[Polarity of the radiological image. Practical applications]

The author considers the polarity "above-below" of a radiological image and makes three remarks: The image perception is better memorized when the axis of "gravity" and of "flow" and identical. When both axis, of gravity and flow, are opposite, it is very difficult to find the best didactic method of teaching the image. Experienced radiologists use since a long time a method of "depolarisation of the image". To improve the elementary perception of ribs, they turn (90 degrees, 180 degrees, 270 degrees) the image of a costal grid.