Probe calib

Stradwin incorporates automatic spatial and temporal calibration systems for 3D ultrasound, derived from the successful Stradx system. The calibration process takes only minutes to perform.

In order to perform probe calibration, you need to do the following things:

Motion sequence for calibration

In what follows, the term 'phantom plane' refers to the bottom of the water bath or the virtual plane swept out by the bar of the Cambridge phantom. Initially position the probe above the phantom plane. This is the starting position for all the movements described below.

(A) Move the probe vertically up and down without changing its orientation.

(B) Rotate the probe from side to side while keeping the scan plane and the phantom plane perpendicular. You should rotate the probe both clockwise and anticlockwise from its starting position.

(C) Rotate the probe towards you and away from you, changing the angle between the scan plane and the phantom plane. Note that the angle of the image of the phantom plane should not change significantly during these moves.

(D) Rotate the probe through 45 degrees in each direction about its vertical axis.

(E) Translate the probe across the phantom plane and perform motions A, B, C and D in a total of three non-collinear locations.

You should aim to acquire about 40 lines.

Suggested motions for spatial calibration

Verification of calibration consistency with the pointer

Wherever possible, you should evaluate your calibration by testing whether or not it is consistent with your pointer calibration.

To do this you need to have a Polaris position sensor that is able to track two things simultaneously. Ensure that the tracked probe and a trackable pointer are both in the active volume of the position sensor. Use the position source dialog from the configuration menu to get Stradwin to track both of them. You should be able to see them both in the 3D window when live display is enabled (using the left-most button on the toolbar).

If the pointer has not already been calibrated, calibrate it as described on the pointer calibration page.

Ensure that the pointer calibration task page is selected. While keeping the probe in the tracked volume, dip it in a water-bath and hold the tip of the pointer in the beam. You should be able to see the reflections from the tip of the pointer in the live B-scan image. If the calibration of the probe and the pointer are consistent, you will also see a dot with a circle around it. The dot indicates the location of the pointer in the B-scan calculated by the system. The radius of the circle indicates the measured distance of the pointer tip from the plane of the B-scan. If the calibrations are consistent, the circle should be small when the pointer tip shows up most strongly in the B-scan image. Note that there is a small delay in displaying the pointer position in the image. This is not a sign of a problem with the system. The goal is to ensure consistency when both the pointer and the ultrasound probe are effectively stationary.

It is important to repeat the consistency check described in the previous paragraph with the pointer tip at various different positions in the B-scan image. You should make sure you check out all parts of the B-scan, not just the centre or one corner.

Controls affecting the line-fitting algorithm

Variance of Gaussian kernel
This controls the smoothing applied to the image in advance of the edge detection process.
Vertical analysis bands
This sets the number of vertical bands in the image that are used for the edge detection process. They are indicated in the image window by vertical red lines drawn down the image.
Gradient threshold
This threshold governs the way the line detection algorithm detects edge elements. The algorithm scans down the image and picks the first local maximum which exceeds the threshold, or the biggest local maximum if none exceed the threshold.
Pixel linearity threshold
The random sample consensus (Ransac) algorithm requires a minimum proportion of the edge elements to be collinear, otherwise the line is not considered reliable. This parameter determines the vertical tolerance (in pixels) that is used when determining collinearity.
Ransac acceptance proportion
This is the proportion of edge elements that must be collinear for the line to be accepted.
Configure for upside down image
It is conventional with some probes (eg. endovaginal) to view the image upside down, so that the probe face is at the bottom of the image and objects distant from the probe at the top of the image. By clicking on the 'Configure for upside down image' button, the edge detector will scan up from the bottom of the image for the first strong edge, instead of down from the top. Click this button when the image is inverted.