lies along a first axis portion b lies along a second axis which is perpendicular to the first axis portions a and b are displayed together the relationship of portions a and b lying at 90 degrees to each other is maintained no matter what the location or orientation of the bidirectional measurement object 350 on the display fig4 shows a bidirectional measurement object 450 as displayed on a medical image 400 medical image 400 and structure 410 correspond to those shown in fig1 fig4 shows bidirectional measurement object 450 as it might first appear when a user superimposes it on a medical image 400 portions a and b are not aligned with any feature on the medical image 400 but remain at 90 degrees to each other a user may click onto the bidirectional measurement object 450 for example at the crossing point of portions a and b and then drag the bidirectional measurement object 450 to a point where a measurement is to be performed alternatively the bidirectional measurement object 450 may first appear with one end of portion a or portion b located at a point that the user selects for example a point on the perimeter of structure 410 i firstly the user clicks and drags bidirectional measurement object 450 to a location such as structure 410 in this example we assume that structure 410 is a tumour the bidirectional measurement object 450 may be configured such that by grabbing the crossing point of the measurement object with a mouse click the user may drag the whole tool around the medical image 400 ii the user then locates a first end of one portion for example portion a on.