MousePoint Window

The `MousePoint' window, accessible via the registration menu, allows the easy acquisition of landmarks from, and calibration of, a tracked pointing device. This is primarily intended for fiducial registration. Like the MouseRecord window, it makes it easy for one person to perform this without having to fiddle with multiple Stradx controls, since all the necessary functions are obtained by pressing one of the three mouse buttons. The mouse can be held in one hand while the tracked pointer is held in the other.

Since calibration is performed within this window, the pointer can be any device which has a position sensor attached, so long as the location of the position sensor relative to the part of the device being used to point remains constant. Calibration is very fast (generally only a few seconds) and requires nothing more than a piece of card or thin foam. It is also possible to use an ultrasound probe calibrated in the normal way, in which case the top centre of the B-scan (and hence centre of the probe face) will be used as the 'point', and this is where landmarks will be placed.

You do not have to go through the preparing to scan process before opening the MousePoint window, however you must have initialised communications with a position sensor, by using the setup menu. When you open the MousePoint window you will see the following message:

Stradx MousePoint Window

If you need to calibrate the pointing device, click the right mouse button, otherwise click the left mouse button to record landmarks. Either way, the mouse pointer will get locked in the window after the click. This allows you to pick up and move the mouse without worrying about whether the pointer is still within the window.

Pointer calibration

To calibrate a pointer, simply hold the pointing end still (for instance by placing it on a fixed piece of card or thin foam), and move the device around this fixed point. It is generally best to include some device twist as well. As the device is moved, the calibration is updated and displayed, in addition to the rms error, and the number of position values which have been used to determine the calibration:

Stradx MousePoint Window

Once the calibration values are fairly static, generally after only a few seconds, the right mouse button can be used to halt the calibration. If you need to re-start (for instance if the pointing end slips), then use the left mouse button. Upon halting, the following is displayed:

Stradx MousePoint Window

If you are happy with the rms error (this should typically be much less than 1mm, but is dependent on which position sensor you are using, how rigid your pointing device is, and the size of the 'point'), then click the middle mouse button to accept the calibration. Note that this calibration replaces the current spatial calibration, and you may want to save this in the normal way, using the calibration menu.

If you are using an optical position sensor, and the position sensor is not visible during calibration, the window will be be drawn in red. In addition, if the calibration did not converge (e.g. due to movement of the point) this will also be displayed at the top of the window:

Stradx MousePoint Window

Landmark recording

Any of the persistent landmarks, 'a' to 'z' can be recorded using this window. Once recorded, they are displayed in both the measurements window and the outline window. If either of these windows are open during recording, they will be dynamically updated as new landmarks are recorded. New landmarks are recorded in alphabetical order, skipping over any which already exist. The landmark which is about to be recorded is displayed at the top of the window:

Stradx MousePoint Window

To record a landmark, hold the pointer (or the face of the ultrasound probe if you have not calibrated a pointing device) against the point of interest, and click the left mouse button. Having done so, the window automatically moves on to the next available landmark:

Stradx MousePoint Window

You can delete the landmark you have just recorded by using the right mouse button, or alternatively clear all the landmarks 'a' to 'z' by using the middle mouse button. You must do one of these if there are no more available landmarks, in which case the window is drawn in yellow:

Stradx MousePoint Window

If you are using an optical position sensor, and the position sensor is not visible during landmark recording, the window will be be drawn in red:

Stradx MousePoint Window