Modeling granular phosphor screens by Monte Carlo methods

• Published in: Medical physics (Lancaster) 2006-12, Vol.33 (12), p.4502-4514
• Main Authors: Liaparinos, Panagiotis F., Kandarakis, Ioannis S., Cavouras, Dionisis A., Delis, Harry B., Panayiotakis, George S.
• Format: Article
• Language: English
• Subjects: ABSORPTION; biomedical equipment; Diagnostic Imaging - methods; diagnostic radiography; Distribution theory and Monte Carlo studies; Equipment Design; fluorescent screens; Gadolinium - chemistry; gadolinium compounds; GRAIN SIZE; granular materials; Image quality; image resolution; Light; light absorption; LIGHT SCATTERING; Mie scattering; modeling; Models, Statistical; Models, Theoretical; Modulation transfer functions; Monte Carlo; MONTE CARLO METHOD; Monte Carlo methods; MTF; Non‐ionizing radiation equipment and techniques; Optical properties; optical transfer function; phosphor screens; phosphors; Phosphorus - chemistry; PHOTONS; probability; Radiographic Image Enhancement; Radiography; RADIOLOGY AND NUCLEAR MEDICINE; REFRACTIVE INDEX; Scattering, Radiation; SIMULATION; SPATIAL DISTRIBUTION; terbium; Terbium - chemistry; TRANSFER FUNCTIONS; X RADIATION; x-ray imaging; X-Ray Intensifying Screens; X-Rays; X‐ray optics; X‐ray scattering
• ISSN: 0094-2405; 2473-4209
• DOI: 10.1118/1.2372217

The intrinsic phosphor properties are of significant importance for the performance of phosphor screens used in medical imaging systems. In previous analytical-theoretical and Monte Carlo studies on granular phosphor materials, values of optical properties, and light interaction cross sections were found by fitting to experimental data. These values were then employed for the assessment of phosphor screen imaging performance. However, it was found that, depending on the experimental technique and fitting methodology, the optical parameters of a specific phosphor material varied within a wide range of values, i.e., variations of light scattering with respect to light absorption coefficients were often observed for the same phosphor material. In this study, x-ray and light transport within granular phosphor materials was studied by developing a computational model using Monte Carlo methods. The model was based on the intrinsic physical characteristics of the phosphor. Input values required to feed the model can be easily obtained from tabulated data. The complex refractive index was introduced and microscopic probabilities for light interactions were produced, using Mie scattering theory. Model validation was carried out by comparing model results on x-ray and light parameters (x-ray absorption, statistical fluctuations in the x-ray to light conversion process, number of emitted light photons, output light spatial distribution) with previous published experimental data on Gd 2 O 2 S : Tb phosphor material (Kodak Min-R screen). Results showed the dependence of the modulation transfer function (MTF) on phosphor grain size and material packing density. It was predicted that granular Gd 2 O 2 S : Tb screens of high packing density and small grain size may exhibit considerably better resolution and light emission properties than the conventional Gd 2 O 2 S : Tb screens, under similar conditions (x-ray incident energy, screen thickness).