Body Registration

The body registration facility allows you to register the recorded scans to the position and orientation of the patient. This means that when you subsequently review the data, you can see the scans correctly located on a model of a human body, aiding interpretation of the data.

You need to perform the body registration either immediately before or after recording the data set, so that the patient does not move between acquisition and registration. Select the `Registration' option from the `Calibration Menu' to begin the process.

You will see two windows pop up. The smaller of these allows you to select the body parts being scanned. Simply click on the appropriate region or regions, making sure you select sufficient regions to fully cover the scanned area. Selected regions are highlighted in black: to de-select a region, click on it again. The example below shows the correct regions for a scan of a man's wrist.

The Body Registration Window

You can also select a female body using the toggle button. The example below shows the correct regions for a scan of a woman's thyroid gland.

The Body Registration Window

You can abort the selection process at any time by pressing `Cancel'. When you select `Done', the selected body parts appear in the larger `Registration' window, as shown below.

The Body Registration Window

The resolution and speed of the body rendering can be controlled using the `CPU & graphics performance' slider in the `Edit setup' panel. You can manipulate the viewpoint using the mouse and the `Scale' slider, in precisely the same way as in the `Outline' window.

You are now ready to start the registration process. To do this, you need to specify three points on the patient's body by holding the middle of the ultrasound probe's face on the patient's skin, and three corresponding points on the rendered body model by clicking with the right mouse button in the `Registration' window. The three points should not be colinear.

To specify a pair of corresponding points, do the following. Hold the ultrasound probe on the patient's skin and then either press the `Read sensor' button or the space bar on the computer's keyboard (these two operations are equivalent). Then manipulate the viewpoint to get a good view of the same point on the rendering in the `Registration' window, and click on that point with the right mouse button. As you enter each point, Stradx desensitizes the corresponding button at the bottom of the `Registration' window, to indicate that the point has been successfully entered.

Repeat this operation for the remaining two pairs of points, and Stradx will estimate the rigid body transformation between the points on the patient's body and the points on the rendering. It will also give you an error measure which indicates how well the registration process went: for example, if you specify colinear points on the patient's body and non-colinear points on the rendering then this is clearly wrong, and you will get a high error measure. The example below shows the `Registration' window after all three points have been entered: note the error measure.

The Body Registration Window

You can adjust the registration at any time by re-entering any or all of the three points. To do this, click the `Clear' button below the appropriate point and then define the new points as before. If you clear more than one point, you can select which one to re-enter first by simply clicking on the appropriate `Point' button. You can also reselect the body parts by clicking the `Select region' button.

Even though full registration requires three point correspondences, an approximate registration is calculated after the first two points have been entered: you can quit the `Registration' window at this stage if you like. The two-point estimate makes use of the surface normal on the rendering and the orientation of the ultrasound probe, so it is essential that the probe is held perpendicular to the patient's skin for this estimate to be any good. The two-point estimate may be preferable when scanning long, thin structures, where it might be difficult to localise three non-colinear points for the three-point procedure.

When you are happy with the current registration and its error measure, click the `Save body registration' button. This notifies all Stradx's modules about the new registration. Should you wish to notify the modules that the body registration is no longer valid, click the `Clear body registration' button.

The body registration information is saved with the 3D ultrasound data, allowing the body to be visualised along with the scans when the data set is reloaded. If you would like to acquire multiple 3D scans with the same body registration, simply ensure that the patient keeps still between scans: the current body registration is preserved when you open a new (empty) file to start recording another data set. If the patient moves, however, you should erase the body registration by opening the `Registration' window and clicking the `Clear body registration' button: failure to do this results in subsequent scans being saved with an invalid body registration, leading to misleading visualisations when the data set is reviewed.

With a valid body registration, you can click on the `Body' button in the `Outline' window to view the patient's body in relation to the recorded scans. Here's an example of a wrist scan.

The Body Registration Window

Note the effect of the `Alpha' transparency slider.

The Body Registration Window

Here's a further example, this time showing a scan of the patient's arm.

The Body Registration Window