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Increase diagnostic confidence with accurate and consistent HU measurements

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 the evaluated accuracy of virtual monoenergetic images for different DECT technologies. IQon Spectral CT demonstrated high accuracy for soft tissue-like materials over the complete energy range, with the smallest Root Mean Square Deviation across the different evaluated DECT technologies.

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.

Measurement of HU’s is part of the routine clinical diagnostic process when using CT imaging. Accuracy levels of these measurements can influence the quality of pathology interpretation, such as management of incidental finding in adrenal glands, that are subjected to guidelines based on HU thresholds.

Unlike conventional CT imaging, where HU values are prone to inherent inaccuracies caused by beam hardening effects from attenuating materials, different kVp settings, or patient size variations, HU values derived from spectral virtual mono-energetic images exhibit increased consistency between patients, leading to better fidelity of measurements. The use of different virtual mono-energetic levels can be utilized for various clinical applications such as enhanced iodine conspicuity at low kilo-electron volt (keV) values or reduced metal artifacts at high keV values.

With the Philips IQon Spectral CT scanner, virtual monoenergetic images ranging from 40keV to 200 keV are available prospectively and retrospectively, alongside other spectral results and a true conventional reconstruction from a single acquisition. These virtual monoenergetic images are created from perfectly matched low and high energy CT sonograms which undergo projection-based material decomposition. In the study, a semi-anthropomorphic abdomen phantom equipped with inserts simulating different tissues was scanned on three different DECT systems with different fat-simulating extension rings to mimic small, medium and large patient sizes, at three different dose levels of 10 mGy, 20 mGy and 30 mGy. The two-cm diameter inserts were comprised of materials mimicking water, adipose, muscle, liver and bone (hydroxyapatite concentration 200 mg/sm³). For each of the technologies, circular ROIs were placed on the different tissue-mimicking materials on virtual monoenergetic images from 40 keV to 140 keV, in steps of 10 keV. Deviation between the observed and theoretical attenuation values of each insert was quantified by an RMS (Root-Mean Square) calculation over the entire evaluated energy range.

For all patient sizes and dose levels, the Philips IQon Spectral CT demonstrated accurate HU values for soft tissue-like materials over the complete energy range. For the adipose-like material, the quantified RMS deviations of IQon Spectral CT were between 1.7 HU and 5.0 HU. For Rapid-kVp-Switching and Dual-Source RMS deviations ranged between 4.7 HU to 10.1 HU and 2.9 HU to 4.4 HU respectively. For the liver-like material, the quantified RMS deviations of IQon Spectral CT were between 2.3 HU and 5.3 HU. For Rapid-kVp-Switching and Dual-Source RMS deviations were ranged between 6.0 HU to 17.1 HU and 2.2 HU to 8.3 HU respectively.

The Philips IQon Spectral CT provides high quality virtual monoenergetic images and demonstrates accurate HUs across energy values, patient sizes and radiation dose levels, allowing the possibility of stable material quantification. Across the various DECT technologies evaluated, the Philips IQon Spectral CT demonstrated smallest average deviation of virtual monoenergetic HUs for soft tissue-like materials.

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

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Oct 25, 2018

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IQon Spectral CT
abdomen, Body, dose, liver, MonoE, Musculoskeletal, phantom, prospective, retrospective, spectral CT

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