Freehand 3D ultrasound data, such as is recorded by Stradwin, is constructed by placing each frame at the location and orientation indicated by the position sensor. There are two potential sources of error in this process that can be reduced by looking at the ultrasound data. Noise in the position sensor readings introduces a sub-millimetre 'jitter' in the frame locations, which can be seen in high-resolution 3D data sets. Variations in probe contact pressure can cause varying deformation of the anatomy which can also corrupt the location of the 3D data, particularly near the skin surface.

This task page can be used to attempt to correct for both these errors by examining the acquired ultrasound data to look for jitter and variations in probe contact pressure. Since this correction is based solely on evidence from the data, it will only work well if:

Since there are so many provisos, pressure correction in Stradwin should always be checked before it is accepted: it is the user's responsibility to ensure that the suggested correction matches the known contact pressure variation during scanning.

Having said this, in most cases pressure correction will result in improvements to the 3D data, and Stradwin has various ways of ensuring that the correction is appropriate:

Pressure correction controls

The first control, 'Auto correct positions during recording', determines whether or not the system will attempt to correct positions (not pressure) for live data. Such corrections will only be allowed if the maximum movement due to the correction is less than 2.0 mm: otherwise the correction will be discarded.

The following two buttons allow the correction of positions only, or positions and pressure, on pre-recorded data. Note that corrections for probe pressure can take up to one minute to complete, dependent on the size of the data set and speed of the computer. These corrections are not applied to the data at this stage. Having calculated the correction, the maximum data movement is displayed, and the correction becomes available for testing using the pressure visualisation. If the maximum movement at the bottom of the ultrasound frames (due mainly to position correction) is over 5 mm, or the maximum movement at the top of the ultrasound frames (due mainly to pressure correction) is over 20 mm, the correction will be discarded.

If the data has been recorded using multiple sweeps, the pressure correction will be calculated individually for each sweep. The 'Results for sweep' slider can then be used to show the maximum data movement for each sweep, and to re-align the pressure visualisation appropriately for each sweep.

Having assessed the correction, the following two buttons either reject or accept the correction. If it is accepted, it will be permanently applied to the data, and you will subsequently need to save the data (to a different file name if you want to keep the un-corrected data too). If any data-based landmarks or object contours have been defined for this data, these too are corrected and will also need to be saved with the data.

Rejecting the correction permanently deletes it, though it can be re-calculated if desired. The correction will also be deleted if the task page is changed after a correction has been calculated but before it has been accepted.