Measurement of Wiener Spectrum of Radiographic Screen–Film Systems

Abstract

Satu kaedah untuk mengukur spektrum Wiener sistem skrin-filem radiografi dalam persekitaran hospital disajikan. Radiograf yang didedahkan kepada sinar–X seragam, diimbas oleh sebuah mikrodensitometer, dan data turun naik ketumpatan optik di sekitar nilai ketumpatan min diperoleh dengan menolak nilai ketumpatan min daripada nilai ketumpatan optik. Data dituras-laluan-rendahkan dengan mempuratakan pasangan piksel, diikuti dengan penurasan frekuensi rendah untuk menyingkirkan komponen frekuensi yang sangat rendah. Suatu surihan celah disintesis dengan mempuratakan surihan-surihan bersebelahan, dan surihan celah itu dibahagikan kepada segmen-segmen 256 data yang bertindih 128 datanya. Data dalam setiap segmen ditingkapkan dan transformasi–Fourier–pantaskan. Pekali–pekali Fourier dikuasa dua dan dinormalkan untuk menghasilkan spektrum kuasa. Pengukuran di lapangan terhadap skrin Lanex Regular dan Lanex Fine memberikan keputusan yang hampir sama dengan hasil yang telah diterbitkan. Kata kunci: Spektrum kuasa; radiografi; sistem skrin-filem; spektrum Wiener A method to measure the wiener spectrum of radiagraphic screen–film systems in a hospital setting is presented. Radiographs exposed to uniform X–rays are scanned by a microdensitometer, and optical density fluctuation data about the mean density are obtained by subtracting the mean density from the density values. The data are low–pass–filtered by averaging pairs of pixels, followed by low–frequency filtering to eliminate very low–frequency components. A slit trace is synthesised by averaging adjacent traces and the trace is segmented to segments of 256 data points per segment with overlap of 128 data points. Data in each segment are windowed and fast Fourier transformed. The Fourier coefficients are squared and normalized to obtain the power spectrum. On site measurement for Lanex Regular and Lanex Fine screens gives results that are comparable with published data. Key words: Noise power spectrum; radiography; screen-film system; Wiener spectrum