A 'constrained' 3-D aquisition device using an ordinary 2-D probe and a device to constrain movement of the probe to one plane. Such devices are simple to make and to interface with the conventional ultrasound system, but are limited to small number of anatomical areas.

Free-hand scanning with position sensors which may be electromagnetic and optical tracking devices. They can be as accurate as to a millimeter in position and better than a degree in slant angle estimation. Passive stereoscopic optical trackers are most robust but are dependent on the line of sight and are expensive. Electromagnetic sensors are cheap but require careful calibration prior to use. These can be attached to the conventional probe without modification to the probe or to the scanning technique used.

* Source of images: Medicamundi, published by Philips Medical Systems ® 43:3, p24, 1999.

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