title: Parallel transmit excitation at 1.5 T based on the minimization of a driving function for device heating.
abstract: OBJECTIVE
  To provide a rapid method to reduce the radiofrequency (RF) E-field coupling and consequent heating in long conductors in an interventional MRI (iMRI) setup.
  
  
  METHODS
  A driving function for device heating (W) was defined as the integration of the E-field along the direction of the wire and calculated through a quasistatic approximation. Based on this function, the phases of four independently controlled transmit channels were dynamically changed in a 1.5 T MRI scanner. During the different excitation configurations, the RF induced heating in a nitinol wire immersed in a saline phantom was measured by fiber-optic temperature sensing. Additionally, a minimization of W as a function of phase and amplitude values of the different channels and constrained by the homogeneity of the RF excitation field (B1) over a region of interest was proposed and its results tested on the benchtop. To analyze the validity of the proposed method, using a model of the array and phantom setup tested in the scanner, RF fields and SAR maps were calculated through finite-difference time-domain (FDTD) simulations. In addition to phantom experiments, RF induced heating of an active guidewire inserted in a swine was also evaluated.
  
  
  RESULTS
  In the phantom experiment, heating at the tip of the device was reduced by 92% when replacing the body coil by an optimized parallel transmit excitation with same nominal flip angle. In the benchtop, up to 90% heating reduction was measured when implementing the constrained minimization algorithm with the additional degree of freedom given by independent amplitude control. The computation of the optimum phase and amplitude values was executed in just 12 s using a standard CPU. The results of the FDTD simulations showed similar trend of the local SAR at the tip of the wire and measured temperature as well as to a quadratic function of W, confirming the validity of the quasistatic approach for the presented problem at 64 MHz. Imaging and heating reduction of the guidewire were successfully performed in vivo with the proposed hardware and phase control.
  
  
  CONCLUSIONS
  Phantom and in vivo data demonstrated that additional degrees of freedom in a parallel transmission system can be used to control RF induced heating in long conductors. A novel constrained optimization approach to reduce device heating was also presented that can be run in just few seconds and therefore could be added to an iMRI protocol to improve RF safety.
authors.affiliation: Department of Biomedical Engineering, Case Western Reserve University, Cleveland, Ohio 44106 and National Heart, Lung and Blood Institute, National Institutes of Health, Bethesda, Maryland 20892.
authors.forename: N
authors.initials: N
authors.lastname: Gudino
authors.affiliation: National Heart, Lung and Blood Institute, National Institutes of Health, Bethesda, Maryland 20892.
authors.forename: M
authors.initials: M
authors.lastname: Sonmez
authors.affiliation: Department of Physics, Case Western Reserve University, Cleveland, Ohio 44106.
authors.forename: Z
authors.initials: Z
authors.lastname: Yao
authors.affiliation: Department of Physics, Case Western Reserve University, Cleveland, Ohio 44106.
authors.forename: T
authors.initials: T
authors.lastname: Baig
authors.affiliation: National Heart, Lung and Blood Institute, National Institutes of Health, Bethesda, Maryland 20892.
authors.forename: S
authors.initials: S
authors.lastname: Nielles-Vallespin
authors.affiliation: National Heart, Lung and Blood Institute, National Institutes of Health, Bethesda, Maryland 20892.
authors.forename: A Z
authors.initials: AZ
authors.lastname: Faranesh
authors.affiliation: National Heart, Lung and Blood Institute, National Institutes of Health, Bethesda, Maryland 20892.
authors.forename: R J
authors.initials: RJ
authors.lastname: Lederman
authors.affiliation: Department of Physics, Case Western Reserve University, Cleveland, Ohio 44106.
authors.forename: M
authors.initials: M
authors.lastname: Martens
authors.affiliation: National Heart, Lung and Blood Institute, National Institutes of Health, Bethesda, Maryland 20892.
authors.forename: R S
authors.initials: RS
authors.lastname: Balaban
authors.affiliation: National Heart, Lung and Blood Institute, National Institutes of Health, Bethesda, Maryland 20892.
authors.forename: M S
authors.initials: MS
authors.lastname: Hansen
authors.affiliation: Department of Biomedical Engineering, Case Western Reserve University, Cleveland, Ohio 44106 and Department of Radiology, University Hospitals of Cleveland, Cleveland, Ohio 44106.
authors.forename: M A
authors.initials: MA
authors.lastname: Griswold
mesh_terms: D000465:Algorithms; D000497:Alloys / Q000737:chemistry; D000818:Animals; D003198:Computer Simulation; D004574:Electromagnetic Fields / Q000009:adverse effects; D006358:Hot Temperature; D008279:Magnetic Resonance Imaging / Q000295:instrumentation* / Q000379:methods*; D008962:Models, Theoretical; D019047:Phantoms, Imaging; D011846:Radio Waves / Q000009:adverse effects*; D034421:Sus scrofa
chemical_list: D000497:Alloys; C013616:nitinol