title: CT substitutes derived from MR images reconstructed with parallel imaging.
abstract: OBJECTIVE
  Computed tomography (CT) substitute images can be generated from ultrashort echo time (UTE) MRI sequences with radial k-space sampling. These CT substitutes can be used as ordinary CT images for PET attenuation correction and radiotherapy dose calculations. Parallel imaging allows faster acquisition of magnetic resonance (MR) images by exploiting differences in receiver coil element sensitivities. This study investigates whether non-Cartesian parallel imaging reconstruction can be used to improve CT substitutes generated from shorter examination times.
  
  
  METHODS
  The authors used gridding as well as two non-Cartesian parallel imaging reconstruction methods, SPIRiT and CG-SENSE, to reconstruct radial UTE and gradient echo (GE) data into images of the head for 23 patients. For each patient, images were reconstructed from the full dataset and from a number of subsampled datasets. The subsampled datasets simulated shorter acquisition times by containing fewer radial k-space spokes (1000, 2000, 3000, 5000, and 10,000 spokes) than the full dataset (30,000 spokes). For each combination of patient, reconstruction method, and number of spokes, the reconstructed UTE and GE images were used to generate a CT substitute. Each CT substitute image was compared to a real CT image of the same patient.
  
  
  RESULTS
  The mean absolute deviation between the CT number in CT substitute and CT decreased when using SPIRiT as compared to gridding reconstruction. However, the reduction was small and the CT substitute algorithm was insensitive to moderate subsampling (≥ 5000 spokes) regardless of reconstruction method. For more severe subsampling (≤ 3000 spokes), corresponding to acquisition times less than a minute long, the CT substitute quality was deteriorated for all reconstruction methods but SPIRiT gave a reduction in the mean absolute deviation of down to 25 Hounsfield units compared to gridding.
  
  
  CONCLUSIONS
  SPIRiT marginally improved the CT substitute quality for a given number of radial spokes as compared to gridding. However, the increased reconstruction time of non-Cartesian parallel imaging reconstruction is difficult to motivate from this improvement. Because the CT substitute algorithm was insensitive to moderate subsampling, data for a CT substitute could be collected in as little as minute and reconstructed with gridding without deteriorating the CT substitute quality.
authors.affiliation: Radiation Physics, Department of Radiation Sciences, Umeå University, Umeå SE-90187, Sweden.
authors.forename: Adam
authors.initials: A
authors.lastname: Johansson
authors.affiliation: Radiation Physics, Department of Radiation Sciences, Umeå University, Umeå SE-90187, Sweden and CJ Gorter Center for High Field MRI, Leiden University Medical Center, Leiden NL-2333 ZA, Netherlands.
authors.forename: Anders
authors.initials: A
authors.lastname: Garpebring
authors.affiliation: Oncology, Department of Radiation Sciences, Umeå University, Umeå SE-90187, Sweden.
authors.forename: Thomas
authors.initials: T
authors.lastname: Asklund
authors.affiliation: Radiation Physics, Department of Radiation Sciences, Umeå University, Umeå SE-90187, Sweden.
authors.forename: Tufve
authors.initials: T
authors.lastname: Nyholm
mesh_terms: D000328:Adult; D000368:Aged; D000465:Algorithms; D005260:Female; D006257:Head / Q000000981:diagnostic imaging / Q000473:pathology; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D008875:Middle Aged; D013997:Time Factors; D014057:Tomography, X-Ray Computed / Q000379:methods*; D055815:Young Adult