title: Multi-echo length and offset VARied saturation (MeLOVARS) method for improved CEST imaging.
abstract: OBJECTIVE
  The aim of this study was to develop a technique for rapid collection of chemical exchange saturation transfer images with the saturation varied to modulate signal loss transfer and enhance contrast.
  
  
  METHODS
  Multi-echo Length and Offset VARied Saturation (MeLOVARS) divides the saturation pulse of length Tsat into N = 3-8 submodules, each consisting of a saturation pulse with length of Tsat /N (∼0.3-1 s), one or more low flip-angle gradient-echo readout(s) and a flip back pulse. This results in N readouts with increasing saturation time from Tsat /N to Tsat without extra scan time.
  
  
  RESULTS
  For phantoms, eight images with Tsat incremented every 0.5 s from 0.5-4 s were collected simultaneously using MeLOVARS, which allows rapid determination of exchange rates for agent protons. For live mice bearing glioblastomas, the Z-spectra for five different Tsat values from 0.5 to 2.5 s were acquired in a time normally used for one Tsat . With the additional Tsat -dependence information, LOVARS phase maps were produced with a more clearly defined tumor boundary and an estimated 4.3-fold enhanced contrast-to-noise ratio (CNR). We also show that enhancing CNR is achievable by simply averaging the collected images or transforming them using the principal component analysis.
  
  
  CONCLUSIONS
  MeLOVARS enables collection of multiple saturation-time-weighted images without extra time, producing a LOVARS phase map with increased CNR.
authors.affiliation: Division of MR Research, The Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins University, Baltimore, Maryland, USA; F.M. Kirby Research Center for Functional Brain Imaging, Kennedy Krieger Institute, Baltimore, Maryland, USA.
authors.forename: Xiaolei
authors.initials: X
authors.lastname: Song
authors.forename: Jiadi
authors.initials: J
authors.lastname: Xu
authors.forename: Shuli
authors.initials: S
authors.lastname: Xia
authors.forename: Nirbhay N
authors.initials: NN
authors.lastname: Yadav
authors.forename: Bachchu
authors.initials: B
authors.lastname: Lal
authors.forename: John
authors.initials: J
authors.lastname: Laterra
authors.forename: Jeff W M
authors.initials: JW
authors.lastname: Bulte
authors.forename: Peter C M
authors.initials: PC
authors.lastname: van Zijl
authors.forename: Michael T
authors.initials: MT
authors.lastname: McMahon
mesh_terms: D000465:Algorithms; D000818:Animals; D001932:Brain Neoplasms / Q000378:metabolism* / Q000473:pathology*; D045744:Cell Line, Tumor; D005909:Glioblastoma / Q000378:metabolism* / Q000473:pathology*; D006801:Humans; D007089:Image Enhancement / Q000379:methods; D008279:Magnetic Resonance Imaging / Q000379:methods*; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D051379:Mice; D016513:Mice, SCID; D009363:Neoplasm Proteins / Q000378:metabolism*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D012815:Signal Processing, Computer-Assisted
chemical_list: D009363:Neoplasm Proteins
keywords: CEST; EPI; Length and Offset VARied Saturation (LOVARS); Multi-echo LOVARS(MeLOVARS); brain tumor imaging