For magnetic resonance imaging design coupled microstrip resonators

For medical and scientific applications of magnetic resonance imaging (MRI) system requires a for a uniform magnetic field of high performance, high power inductor. Transverse electromagnetic (TEM) resonators 1 as the required field strength 4.7 and MRI 9.4T application standard AC birdcage coil 2 Advanced alternative recently popular concern. For example, the operating frequency of 200 and 400MHz, transverse electromagnetic (TEM) resonator can be achieved than an equivalent birdcage coil for MRI images due to improved quality better magnetic field in the same direction and higher quality factor (Q). In order to support object-oriented based on coupled microstrip line MRI applications high Q TEM resonators analysis and design, the author proposes a finite element method based on the use of effective ways.

TEM resonators and birdcage coil for the main difference is the cylindrical shield.

Shield as part of an activity in the system, in the internal wires to provide a return path for current. The birdcage coil, the shield is a and internal component broken separate entities, only have an impact on the internal coil in order to prevent excessive radiation loss. Due to the TEM resonators shield design, it looks like a standing wave is able to support the high-frequency longitudinal multi-line transmission lines. And birdcage coil, TEM resonators internal wire is not connected to the adjacent wire, and most, but by the capacitor components are connected directly and shielding enclosures. Through induction internal conductor realization mutual inductance between resonance mode. Because all conductors using the adjustable capacitor components and shielding enclosures attached, so you can adjust the field to achieve the best in the same direction.

5 the reference, the authors successfully adjusted reference 6 numerical methods in order to fit for analysis and design of an n-element no load coupled microstrip line TEM resonators.

This change of numerical methods allow the determination of the main arguments: inductance and capacitance coefficient matrix, [L] and [C], consider using FEM analysis of TEM resonators of geometrical parameters. These results and reference 4 FEM using boundary element method (BEM) results of comparison between shows a 12-element no load coupled microstrip line TEM resonators good relevance. In order to verify that the appropriate numerical method, $ 8 for a no load coupled microstrip line TEM resonators for design and analysis. This resonator with-minimum reflectance 63.33dB and no load quality factors (Qo) 400 (in 200MHz).

Figure 1 is no load diagram of TEM resonators.

TEM resonators basic component is n internal coupled microstrip line, which leads to cylindrical mode distribution and end points through capacitor connected to the cylindrical outer shielding enclosures.

Figure 2 shows the coupled microstrip line TEM resonators of the interactive portions of the radius of rB external shielding and w w ¢ t thick n microstrip line (these conductors cylindrical RADIUS rR).

Coupled microstrip line TEM resonators EM properties you can use the main parameter [L], [C] and secondary parameters no load quality factors Qo.

By Laplace equation solving one based on two-dimensional static problems can get the matrix of coefficients.

Where: V = 1V in the ith wire surface, V = 0 in other conductors.

In this article, by using FEM analysis solution of equation 1.

This solution represents a structure different network nodes at the distribution of voltage V. When the voltage v is known, from the charge for each wire you can calculate the [C] matrix I in line.

Here:

Lj for j a wire around the contour, EN-dielectric constant electric field.

Matrix [C] shows all the metal conductor capacitance between, describes coupled microstrip line TEM resonators in electric energy storage.

Inductance matrix [L] contains a diagonal wire inductance coefficient and diagonal outside wire between inductive coefficient, which defines the magnetic field energy storage.

In the high-frequency limit, that is, the skin depth is small enough so that the current surface only in traverse, you can use matrix [C] is inductive coefficient matrix [L]. [C] that [L]:

When the matrix [L] and [C] is OK, you can use an appropriate numeric model shown in Figure 3 resonance frequency resonator (S11).

This article details the MRI resonator including length I of the shield of coupled microstrip lines that match the capacitor capacitance CM, termination, CSi and CLI (I = 1 to n)

Can sweep of reflection parameters (S11) estimated resonator without quality factors (Qo).

Here: fr-circuit resonant frequency, fu-higher than the resonance frequency of 3dB frequency, fl to below the resonance frequency of 3dB frequency.

The author uses a modified FEM digital tools to take advantage of coupled microstrip lines of MRI resonator for analysis and design.

FEM method can design simulation to determine whether a given constraints of some possible resonator.

In order to design a MRI Resonators, author of Figure 2 shows the structure of the analysis.

The structure has eight internal microstrip line and the following characteristics:

* An external cylinder with a RADIUS (rB) 52.5mm

* An internal cylinder with a RADIUS (rR) 36.25mm

* Width (w) 17mm

* Strip thickness (t) 0.5mm

* Polyelectrolyte constants (ε r) 1

Using FEM settlement of voltage distribution, as shown in Figure 4.

Once you determine, you can calculate in TEM resonators arbitrary point voltage. Figure 5 shows the different boundary conditions under voltage distribution (table 1).

As discussed above, through the conductor at the standard flux contour integrated determines the unit length parameter matrix.

Table1 cited [L] and [C] the first column of the matrix. This information is sufficient for the reconstruction of the entire matrix, because they are the number of circulating ranks.

Finally, as shown in Figure 3 MRI coupled microstrip line resonator design has the following characteristics: resonance length I match for 37.5cm; 19.14pF capacitance CM for the source and load Terminal Capacitor respectively CS and CL values are 2.415pF.

Figure 6 shows the S11 in MRI resonator RF port emulation frequency response.

The curve shows the resonance frequency (i.e., 200MHz) at a minimum.

Coupled microstrip line TEM resonators reflecting the minimum value in the Office of small resonant frequency (-63.33dB). Use equation 4 can determine Qo is equal to 400.

With the recent 8 element no load coupled coaxial TEM resonators quality factors (Qo = 260) compared to $ 8 from above without load coupled microstrip line TEM resonators geometric and electrical parameters of the no load quality factors (Qo = 400) very attractive.

This article describes for 4.7T (i.e. 200MHz) magnetic resonance images of 8 element no load coupled microstrip line TEM resonators analysis and design, have a very high quality factors (Qo = 400).

To this end, it is necessary to determine the TEM resonators electromagnetic parameters. In this issue at 200MHz can use Laplace equation estimation results. Using finite element method to obtain the results, so we can determine the inductance and capacitance matrix ([L] and [C] matrix). When the [L] and [C] matrix has determined that the design of TEM resonators RF port S11 for frequency response simulation. Thus you can estimate the MRI resonator without quality factors (Qo).

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