title: New method to characterize and correct with sub-µs precision gradient delays in bipolar multispoke RF pulses.
abstract: OBJECTIVE
  Small gradient delays with respect to radiofrequency (RF) events can have disastrous effects on the performance of bipolar spokes RF pulses used in parallel transmission (pTx). In this work, we propose a new method to characterize and correct this delay with sub-µs precision.
  
  
  METHODS
  By determining experimentally the phase Δφ producing a 0 ° flip angle excitation in a α0°-α180°+Δφ bipolar two-spoke pulse configuration at multiple slice locations, we demonstrate the possibility of deducing the underlying gradient delay with precision. The technique also suggests prospectively compensating for the same delay by altering the phase of the second pulse. The approach was tested with a multislice gradient echo sequence on a phantom and on one healthy volunteer at 7 Tesla.
  
  
  RESULTS
  Application of the method returned an accuracy of approximately 50 ns on the gradient delay measurement, a performance shown in fact to be desirable for high-performance pTx 2D applications. Phase corrections of up to 180 ° on the second spoke RF pulse in the bipolar configuration allowed us to obtain similar performance as for unipolar designs, yet with significantly shorter excitations.
  
  
  CONCLUSIONS
  A simple and accurate gradient-delay calibration method was proposed that offers the possibility of using bipolar multispoke pulses in multislice protocols. Magn Reson Med 78:2194-2202, 2017. © 2017 International Society for Magnetic Resonance in Medicine.
authors.affiliation: CEA, DRF, I2BM, NeuroSpin, Unirs, Gif-sur-Yvette, Cedex, France.
authors.forename: Vincent
authors.initials: V
authors.lastname: Gras
authors.affiliation: CEA, DRF, I2BM, NeuroSpin, Unirs, Gif-sur-Yvette, Cedex, France.
authors.forename: Alexandre
authors.initials: A
authors.lastname: Vignaud
authors.affiliation: CEA, DRF, I2BM, NeuroSpin, Unirs, Gif-sur-Yvette, Cedex, France.
authors.forename: Alexis
authors.initials: A
authors.lastname: Amadon
authors.affiliation: Siemens Healthcare, 40 Avenue des Fruitiers, Saint-Denis, France.
authors.forename: Franck
authors.initials: F
authors.lastname: Mauconduit
authors.affiliation: CEA, DRF, I2BM, NeuroSpin, Unirs, Gif-sur-Yvette, Cedex, France.
authors.forename: Denis
authors.initials: D
authors.lastname: Le Bihan
authors.affiliation: CEA, DRF, I2BM, NeuroSpin, Unirs, Gif-sur-Yvette, Cedex, France.
authors.forename: Nicolas
authors.initials: N
authors.lastname: Boulant
mesh_terms: D000328:Adult; D000465:Algorithms; D001921:Brain / Q000000981:diagnostic imaging; D002138:Calibration; D064368:Healthy Volunteers; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008279:Magnetic Resonance Imaging; D019047:Phantoms, Imaging; D011846:Radio Waves; D015203:Reproducibility of Results; D014867:Water / Q000737:chemistry
chemical_list: D014867:Water
keywords: bipolar spokes; gradient delay; parallel transmission