Relative quantitation of drug levels within caseum and cellular lesion regions was performed using MSiReader 37 . The proprietary Thermo * RAW data format was converted to imzML using Raw to imzML converter to enable importation into MSiReader 38 . Regions of interest ( ROIs ) were drawn in each image covering either cellular lesion or caseum and the mean drug signal intensity within that region ( normalized to the corresponding drug internal standard ) was calculated following exportation of the ROI peak list into Excel .