Calibration Menu

Calibration Menu

The first three items on the calibration menu relate to spatial calibration. They permit you to load and save spatial calibration files and generate new calibrations using the automatic spatial calibration facility. A spatial calibration is nothing more than the coordinate transformation from the ultrasound scan plane to the receiver of the position sensing device. It also includes the scales (in cm/pixel) in the ultrasound image. After generating a new spatial calibration, it is important to save it and also to save the current setup (see the section on the `Setup' menu). This is because the calibration is calculated relative to the corner of the cropped ultrasound image. Thus it is important to record the setup cropping information in case you need to use the calibration again.

The fourth item on the menu allows you to set the value of some calibration controls. These affect the line-fitting algorithms used in both spatial and temporal calibration. They can also be used to change the current estimate of probe shape, which affects both the spatial calibration and the image registration facility. This window also displays the current values of all the spatial calibration transformations and scales.

The fifth item on the menu permits you to automatically reset the temporal calibration parameters to suit your position sensor and ultrasound machine. Stradx matches position information with the images using their time stamps. Different ultrasound machines and position sensors take different amounts of time to deliver their images and readings. It is therefore necessary to include a fixed, sensor-dependent delay in the timing calculations. Stradx has sensible built-in delays for all the position sensors. The temporal calibration facility is provided to enable you to fine tune this if you want to. Once temporal calibration parameters have been found, they can be saved using `Save setup'. They are not saved by `Save calibration', which is for spatial calibration only.

The sixth item on the menu allows you to calibrate the rate of speckle decorrelation from an RF echo signal. This is useful for researching elevational distance estimation by speckle decorrelation. Full details of this facility can be found here.

Suggested calibration order

A variety of different calibration procedures are required to set up an accurate acquisition system. For optimal accuracy, you should perform the various calibrations in the following order:

  • Set up the ultrasound machine with the depth setting you expect to use most.
  • Estimate the probe shape. This will allow you to change the zoom and depth settings later.
  • Perform a temporal calibration. This will make the subsequent spatial calibration more accurate.
  • Perform a spatial calibration, and save the calibration (.sxc) file.
  • Save the setup file: this will include the name of the calibration file, the temporal calibration and the various video clipping parameters, amongst other things.
  • When you want to scan, just load the setup file, and this will automatically load the right calibrations. If you want to change the depth setting, just re-estimate the probe shape, and save the new calibration file with a new name.