Spatial Calibration

Stradx offers two methods for spatial calibration. For the quickest and most accurate calibration, you'll need to manufacture or purchase a mechanical alignment instrument, as described in this technical report. The Stradx features which support this technique are described here.

Most users, however, will use Stradx's alternative spatial calibration technique, for which you need only a flat-bottomed waterbath (or a Cambridge phantom for more accuracy). This technique is still very accurate and takes only a few minutes to perform. Details of how it works can be found in this technical report. To perform spatial calibration this way, you need to do the following things:

Automatic spatial calibration window

Sequence of Probe Movements for Automatic Calibration

There are some movements of the probe which it is vital to perform in order to sufficiently constrain the calibration process. A suggested procedure for calibration is shown in the diagram below.

Suggested motions for spatial calibration

Motions (A), (B) and (C) must be performed in order to constrain the six positional parameters of the B-scan relative to the position sensor transmitter (three angles and three translations) and the two scale parameters (in the plane of the B-scan).

However, the location of the plane being imaged by the probe is also unknown and must be estimated by the spatial calibration process. Repeating motions (A), (B) and (C) in three different locations will provide sufficient constraints for the location of this plane to be determined.

An additional rotation about the vertical axis between each of these three measurement locations (as shown in the diagram above) is not strictly necessary, but will ensure that the calibration process will not be degraded by a poor choice of orientation of the position sensor transmitter with respect to the receiver.

You may need to adapt this sequence to satisfy constraints imposed by your position sensor or water bath, but you must ensure that you incorporate all the motions in several different positions on the calibration plane.

The full set of unknowns that the system solves for are as follows:

  1. Z translation parameter of transformation from datum of position sensor to phantom plane. This value is discarded.
  2. Elevation parameter of transformation from datum of position sensor to phantom plane. This value is discarded.
  3. Roll parameter of transformation from datum of position sensor to phantom plane. This value is discarded.
  4. Azimuth parameter of transformation from the ultrasound scan plane to the sensing element of the position sensor.
  5. Elevation parameter of transformation from the ultrasound scan plane to the sensing element of the position sensor.
  6. Roll parameter of transformation from the ultrasound scan plane to the sensing element of the position sensor.
  7. X translation parameter of transformation from the ultrasound scan plane to the sensing element of the position sensor.
  8. Y translation parameter of transformation from the ultrasound scan plane to the sensing element of the position sensor.
  9. Z translation parameter of transformation from the ultrasound scan plane to the sensing element of the position sensor.
  10. Scaling parameter of the ultrasound scan plane in the X direction.
  11. Scaling parameter of the ultrasound scan plane in the Y direction.