Design of a forward view antenna for prostate imaging at 7 Tesla

Bart Steensma¹, Dennis Klomp¹, Nico van den Berg¹, Peter Luijten¹, Abe van der Werf², and Alexander Raaijmakers¹

¹University Medical Centre Utrecht, Utrecht, Netherlands, ²Machnet B.V., Maarn, Netherlands

Synopsis

The forward view antenna has been introduced as a novel antenna for ultrahigh field imaging. This study has investigated its potential for prostate imaging where the antenna is placed between the legs, to contribute as an additional element of an existing dipole antenna transceiver array. A significant increase in signal-to-noise ratio is expected because of the generally smaller distance towards the prostate from this side. Numerical simulations and in vivo scans show that signal-to-noise ratio in the prostate region increases as a result of adding the forward view antenna to the dipole antenna array.

Purpose

The conventional setup for prostate imaging at 7 Tesla is using a transceive surface array with the elements aligned in a belt-like fashion around the pelvis^1-6. The B₁⁺-signal is transmitted through the transverse plane towards the prostate. A possible addition to this setup can be made by positioning an antenna between the legs of a patient against the perineum (figure 1). The distance from such an antenna towards the prostate is lower than through the transverse plane. This decrease in distance is expected to result in significantly larger sensitivity/efficiency. In contrast to conventional RF coils and antennas, such an antenna needs to radiate B₁⁺-signal towards the prostate along the longitudinal rather than the transverse axis (figure 1). Such antennas have been explored by means of a simulation study^7,8. They are called ‘forward view antennas’. In this study we will realize a dielectric waveguide forward view antenna for prostate imaging at 7 Tesla. The basic concept of this antenna is a waveguide filled with dielectric material^9,10. In this waveguide a circularly polarized wave is emitted towards the prostate by two dipole antennas. We will evaluate the benefits of such an antenna as an extra element in an existing array of fractionated dipole antennas⁵for prostate imaging at 7 Tesla.

Methods

Finite-difference time domain simulations have been performed in Sim4Life (ZMT, Zurich, CH) to show potential benefits of the forward view antenna. Two antenna setups are simulated on the Virtual Family human model Duke. The first antenna setup consists of the fractionated dipole antenna array only, in the second setup the forward view antenna is added. Both simulations are evaluated and compared in terms of B₁⁺ field strength in the prostate and 10g averaged spatial peak SAR. A first prototype of the forward view antenna was built using basic lab equipment (figure 2). The prototype consists of two orthogonally positioned dipole antennas that are driven in quadrature, these antennas are used to transmit and receive circularly polarized electromagnetic fields along the longitudinal axis. The forward view antenna was tested in the scanner on two male volunteers. The dipole array was used in transmit and receive mode, while the forward view antenna was used in receive mode. Results were analyzed in terms of signal-to-noise-ratio and image quality.

Results

Figure 3a shows a sagittal slice of the voxelized Sim4Life model that was used in the FDTD simulations. The forward view antenna is positioned between the legs, the prostate is shown in blue. Figure 3b shows the B₁⁺-field distribution. Adding the forward view antenna to the fractionated dipole antenna array leads to an increase of average B₁⁺ in the prostate of 42% (figure 4a). However, the SAR of the forward view antenna is 200% higher than the SAR of the dipole array (figure 4b). Figures 5a-c show results of gradient echo scans on a male volunteer. The SNR in the prostate region increases by an average of 19.6% when the forward view antenna was added to the setup.

Discussion

Numerical simulations show that using the forward view antenna in addition to the fractionated dipole antenna array can lead to a substantial increase in B₁⁺ in the prostate. This implies that SNR can increase at the same rate, because a similar increase is expected for B₁^-. Because of the relatively high SAR level of the forward view antenna, this antenna has been used in receive mode only during volunteer scans. Results from the volunteer scans show that the forward view antenna enhances image quality in the perineum region. Already with the first prototype and in the limited set of volunteers that were used in this study, SNR is shown to be increased in the prostate region when the forward view antenna is used. The SNR benefit that is reached in practice does not yet compare to the SNR benefit that is shown in simulations. Further SNR increase as shown in simulations may be anticipated when optimizing the dielectric layer between the forward view antenna and the perineum that currently comprises a large air gap. A subsequent version will need an adapted shape of the waveguide ending to better adapt to the body curvature.

Conclusion

The forward view antenna provides a non-invasive method of improving image quality of prostate MR images at 7 Tesla. It is shown both theoretically and in practice that the forward view antenna can improve SNR and image quality in the prostate region.

Acknowledgements

The forward view antenna has been developed and patented in collaboration with Machnet B.V. (Maarn, The Netherlands).

References

1. Metzger, G.J., et al., Local B1+ shimming for prostate imaging with transceiver arrays at 7T based on subject-dependent transmit phase measurements. Magnetic Resonance in Medicine, 2008. 59(2): p. 396-409.

2. Vaughan, J.T., et al., Whole-body imaging at 7T: Preliminary results. Magnetic Resonance in Medicine, 2009. 61(1): p. 244-248.

3. Raaijmakers, A.J.E., et al., Design of a radiative surface coil array element at 7 T: The single-side adapted dipole antenna. Magnetic Resonance in Medicine, 2011. 66(5): p. 1488-1497.

4. Raaijmakers, A.J.E. and C.A.T. van den Berg, Antennas as Surface Array Elements for Body Imaging at Ultrahigh Field Strengths, in eMagRes2007, John Wiley & Sons, Ltd.

5. Raaijmakers, A.J.E., et al., The fractionated dipole antenna: A new antenna for body imaging at 7 Tesla. Magnetic Resonance in Medicine, 2015

6. Kraff, O., Bitz, A.K., Kruszona, S., Orzada, S., Schaefer, L.C., Theysohn, J.M., Maderwald, S., Ladd, M.E., Quick, H.H., A flexible 8-channel transmit/receive body coil for 7 T human imaging. Invest Radiol., 2009. 44(11): p. 734-40.

7. Raaijmakers, A.J.E. Dipoles & Traveling Waves. in ISMRM. 2015. Toronto

8. Raaijmakers, A.J.E., et al. Helix antennas: approaching the target from a different angle. in ISMRM. 2014. Milan.

9. Andreychenko, A., et al., Coaxial waveguide for travelling wave MRI at ultrahigh fields. Magnetic Resonance in Medicine, 2013. 70(3): p. 875-884.

10. Brunner, D.O., et al., Travelling-wave nuclear magnetic resonance. Nature, 2009. 457(7232): p. 994-998.

Figures

Figure 1: Schematic figure of the forward view antenna positioned against the perineum of a human in the scanner. Circularly polarized fields are transmitted and received along the longitudinal axis.

Figure 2: a. Schematic figure showing the setup of the first forward view antenna prototype. The model consists of a water-filled rectangular conductive waveguide. Two orthogonal TE01-modes are excited in this waveguide by a cross-dipole antenna. b. Photo of the first forward view antenna prototype

Figure 3: a. Schematic figure of the combined setup on human model Duke. b. B1+-fields induced by the combined setup. Prostate is shown in blue. TE01-mode is visible inside the forward view antenna waveguide.

Figure 4: a. B1+ in prostate increases by 42% by using the combined setup. b. The peak SAR of the forward view antenna is 200% higher than the peak SAR of the antenna array, therefore the forward view antenna may predominantly be optimal in receive mode.

Figure 5: Coronal slices of volunteer trunk, obtained with the following setups: a. Fractionated dipole array in transceiver mode. b. Fractionated dipole array transmitting and the forward view antenna receiving. c. Fractionated dipole array transceiving and the forward view antenna receiving. SNR in the prostate region increases by 19.6%.

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

2126