A comparison of modulation transfer function of indirect digital radiography systems

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Banjong Kheonkaew
Kridsanapan Srimongkon
Thawatchai Prabsattroo
Wichai Witchathorntragul


Background: There are several parameters to characterize the quality of digital image. Resolution is one of the main parameters of an image quality. Modulation transfer function (MTF) is a quantitative measurement describes image resolution properties of an imaging system as a function of the spatial frequency. Several reports compared the spatial resolution between direct and indirect digital radiography (DR) systems proved that direct DR systems had better spatial resolution. Moreover, they also compared the different phosphor detectors of indirect DRs. However, to our knowledge, there is no report that compares the same gadolinium oxysulfide (GOS) phosphor detectors of indirect DR from different DR system manufacturers.

Objectives: To compare MTF of flat panel detectors (FPDs) indirect conversion using GOS phosphor with fixed focal spot size under radiation beam condition according to International Electrotechnical Commission (IEC) RQA5 standard.

Materials and methods: Three indirect FPDs from 3 DR manufacturers i.e. detector A, detector B and detector C were used in this study. Measurement tests for spatial resolution evaluating were performed by means of a set of 30 groups of bar patterns with different spatial frequencies which vary increasing order and express as line pairs per unit distance (lp/mm). MTF can be performed from a bar pattern within the image file by elaboration software AutoPia (Auto Phantom image analysis). Frequency at the 0.1 point of MTF was applied with this limiting spatial resolution.

Results: Three FPDs had similar MTF shape. All of MTF values from detectors were decreased with increasing spatial frequency from detector A, B, and C, respectively. This can be sorted in descending order as follows. MTF showed that detector A demonstrated both the highest contrast resolution and spatial resolution. Nevertheless, detector B and C had the same contrast resolution, yet the spatial resolution of detector B was better than that of detector C. Spatial frequency reflects the limiting spatial resolution (MTF=0.1) of detector A, B, and C, at 4.40, 4.02, and 3.77 lp/mm, respectively.

Conclusion: The bar pattern method with an automatic software analysis can be simply obtained MTF result. Test of MTF in beam quality as recommended in IEC RQA5 standard of three FPDs show that spatial resolution sorted in descending order were through detector A, B and C, respectively.


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Kheonkaew, B., Srimongkon, K., Prabsattroo, T., & Witchathorntragul, W. (2020). A comparison of modulation transfer function of indirect digital radiography systems. Journal of Associated Medical Sciences, 53(3), 1–7. Retrieved from https://he01.tci-thaijo.org/index.php/bulletinAMS/article/view/241259
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