**3.1 Accuracy and spatial linearity of measurement**

Most of the equipment allows the using electronic callipers, to carry out measurements of depth, dimensions of objects, areas and volumes, necessary to ensure a correct diagnosis.

Accuracy is a parameter that provides an indication of the error made in the measurement of distances and is evaluated by comparing the distance measured between two targets in the phantom with the known distance declared by the manufacturer of the phantom. By determining the relationship between the true dimensions and the measured ones, spatial linearity is obtained: this can be evaluated both horizontally and vertically. Several test objects and phantoms available on the market are equipped with a horizontal series (at a given depth) of equidistant points and a vertical series. By representing the measured distances on a Cartesian plane, on the x-axis and the true distances on the y-axis, it is possible to evaluate both the linearity of the calliper's response (statistical p-value of regression) and its accuracy (angular coefficient of the straight line close to 1 i.e. 45°). No particular problems were found in performing this test and the results obtained were quite often excellent (p very low, angular coefficient very close to 1, typical 0.98–0.99).
