title: A fast regularized least-squares method for retinal vascular oxygen tension estimation using a phosphorescence lifetime imaging model.
abstract: BACKGROUND
  Monitoring retinal oxygenation is of primary importance in detecting the presence of some common eye diseases. To improve the estimation of oxygen tension in retinal vessels, regularized least-squares (RLS) method was shown to be very effective compared with the conventional least-squares (LS) estimation. In this study, we propose an accelerated RLS estimation method for the problem of assessing the oxygenation of retinal vessels from phosphorescence lifetime images.
  
  
  METHODS
  In the previous work, gradient descent algorithms were used to find the minimum of the RLS cost function. This approach is computationally expensive, especially when the oxygen tension map is large. In this study, using a closed-form solution of the RLS estimation and some inherent properties of the problem at hand, the RLS process is reduced to the weighted averaging of the LS estimates. This decreases the computational complexity of the RLS estimation considerably without sacrificing its performance.
  
  
  RESULTS
  Performance analyses are conducted using both real and simulated data sets. In terms of computational complexity, the proposed RLS estimation method is significantly better than RLS methods that use gradient descent algorithms to find the minimum of the cost function. Additionally, there is no significant difference between the estimates acquired by the proposed and conventional RLS estimation methods.
  
  
  CONCLUSIONS
  The proposed RLS estimation method for computing the retinal oxygen tension is computationally efficient, and produces estimates with negligible difference from those obtained by iterative RLS methods. Further, the results of this study can be applied to other lifetime imaging problems that have similar properties.
authors.forename: Gokhan
authors.initials: G
authors.lastname: Gunay
authors.affiliation: Electrical and Electronics Engineering Department, Istanbul Technical University, 34469 Istanbul, Turkey. iyildirim@itu.edu.tr.
authors.forename: Isa
authors.initials: I
authors.lastname: Yildirim
mesh_terms: D000818:Animals; D016018:Least-Squares Analysis; D008163:Luminescent Measurements; D008954:Models, Biological; D061848:Optical Imaging; D010100:Oxygen / Q000378:metabolism*; D051381:Rats; D012171:Retinal Vessels / Q000378:metabolism*; D013997:Time Factors
chemical_list: D010100:Oxygen