Theme: Biomedical Engineering
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The Enhanced Meander Dipole Radiofrequency Coil for 7 Tesla Magnetic Resonance Imaging Machines
Authors: Najla Alashban
Fatima Sibaii
Fatma Hegazi
Haneen Alkhateeb
Mariam Hegazi
Gameel Saleh*
Institutions: *Biomedical Engineering Department, College of Engineering, Imam Abdulrahman Bin Faisal Universit
 
Background

RF coils perform the function of antennas which propagate electromagnetic radiations into the patient [1]. They are used in the MRI system to either transmit, receive or both [2]. The purpose of transmit RF coils is to send the RF pulses at Larmor frequency creating a uniform rotating magnetic field B1 in a pulse sequence. Additionally, they detect the transverse magnetization as it presses in the xy plane [3]. RF coils radiate electromagnetic pulses into the body, generating a nuclear magnetic resonance. This resonance is then detected by the receiving coils [4].

Summary of Work

In this abstract, different parameters of an established meander dipole radio frequency (RF) coil is altered in an attempt to improve its performance by either reducing the electric field and/or maximizing the magnetic field for 7 Tesla magnetic resonance imaging (MRI) [5]. 

The reference design is a meandered dipole coil used at the Erwin L. Hahn Institute for 7 Tesla MRI. It resonates at 297 MHz which is achieved using a matching and tuning circuit. The design parameters are shown in Figure 1.

Fig.1      

Figure 1: The reference existing meander RF coil in 2D (left-top) in 3D with phantom (left-bottom). E-Field 1D distribution curve  (right-top), H-Field 1D distribution curve (right-bottom)

By using CST-MWS, new designs were experimented by changing the following parameters: 

  • Length of meander turns    
  • Width of the turns    
  • Separation between turns

A new design was created and fabricated using the best values of the aforementioned variables.

Summary of Results

To compare the results with the reference design, Figure of Merit was obtained by dividng the maximum H-field value by the maximum E-field value. 

  • Length of meander turns

It was observed that the increase in length resulted in increasing FoM, and the greatest ratio yielded was at a meander length of 90 mm which can be seen in Figure 2 . 

Figure 2: (Left) Different FoM values for various lengths. E-Field 1D distribution curver (right-top) and H-Field 1D distribution at 80 mm (right-bottom)

  • Width of the turns

Similar to the length of the meander, as the width decreased from its original value of 2 mm, the E-field increased. The width value that produced the best FoM value was 3 mm as seen in Figure 4.17.

Figure 3: Different FoM values for various widths. E-Field and H-Field  at 3 mm (right)

  • Separation between turns

It was discovered that when the separation between the loops was decreased, the E-field increased. However, when the separation was made larger, the H-field decreased. Although enlarging the spacing displayed an undesirable reduction in the H-field, the FoM displayed favourable results for larger separation distances. The separation that provided the best FoM was 3.5 mm as seen in Figure 4.18.

Figure 4: Different FoM values for various separation. E-Field and H-Field at 3.5 mm (right)

 

From the table below, changing the meander width by increasing or decreasing it, increasing the length of meander turns, and increasing the separation between the meander loops had a desirable effect. These three changes yielded a lessening of the E-field and an enhancement to the H-field. Combining the optimal values of the parameters with separation of 3.5 mm, length of 80 mm, and width of 3 mm showed the best result thus far. The electric field had an improved value of 42.7 V/m compared to the reference meander dipole coil and a magnetic field of 0.988 A/m. The FoM of the reference meander dipole coil had a value of 0.0208 A/V. Although the magnetic field decreased, the FoM of the combined meander dipole coil had a value of 0.0231 A/V, yielding an 11% increase in FoM. 

Table 1: Values of H-fields and E-fields

  • Fabrication

The CST design of the meander dipole coil was exported as a Gerber file. The design was later laser-printed on transparent papers in order to transfer them by ironing the papers on a single-sided FR4 board. Ferric Chloride, which is the chemical substance used for etching was heated in the range of 40oC to 45oC. The PCB board was then inserted into the etching tank for approximately 15 minutes. After the etching process, the remaining ink, that was shielding the design, was removed by rubbing it with a metal sponge. The excess ink left on the board was then cleaned with acetone. Four holes were drilled on each corner of the FR4 board and connected with bolts 20 mm above the reflector for support as seen in Figure 3.10. Although plastic bolts should be used to prevent interference; since they were unavailable, ferrous bolts were used instead. Two holes were then drilled on the centre of each width of the FR4 board in order to insert the end capacitors which were then soldered between the FR4 board and the reflector. The fabricated meander dipole coil can be seen in figure 5.

Figure 5: The fabricated meander dipole coil

Conclusion

In conclusion, decreasing the length of meander turns did not show any good results. However, increasing the length of meander turns, the separation distance between turns, and changing the width of the meander yielded a lessening of the E-field and an enhancement to the H-field.  Combining the optimal parameters showed a good improvement.

References

[1] A. Dabirzadeh, "RF Coil Design for Multi-Frequency Magnetic Resonance Imaging and Spectroscopy," MASTER OF SCIENCE, Office of Graduate Studies of Texas A&M University, Texas A&M University, Texas, 2008.

[2] B. D. Muzio, J. R. Ballinger, and e. al. Radiofrequency Coils. Available: http://radiopaedia.org/articles/radiofrequency-coils-1

[3] J. P. Hornak. (2010). The Basics of MRI. Available: https://www.cis.rit.edu/htbooks/mri/inside.htm

[4] K. Thiyagarajan, T. Kesavamurthy, and G. Bharathkumar, "Design and Analysis of Microstrip-Based RF Birdcage Coil for 1.5 T Magnetic Resonance Imaging," Applied Magnetic Resonance, vol. 45, pp. 255-268, 2014/03/01 2014.

[5] S. Orzada et al., Proc. Intl. Soc. MRM 16 (2008).

[6] Saleh, G.; Solbach, K.; Rennings, A., "SAR Reduction for Dipole RF Coil Element at 7 Tesla by using Dielectric Overlay", LAPC2012, Loughborough, England-UK, Nov. 2012.

Background
Summary of Work
Summary of Results
Conclusion
References
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