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:

Fill a flat-bottomed water bath with water. The water should be at room temperature. Ideally the temperature of the water should be fairly constant during the calibration process.
Mount your position sensor rigidly on the ultrasound probe. A Polaris or Bird sensor may usually be mounted close to the body of the probe, but a Fastrak should be fixed about 10 cm away using some sort of rigid plastic strut.
If you have a Cambridge phantom available, mount the probe in the phantom, ensuring that the brass strip in the phantom is at all times in the centre of the ultrasound beam. If you don't have a Cambridge phantom, ensure that the bottom of the water bath is flat and roughen it uniformly using small circular motions with very fine emery cloth or sand paper. This will improve the reflection of ultrasound at oblique angles.
Check that you have Stradwin running with both images and positions available. This information is displayed on the 'Record' task page. The configuration dialogs for both images and positions can also be accessed from this page. Use the 'image source' dialog to select the image source you wish to use and the area of the ultrasound scanner screen that you want to record. When you are happy with the image cropping you should save it to a template file using the file menu, so you can use it with other calibrations if required.
On the 'Record' task page, set the 'Video Rate' to 'Full Speed'. After changing this value, select the 'Probe Calib' task page.
Switch on live display mode using the (leftmost) button on the toolbar. Configure the controls of the ultrasound machine so that the line feature appears clearly in the B-scan image.
If you have a curvilinear probe, use the 'Define region of interest' button to outline the region of the image occupied by the scan. The left mouse button is used to specify each point round the edge of the region, and the right button is used to indicate the final point.
Set the controls of the ultrasound machine so that the line detector locks on to the bottom of the water bath (or bar of the Cambridge phantom).
Select the 'Probe Calib' task page and press the 'Toggle temporal calibration' button. Stradwin is now in temporal calibration mode. Follow the instructions at the top of the task page. You will be asked to hold the probe steady while imaging the bottom of the water bath. You will then be asked to move the probe up and down while Stradwin tracks the bottom of the water bath in the image. By matching the image motion with the readings from the position sensor, Stradwin can estimate the temporal calibration, which is the relative lag between the position and image streams. The result is presented in terms of the lag, and also a confidence value, representing how how well the image and position streams correlated. You should repeat the temporal calibration a few times (by pressing the 'Toggle temporal calibration' button again), and obtain a few results with confidences exceeding 95%. Keep repeating this process until Stradwin displays a temporal calibration result that is in line with the >95% confidence consensus, then move on to the next step.
Measure the temperature of the water in Celsius (degrees Centigrade), enter the value in the box on the 'Probe Calib' task page and press the 'Set' button. Note that the temperature compensation strategy is not completely effective for probes with very high curvature, such as rectal probes. For these probes, it will be more accurate to calibrate with warm water as close as possible to 48 Celsius.
This next step involves estimating the x and y image scales (mm per pixel). You should skip this step if you have an RF image source, or are using a Terason or Sonix, in which case the scales are implicit. Most ultrasound machines have facilities for measuring distances in the image. Use a facility of this sort to display either a line of a known length, or two marks a known distance apart. You need to be able to see both marks, or both ends of the line, in the live Stradwin image window. For maximum accuracy, the line should not be short: it should occupy a good portion of the visible ultrasound image. Click the button 'Mark ends of first line', and click the left mouse button on one end of the line and the right mouse button on the other end. Then click the 'Finished locating ends of line' button. Set the slider as close as you can to the correct length of the line in millimetres. Repeat this process for a second line (use the 'Mark ends of second line' button), which should be roughly perpendicular to the first line.
Go through the sequence of movements listed below and, when you are happy that the line detector is locked onto the bottom of the water bath in each position, click the 'Accept line for calibration' button. If the line detector is not happy with the line it will display 'No valid line' on this button, but you also need to exercise judgement to avoid capturing incorrect lines. If you mistakenly accept a line that is not an image of the bottom of the bath (or bar of the Cambridge phantom), then you should click the 'Delete all accepted lines' button and start again.
You should aim to acquire about 40 lines, as described below.
Press the 'Solve for spatial calibration' button to run the calibration solution algorithm. This algorithm uses the recorded positions of those lines which were accepted.
Enter the name of the ultrasound probe and the depth setting in the two boxes. Since a calibration applies only to a specific probe and depth setting, it is very important that you correctly identify these at this stage. You need to press 'enter' in both boxes before the 'Accept calibration' button is enabled.
Press the 'Accept calibration' button. The effect of the new calibration is now apparent in all of Stradwin's windows. However, the calibration is not yet saved to file.
On the 'Record' task page, set the 'Video rate' to the value that you will require when you reload the template to use it for recording data. The recommended value for this is the 'Gated' setting.
Save the calibration to a template file using the file menu.

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.

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.