title: A fast, analytically based method to optimize local transmit efficiency for a transmit array.
abstract: OBJECTIVE
  To develop an analytically based algorithm for rapid optimization of the local radiofrequency magnetic (B1+) field intensity for a given radiofrequency power through a transmit array. The analytical nature of the method will yield insight to optimization requirements and provides a valuable reference for numerically based searches.
  
  
  METHODS
  With the knowledge of the B1+ field distribution generated by each single coil of the array, both the phases and the amplitudes of each coil current are optimized to maximize the magnitude of the B1+ field in a specific location of the body per unit of power transmitted through the array and, consequently, minimizing the whole body specific absorption rate for a given pulse sequence.
  
  
  RESULTS
  Simulations considering the human body show that the proposed method can reduce the whole-body specific absorption rate for a given B1+ magnitude at the location of interest by a factor of about 6.3 compared to the classic birdcage current configuration, and by a factor of 3.2 compared to phase-only shimming in a case with significant coupling between the elements of the array.
  
  
  CONCLUSIONS
  The proposed method can rapidly provide valuable information pertinent to the optimization of field distributions from transmit arrays.
authors: Department of Radiology, New York University, New York, New York, USA.
authors: Giuseppe
authors: G
authors: Carluccio
authors: Christopher M
authors: CM
authors: Collins
authors: Danilo
authors: D
authors: Erricolo
mesh_terms: D017076:Computer-Aided Design; D004867:Equipment Design; D019544:Equipment Failure Analysis; D006801:Humans; D007089:Image Enhancement / Q000295:instrumentation*; D008279:Magnetic Resonance Imaging / Q000295:instrumentation*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D014159:Transducers; D051598:Whole Body Imaging / Q000295:instrumentation*
keywords: magnetic resonance imaging; power; radiofrequency; shimming; specific absorption rate; spectroscopy