Stepped Impedance Resonators for High-Field Magnetic Resonance Imaging

Multi-element volume radio-frequency (RF) coils are an integral aspect of the growing field of high-field magnetic resonance imaging. In these systems, a popular volume coil of choice has become the transverse electromagnetic (TEM) transceiver coil consisting of microstrip resonators. In this paper, to further advance this design approach, a new microstrip resonator strategy in which the transmission line is segmented into alternating impedance sections, referred to as stepped impedance resonators (SIRs), is investigated. Single-element simulation results in free space and in a phantom at 7 T (298 MHz) demonstrate the rationale and feasibility of the SIR design strategy. Simulation and image results at 7 T in a phantom and human head illustrate the improvements in a transmit magnetic field, as well as RF efficiency (transmit magnetic field versus specific absorption rate) when two different SIR designs are incorporated in 8-element volume coil configurations and compared to a volume coil consisting of microstrip elements.

[1]  J T Vaughan,et al.  High resolution neuroimaging at 4.1T. , 1995, Magnetic resonance imaging.

[2]  Michael B. Smith,et al.  Central brightening due to constructive interference with, without, and despite dielectric resonance , 2005, Journal of magnetic resonance imaging : JMRI.

[3]  Chih-Ming Tsai,et al.  New cross-coupled filter design using improved hairpin resonators , 2000 .

[4]  J. Thomas Vaughan,et al.  Ultra High Field MRI: High-Frequency Coils , 2006 .

[5]  P. Schmalbrock,et al.  Electromagnetic perspective on the operation of RF coils at 1.5–11.7 Tesla , 2005, Magnetic resonance in medicine.

[6]  T. Ibrahim,et al.  Effect of RF coil excitation on field inhomogeneity at ultra high fields: a field optimized TEM resonator. , 2001, Magnetic resonance imaging.

[7]  G. Metzger,et al.  Local B1+ shimming for prostate imaging with transceiver arrays at 7T based on subject‐dependent transmit phase measurements , 2008, Magnetic resonance in medicine.

[8]  L. Bolinger,et al.  Mapping of the Radiofrequency Field , 1993 .

[9]  H. A. Wheeler.  Transmission-Line Properties of Parallel Strips Separated by a Dielectric Sheet , 1965 .

[10]  Ke Wu,et al.  Corrections to "accurate circuit model of interdigital capacitor and its application to design of new quasi-lumped miniaturized filters with suppression of harmonic resonance" , 2000 .

[11]  M. Makimoto,et al.  Bandpass Filters Using Parallel Coupled Strip-Line Stepped Impedance Resonators , 1980 .

[12]  B. Beck,et al.  Tissue‐equivalent phantoms for high frequencies , 2004 .

[13]  P. Börnert,et al.  Transmit SENSE , 2003, Magnetic resonance in medicine.

[14]  M. Makimoto,et al.  Bandpass Filters Using Parallel Coupled Strip-Line Stepped Impedance Resonators , 1980, 1980 IEEE MTT-S International Microwave symposium Digest.

[15]  Ravi S. Menon,et al.  Imaging at high magnetic fields: initial experiences at 4 T. , 1993, Magnetic resonance quarterly.

[16]  P. Silvester,et al.  TEM wave properties of microstrip transmission lines , 1968 .

[17]  J. Lagendijk,et al.  7 T body MRI: B1 shimming with simultaneous SAR reduction , 2007, Physics in medicine and biology.

[18]  K. Uğurbil,et al.  Analysis of wave behavior in lossy dielectric samples at high field , 2002, Magnetic resonance in medicine.

[19]  R. W. Lau,et al.  The dielectric properties of biological tissues: II. Measurements in the frequency range 10 Hz to 20 GHz. , 1996, Physics in medicine and biology.

[20]  K. Uğurbil,et al.  Microstrip RF surface coil design for extremely high‐field MRI and spectroscopy , 2001, Magnetic resonance in medicine.

[21]  T. Vaughan,et al.  Calibration Tools for RF Shim at Very High Field with Multiple Element RF Coils : from Ultra Fast Local Relative Phase to Absolute Magnitude B 1 + Mapping , 2007 .

[22]  Peter Andersen,et al.  9.4T human MRI: Preliminary results , 2006, Magnetic resonance in medicine.

[23]  Steen Moeller,et al.  B1 destructive interferences and spatial phase patterns at 7 T with a head transceiver array coil , 2005, Magnetic resonance in medicine.

[24]  R. Goebel,et al.  7T vs. 4T: RF power, homogeneity, and signal‐to‐noise comparison in head images , 2001, Magnetic resonance in medicine.

[25]  K. Uğurbil,et al.  Transmit and receive transmission line arrays for 7 Tesla parallel imaging , 2005, Magnetic resonance in medicine.

[26]  R. W. Lau,et al.  The dielectric properties of biological tissues: III. Parametric models for the dielectric spectrum of tissues. , 1996, Physics in medicine and biology.

[27]  F. Shellock,et al.  Effects of coil dimensions and field polarization on RF heating inside a head phantom. , 2005, Magnetic resonance imaging.

[28]  Peter M Jakob,et al.  Myocardial perfusion measurements by spin-labeling under different vasodynamic states. , 2004, Journal of cardiovascular magnetic resonance : official journal of the Society for Cardiovascular Magnetic Resonance.

[29]  A. Gopinath.  Maximum Q-Factor of Microstrip Resonators , 1981 .

[30]  Christopher M Collins,et al.  Numerical field calculations considering the human subject for engineering and safety assurance in MRI , 2009, NMR in biomedicine.