The grating interferometer, developed over CIA-CT project by NBI, was tested with respect to stabilities. As the grating was stepped with submicron precision, stability was a key issue. It turned out that the instabilities mainly were coming from temperature variations. After careful stabilization of the room temperature, the fluctuations were much reduced.
The task of correcting instabilities in the grating interferometer setup at the NBI was solved using two different approaches. The first approach was the straight forward work to minimize the mechanical instabilities of the setup. The major contribution to the mechanical instability was due to air conditioning effects. A new control unit greatly minimized the mechanical instability reflecting in significantly reduced time variations in measurements. The second approach was a computational data analysis approach. Rather than assuming perfectly constant mechanical circumstances, which will in reality never be the case, a new algorithm for grating interferometer data analysis was developed.
Yxlon established a test unit for measuring the long term stability of the beam using dedicated software and several kinds of external detectors. The stability was measured for CT scanners available at Novo Nordisk and DTI. Some variations were identified and analyzed by NBI with the purpose of software compensation. No systematic variations could be identified, and a numerical elimination or compensation for the variations was therefore not possible. In Figure 1, the Novo Nordisk CT scanner and the variations in dose are shown as an example.
Figure 1: Stability test. Position of detector in scanner (left) and typical results (right).
Since commercial CT-scanners take variations into account by repeating the measurements and average over time and position on detector, there is no clear influence on the quality of measurements carried out on the tested scanners. Besides the test unit, Yxlon has during the entire project worked on an improved power supply for the system. The power supply is more stable and is expected to be a part of future products from Yxlon.