ABSTRACT
Three-dimensional (3D) imaging techniques such as computed tomography, magnetic resonance imaging and positron emission tomography will be familiar to the modern imaging specialist. The terms ‘one-dimensional (1D)’, ‘two-dimensional (2D)’, ‘3D’ and ‘four-dimensional (4D)’ are used. The ‘D’ in every case refers to ‘dimension’. ‘1D’ is one spatial dimension, in other words a line; for example a 1D transducer consists of a line of elements. ‘2D’ is two spatial dimensions, which is an area. A 2D transducer consists of a matrix of elements. ‘3D’ is three spatial dimensions, in other words a volume. 3D scanning refers to the collection of 3D volume data. ‘4D’ is three spatial dimensions and one time dimension. In freehand 3D scanning a conventional ultrasound transducer is swept across the patient's skin. In this system knowledge of the transducer position in space is obtained using a tracking device.
