Light Field Alignment

It is beneficial to pay attention to good light field alignment. When Light field setup is OK replacing a bulb will take no more than ten minutes. Wrong setup results in low light intensity, bad correlation between light and Xray dimensions, bad field size calibration, moving cross hairs on isocentre or on the floor.

We use the following procedure with great success.

• Place the bulb in the centre of the optics. (With the power switched off you can see the filament through the lenses). This results in maximum light intensity.(non MLC)
• Place the table at about the isocentre height. Fix a rule at the edge of the table in a way that light from the light field passes through the hole of the rule. Place a piece of paper on the floor. Rotate the diaphragm 4 times 90 degrees and mark the projection of the light spot. Determine the centre of the four marks. Carefully move the light assembly to match with the centre. Rotate diaphragm and check if the light spot stays at the same place. If so than the light focus is on the rotation axis of the diaphragm.
• Check projection of cross hairs at isocentre and re-adjust with the small mirror (better than 1/2 mm). If OK then check at the floor (Max 2mm). Check if cross hairs are parallel to blocks (remove trimmers)
• Ensure if blocks are calibrated. (The easiest and fastest way of block calibration can be done in a water phantom). Re-position trimmers to match field size.
• Check magnification factor. Lower table position for example 20cm. Light field dimensions must be 20% larger.