, 14 had confirmed CECS and 28 had lower extremity anomalies attributable to other causes. Ratios of relative T2-weighted signal intensities were calculated for exercise and recovery images compared to baseline after processing of images, including re-registration for motion, smoothing and segmentation to remove bone and pulsation artifacts from blood vessels. Receiver operating characteristic (ROC) analysis showed a threshold for the ratio of relative T2-weighted signal intensity of 1.54 to have a sensitivity of 96%, specificity of 90% and accuracy of 96% for CECS. Patients with CECS had their peak ratio of signal intensity compared with baseline during the first recovery period after isometric dorsi flexion, whereas unaffected subjects and patients with other causes of exercise-induced lower extremity pain reached their peak values during exercise (P<0.001). We have developed the first in-scanner MRI exercise protocol for the assessment of patients with suspected CECS. The technique shows high accuracy, sensitivity and specificity for diagnosis in this small cohort of patients with CECS. Further study may allow this non-invasive test to be used as a triage tool for invasive intracompartmental pressure measurements in patients with suspected CECS.

Comparison and evaluation of mouse cardiac MRI acquired with open birdcage, single loop surface and volume birdcage coils

Although the quality and speed of MR images have vastly improved with the development of novel RF coil technologies, the engineering expertise required to implement them is often not available in many animal in vivo MR laboratories. We present here an open birdcage coil design which is easily constructed with basic RF coil expertise and produces high quality images. The quality and advantages of mouse cardiac MR images acquired with open birdcage coils were evaluated and compared to images acquired with a bent single loop surface, and standard birdcage coils acquired at 4.7 Tesla. Two low pass open birdcage coils, two single loop surface coils, and a low pass volume birdcage coil were constructed and their B1 distributions were evaluated and compared. The calculated average signal-to-noise ratio for the left ventricular wall was 10, 23 and 32 for the volume birdcage coil, single loop surface coil and open birdcage coil, respectively. The results demonstrate that the open birdcage coil provides greater sensitivity than the volume coil and a higher signal/contrast-to-noise ratio and B1 homogeneity than the single loop surface coil. The open birdcage coil offers easy access and better quality mouse cardiac imaging than both the single loop surface coil and volume birdcage coil and does not require extensive RF engineering expertise to construct.

Analytical approach to noncircular section birdcage coil design: Verification with a Cassinian oval coil

A general analytical framework is presented for the design of birdcage radiofrequency resonators on cylindrical formers having arbitrary cross-sectional shape. The primary objective of such shapes would be to improve the sensitivity of the NMR experiment to noncircular regions of the human anatomy while maintaining field homogeneity and quadrature polarization comparable to those of standard circular birdcage coils. The shape of the corresponding radiofrequency screen, which is required to decouple the coil from the rest of the NMR system and which is key to the performance, is also provided by this methodology. The theory was tested by constructing a 3-T, quadrature, proton coil on a shape conforming to the anthropomorphic mean of the human head, namely, the oval of Cassini. Both bench tests (Q) and in vivo spectral and imaging comparisons of the Cassinian coil with an equivalently dimensioned and constructed circular birdcage coil, respectively, predicted and demonstrated in vivo an improvement in SNR of approximately 24% over the circular section coil. The experimental RF field homogeneity and quadrature performance were comparable for both coil geometries, with the circular coil being marginally superior.

2‐D full wave solution for the analysis and design of birdcage coils

AbstractA simple 2‐D full wave model for the birdcage coil is described that includes the effects of shields, lossy layered media, and finite leg widths. This model has been implemented in Matlab, which provides a simple user interface. Using this model, various coil parameters such as signal‐to‐noise ratio (SNR), magnetic field homogeneity, magnetic field magnitude, etc. can be computed quickly. The efficiency of this model allows optimization of the coil geometry for particular applications. A method for finding the optimal coil radius in terms of SNR and homogeneity, given shield and sample radii, is presented. © 2003 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 18B: 15–23, 2003.

A fast and accurate simulator for the design of birdcage coils in MRI

A fast and accurate simulator for the design of birdcage coils in MRI

BirdcageBuilder: Design of specified‐geometry birdcage coils with desired current pattern and resonant frequency

A simple method for birdcage coil design (high pass, low pass, and band pass) is presented. Rather than iteratively approaching the Larmor resonant frequency with known capacitances and calculated inductances, a more versatile approach to birdcage coil design is developed and validated, in which the necessary capacitances are calculated using a desired current pattern, a calculated inductance, and a predetermined resonant frequency. In order to expedite coil design for experienced and novice coil builders, a computer program (BirdcageBuilder) is also implemented based on this method. Experimental results show that the calculated capacitances and actual capacitances for several existing birdcage coils are in good agreement.

A method to create an optimum current distribution and homogeneous B1 field for elliptical birdcage coils.

For a birdcage coil with elliptical cross-section, a sinusoidal current pattern does not provide a homogeneous B1 field. A simple theory was developed to create an optimized current distribution for elliptical birdcage coils. This optimized current pattern can create a perfectly homogeneous B1 field inside any elliptical shape. To verify the theory, a 16-element high-pass elliptical birdcage coil was built inside a circular RF shield. The current was optimized by using the inductance characteristics of the coil components to calculate the end-ring capacitances. The B1 field was theoretically calculated and experimentally mapped for the optimized elliptical bird-cage coil and a nonoptimized coil. The results demonstrate that by optimizing the current distribution, a very homogeneous B1 field is produced. This method can be directly applied in design and construction of elliptical birdcage coils for imaging of the naturally occurring elliptical cross-sectional geometries in the human body.

On the field inhomogeneity of a birdcage coil.

A mathematical model is developed for the numerical analysis of birdcage coils constructed using capacitors having different capacitance. With the aid of this model, it is shown that the deviation of some capacitance introduced by tune, balance, and drive mechanisms can cause inhomogeneity in the B1 field produced by the coil. Useful curves and formulae are developed to provide a guideline for the design of birdcage coils.

Implementation of a modified birdcage resonator for 19F/1H MRI at low fields (0.14 T)

Fluorine-19 nuclear magnetic resonance of perfluorinated blood substitute materials provides a method for determination of oxygen tension (pO2) in vivo. Use of a double resonant 19F/1H radio frequency coil allows convenient correlation between the high resolution anatomic presentation of proton images and the fluorine distribution. However, quantitative 19F measurements require an RF coil with good H1 field homogeneity over the image volume and a high quality factor (Q) to minimize errors caused by the low signal-to-noise levels available in in vivo imaging and image nonuniformities introduced by the large chemical shift of fluorocarbons. The birdcage coil design provides a high Q structure with optimum H1 field uniformity and fill factor. However, at low resonance frequencies, the inherently low inductance of the birdcage geometry requires the use of a large number of chip capacitors giving rise to unwieldy coil fabrication and increased cost. This communication describes a modification to the birdcage design that reduces the chip capacitor requirement by at least a factor of 4 for a given dimension, yet retains the essential characteristics of the birdcage design. The modified structure was tuned for double resonance at 5.7/6.0 MHz for 19F/1H magnetic resonance imaging at 0.14 T. For a coil with a length to diameter ratio of 1.67, an H1 uniformity of +/- 2% for the 19F resonance was obtained over a cylindrical region with radius approximately 0.6r (r = radius of coil) and length approximately 1.8r within the coil.

Biplanar Radiofrequency Coil Design

A novel geometry for radiofrequency coil design is described. In this geometry, longitudinal wires of the coil lie on two parallel planes. The currents in the wires of one plane run in the direction opposite to those of the other plane. An analytic solution is provided for the field produced by infinite surface currents running in the biplanar geometry. For the case of discrete wires, computer-generated field maps imply that the homogeneity and sensitivity of the biplanar design are superior to those of a saddle coil, but worse than those obtained in an equivalent discrete cosine or birdcage coil design. Optimization of this coil design was performed using computer simulations. The measured B1 map of an optimized, single-tuned biplanar coil compares favorably to that of an equivalent discrete cosine coil, demonstrating excellent homogeneity in the central region of the coil. A 30 × 24 × 40 cm biplanar coil has been coupled to a 1.5 T imaging system. Images of the human abdomen generated with this coil demonstrate a high degree of homogeneity across nearly all of the sensitive region of the coil.