2D radially compensating excitation pulse in combination with an internal transceiver antenna for 3D MRI of the rectum at 7 T. Issue 7 (21st June 2016)

• Record Type: Journal Article
• Authors: van Kalleveen, I. M. L.; Kroeze, H.; Sbrizzi, A.; Boer, V. O.; Reerink, O.; Philippens, M. E. P.; van de Berg, C. A. T.; Luijten, P. R.; Klomp, D. W. J.
• Is Part Of: Medical physics. Volume 43:Issue 7(2016)
• Page Start: 4375
• Page End: 4382
• Publication Date: 2016-06-21
• Subjects: biomedical MRI -- cancer -- image reconstruction -- image resolution -- image sequences -- medical image processing -- phantoms; Magnetic resonance imaging -- Cancer -- Spatial resolution -- Reconstruction; Involving electronic [emr] or nuclear [nmr] magnetic resonance, e.g. magnetic resonance imaging -- Biological material, e.g. blood, urine; Haemocytometers -- Digital computing or data processing equipment or methods, specially adapted for specific applications -- Image data processing or generation, in general; 7T -- RF pulse -- rectum -- monopole antenna -- 2D RACE; Antennas -- Magnetic resonance imaging -- Cancer -- Coils -- Three dimensional image processing -- Medical image spatial resolution -- Frequency modulation -- Spatial resolution -- Computer software; Medical physics -- Periodicals; Medical physics; Geneeskunde; Natuurkunde; Toepassingen; Biophysics; Periodicals; Periodicals; Electronic journals; 610.153
• DOI: 10.1118/1.4954204
• Languages: English
• ISSNs: 0094-2405

Abstract

Abstract : Purpose: The high precession frequency in ultrahigh field MRI coincides with reduced RF penetration, increased RF power deposition and consequently can lead to reduced scan efficiency. However, the shorter wavelength enables the use of efficient antennas rather than loop coils. In fact, ultrathin monopole antennas have been demonstrated at 7 T, which fit in natural cavities like the rectum in the human body. As the RF field generated by the antenna provides an extremely nonuniform B 1 field, the use of conventional RF pulses will lead to severe image distortions and highly nonuniform contrast. However, using the two predominant dimensions (orthogonal to the antenna), 2D RF pulses can be designed that counteract the nonuniform B 1 into uniform flip angles. In this study the authors investigate the use of an ultrathin antenna not only for reception, but also for transmission in 7 T MRI of the rectum. Methods: The 2D radially compensating excitation (2D RACE) pulse was designed inmatlab . SAR calculations between the 2D RACE pulse and an adiabatic RF pulse (BIR‐4) have been obtained, to visualize the gain in decreasing the SAR when using the 2D RACE pulse instead of an adiabatic RF pulse. The authors used the 7 T whole body MR system in combination with an internally placed monopole antenna used for transceiving and obtained 3D gradient echo images with a conventional sinc pulse and with the 2D RACE pulse. For extra clarity, they also reconstructed an image where the receive field of the antenna was removed. Results: Comparing the results of the SAR simulations of the 2D RACE pulse with a BIR‐4 pulse shows that for low flip angles ( θ < 41°) the SAR can be decreased with a factor of 4.8 or even more, when using the 2D RACE pulse. Relative to a conventional sinc excitation, the 2D RACE pulse achieves more uniform flip angle distributions than a BIR‐4 pulse with a smaller SAR increase (16 × versus 64 ×). Conclusions: The authors have shown that the 2D RACE pulse provides more homogeneous flip angles for gradient echo sequences when compared to a conventional sinc pulse albeit at increased SAR. However, when compared to adiabatic RF pulses, as shown by simulations, the SAR of the 2D RACE pulse can be an order of magnitude less. Phantom and in vivo human rectum images are obtained to demonstrate that the 2D RACE pulse can provide a uniform excitation while transmitting with a single ultrathin endorectal antenna at 7 T. The combination of thin rectal antennas with efficient uniform transmit can open up new possibilities in high resolution imaging of rectal cancer.