The Wireless Inclinometer
The Inclinometer generates real-time information about the current beam angle. With the angle information, the Control Unit is able to rotate the Rotation Unit so that beam incidence is always perpendicular to the 2D Array. The Inclinometer transmits the signal (angle increments, not the absolute angle) via Bluetooth to a small receiver box, which is plugged into the back side of the Control Unit.
The Inclinometer has to be attached to the gantry, ideally at a place close to the rotation axis. Velcro-like fasteners ("DualLock" from 3M) are used for this. One side remains on the device, the other side is glued to the gantry.
It is not necessary that the Inclinometer is perfectly levelled when it is fastened to the gantry. This will not generate an angl error. The zero angle is established by performing a reference run (see below).
In the following picture, the right side is already "armed":
The question where to put the inclinometer is a matter of taste. It depends on how you regard your treatment unit: if you want to keep your machine pristine,
and do not want to spoil the front side, it is probably better to look for a place under the cover. But this also means that you will have to remove the cover every time you set up the phantom. To avoid radiation damage, the Inclinometer should be taken off after each measurement session.
We identified two places which work well and which are not too far away from the rotation axis. We used the place below the LaserGuard for several months, and it works well:
However, removing and reattaching the front cover until it engages and locks into place can be rather cumbersome, because the quality of the TrueBeam covers is rather "suboptimal".
Another potential place under the cover is the front side of the heavy-metal shielding of the motion control nodes which control the imaging arms (to access the nodes it can be folded down, which is why we call it "the secretary"):
We did not try it out, but it should work fine.
The mounting place should not be too far away from the gantry rotation axis. Both places are within the 40 cm radius PTW recommends for placing the Inclinometer:
If you regard your TrueBeam as a tool (like we do) and short setup time is at a premium, the better place to attach the Inclinometer is directly on the cover.
Another advantage of this place is that the device can be positioned exactly on the rotation axis (see lasers):
At the beginning of the following video, one can hear that the Inclinometer is firmly secured to the gantry cover by the velcro strips: