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Calibration of an orientation sensor for freehand 3D ultrasound and its use in a hybrid acquisition system.

Housden RJ, Treece GM, Gee AH, Prager RW - Biomed Eng Online (2008)

Bottom Line: In comparison, six degree-of-freedom drift correction was shown to produce excellent reconstructions.A hybrid system incorporating the MT9-B offers an attractive compromise between quality and ease of use.The position sensor is unobtrusive and the system is capable of faithful acquisition, with the one exception of linear drift in the elevational direction.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Engineering, University of Cambridge, Trumpington Street, Cambridge CB2 1PZ, UK. rjh80@eng.cam.ac.uk

ABSTRACT

Background: Freehand 3D ultrasound is a powerful imaging modality with many potential applications. However, its reliance on add-on position sensors, which can be expensive, obtrusive and difficult to calibrate, is a major drawback. Alternatively, freehand 3D ultrasound can be acquired without a position sensor using image-based techniques. Sensorless reconstructions exhibit good fine scale detail but are prone to tracking drift, resulting in large scale geometrical distortions.

Method: We investigate an alternative position sensor, the Xsens MT9-B, which is relatively unobtrusive but measures orientation only. We describe a straightforward approach to calibrating the sensor, and we measure the calibration precision (by repeated calibrations) and the orientation accuracy (using independent orientation measurements). We introduce algorithms that allow the MT9-B potentially to correct both linear and angular drift in sensorless reconstructions.

Results: The MT9-B can be calibrated to a precision of around 1 degrees . Reconstruction accuracy is also around 1 degrees . The MT9-B was able to eliminate angular drift in sensorless reconstructions, though it had little impact on linear drift. In comparison, six degree-of-freedom drift correction was shown to produce excellent reconstructions.

Conclusion: Gold standard freehand 3D ultrasound acquisition requires the synthesis of image-based techniques, for good fine scale detail, and position sensors, for good large scale geometrical accuracy. A hybrid system incorporating the MT9-B offers an attractive compromise between quality and ease of use. The position sensor is unobtrusive and the system is capable of faithful acquisition, with the one exception of linear drift in the elevational direction.

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Related in: MedlinePlus

MT9-B reconstructions. The graphs show the roll, yaw and tilt measured by the sensor, compared with the same measurements deduced from the controlling mechanism. The left column shows examples of the worst drift in each angle and the right column shows examples of the best. Graphs (a-c) were derived form the roll-yaw experiment, which had some nearby ferromagnetic material. Graphs (d-f) are from the tilt-yaw experiment, where the ferromagnetic material was more distant.
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Figure 5: MT9-B reconstructions. The graphs show the roll, yaw and tilt measured by the sensor, compared with the same measurements deduced from the controlling mechanism. The left column shows examples of the worst drift in each angle and the right column shows examples of the best. Graphs (a-c) were derived form the roll-yaw experiment, which had some nearby ferromagnetic material. Graphs (d-f) are from the tilt-yaw experiment, where the ferromagnetic material was more distant.

Mentions: Figure 5 shows some representative examples of the orientations measured using the MT9-B, compared to the actual orientations set by the mechanism. For each of the three rotation axes, the graphs are for a sequence of frames selected from the grid of orientations so that there is no rotation about the other two axes. The most notable feature of these graphs is the yaw drift in the first column of the figure. This is due to the magnetic field distortion caused by the small ferromagnetic parts of the controlling mechanism. The better yaw result in the second column is from the tilt-yaw experiment, where the sensor was mounted further away from the ferromagnetic part. In comparison, the roll and tilt graphs, which do not depend significantly on the MT9-B's magnetometer readings, show no sign of drift.


Calibration of an orientation sensor for freehand 3D ultrasound and its use in a hybrid acquisition system.

Housden RJ, Treece GM, Gee AH, Prager RW - Biomed Eng Online (2008)

MT9-B reconstructions. The graphs show the roll, yaw and tilt measured by the sensor, compared with the same measurements deduced from the controlling mechanism. The left column shows examples of the worst drift in each angle and the right column shows examples of the best. Graphs (a-c) were derived form the roll-yaw experiment, which had some nearby ferromagnetic material. Graphs (d-f) are from the tilt-yaw experiment, where the ferromagnetic material was more distant.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2268692&req=5

Figure 5: MT9-B reconstructions. The graphs show the roll, yaw and tilt measured by the sensor, compared with the same measurements deduced from the controlling mechanism. The left column shows examples of the worst drift in each angle and the right column shows examples of the best. Graphs (a-c) were derived form the roll-yaw experiment, which had some nearby ferromagnetic material. Graphs (d-f) are from the tilt-yaw experiment, where the ferromagnetic material was more distant.
Mentions: Figure 5 shows some representative examples of the orientations measured using the MT9-B, compared to the actual orientations set by the mechanism. For each of the three rotation axes, the graphs are for a sequence of frames selected from the grid of orientations so that there is no rotation about the other two axes. The most notable feature of these graphs is the yaw drift in the first column of the figure. This is due to the magnetic field distortion caused by the small ferromagnetic parts of the controlling mechanism. The better yaw result in the second column is from the tilt-yaw experiment, where the sensor was mounted further away from the ferromagnetic part. In comparison, the roll and tilt graphs, which do not depend significantly on the MT9-B's magnetometer readings, show no sign of drift.

Bottom Line: In comparison, six degree-of-freedom drift correction was shown to produce excellent reconstructions.A hybrid system incorporating the MT9-B offers an attractive compromise between quality and ease of use.The position sensor is unobtrusive and the system is capable of faithful acquisition, with the one exception of linear drift in the elevational direction.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Engineering, University of Cambridge, Trumpington Street, Cambridge CB2 1PZ, UK. rjh80@eng.cam.ac.uk

ABSTRACT

Background: Freehand 3D ultrasound is a powerful imaging modality with many potential applications. However, its reliance on add-on position sensors, which can be expensive, obtrusive and difficult to calibrate, is a major drawback. Alternatively, freehand 3D ultrasound can be acquired without a position sensor using image-based techniques. Sensorless reconstructions exhibit good fine scale detail but are prone to tracking drift, resulting in large scale geometrical distortions.

Method: We investigate an alternative position sensor, the Xsens MT9-B, which is relatively unobtrusive but measures orientation only. We describe a straightforward approach to calibrating the sensor, and we measure the calibration precision (by repeated calibrations) and the orientation accuracy (using independent orientation measurements). We introduce algorithms that allow the MT9-B potentially to correct both linear and angular drift in sensorless reconstructions.

Results: The MT9-B can be calibrated to a precision of around 1 degrees . Reconstruction accuracy is also around 1 degrees . The MT9-B was able to eliminate angular drift in sensorless reconstructions, though it had little impact on linear drift. In comparison, six degree-of-freedom drift correction was shown to produce excellent reconstructions.

Conclusion: Gold standard freehand 3D ultrasound acquisition requires the synthesis of image-based techniques, for good fine scale detail, and position sensors, for good large scale geometrical accuracy. A hybrid system incorporating the MT9-B offers an attractive compromise between quality and ease of use. The position sensor is unobtrusive and the system is capable of faithful acquisition, with the one exception of linear drift in the elevational direction.

Show MeSH
Related in: MedlinePlus