Volker Heer

Comparison of reversible methods for data compression

Widely differing methods for data compression described in the ACR-NEMA draft are used in medical imaging. In our contribution we will review various methods briefly and discuss the relevant advantages and disadvantages. In detail we evaluate 1st order DPCM pyramid transformation and S transformation. We compare as coding algorithms both fixed and adaptive Huffman coding and Lempel-Ziv coding. Our comparison is performed on typical medical images from CT MR DSA and DLR (Digital Luminescence Radiography). Apart from the achieved compression factors we take into account CPU time required and main memory requirement both for compression and for decompression. For a realistic comparison we have implemented the mentioned algorithms in the C program language on a MicroVAX II and a SPARC station 1. 2.© (1990) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Image quality of a large-area dynamic flat detector: comparison with a state-of-the-art II/TV system

The purpose of this research is to establish the technical and clinical image quality of a 30 X 40 cm2 dynamic flat detector (FD) compared to state-of-the-art IITV technology. A Trixell detector for vascular and RF applications is designed for a mixed use of fluoroscopy as well as exposure series and a range of radiographic applications. An RF system has been built which comprises both the FD as well as an IITV detector. This system enables a direct comparison of technical image quality measurements and patient images under exactly the same X-ray conditions. Image quality measurements comprise Detective Quantum Efficiency including transfer characteristics, Modulation Transfer Function, Noise Power Spectrum, lag, Low Frequency Drop and residual signals. Observation tests, using Threshold Contrast Detail Detectability (TCDD) techniques, are performed in order to confirm the results of the technical measurements. Results show a DQE (f) of the flat detector that is higher compared to IITV and above all constant over a wide dose range, the IITV DQE (f) drops at higher dose range due to fixed structure. Furthermore the Low Frequency Drop is substantially smaller in the FD-based system. The TCDD subjective tests show improved system performance in favor of the FD system.

Photodiode gain calibration of flat dynamic x-ray detectors using reset light

Due to spatial gain differences of the photo diodes and inhomogeneities in the converter (CsI) a gain calibration is usually applied for flat dynamic X-ray detectors. This calibration is calculated from X-ray images. Using the reset light, integrated in the detector, a calibration of the photo diode gain is possible. Since neither the reset light intensity nor the X-ray field distribution in combination with the converter efficiency are spatially homogeneous the ratio of these two effects has to be measured and stored once in an X-ray reset-light map. In a reset light calibration the photo diode gain will be estimated and the final calibration is then calculated from this gain image and the stored X-ray reset-light map. The reset light gain image contains the same information as the X-ray image except the influence of the scintillator which should be very stable over time. Changes in the photo diode gain can easily and automatically be corrected using the reset light calibration. Defect pixels can be determined from the reset light gain images. This method would allow a continuous calibration during the lifetime of the detector without the need for any user interaction.