Abstract:
HTML
Purpose: We are performing simulation studies to investigate the extent to which a layer of microlenses sandwiched between a phosphor screen and a photodetector improves the image quality of indirect-detection x-ray imagers. In this study, we examined the effects of the microlens layer on the Swank information factor.
Methods and Materials: We have developed a simulation package for calculating light collection for indirect-detection x-ray imagers based on models for the screen, microlens array and photodetector. The screen model includes the angular-dependent radiance, modulation transfer function, emission spectrum, and light photon count distribution of the phosphor. The microlens model tracks photons through conical lenses; while the photodetector model maps the light sensitive regions in the detector. In this study, a digital x-ray imager combining a MinR phosphor screen, a microlens layer, and a 127-μm pixel pitch photodetector (optical fill factor of 57%) was modeled. The imager had a single microlens matched to each photodetector pixel. Monte Carlo simulations using DETECT-II of a 19 keV monoenergetic x-ray beam oriented perpendicular to the imager plane and located either in the center of a pixel (center) or at the corner between 4 adjacent pixels (corner) were conducted. The number of collected light photons for each interacting x-ray were tracked and used to calculate the Swank factors, As(center) and As(corner).
Results: The maximum achievable As for 19 keV x-rays and a MinR screen was calculated to be 0.722. The Swank factors fell to As(center)=0.642 and As(corner)=0.641 for an imager with the screen in direct contact with the photodetector (no microlens layer) taking into account the 57% fill factor of the photodetector. Values of As(center)=0.644 and As(corner)=0.650 were found with the microlens focusing layer included. This may correspond to a small increase over the configuration without the microlens layer. In addition, we observed a 24.4% and 46.4% increase in collected light using a microlens layer for the center and corner locations, respectively.
Conclusion: This study shows that improvement in light collection due to the microlens array comes without degradation of the information content in terms of the Swank factor. This indicates that the microlens layer is operating as a light amplifier at this x-ray energy and for this screen model. We will report results for higher and lower x-ray energies, and for different screen models.
Petrick PhD, N,
Digital Indirect Detection X-ray Imaging with a Prototype Microlens Focusing Layer between the Screen and Photodetector: Effects on Swank Information Factor. Radiological Society of North America 2003 Scientific Assembly and Annual Meeting, November 30 - December 5, 2003 ,Chicago IL.
http://archive.rsna.org/2003/3106847.html
