Pressure

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:

• The frames in the acquired data are fairly closely spaced: specifically, each frame should be considerably closer to the surrounding frames than the elevational resolution of the ultrasound machine.
• There is plenty of information in the data. Frames with very little in them (for instance if the probe was lifted off the skin surface, or if the image is mostly of water or a non-echoic fluid) should not be used.
• The data contains speckle. All real ultrasound data will do so, but other data, or ultrasound data of non tissue-mimicking phantoms, may not.
• Both the subject and fixed part of the position sensing device remain stationary during the scan (other then movement due to varying pressure or breathing).
• At some point during the scan, the probe is not pressed hard to the skin surface: data can only be corrected to the minimum applied pressure during the scan.
• The recorded image in Stradwin is only of the ultrasound data, not of the surrounding information displayed on the ultrasound machine. If necessary, recorded data can be masked using the select tool in order to remove any redundant information in each image.
• The ultrasound image has the skin surface at the top.
• The position correction will work for all probes. The pressure correction will work best for linear probes, though some correction is also possible for convex probes.

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:

• When performing position correction, Stradwin automatically optimises the corrections to the original position sensor readings, then displays the maximum that any data has moved as a result of the correction.
• For pressure correction, Stradwin will always leave the data at the base of each ultrasound frame in the same location, and only de-compress data higher up the frame.
• Corrections can be visualised using the pressure visualisation before permanently applying them to the data.
• Corrections are only possible on B-scan (not RF) data which has been recorded with positions.
• It is not possible to correct the same data twice.

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.