Biomedical Engineering Reference
In-Depth Information
5.5.1.2.1.3 Calibration of Sensor
In order to optimize sensor accuracy, a rela-
tionship must be established between the position of the sensor on the trans-
ducer and the ultrasound data. This is referred to as calibration and if done
properly will mean that a given feature in the scanned volume will appear at
the same position relative to a fixed reference point wherever the transducer
is. The accuracy obtainable with the transducer-sensor system should equal
that of the sensor alone. Any errors in the calibration process will degrade the
sensor accuracy and therefore give rise to artifacts described above. A review
of calibration methods is given in Prager et al.
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5.5.1.2.2
Sensor Design-Specific Errors
5.5.1.2.2.1 Electromagnetic Position Sensors
These devices function by mea-
suring the strength of magnetic fields generated by sending current through
three small wire coils which are mutually orthogonal. There are two distinct
types using a.c. and d.c. fields.
Birkfellner et al.
67
investigated errors introduced by transducers and metal
objects for both a.c. (Isotrak II, Polhemus Inc., Colchester, Vermont, U.S.)
and dc (Flock of Birds, Ascension Inc., Burlington, Vermont, U.S.) sensors.
They found that the Isotrak was affected by all metal placed within the use-
ful range of the sensor and also by mounting it directly on the transducer
casing. The Flock of Birds sensor was only affected by ferromagnetic metal
and was relatively unaffected by directly mounting it on the transducer cas-
ing. However, the fundamental accuracy of the Isotrak was better than that
of the Flock of Birds. In a clean environment, both devices performed
according to the manufacturers' specifications. Errors can be minimized by
removing all metal (Isotrak) or ferrous metal (Flock of Birds) from the sen-
sor and mounting the sensor on an insulating rod attached to the trans-
ducer, rather than on the transducer casing itself (Isotrak). According to
manufacturers' specifications, the fundamental accuracy of the Isotrak is
about 0.75 mm and for the Flock of Birds is about 1.8 mm. The maximum
measurement rate for the Isotrak is 120 measurements per second and for
the Flock of Birds is 144 measurements per second. The average latency is
approximately equal to half the minimum measurement interval. This
equates to 4 ms for the Isotrak and 3.5 ms for the Flock of Birds.
Further information on errors produced by electromagnetic sensors may
be found in references 68 through 71.
5.5.1.2.2.2 Optical Position Sensors
These devices function by tracking the
position of a number of infrared markers on the transducer. The accuracy is
greater than for electromagnetic systems and there are no errors introduced by
electromagnetic interference, but the system tends to be more bulky. The main
problem with optical systems is that line of sight must be maintained at all times,
which places a restriction on the type of scanning with which they can be
used. Sato et al. describe a system based around the OPTOTRAK optical sen-
sor (NDI Inc., Waterloo, Ontario, Canada).
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The latency of optical systems
