Elsevier

Academic Radiology

Volume 21, Issue 4, April 2014, Pages 538-545
Academic Radiology

Laboratory Investigation
A Free-response Evaluation Determining Value in the Computed Tomography Attenuation Correction Image for Revealing Pulmonary Incidental Findings: A Phantom Study

https://doi.org/10.1016/j.acra.2014.01.003Get rights and content

Rationale and Objectives

The purpose of this study was to compare lesion-detection performance when interpreting computed tomography (CT) images that are acquired for attenuation correction when performing single photon emission computed tomography/computed tomography (SPECT/CT) myocardial perfusion studies. In the United Kingdom, there is a requirement that these images be interpreted; thus, it is necessary to understand observer performance on these images.

Materials and Methods

An anthropomorphic chest phantom with inserted spherical lesions of different sizes and contrasts was scanned on five different SPECT/CT systems using site-specific CT protocols for SPECT/CT myocardial perfusion imaging. Twenty-one observers (0–4 years of CT experience) searched 26 image slices (17 abnormal, containing 1–3 lesions, and 9 normal, containing no lesions) for each CT acquisition. The observers marked and rated perceived lesions under the free-response paradigm. Four analyses were conducted using jackknife alternative free-response receiver operating characteristic (JAFROC) analysis: (1) 20-pixel acceptance radius (AR) with all 21 readers, abbreviated to 20/ALL analysis, (2) 40-pixel AR with 21 readers (40/ALL), (3) 20-pixel AR with 14 readers experienced in CT (20/EXP), and (4) 20-pixel AR with 7 readers with no CT experience (20/NOT). The significance level of the test was set so as to conservatively control the overall probability of a type I error to <0.05.

Results

The mean JAFROC figure of merit (FOM) for the five CT acquisitions for the 20/ALL study were 0.602, 0.639, 0.372, 0.475, and 0.719 with a significant difference in lesion-detection performance evident between all individual treatment pairs (P < .0001) with the exception of the 1-2 pairing, which was not significant (these differed only in milliamp seconds). System 5, which had the highest performance, had the smallest slice thickness and the largest matrix size. For the other analyses, the system orderings remained unchanged, and the significance of FOM difference findings remained identical to those for 20/ALL, with one exception: for 20/EXP analysis the 1-2 difference became significant with the higher milliamp seconds superior. Improved detection performance was associated with a smaller slice thickness, increased matrix size, and, to a lesser extent, increased tube charge.

Conclusions

Protocol variations for CT-based attenuation correction (AC) in SPECT/CT imaging have a measurable impact on lesion-detection performance. The results imply that z-axis resolution and matrix size had the greatest impact on lesion detection, with a weaker but detectable dependence on the product of milliamp and seconds.

Section snippets

Image Acquisition

Because it would not be desirable from ethical and practical considerations to image enough patients in all five modalities to generate sufficient numbers of normal and abnormal cases for the observer study, a phantom study was indicated. Phantom simulation allows the production of reliable system-matched images without concerns over radiation dose.

Spherical simulated lesions with diameters 3, 5, 8, 10, and 12 mm and densities −800, −630, and +100 Hounsfield units (HU), for a total of 15

Results

Table 2 summarizes the results of the four analyses conducted (for AR = 20, 40, CT experienced, and no CT experience): it lists the F-statistic, and in parenthesis the numerator and denominator degrees of freedom, the P-value, the average number of NL marks per normal slice, the corresponding number per abnormal slice, and the average number of LL marks per abnormal slice. For 20-pixel AR and all 21 readers, Figure 2a displays the JAFROC FOMs and 95% confidence intervals for the five SPECT/CT

Discussion

This study evaluated lesion detectability in the low-resolution CT images acquired for AC as part of the SPECT/CT MPI technique. The diagnostic value of these images has been in question, but the work of Goetze et al. (14) has suggested that there is value in reporting interpretations from these images. Legislative pressures in the United Kingdom also require a formal record of each exposure to be created.

The statistically significant differences observed in this study, which were especially

Conclusions

Protocol variations in operation for CT-based AC have a significant impact on lesion-detection performance. The results imply that z-axis resolution and matrix size had the greatest impact on lesion detection, with a weaker but detectable dependence on the product of milliamp and seconds.

Acknowledgment

The authors would like to acknowledge the extensive work and development of the DLL module used in the analysis of variance at the University of Iowa by Professor Kevin Berbaum, Dr Stephen Hillis, and Dr Kevin Schartz 18, 19, 20, 21, 22, 23, 24.

We thank an anonymous referee for making several constructive suggestions on the analysis.

The authors would like to thank the University of Cumbria for the kind loan of the LUNGMAN N1 Multipurpose Chest Phantom.

Preliminary results of this study were

References (30)

  • A.K. Buck et al.

    SPECT/CT

    J Nucl Med

    (2008)
  • B. Bybel et al.

    SPECT/CT imaging: clinical utility of an emerging technology

    Radiographics

    (2008)
  • J.A. Patton et al.

    SPECT/CT physical principles and attenuation correction

    J Nucl Med Technol

    (2008)
  • K. Willowson et al.

    Quantitative SPECT reconstruction using CT-derived corrections

    Phys Med Biol

    (2008)
  • S. Goetze et al.

    Attenuation correction in myocardial perfusion SPECT/CT: effects of misregistration and value of reregistration

    J Nucl Med

    (2007)
  • Cited by (0)

    Financial Disclosures: D.P.C. was supported in part by grants from the Department of Health and Human Services, National Institutes of Health, R01-EB005243 and R01-EB008688.

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