title: Crushed rephased orthogonal slice selection (CROSS) for simultaneous acquisition of two orthogonal proton resonance frequency temperature maps.
abstract: OBJECTIVE
  To evaluate a novel imaging sequence termed crushed rephased orthogonal slice selection (CROSS) that uses the available time in long echo time (TE) gradient echo (GRE) imaging-as employed for proton resonance frequency (PRF) shift thermometry-to simultaneously acquire two orthogonal magnetic resonance imaging (MRI) temperature maps around the target region.
  
  
  METHODS
  The CROSS sequence encodes a second orthogonal slice between excitation and data readout in long-TE imaging and applies dedicated crusher (CR) gradients to separate the signals from the two slices. Numerical simulations of the Bloch equations and phantom experiments were performed to analyze the MR signal. In phantom and in vivo experiments with two domestic pigs, the applicability of the CROSS sequence for temperature mapping of thermal therapies with focused ultrasound and laser was studied.
  
  
  RESULTS
  A successful separation of the signals from the two slices was achieved for CR dephasing lengths approaching the in-plane resolution. In the two animal experiments, CROSS temperature mapping could be successfully demonstrated at a temporal resolution of 2-3 seconds and a temperature uncertainty of 3-4K.
  
  
  CONCLUSIONS
  At the expense of a reduced signal in the overlap of the two slices, the CROSS sequence achieves an improvement of temporal resolution by 50%, without requiring further acceleration techniques such as parallel imaging, over conventional sequential GRE sequences employing the same repetition time as the CROSS sequence acquires two slices within one repetition interval.
authors.affiliation: Division of Medical Physics in Radiology, German Cancer Research Center (DKFZ), Heidelberg, Germany; Department of Radiological Sciences, St. Jude Children's Research Hospital, Memphis, Tennessee, USA.
authors.forename: Axel J
authors.initials: AJ
authors.lastname: Krafft
authors.forename: Jaane
authors.initials: J
authors.lastname: Rauschenberg
authors.forename: Florian
authors.initials: F
authors.lastname: Maier
authors.forename: Jürgen W
authors.initials: JW
authors.lastname: Jenne
authors.forename: Michael
authors.initials: M
authors.lastname: Bock
mesh_terms: D000465:Algorithms; D000818:Animals; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D016247:Information Storage and Retrieval / Q000379:methods; D008279:Magnetic Resonance Imaging / Q000295:instrumentation / Q000379:methods*; D018482:Muscle, Skeletal / Q000473:pathology / Q000503:physiopathology* / Q000601:surgery*; D019047:Phantoms, Imaging; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D025321:Surgery, Computer-Assisted / Q000295:instrumentation / Q000379:methods*; D013552:Swine; D013817:Thermography / Q000295:instrumentation / Q000379:methods*
keywords: MR thermometry; MRgFUS; MRgLITT; PRF temperature mapping; interventional MRI