This window contains controls which affect the line-fitting algorithms used in spatial and temporal calibration. It also allows the probe shape to be estimated: i.e. the region within the acquired rectangular image which actually contains ultrasound data. The probe shape can be used to update the current calibration if a new depth setting has been selected.
This window also displays the current values of all the spatial calibration transformations and scales:
To calculate the probe shape, first ensure that the gain on the ultrasound machine is set high, so that you can clearly see the area containing ultrasound data. In order to calculate the shape correctly, you must be able to see at least a part of the top of the ultrasound data, and both of the sides. This means that, although the procedure can theoretically handle depth changes, panning and zooming of the image, it is not possible to pan or zoom such that the edges of the ultrasound data can no longer be seen.
The following is an example of an acceptable image, seen in the preview window, for a convex probe:
Now use the segmentation tresholds to select a grey-scale threshold which includes most of the ultrasound data, and very little outside. The segmentation does not have to be particularly good; as long as most of the ultrasound data is included, the probe shape should be calculated correctly. For instance, the following segmentation for a convex probe is perfectly adequate:
The probe shape parameters are all stored in the calibration (.sxc) file, hence you will need to save this file again if the calibration has been updated by changing the probe shape.
The facility will also work with linear array probes:
To perform sensorless freehand 3D ultrasound it is necessary to have access to the ultrasound envelope intensity signal direct from the scanner. If, as is normally the case, you are using B-scan data then this will have undergone logarithmic compression which changes the statistical properties of the signal. To undo this process we need to estimate the key parameter of the compression that has been performed. If you are using RF data then this step is not required. In this case the system is performing the image intensity compression directly, and thus knows the correct decompression parameter to use.
To estimate the decompression parameter you should first, segment out about 40 patches of "fully developed speckle" in some of the ultrasound images. An example of two such patches is shown below.
A good strategy is to segment out a couple of patches in roughly twenty different images. Then press the "Find decompression parameter" button. The system will compare the statistics of the compressed speckle patches with the known statistics of perfect speckle and come up with an estimate for the decompression parameter.
This window displays information about the average width of the ultrasound beam in the elevational direction for the top, middle and bottom thirds of the beam. (The top third of the beam is the third of the beam closest to the probe.) It is assumed that the beam has a Gaussian profile in the elevational direction, and the dimension given is one standard deviation of this Gaussian profile.
This beam-width information may be saved in and loaded from the calibration (.sxc) files and registration (.sxr) files. It is set and used by the Image registration window, particularly when performing sensorless freehand 3D ultrasound.