title: 2D radially compensating excitation pulse in combination with an internal transceiver antenna for 3D MRI of the rectum at 7 T.
abstract: OBJECTIVE
  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 B1 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 B1 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 in matlab. 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.
authors.affiliation: Department of Radiology, UMC Utrecht, Utrecht 3584 CX, The Netherlands.
authors.forename: I M L
authors.initials: IM
authors.lastname: van Kalleveen
authors.affiliation: Department of Radiology, UMC Utrecht, Utrecht 3584 CX, The Netherlands and Department of Medical Technology, UMC Utrecht, Utrecht 3584 CX, The Netherlands.
authors.forename: H
authors.initials: H
authors.lastname: Kroeze
authors.affiliation: Department of Radiology, UMC Utrecht, Utrecht 3584 CX, The Netherlands.
authors.forename: A
authors.initials: A
authors.lastname: Sbrizzi
authors.affiliation: Department of Radiology, UMC Utrecht, Utrecht 3584 CX, The Netherlands.
authors.forename: V O
authors.initials: VO
authors.lastname: Boer
authors.affiliation: Department of Radiotherapy, UMC Utrecht, Utrecht 3584 CX, The Netherlands.
authors.forename: O
authors.initials: O
authors.lastname: Reerink
authors.affiliation: Department of Radiotherapy, UMC Utrecht, Utrecht 3584 CX, The Netherlands.
authors.forename: M E P
authors.initials: ME
authors.lastname: Philippens
authors.affiliation: Department of Radiotherapy, UMC Utrecht, Utrecht 3584 CX, The Netherlands.
authors.forename: C A T
authors.initials: CA
authors.lastname: van de Berg
authors.affiliation: Department of Radiology, UMC Utrecht, Utrecht 3584 CX, The Netherlands.
authors.forename: P R
authors.initials: PR
authors.lastname: Luijten
authors.affiliation: Department of Radiology, UMC Utrecht, Utrecht 3584 CX, The Netherlands.
authors.forename: D W J
authors.initials: DW
authors.lastname: Klomp
mesh_terms: D000465:Algorithms; D003198:Computer Simulation; D004867:Equipment Design; D006801:Humans; D021621:Imaging, Three-Dimensional / Q000295:instrumentation* / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000295:instrumentation* / Q000379:methods*; D008953:Models, Anatomic; D019047:Phantoms, Imaging; D012007:Rectum / Q000000981:diagnostic imaging*; D012984:Software