Apparatus for use in selecting a point or plane in an object translatable along a path, and for indicating the distance of the selected point or plane from a predetermined position in the path. A reference point in the path is established by a line of light projected onto the translatable object. When the line of light identifies a point or plane of interest in the object, a reset signal is transmitted to a computer. The reset signal causes a number, a reset quantity representing the known distance between the reference point or line of light and the predetermined position, to be recorded in computer memory. As the object then is translated along the path, an encoder supplies signals to a counter that is incremented or decremented in response to movement of the object toward or away from the predetermined position. The reset quantity initially has the count both added to it and subtracted from it and the result, the reset quantity, is displayed by the computer. As the count in the counter changes during movement of the object toward or away from the predetermined position, the displayed result changes accordingly. The apparatus may be used in properly positioning a patient in a medical imaging system.