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Comparison of visual grading analysis and determination of detective quantum efficiency for evaluating system performance in digital chest radiography

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Abstract

A study was conducted to compare physical and clinical system performance in digital chest radiography. Four digital X-ray modalities, two storage-phosphor based systems and two generations of a CCD-based system, were evaluated in terms of both their imaging properties (determination of presampling MTF and DQE) and clinical image quality (grading of the reproduction of anatomical details of 23 healthy volunteers using both absolute and relative visual grading analysis). One of the two storage-phosphor systems performed best in both evaluations and the first generation of the CCD-based system was rated worst; however, the other two systems were ranked differently with the two methods. The newest CCD-based system yielded a higher clinical image quality than the second storage-phosphor system, although the latter presented a DQE substantially higher than the former. The results show that clinical performance cannot be predicted from determinations of DQE alone, and that a system with lower DQE, under the quantum-saturated conditions in chest radiography, can outperform a system with higher DQE if the image processing used on the former is more effective in presenting the information in the image to the radiologist.

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Acknowledgements

The authors thank the following radiologists, technicians, engineers and physicists for their participation in the study: A. Flinck, B. Gottfridsson and U. Tylén for reading the images; L. Björneld and M. Widell for taking care of the X-ray exposures and numerous other practical matters; A. Karlsson for writing the software for the soft-copy evaluation; and M. Håkansson for characterising the grids. Many persons at IMIX ADR Oy, Fuji Photo Film and Agfa-Gevaert contributed by helping us with equipment and valuable advice.

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Authors and Affiliations

  1. Department of Radiation Physics, Göteborg University, 41345 Göteborg, Sweden

    Patrik Sund, Magnus Båth & Lars Gunnar Månsson

  2. Department of Radiology, Göteborg University, 41345 Göteborg, Sweden

    Susanne Kheddache

  3. Department of Medical Physics and Biomedical Engineering, Sahlgrenska University Hospital, 41345 Göteborg, Sweden

    Patrik Sund, Magnus Båth & Lars Gunnar Månsson

  4. Department of Radiology, Sahlgrenska University Hospital, 41345 Göteborg, Sweden

    Susanne Kheddache

Authors
  1. Patrik Sund
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  2. Magnus Båth
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  3. Susanne Kheddache
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  4. Lars Gunnar Månsson
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Corresponding author

Correspondence to Magnus Båth.

Appendix

Appendix

A simple analysis of the effect of using a grid can be performed in the following way: Assume that a signal S0 photons/mm2 of size A mm2 is overlaid on a homogeneous background of primary radiation B photons/mm2 and on an amount of scattered radiation R photons/mm2. The signal-to-noise ratio (SNR) of the signal without a grid can then be expressed as:

$${SNR_{{w/o}} = {{S_{0} A} \over {{\sqrt {{\left( {B + R} \right)}A} }}}.}$$
(7)

If the grid is characterised by a transmission of primary radiation of Tp and a transmission of scattered radiation of Ts, the SNR of the signal with the grid becomes:

$${SNR_{w} = {{T_{p} S_{0} A} \over {{\sqrt {{\left( {T_{p} B + T_{s} R} \right)}A} }}}.}$$
(8)

Combining Eqs. (7) and (8) and writing B/R=K leads to a relative increase in the SNR for the object when the grid is used by an amount:

$$ \Delta _{{SNR,rel}} = \frac{{SNR_{w} }} {{SNR_{{w/o}} }} - 1 = \frac{{T_{p} {\sqrt {K + 1} }}} {{{\sqrt {T_{p} K + T_{s} } }}} - 1. $$
(9)

The determined values of Tp and Ts were 0.705 and 0.131 for the IMIX/IMIX 2000 grids, 0.627 and 0.095 for the Agfa grid and 0.643 and 0.120 for the Fuji grid. The scattered fraction (SF) can be written as (neglecting the influence of the signal):

$${SF = {R \over {B + R}},}$$
(10)

which leads to a relationship between K and SF given by:

$$ {K = {1 \over {SF}} - 1.} $$
(11)

A value of 0.90 for the scattered fraction leads to a K value of 0.11, which, if inserted into Eq. (9), leads to values of ΔSNR,rel of 0.63 for the IMIX/IMIX 2000 and Agfa grids and 0.55 for the Fuji grid. Using SF values of 0.50 and 0.60 for the calculations and averaging the results leads to values of ΔSNR,rel of 0.13 for the IMIX/IMIX 2000 grids, 0.09 for the Agfa grid, and 0.08 for the Fuji grid.

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Sund, P., Båth, M., Kheddache, S. et al. Comparison of visual grading analysis and determination of detective quantum efficiency for evaluating system performance in digital chest radiography. Eur Radiol 14, 48–58 (2004). https://doi.org/10.1007/s00330-003-1971-z

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