To measure Gd-DTPA concentrations directly from gray scale T1-weighted MR images, standard solutions of Gd-DTPA were scanned along with the rabbit eye. The following concentrations of Gd-DTPA were prepared in a 2% hydroxypropyl methylcellulose (Methocel; Dow Chemical Co., Midland, MI) solution: 1.0 × 10−1 M, 0.5 × 10−1 M, 1.0 × 10−2 M, 0.5 × 10−2 M, 0.25 × 10−2 M, 1.0 × 10−3 M, 0.5 × 10−3 M, 1.0 × 10−4 M, 0.5 × 10−4 M, 1.0 × 10−5 M, 0.5 × 10−5 M, and 1.0 × 10−6 M. The 12 standard solutions were poured into 12 individual wells cut from a standard 96-well plate culture chamber and sealed with a silicone adhesive. Gray scale MR images of the standard solutions were developed and average intensity value of each concentration was determined using ImageJ (ver 1.27z; available by ftp at zippy.nimh.nih.gov/ or at http://rsb.info.nih.gov/nih-imageJ; developed by Wayne Rasband, National Institutes of Health, Bethesda, MD).