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Increase diagnostic confidence for abdominal CT imaging

Abstract
Philips CT Clinical Science Philips Healthcare • USA

Purpose of study

This study evaluated the imaging performance of various dual energy CT technologies: Spectral detector, dual source and rapid kVp switching. Researchers from the Technical University of Munich, University Hospital of Cologne and Massachusetts General Hospital measured the accuracy of iodine quantification, accuracy of Hounsfield units (HU) of virtual monoenergetic images and image noise while varying patient size and radiation dose. This summary focuses on noise of virtual monoenergetic images across a range of energies generated by the different DECT technologies. The Philips IQon Spectral CT demonstrated the lowest levels of noise in virtual mono energetic images across the evaluated systems. The low noise, together with the high image contrast at low energy levels, may improve diagnostic information and confidence in abdominal CT.

The following is a summary of the study, “Dual energy CT: A phantom comparison of different platforms for abdominal imaging,” published in European Radiology, February 2018.

Overview

The detection, delineation and characterization of iodine enhancing pathologies such as lesions, hemorrhages and endo-leaks pose a diagnostic challenge in routine CT imaging. Virtual monoenergetic images, at low kilo-electron volt (keV) levels, have the potential to increase iodine conspicuity. Images generated at high keV levels are less prone to beam hardening artifacts and have the potential to reduce metal artifacts. However, while enhancing the signal at a low KeV, the noise has to be maintained at low values in order to achieve an overall improvement of image quality as expressed by an improved contrast-to-noise ratio.

With the Philips IQon Spectral CT scanner, virtual monoenergetic images ranging from 40 keV to 200 keV are available prospectively and retrospectively, alongside other spectral results and a true conventional reconstruction from a single acquisition. All virtual monoenergetic images created through material decomposition process amplifies noise. However, the noise generated by detector-based technologies has the special property of being anti-correlated between the two components of the 2-base model, i.e. when the noise is positive in one component, it is negative in the other. Since the design of the IQon Spectral CT scanner enables the utilization of spectral data sets that are acquired simultaneously and are perfectly aligned in time and space, IQon's spectral reconstruction includes projection-based material decomposition followed by a dedicated spectral de-nosing algorithm which properly accounts for and removes this anti-correlated noise.

In this study, a semi-anthropomorphic abdomen phantom equipped with inserts mimicking different tissues was scanned on various DECT systems with fat simulating extension rings to mimic, small medium and large patient sizes and dose levels of 10 mGy, 20 mGy, and 30 mGy. The 2 cm diameter inserts were comprised of materials mimicking water, adipose, muscle, liver and bone (hydroxyapatite concentration 200 mGy/cm3). For each of the scans, circular ROIs were placed on the different tissue-mimicking materials on virtual monoE images from 40 keV to 140 keV, in steps of 10 keV. The maximal energy value of 140 keV, upper limit for rapid kVp switching was selected as the end point.

The noise for each material, at each energy level, for each phantom size and dose level was determined as the standard deviation of HU over all voxels within the ROI.

Results

The Philips IQon Spectral CT demonstrated near steady noise levels across all energy levels, far all patient sizes and dose levels. Across patient sizes, the evaluated average noise increase for virtual monoenergetic images from 140 keV to 40 keV was approximately 14% for IQon Spectral CT, 363% for rapid kVp switching and 265% for dual source, as measured on 20 mGy scans (see images below).

Conclusion

The Philips IQon Spectral CT demonstrated consistently low noise levels across all patient sizes, radiation dose levels and material types for the entire energy range of virtual monoE images. Both the rapid kVp switching and dual source-derived virtual monoE images suffer from substantial noise increase, particularly at the lower energy range.

In abdominal diagnostics, the low virtual monoenergetic results provide improved tissue contrast which, in combination with lower noise, improves the image quality through better contrast-to-noise ratio. This is especially important in oncological imaging, where detectability of lesions and diagnostic confidence are crucial.


Results from case studies are not predictive of results in other cases. Results in other cases may vary


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Apr 9, 2018

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Abstract
IntelliSpace Portal, IQon Spectral CT
abdomen, artifacts, Body, Dual Energy, image quality, liver, metal artifact reduction, MonoE, phantom, prospective, retrospective, spectral CT
 

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