In accordance with the invention, a dental implant comprises a body portion and a head portion. A pixel array includes pixels each including a photoelectric transducer and pixel transistors and outputs analog pixel signals. The timing control block can include an SR latch and first and second delay blocks. The magnet used for focusing the electron beam is closed at the multistage depressed collector so that no magnetic flux reversals are present to affect the beam dispersal, due to electrostatic space charge forces, onto the multistage depressed collector. The height of the tower is therefore not limited by the availability of suitable cranes so that a greater height of tower can be erected to take advantage of winds at greater elevation.