A new monopole intravascular coil with three parasitic elements optimized for MRI 1.5 T
mohammad mohammadzadeh1,2 and alireza ghasempour Shirazi1

1ICT, University of applied science and technology, Tehran, Iran, 2Shahid Beheshti, Tehran, Iran

Synopsis

Monopole coil has a thin and flexible structure provides high-resolution MR images from the internal vessels such as aorta and coronary arteries. However, its SNR homogeneity decreases at higher Tesla MRI systems, leading to increasing the image artifact caused by the wall movement which is not fully compensated using the post processing algorithms. In this study, we introduced a monopole coil with tree parasitic elements and compared its ISNR (Intrinsic SNR) magnitude and distribution homogeneity to a conventional monopole coil with one parasitic element for MRI 1.5 T at 64MHz . We optimized the coil geometry using a fast genetic algorithm written in MATLAB and performed the simulation of the ISNR indices by Involving HFSS and MATLAB inside a saline phantom.

Purpose

Design a new intravascular coil with high uniform SNR distribution to reduce the MRI image artifact of vessel walls caused by the blood pulsed movement.

Theory and Methods

According to the antenna theory, adding parasitic elements to the RF coil design can increase the homogeneity of magnetic fields produced around the coil [1]. Developing this idea, we introduced a new monopole MR antenna with three parasitic elements. The proposed coil has 4cm length and a diameter of 2.2 mm (See Fig. 1a). Lengths of the coil parasitic elements are optimized at 63.875 MHz using a genetic algorithm implemented by involving MATLAB and HFSS. They are obtained as 1, 33, 1 mm respectively. The optimization process has performed by defining a cost function;$$$ Cost = Total ISNR × Homogeneity$$$. Intrinsic SNR (ISNR) is a reliable measure to quantify the role of coil parameters in the MRI image SNR which is defined by $$$ ISNR=k×H+/√(1-s112) $$$ where, K is a constant coefficient for a given sample, S11 is the ratio of the reflected power to the input power at the coil’s feed point (point A at Fig. 1a,b) H+ is the right-hand circularly polarized magnetic field component, and R is the real part of the input impedance seen across the coil input terminals. Total ISNR is the sum of the ISNR data throughout of the phantom and Homogeneity is defined as the inverse variance of the ISNR data of the phantom which is defined as $$$ Homogeneity=1/ISNRSTD $$$.

Results

Figures 2 and 3 show 2D ISNR maps of the introduced coil (Fig. 1a) and a previously presented monopole coil with one parasitic element [3](Fig. 1b) in a sagittal plane (Fig. 1c) inside a saline phantom (Fig. 1d). Decreasing the signal intensity but increasing the signal homogeneity are resulted in the introduced coil maps. Figure 4 illustrate the ISNR variation of the proposed coil and the coil with one parasitic elements along a longitudinal line placed 0.5 cm away from the z axis. Increasing the ISNR homogeneity and decreasing the maximum ISNR index of the introduced coil are highlighted in this figure. Table 1 verifies that the Maximum ISNR and Total_ISNR of the introduced coil have decreased by less than 2 times. However, the ISNR homogeneity of and then the cost function of the introduced coil has increased by 2.9 and 1.24 times respectively.

Conclusion

In this paper we introduced a monopole coil with three parasitic elements and assessed the ISNR values and homogeneity of the coil inside a saline phantom. Results show that adding parasitic elements to the coil design increases the uniformity of ISNR distribution, which in turn, it will increase MRI image contrast to noise ratio (CNR) by decreasing the vessel wall movement artifact.

Acknowledgements

No acknowledgement found.

References

[1] C. A. Balanis, 2005, Antenna theory and design, analysis and design, Wiley: New jersey.

[2] D. I. Hoult and et. al., JMR., vol. 24, pp. 71-85, 1995.

[3] M. Mohammadzadeh et al., Meas. Sci. Technol., vol. 17, pp. 1987–1994, 2006. nce experiment,” JMR., vol. 24, pp. 71-85, 1995.

Figures

Fig. 1. Design of monopole coils with a) three parasitic elements and b) one parasitic element. c) A sagittal plane placed at x = -20 to 20 mm, y = 0 mm, and z = 0 to 40 mm accompany with a longitudinal line placed at x= 5 mm, y=0 mm, and z = 0 to 40 mm. d) A saline phantom with a coil placed at its center. The both sizes of radius and height of the phantom are 10 cm and its electromagnetic characteristics have been assumed μr=1, εr =80, σ =0.8 S/m

Fig. 2. 2D ISNR map of the coil with one parasitic element.

Fig. 3. 2D ISNR map of the introduced coil with three parasitic elements.

Fig. 4. 1D ISNR curves of the introduced coil and the coil with one parasitic element along with the longitudinal line( see Fig. 1c).

Table 1. ISNR Indices of the introduced coil and the monopole coil with one parasitic element at 64MHz



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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