A method of mapping a tissue characteristic in a tissue sample comprising, for each of a plurality of pixels in a two dimensional array: gathering infrared absorption data from the sample at selected wavelengths; determining, from the infrared absorption data, a first measure of the amount of energy or power absorbed attributable to an amide moiety and a second measure of the amount of energy or power absorbed attributable to a phosphate moiety; determining a ratio of the first measure and the second measure to establish a histological index, to thereby generate a dataset corresponding to pixel values of a spatial map of the sample, wherein the histological index, PA, is derived according to the expression PA = [X M(λ3) - M(λ4)] / [X M(λ1) - M(λ2)], where: M(λn) is a measure of the absorbed energy or power at λn; λ1 is a wavelength corresponding to a peak absorption value attributable to an amide moiety; λ2 is a wavelength corresponding to a baseline absorption value attributable to an amide moiety; λ3 is a wavelength corresponding to a peak absorption value attributable to a phosphate moiety; λ4 is a wavelength corresponding to a baseline absorption value attributable to a phosphate moiety; X is numerical factor ≥ 1 which is set to a value sufficient to ensure that the measure M for a peak absorption values λ3 and λ1 is always greater than the measure M for the corresponding baseline absorption values λ4 and λ2 for all measurements; and λ1 = 6.0 ± 0.1 μm; λ2 = 6.5 ± 0.1 μm; λ3 = 8.13 ± 0.1 μm; or 9.26 ± 0.1 μm; λ4 = 8.57 ± 0.1 μm or 10.0 ± 0.1 μm, in which the value X is set according to the signal-to-noise ratio of the measurement system. gathering infrared absorption data from the sample at selected wavelengths; determining, from the infrared absorption data, a first measure of the amount of energy or power absorbed attributable to an amide moiety and a second measure of the amount of energy or power absorbed attributable to a phosphate moiety; determining a ratio of the first measure and the second measure to establish a histological index, to thereby generate a dataset corresponding to pixel values of a spatial map of the sample, wherein the histological index, PA, is derived according to the expression PA = [X M(λ3) - M(λ4)] / [X M(λ1) - M(λ2)], where: M(λn) is a measure of the absorbed energy or power at λn; λ1 is a wavelength corresponding to a peak absorption value attributable to an amide moiety; λ2 is a wavelength corresponding to a baseline absorption value attributable to an amide moiety; λ3 is a wavelength corresponding to a peak absorption value attributable to a phosphate moiety; λ4 is a wavelength corresponding to a baseline absorption value attributable to a phosphate moiety; X is numerical factor ≥ 1 which is set to a value sufficient to ensure that the measure M for a peak absorption values λ3 and λ1 is always greater than the measure M for the corresponding baseline absorption values λ4 and λ2 for all measurements; and λ1 = 6.0 ± 0.1 μm; λ2 = 6.5 ± 0.1 μm; λ3 = 8.13 ± 0.1 μm; or 9.26 ± 0.1 μm; λ4 = 8.57 ± 0.1 μm or 10.0 ± 0.1 μm, in which the value X is set according to the signal-to-noise ratio of the measurement system. M(λn) is a measure of the absorbed energy or power at λn; λ1 is a wavelength corresponding to a peak absorption value attributable to an amide moiety; λ2 is a wavelength corresponding to a baseline absorption value attributable to an amide moiety; λ3 is a wavelength corresponding to a peak absorption value attributable to a phosphate moiety; λ4 is a wavelength corresponding to a baseline absorption value attributable to a phosphate moiety; X is numerical factor ≥ 1 which is set to a value sufficient to ensure that the measure M for a peak absorption values λ3 and λ1 is always greater than the measure M for the corresponding baseline absorption values λ4 and λ2 for all measurements; and λ1 = 6.0 ± 0.1 μm; λ2 = 6.5 ± 0.1 μm; λ3 = 8.13 ± 0.1 μm; or 9.26 ± 0.1 μm; λ4 = 8.57 ± 0.1 μm or 10.0 ± 0.1 μm, in which the value X is set according to the signal-to-noise ratio of the measurement system.