A method for determining a lower-bound quantity per unit volume of a physical property associated with a body, the property being susceptible to measurement in a plurality of directions. In general, the method comprises the steps of scanning a volume of interest in a predetermined number of directions so as to derive for each direction an area having a plurality of projections thereon each representing a cumulative value of said property within the volume of interest along a respective one of the directions, and determining an estimated volume of the body as the intersection of all volume elements bounded by the derived areas. For each area the sum of all the projections are compared and correction is applied as required so as to remove any disparity between the sums so as to produce a uniform sum in each of the directions. The lower-bound quantity per unit volume of the property of the body is then calculated as the quotient of the uniform sum divided by the estimated volume. A system is also described for determining the lower-bound density of nitrogen using the method according to the invention.