Cal with pointer

This is the second of two options which Stradwin offers for calibrating the ultrasound probe. This technique requires the tracking of an additional pointer (only available with the Polaris or trackSTAR systems). For ideal results, this should be a "Cambridge Pointer" device.

In order to calibrate using this method, you need to do the following things:

• Fill a small 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. You should wait five or ten minutes for air bubbles in the water to dissipate.
• 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.
• Mount the probe on the edge of the water bath, such that the probe face is just under the water, and the probe is as near horizontal as is practical. This will mean that most of the B-scan is close to the water surface, even at large B-scan 'depths'.
• Make sure that you have calibrated a pointer (or preferably a Cambridge Pointer) carefully using the Pointer Calibration task. Cambridge Pointers must be connected to the third port on the Polaris system.
• 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 'Cal with pointer' task page.
• Switch on live display mode using the (leftmost) button on the toolbar. Configure the controls of the ultrasound machine so that the time-gain controls are relatively constant with depth, and the gain is fairly low. Under these conditions the pointer should be the main feature in the image, and should appear with relatively uniform gain over depth.
• 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.
• If you have a curvilinear probe, you might need to 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.
• Position the pointer in the ultrasound beam, so you can see it in the B-scan. If you are using a conventional pointer, you need to attempt to locate the tip at the elevational centre of the B-scan beam. If you are using a Cambridge Pointer, then move this in and out of the beam until the circles from the cones are as small as possible in the B-scan. During this, Stradwin displays a green line, which should pass through the middle of the pointer tip in the direction of insonification. Stradwin also displays a red dot which should be at the top of the pointer bar.
• Once you are happy that the pointer is correctly located in the elevational centre of the beam, and that the green line and red circle are also correctly located, press the 'accept' button. Then repeat this operation ten or twenty times, ensuring you cover the extremities of the B-scan, and that you rotate the pointer to various angles in doing so. The calibration quality will be better if you keep the pointer at right-angles to the plane of the B-scan.
• Press the 'Solve for spatial calibration' button to run the calibration solution algorithm. This algorithm uses the recorded positions of those points 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.