title: Iterative separation of transmit and receive phase contributions and B1(+)-based estimation of the specific absorption rate for transmit arrays.
abstract: OBJECTIVE
  The specific absorption rate (SAR) can be determined from radiofrequency transmit fields measured via magnetic resonance imaging.
  
  
  METHODS
  The proposed method estimates the SAR solely from the complex transmit field (B1(+)) by taking into account the particular properties of the electromagnetic field generated by an 8-channel transmit array. It is further based on an iterative consistency check between the measured B1(+) magnitude and an appropriate field estimate fulfilling Maxwell's equations. For testing the method, simulations and phantom experiments were performed for a multi-transmit array at 3T using a cylindrical phantom.
  
  
  RESULTS
  The method's robustness with respect to the assumptions made about electric tissue properties as well as its stability under different initial conditions regarding the signal phase was shown. A high sensitivity to signal noise was found. Robust reconstruction results were achieved including information from more than two transmit elements. The validity of the experimental results was confirmed by a qualitative comparison to simulated electromagnetic fields.
  
  
  CONCLUSIONS
  The method allows the determination of the SAR as well as the transmit phase of the individual channels of a multi-transmit array. With additional B0 inhomogeneity measurements, a reconstruction of the receive phase is feasible independent of the receive coil type in use.
authors: Medical Physics, Department of Radiology, University Medical Center Freiburg, Breisacher Str. 60a, 79106, Freiburg, Germany, stefanie.buchenau@uniklinik-freiburg.de.
authors: Stefanie
authors: S
authors: Buchenau
authors: Martin
authors: M
authors: Haas
authors: Daniel Nicolas
authors: DN
authors: Splitthoff
authors: Jürgen
authors: J
authors: Hennig
authors: Maxim
authors: M
authors: Zaitsev
mesh_terms: D000042:Absorption; D000465:Algorithms; D003198:Computer Simulation; D004553:Electric Conductivity; D004574:Electromagnetic Fields; D004867:Equipment Design; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008280:Magnetics; D019047:Phantoms, Imaging; D011846:Radio Waves; D014159:Transducers