Biomedical Engineering Reference
In-Depth Information
will increase with the number of markers that are tracked. When the
OPTOTRAK system is used for tracking probe plus object, the latency will be
40 to 50 ms.
5.5.1.2.2.3 Acoustic Position Sensors
These are broadly the same as the opti-
cal devices with the infrared markers replaced by acoustic receivers. Since
the speed of sound is very much less than that of light, the latency is much
greater. King et al. describe a system based on the GP 8-3D acoustic sensor
(Science Accessories Corporation, Stratford, Connecticut, U.S.) in which
they evaluate accuracy of linear, angular, and volume measurements.
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Hata
et al. describe a system for US
CT registration based on a homemade acous-
tic sensor.
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5.5.1.2.2.4 Mechanical Arms
A system which uses a robot arm (Faro Medi-
cal Technologies, Lake Mary, Florida, U.S.) has recently been described.
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They quote a “sub-mm” accuracy that yields a patient-image coordinate
mapping accuracy of 2 to 3 mm. With such a mechanical system there are
restrictions imposed in the scanning process due to the physical connection
between the transducer and scanner. However no errors due to electromag-
netic interference occur nor is line of sight required.
5.5.1.3 3D Acquisition Process
The 3D acquisition process consists of moving an ultrasound beam through
a tissue volume which itself may be moving. Errors are therefore introduced
by both the transducer and tissue motion. Also, freehand acquisition tech-
niques introduce geometric distortions by collecting voxels in a spatially
irregular fashion.
5.5.1.3.1 Transducer Motion
When a 3D ultrasound image is being acquired, the transducer is moving
continuously. Therefore the position of the transducer when a pulse is trans-
mitted is not the same as when the pulse is received. This causes the image
plane to become distorted, since the transmitter positions (which are fired in
sequence) will not lie on a straight line. If the slices are approximately paral-
lel, the effect of these distortions is small, provided the interslice spacing is
less than the slice beam width. However, for scanning geometries which have
a rotational element, the effect may be more significant. Such scanning geom-
etries could be a freehand acquisition where the transducer is “rocked” (i.e.,
rotated about a fixed point on the skin surface) or a transducer rotating under
computer control. Rotating transducer systems have been described.
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The
magnitude of these errors can be calculated for particular scanning condi-
tions and their effects can be determined.
Slices produced by the freehand scanning process will have a variable spac-
ing and are unlikely to be parallel. This can cause a significant increase in the
