Isocentre

This task page enables you to align a coordinate system inside Stradwin with an external coordinate system. The isocentre visualisation page can then be used to generate reslice images aligned with this coordinate system.

The external coordinate system is defined by the location of a rectangular box. Ideally, this box should have all dimensions in the range 250 mm to 450 mm. The origin of the coordinate system will be located at the centre of the box and the axes will be perpendicular to its faces.

• Check that a tracked pointer is available and is being tracked by Stradwin. It should be visible in the 3D window (when the 3D window is running live). If you don't yet have a tracked pointer, press the button on the isocentre task page labelled 'Configure position source', which will bring up the 'Position source' dialog from the configuration menu. Use this to set up your position sensor appropriately to track the pointer.
• Check that your pointer is correctly calibrated. If you need to recalibrate the pointer, this can be done using the 'Pointer' task page.

The main section of this task page consists of five buttons. The diagram on the buttons symbolises a sheet of cardboard that might be folded to form the sides and top of a rectangular box. Each of the four outer buttons are associated with one of the sides of the box. They each have a red blob showing the side to which they relate. The middle button is associated with the top of the box and has a corresponding red blob to show this.

The idea is that you put a box in the radiotherapy simulator at the isocentre, turn on the lasers, and the lasers then reflect on the sides of the box. You locate the pointer on each side of the box where the lasers cross and press the appropriate button on the task page. The reslices that you eventually generate using the isocentre visualisation page will be parallel to the plane defined by the pointer positions when the left, middle and right buttons are pressed.

It is possible to relate the coordinate system that is defined inside Stradwin to the location of the pointer when each button is pressed. The 'x' axis is in the direction from the left button to the right button. The 'y' axis is in the direction from lower (invisible) surface of the box to the middle button location. The 'z' axis is in the direction from the top button to the bottom button.

You need to locate at least four of the box faces before the system will solve for the isocentre calibration. When you are ready, you can click the 'Calculate isocentre' button to do this.

When you quit Stradwin, the isocentre calibration is saved in the stradwin.ini file. By default, each user has a separate copy of this file which is saved in their home folder and reloaded when the program starts up. Details of how to change this behaviour are given on the installing Stradwin page.