title: Automated Registration of Multimodal Optic Disc Images: Clinical Assessment of Alignment Accuracy.
abstract: OBJECTIVE
  To determine the accuracy of automated alignment algorithms for the registration of optic disc images obtained by 2 different modalities: fundus photography and scanning laser tomography.
  
  
  METHODS
  Images obtained with the Heidelberg Retina Tomograph II and paired photographic optic disc images of 135 eyes were analyzed. Three state-of-the-art automated registration techniques Regional Mutual Information, rigid Feature Neighbourhood Mutual Information (FNMI), and nonrigid FNMI (NRFNMI) were used to align these image pairs. Alignment of each composite picture was assessed on a 5-point grading scale: "Fail" (no alignment of vessels with no vessel contact), "Weak" (vessels have slight contact), "Good" (vessels with <50% contact), "Very Good" (vessels with >50% contact), and "Excellent" (complete alignment). Custom software generated an image mosaic in which the modalities were interleaved as a series of alternate 5×5-pixel blocks. These were graded independently by 3 clinically experienced observers.
  
  
  RESULTS
  A total of 810 image pairs were assessed. All 3 registration techniques achieved a score of "Good" or better in >95% of the image sets. NRFNMI had the highest percentage of "Excellent" (mean: 99.6%; range, 95.2% to 99.6%), followed by Regional Mutual Information (mean: 81.6%; range, 86.3% to 78.5%) and FNMI (mean: 73.1%; range, 85.2% to 54.4%).
  
  
  CONCLUSIONS
  Automated registration of optic disc images by different modalities is a feasible option for clinical application. All 3 methods provided useful levels of alignment, but the NRFNMI technique consistently outperformed the others and is recommended as a practical approach to the automated registration of multimodal disc images.
authors.affiliation: *University Hospital Wales †School of Computer Sciences and Informatics §Cardiff Centre for Vision Sciences, Cardiff University, Cardiff ‡Royal Glamorgan Hospital, Llantrisant, Wales, UK.
authors.forename: Wai Siene
authors.initials: WS
authors.lastname: Ng
authors.forename: Phil
authors.initials: P
authors.lastname: Legg
authors.forename: Venkat
authors.initials: V
authors.lastname: Avadhanam
authors.forename: Kyaw
authors.initials: K
authors.lastname: Aye
authors.forename: Steffan H P
authors.initials: SH
authors.lastname: Evans
authors.forename: Rachel V
authors.initials: RV
authors.lastname: North
authors.forename: Andrew D
authors.initials: AD
authors.lastname: Marshall
authors.forename: Paul
authors.initials: P
authors.lastname: Rosin
authors.forename: James E
authors.initials: JE
authors.lastname: Morgan
mesh_terms: D000465:Algorithms; D003952:Diagnostic Imaging / Q000379:methods; D005901:Glaucoma / Q000175:diagnosis*; D006801:Humans; D007089:Image Enhancement / Q000379:methods; D007090:Image Interpretation, Computer-Assisted; D007429:Intraocular Pressure; D009898:Optic Disk / Q000473:pathology*; D009901:Optic Nerve Diseases / Q000175:diagnosis*; D010781:Photography; D015203:Reproducibility of Results