The radio frequency (RF) coil is one of the key components of the magnetic resonance imaging (MRI) system. It has a significant impact on the performance of the nuclear magnetic resonance (NMR) detection. Among numerous practical designs of RF coils for NMR imaging, the birdcage RF coil is the most popular choice from low field to ultra-high field MRI systems. In the transmission mode, it can establish a strong and homogeneous transverse magnetic field B1 for any element at its Larmor frequency. Similarly, in the reception mode, it exhibits extremely high sensitivity for the detection of even faint NMR signals from the volume of interest. Despite the sophisticated 3D structure of the birdcage coil, the developments in the design, analysis, and implementation technologies during the past decade have rendered the development of the birdcage coils quite reasonable. This article provides a detailed review of the recent progress in the birdcage RF coil technology for the MRI system.

A novel analytical solution for the designing of the birdcage RF coil has been demonstrated in this paper. A new concept of dominant resonance path has been introduced in this paper which is used to identify the specific closed current loop in the birdcage RF coil which is responsible for the dominant resonance frequency mode. This concept is used to determine the precise numerical values of the lumped capacitance deployed in the legs and/or end-rings of the birdcage RF coil for its proper operation at the desired resonance frequency. The analytical solution presented in this paper has been established by performing the two-port network based equivalent circuit modeling of the birdcage RF coil. The proposed analytical solution uses T-matrix theory and develops a relationship between the input impedance of the birdcage coil and the impedances of its leg and end-ring segments. The proposed analytical solution provides the information about the resonance frequency spectrum of the birdcage RF coil and solves the issue of its interfacing with external circuits without affecting its resonance characteristics. Based upon the proposed analysis and designing strategy presented in this paper, the low pass, high pass and band pass configurations of the birdcage RF coil were successfully implemented with FPCB (Flexible Printed Circuit board) technique for small volume NMR imaging applications at 1.5 T and 3.0 T MRI system. The results obtained for the implemented birdcage coils using the proposed analysis and designing technique are in closed agreement with already established methods.

An algorithm for measurement of the cutting blade of the ultrasonic vibration cutting machine used in an automation machine is proposed in this paper. The proposed algorithm, which is based upon a multi-step detection method, is developed for the accurate measurement of the cutting blade by exactly determining its rotation angle, length, and thickness. Instead of the commonly used Otsu method, we propose a new curvature-based adaptive binarization method, which provides more accurate details about the dimensions of the cutting blade. A region of interest containing the cutting blade from the acquired image is first extracted in the multi-step detection method, which is further processed to remove the noise, which increases the measurement reliability. An important feature of the proposed process is the restoration of the cutting blade’s tip data, which used to be lost during the fine noise-filtering process. The rotation angle and length are measured using the minimum rotated rectangle while the line fitting based upon the least square method is applied to increase the reliability of the thickness measurement. Experimental results validate the superiority of the proposed method over the conventional Otsu method.

A novel method has been established to mitigate the complexity of a birdcage coil design process for use in a MRI (Magnetic Resonance Imaging) system. It can reduce the time consumptions of the typical coil design process based on 3D (three-dimensional) simulations. The method combines an equivalent circuit model and a 3D electromagnetic simulation. It makes it possible to understand the coil properties intuitively and to solve the major engineering problems by using just the circuit model not by using the complex 3D simulation. The concept of the dominant resonance path length of the coil has also been introduced, which facilitates the design process by considering the skin effect of the coil. We have verified that the skin effect of the birdcage coil can be evaluated through a simple circular loop circuit instead of the whole 3D geometry. Base on the design proposed method, we also have successfully demonstrated that the birdcage coil based on a FPCB (Flexible Printed Circuit board) technique can be used in high filed (3.0 T and 4.7 T) MRI systems for small animal NMR (Nuclear Magnetic Resonance) imaging.

A novel birdcage type Radio Frequency (RF) receiver coil sensor with passive type detuning circuits integrated on the end-rings of the coil sensor has been implemented. A simulation model of the RF receiver coil sensor with the passive type detuning circuits has also been proposed. The simulation model completely explains the working principle of the RF receiver coil sensor with the passive type detuning circuits. The RF coil sensor was designed to perform the Nuclear Magnetic Resonance (NMR) imaging of small animal at 3T Magnetic Resonance Imaging (MRI) system. The integrated detuning circuits of the RF coil sensor eliminate the noises induced by the mutual inductance between receiving and transmitting coils. We have confirmed the performances of the RF coil sensor through experiments, which agreed well with those of the simulations.

A novel simple design of birdcage type RF receiver coil for using in the ¹H NMR (Nuclear Magnetic Resonance) imaging system operating at 1.5 T and 3.0 T has been demonstrated. The RF coil is a band pass type coil composed of legs and end rings and both legs and end rings are made of epoxy-etched copper strips on a single flexible circuit board. The important feature of the coil with multiple resonance frequency is the use of multiple capacitors in each leg of the coil. The coil is capable to resonate at 64 MHz (1.5 T) and 127 MHz (3.0 T) simultaneously. We have analyzed the performance of the coil through simulations and experimental measurements.