title: A method for dynamic subtraction MR imaging of the liver.
abstract: BACKGROUND
  Subtraction of Dynamic Contrast-Enhanced 3D Magnetic Resonance (DCE-MR) volumes can result in images that depict and accurately characterize a variety of liver lesions. However, the diagnostic utility of subtraction images depends on the extent of co-registration between non-enhanced and enhanced volumes. Movement of liver structures during acquisition must be corrected prior to subtraction. Currently available methods are computer intensive. We report a new method for the dynamic subtraction of MR liver images that does not require excessive computer time.
  
  
  METHODS
  Nineteen consecutive patients (median age 45 years; range 37-67) were evaluated by VIBE T1-weighted sequences (TR 5.2 ms, TE 2.6 ms, flip angle 20 degrees , slice thickness 1.5 mm) acquired before and 45s after contrast injection. Acquisition parameters were optimized for best portal system enhancement. Pre and post-contrast liver volumes were realigned using our 3D registration method which combines: (a) rigid 3D translation using maximization of normalized mutual information (NMI), and (b) fast 2D non-rigid registration which employs a complex discrete wavelet transform algorithm to maximize pixel phase correlation and perform multiresolution analysis. Registration performance was assessed quantitatively by NMI.
  
  
  RESULTS
  The new registration procedure was able to realign liver structures in all 19 patients. NMI increased by about 8% after rigid registration (native vs. rigid registration 0.073 +/- 0.031 vs. 0.078 +/- 0.031, n.s., paired t-test) and by a further 23% (0.096 +/- 0.035 vs. 0.078 +/- 0.031, p < 0.001, paired t-test) after non-rigid realignment. The overall average NMI increase was 31%.
  
  
  CONCLUSIONS
  This new method for realigning dynamic contrast-enhanced 3D MR volumes of liver leads to subtraction images that enhance diagnostic possibilities for liver lesions.
authors.affiliation: Dipartimento di Ingegneria Biomedica, Politecnico di Milano, Italy. luca.mainardi@biomed.polimi.it
authors.forename: Luca T
authors.initials: LT
authors.lastname: Mainardi
authors.forename: Katia M
authors.initials: KM
authors.lastname: Passera
authors.forename: Agnese
authors.initials: A
authors.lastname: Lucesoli
authors.forename: Paolo
authors.initials: P
authors.lastname: Potepan
authors.forename: Ernesto
authors.initials: E
authors.lastname: Setti
authors.forename: Renato
authors.initials: R
authors.lastname: Musumeci