A practical method to measure the MTF of CT scanners

Medical Physics (Impact Factor: 3.01). 01/1982; 9(5):758-60. DOI: 10.1118/1.595124
Source: PubMed

ABSTRACT A method has been devised which enables the modulation transfer function (MTF) of computerized tomographic (CT) scanners to be simply and rapidly determined. The method relies upon the measurement of the standard deviation of the pixel values within the image of cyclic bar patterns. The method is shown to produce results which are indistinguishable from those derived by less practical conventional methods.

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    • "), a line spread function (LSF) (Boone 2001), a point spread function (PSF) (Nickoloff and Riley 1985) or methods like the ones proposed by Droege (Droege and Morin 1982), Friedman (Friedman et al 2013) and Nakaya (Nakaya et al 2012). Extensive work has already been done to compare and choose the metric (either ESF or LSF or even PSF) the most adapted to our experimental conditions when computing the MTF (Miéville et al 2010, Samei et al 2006). "
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    ABSTRACT: The state of the art to describe image quality in medical imaging is to assess the performance of an observer conducting a task of clinical interest. This can be done by using a model observer leading to a figure of merit such as the signal-to-noise ratio (SNR). Using the non-prewhitening (NPW) model observer, we objectively characterised the evolution of its figure of merit in various acquisition conditions. The NPW model observer usually requires the use of the modulation transfer function (MTF) as well as noise power spectra. However, although the computation of the MTF poses no problem when dealing with the traditional filtered back-projection (FBP) algorithm, this is not the case when using iterative reconstruction (IR) algorithms, such as adaptive statistical iterative reconstruction (ASIR) or model-based iterative reconstruction (MBIR). Given that the target transfer function (TTF) had already shown it could accurately express the system resolution even with non-linear algorithms, we decided to tune the NPW model observer, replacing the standard MTF by the TTF. It was estimated using a custom-made phantom containing cylindrical inserts surrounded by water. The contrast differences between the inserts and water were plotted for each acquisition condition. Then, mathematical transformations were performed leading to the TTF. As expected, the first results showed a dependency of the image contrast and noise levels on the TTF for both ASIR and MBIR. Moreover, FBP also proved to be dependent of the contrast and noise when using the lung kernel. Those results were then introduced in the NPW model observer. We observed an enhancement of SNR every time we switched from FBP to ASIR to MBIR. IR algorithms greatly improve image quality, especially in low-dose conditions. Based on our results, the use of MBIR could lead to further dose reduction in several clinical applications.
    Physics in Medicine and Biology 07/2014; 59(15):4047-4064. DOI:10.1088/0031-9155/59/4/4047 · 2.92 Impact Factor
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    • "where the full details of the series are given in Coltman (1954). In practice, the later terms rapidly become insignificant (Droege and Morin 1982). A third approach is based on the observation that the MTF is equivalent to the Fourier transform of the 1D point-spread function (or line-spread function, LSF(x)). "
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    ABSTRACT: Optical computed tomography (CT), in conjunction with radiochromic gels and plastics, shows great potential for radiation therapy dose verification in 3D. However, an effective quality assurance (QA) regime for the various scanners currently available still remains to be developed. We show how the favourable properties of the PRESAGE® radiochromic polymer may be exploited to create highly sophisticated QA phantoms. Five 60 mm diameter cylindrical PRESAGE® samples were irradiated using the x-ray microbeam radiation therapy facility on the ID-17 biomedical beamline at the European Synchrotron Radiation Facility. Samples were then imaged on the University of Surrey parallel-beam optical CT scanner. The sample irradiations were designed to allow a variety of tests to be performed, including assessments of linearity, modulation transfer function (three independent measurements), geometric distortion and the effect of treatment fractionation. It is clear that, although the synchrotron method produces extremely high-quality test objects, it is not practical on a routine basis, because of its reliance on a highly specialized radiation source. Hence, we investigated a second possibility: three PRESAGE® samples were illuminated with ultraviolet light of wavelength 365 nm, using cheap masks created by laser-printing patterns onto overhead projector acetate sheets. There was good correlation between optical densities measured by the CT scanner and the expected UV 'dose' delivered. The results are encouraging and a proposal is made for a scanner test regime based on calibrated and well-characterized PRESAGE® samples.
    Physics in Medicine and Biology 06/2011; 56(14):4177-99. DOI:10.1088/0031-9155/56/14/001 · 2.92 Impact Factor
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    • "Further study is needed to validate our work by determining the MTF using radiation qualities of the standard MTF measurement and comparing it with published MTF results. Droege and Morin [7] have suggested a method to determine the MTF of CT scanners that relies upon the measurement of the standard deviation of the pixel values within the image of cyclic bar patterns. This method has not been extented to conventional screen-film radiography due to the fact that relatively large and many different areas of film have to be digitized by the microdensitometer before Fig. 2 – A close up of Figure 1 showing the MTF curves at low spatial frequencies fluctuate with the number of terms used in the calculation. "
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    ABSTRACT: Two ways to calculate the modulation transfer function (MTF) of radiographic screen-film systems from the measured square wave response function (SWRF) data were investigated with an interactive curve fitting software. The measured SWRF data obtained by digitising a radiographic image of a bar pattern test object were fitted to a curve, and the fitted curve was used to calcul ate the MTF. Satisfactory MTF was obtained by using 12 terms in the calculation. A second version of the calculation included a correction for the normalization at 0.25 cycles/mm of the SWRF data. Measurements from a screen-film combination showed that the MTF of the first version was higher than the second by an average amount of 0.02 units for spatial range 0-3.5 cycles/mm, and on average the MTF of the first version was higher than the second by 10%. Both the SWRF data fitting and the MTF calculati on were done within an interactive curve fitting software which made the calculation relatively easy to perform.
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