Christian Steiding MSc, Abstract Co-Author: Employee, CT Imaging GmbH

Daniel Kolditz PhD, Presenter: Employee, CT Imaging GmbH

Felix Althoff, Abstract Co-Author: Nothing to Disclose

Willi A. Kalender PhD, Abstract Co-Author: Consultant, Siemens AG Consultant, Bayer AG Founder, CT Imaging GmbH Scientific Advisor, CT Imaging GmbH CEO, CT Imaging GmbH

Single x-ray photon counting detector (PCD) concepts may offer superior imaging performance. However, to date the manufacturing of large-area homogeneous high-Z PCDs for CT still remains a challenge. Tiled PCD arrays usually suffer from complex geometric detector alignment errors and artifacts may occur in the CT volumes if not corrected. The aim of this work was to introduce and validate a novel approach for the fully automated geometric calibration of CT systems equipped with a tiled PCD array.

To estimate the actual imaging geometry, we developed a two-step calibration approach utilizing genetic algorithms: (1) identification of errors for each single PCD tile; (2) calculation of the rotational source-detector geometry. The proposed calibration approach was verified in simulation studies; realistic alignment errors of position and orientation for both the x-ray source and the PCD tiles were used. The ideal case mimicking the specified system geometry was set as a reference. The experimental validation was carried out on a breast CT prototype equipped with a tiled cadmium telluride detector array with 100 µm pixel size. The 10% modulation transfer function (MTF) value, the slice sensitivity profile’s full width at tenth maximum (FWTMSSP), and the maximum contrast were assessed in the reconstructed volumes for centrally and peripherally positioned small wires and thin disks, respectively. Results were compared for non-calibrated, calibrated, and ideal geometry data.

Measurements on the prototype revealed the following: With the use of the non-calibrated geometry data, maximum contrast dropped by 65-80%, MTF was no longer evaluable, and FWTMSSP fell by about 50%. The maximum error in misalignment estimation was 0.02 mm for the proposed calibration approach; i.e., accuracy was equal to a fifth of the detector pixel. 3D resolution and contrast was above 95% of the values obtained for the assumed ideal geometry for all phantom positions.

The proposed calibration approach provided accurate and fully automated geometry estimation for a tiled PCD array and is applicable for arbitrary CT systems and scan trajectories.

The proposed approach ensures undisturbed image quality and thereby allows for applying tiled PCD concepts in clinical CT.

Steiding, C, Kolditz, D, Althoff, F, Kalender, W, Fully Automated Geometric Calibration of a Tiled Directly-converting Single X-ray Photon Counting Detector Array for CT.  Radiological Society of North America 2014 Scientific Assembly and Annual Meeting, - ,Chicago IL. http://archive.rsna.org/2014/14013411.html