RF Calibration

The RF calibration window is used to record speckle decorrelation curves in the axial, lateral and elevational directions. This is a prerequisite for research into elevational distance estimation by speckle decorrelation. The starting point is a 3D RF data set of a suitable speckle phantom. The B-scans should be parallel and separated by a fixed offset: you can acquire suitable data by mounting the probe on a screw thread and measuring its displacement with a dial guage. With such a data set loaded into Stradx, the RF calibration window can be started via the calibration menu.

The RF Calibration Window

Stradx records the decorrelation of the RF echo envelope amplitude (not intensity). To measure the decorrelation curves, it is first necessary to tell Stradx the elevational separation of the B-scans (top left slider) and the dimensions of the patch grid (two sliders just beneath). Separate decorrelation curves are calculated for each patch, thus capturing any variations across and down the B-scan. Stradx calculates the correlation in each direction in ten steps. So, by default, the axial decorrelation is recorded at shifts of 0 to 9 samples, the elevational decorrelation at shifts of 0 to 9 B-scans and the lateral decorrelation at shifts of 0 to 9 vectors. You might find that the signal is not completely decorrelated by 9 shifts, in which case you should adjust the two `skip factor' sliders appropriately. So, for example, if you set the axial skip to 2, Stradx will calculate axial decorrelation at shifts of 0, 2, 4 ... 18 samples. To get these sliders right, click the `Quick check' button and then `Calculate' to process a small number of B-scans (not the whole data set). You can then review the decorrelation curves in the bottom half of the window, and adjust the skip sliders if necessary. When you're happy, switch off the `Quick check' button, press the `Clear' button and press `Calculate' again.

The RF Calibration Window

Stradx will now calculate as many different decorrelation curves as it can, using all the B-scans in the data set, before averaging them together to get one set of curves per patch. This might take a while, and Stradx is unresponsive while calculating. The progress bar lets you know how much longer you have to wait!

The RF Calibration Window

When complete, you can view the decorrelation curves for any particular patch by clicking on the patch with the left mouse button. The axial decorrelation curve is displayed in red, with the scale (in samples) in red below the x-axis. The elevational decorrelation curve is displayed in green, with the scale (in millimetres) in green below the x-axis. The lateral decorrelation curve is displayed in cyan at a fixed scale of one vector per step, or 9 vectors to the last point on the curve (the lateral skip is not adjustable).

The RF Calibration Window

It may not be possible to estimate decorrelation curves for all patches in the grid. In the example above, the top two rows are affected by the phantom's scanning window, while the bottom of the B-scan is corrupted by noise. You can mark invalid patches using the right mouse button. Invalid patches are indicated by a red cross. You can toggle the status back to `valid' by clicking again with the right mouse button.

Finally, when you're happy with the decorrelation curves, you can save all the information in a .sxe file using the `Save' button. You can reload previously saved calibration files using the `Load' button, but note that the .sxe file must be compatible with the current data set (same number of samples and vectors, same width and height). The format of the .sxe data file is described