Prog Med Phys.  2016 Jun;27(2):55-63. 10.14316/pmp.2016.27.2.55.

Modulation Transfer Function with Aluminum Sheets of Varying Thickness

• Rhee DJ
• Kim MY
• Moon YM
• Jeong DH

• KMID: 2328189
• DOI: http://doi.org/10.14316/pmp.2016.27.2.55

Abstract

We studied the method to gain a clear LSF using a thick aluminum sheet and to acquire the spatial resolution value with a high accuracy for a low spatial resolution imaging modality. In this study, aluminum sheets with thicknesses varying from 0.3 mm to 1.2 mm were tested to derive a modulation transfer function (MTF) for the oversampling and non-oversampling methods. The results were evaluated to verify the feasibility of the use of thick sheets for periodic quality assurance. Oversampling was more accurate than non-oversampling, and an aluminum sheet with a correction factor less than 2 at the cut-off frequency, which was less than 0.8 mm in this case, was confirmed to be suitable for MTF measurements. Therefore, MTF derivation from a thick aluminum sheet with thickness correction is plausible for a medical imaging modality.

Keyword

Modulation transfer functions; Image spatial resolution; Oversampling method

MeSH Terms

Aluminum*
Diagnostic Imaging
Methods
Aluminum

Figure

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  Fig. 1. Aluminum sheet images in the transverse plane for parallel non-oversampling LSF (left) and tilted oversampling LSF (right). The LSF was determined by the sum of all the vertical projections across the plane within the regions of interest (red boxes). Phase correction was applied for the oversampled LSF.

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  Fig. 2. Correction factors as a function of spatial frequency with different slab thicknesses. As the sheet becomes thicker and the spatial frequency becomes higher, the correction factor increases.

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  Fig. 3. 2D PSF reconstructed from LSF was convolved with the object to compute the constructed image. The constructed image was compared with the real CT image of the object by subtraction. The region of interest (ROI) was determined as the size of the object, with background as the remained of the image. The standard deviation of the subtracted images were evaluated both separately and in combination.

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  Fig. 4. Normalized standard deviation of the region of interest (ROI) in the subtracted image. Parallel 1 (triangle) and Tilt 1 (circle) are compared in (a), Parallel 2 (inverted triangle) and Tilt 2 (square) are compared in (b), Parallel 1 and 2 are compared in (c), and Tilt 1 and 2 are compared in (d).

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  Fig. 5. Normalized standard deviation of the background (BKG) region in the subtracted image.

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  Fig. 6. Normalized standard deviation of the subtracted image including both ROI and BKG.

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  Fig. 7. MTFs as a function of sheet thickness. MTF 50 (a), MTF 10 (b), and MTF 5 (c) for Tilt 1, Tilt 2, Parallel 1, and Parallel 2 cases are presented.

Reference

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