Slide 9 of 15
Notes:
Gantry vibration can result in movement of the focal spot relative to the detectors, in the z-direction, during rotation.
If the beam width was only large enough to cover the active detectors, then any movement of the focal spot relative to the detectors (in z) would mean that PARTS of the active detector width would NOT be irradiated throughout ALL of the rotation.
GE’s original solution to this problem was to irradiate widths, which were up to 7mm larger than the active detector width (diagram on left). Obviously, this was not good for dose efficiency.
GE’s new solution is to use what they call ‘focal spot tracking’ (diagram on right). That is, they move the x-ray beam collimation WITH the focal spot, thereby restricting the beam to fall within the active detector width throughout the rotation. Thus, uniform coverage is achieved for reduced irradiated slice widths.
