title: Effect of shot number on the calculated apparent diffusion coefficient in phantoms and in human liver in diffusion-weighted echo-planar imaging.
abstract: OBJECTIVE
  To show the signal intensity varies with shot number in diffusion-weighted (DW) echo-planar imaging (EPI) and affects apparent diffusion coefficient (ADC) calculation.
  
  
  METHODS
  This prospective study was performed on 35 adult patients and 20 volunteers. Measurements were made on a 3T scanner using a breathhold DW spin-echo EPI (SE EPI) sequence. Three protocols were used: A) eight consecutive shots at a fixed b-value of 0 seconds/mm(2) with TR = 1000 and 3000 msec; B) seven consecutive shots at b-values = 0, 1000, 750, 500, 250, 100, 0 seconds/mm(2) (in that order) with TR = 3500 msec; and C) seven consecutive shots (as in B) with TR = 1000, 1750, and 7000 msec.
  
  
  RESULTS
  For protocol A, signal intensity decreased significantly from the first to second shot (P<0.0001) and thereafter remained constant. For protocol B, the ADC depended on which b = 0 seconds/mm(2) image was used. Using the first b = 0 seconds/mm(2), the mean ADC was 15% higher than using the second b = 0 seconds/mm(2) (P<0.0001). For protocol C, the difference between ADC using the first b = 0 seconds/mm(2) and the second b = 0 seconds/mm(2) decreased as the TR increased.
  
  
  CONCLUSIONS
  The signal intensity can vary with shot number in SE EPI. For TR > or = 3000 msec, steady-state is attained after one shot. Using data acquired prior to steady-state confounds the calculation of ADC values.
authors.affiliation: Department of Radiology, University of California, San Diego, California, USA.
authors.forename: Masoud
authors.initials: M
authors.lastname: Shiehmorteza
authors.forename: Claude B
authors.initials: CB
authors.lastname: Sirlin
authors.forename: Tanya
authors.initials: T
authors.lastname: Wolfson
authors.forename: Anthony
authors.initials: A
authors.lastname: Gamst
authors.forename: Andre E
authors.initials: AE
authors.lastname: Soumekh
authors.forename: Shay P
authors.initials: SP
authors.lastname: Heaton
authors.forename: Chad A
authors.initials: CA
authors.lastname: Kohl
authors.forename: Nicholas
authors.initials: N
authors.lastname: Pinto
authors.forename: Mark
authors.initials: M
authors.lastname: Bydder
mesh_terms: D000328:Adult; D000368:Aged; D006528:Carcinoma, Hepatocellular / Q000473:pathology*; D003430:Cross-Sectional Studies; D038524:Diffusion Magnetic Resonance Imaging / Q000379:methods*; D017352:Echo-Planar Imaging / Q000379:methods*; D005260:Female; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods; D008099:Liver / Q000473:pathology*; D008107:Liver Diseases / Q000473:pathology*; D008113:Liver Neoplasms / Q000473:pathology*; D008297:Male; D008875:Middle Aged; D019047:Phantoms, Imaging; D011446:Prospective Studies; D055815:Young Adult