Metal artifacts from sternal wires: evaluation of virtual monoenergetic images from spectral-detector CT for artifact reduction

Journal Pre-proof Metal artifacts from sternal wires: evaluation of virtual monoenergetic images from spectral-detector CT for artifact reduction

Kai Roman Laukamp, Nils Große Hokamp, Omar Alabar, Verena Carola Obmann, Simon Lennartz, David Zopfs, Robert Gilkeson, Nikhil Ramaiya, Amit Gupta PII:

S0899-7071(19)30286-4

DOI:

https://doi.org/10.1016/j.clinimag.2019.12.018

Reference:

JCT 8810

To appear in:

Clinical Imaging

Received date:

7 May 2019

Revised date:

22 December 2019

Accepted date:

27 December 2019

Please cite this article as: K.R. Laukamp, N.G. Hokamp, O. Alabar, et al., Metal artifacts from sternal wires: evaluation of virtual monoenergetic images from spectral-detector CT for artifact reduction, Clinical Imaging(2020), https://doi.org/10.1016/ j.clinimag.2019.12.018

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© 2020 Published by Elsevier.

Journal Pre-proof

Metal artifacts from sternal wires: Evaluation of virtual monoenergetic images from spectral-detector CT for artifact reduction Authors: Kai Roman Laukamp, MD1,2,3, MD; Nils Große Hokamp, MD1,2,3, MD; Omar Alabar1,2, MD; Verena Carola Obmann1,2, MD; Simon Lennartz3, MD; David Zopfs3, MD; Robert Gilkeson

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, MD; Nikhil

Ramaiya1,2, MD; Amit Gupta1,2, MD

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Affiliation: 1

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Department of Radiology, University Hospitals Cleveland Medical Center Cleveland, OH, USA

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Department of Radiology, Case Western Reserve University, Cleveland, OH, USA

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Institute for Diagnostic and Interventional Radiology, University Hospital Cologne, Cologne, Germany

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Corresponding author:

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Dr. Kai Roman Laukamp University Hospitals Cleveland Medical Center, Department of Radiology 11000 Euclid Ave Cleveland, OH 44106 telephone: +1 216 844 7519 fax: + 1 216 983 0798 [email protected] [email protected] Conflicts of interest: NGH: On the speaker’s bureau of Philips Healthcare. The other authors state that they have nothing to disclose. Funding: This research was supported in part by Philips Healthcare under a research agreement with University Hospitals Cleveland Medical Center and Case Western Reserve University. Colors should be used for Figure 3-5.

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Journal Pre-proof Abstract

Purpose: In chest imaging, sternal wires can cause metal artifacts that hamper depiction of surrounding soft tissue and bone. This study investigated if these artifacts may be reduced by means of virtual monoenergetic images (VMI) obtained from a novel detector-based spectral CT scanner (SDCT) in comparison to conventional CT images (CI).

Materials and Methods: 30 patients with clinically indicated SDCT scans of the chest exhibiting

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artifacts due to sternal wires were included in this IRB-approved study. CI and VMI (40-200keV,

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10keV increment) were reconstructed. Quantitative image analysis was conducted by ROI-based

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measurement of attenuation (HU) and standard deviation within the most pronounced hypo- and hyperdense artifacts. Visually, artifact reduction and diagnostic assessment of surrounding soft

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tissue as well as sternal bone were independently rated by two radiologists on 5-point Likert-scales.

Results: In comparison to CI, high-keV VMI showed an effective reduction of hypo- and

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hyperattenuating parasternal artifacts, as corrected HU-values approximated their true expected values (hypodense: CI -66.2±70.8; VMI200keV 2.4±29.2; hyperdense: CI 156.7±70.8HU; VMI200keV

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76.9±45.4, both p<0.05). In addition, image noise was significantly lower in high-keV VMI compared to CI. Subjective analysis confirmed that VMI of ≥100keV significantly reduced artifacts and improved diagnostic assessment of surrounding soft tissue and bone. Interrater-agreement was excellent (intraclass-correlation-coefficient=0.83).

Conclusions: High-keV VMI yielded a significant reduction of artifacts from sternal wires and improved assessment of surrounding structures.

Keywords: X-Ray Computed Tomography; Artifacts; Sternal wires; Neoplasm staging; Thoracic surgery

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Journal Pre-proof Abbreviations: CI: Conventional CT imaging/images SDCT: Spectral-detector CT VMI: Virtual monoenergetic images SD: Standard deviation

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HU: Hounsfield Unit

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Journal Pre-proof 1. Introduction

In CT imaging, metal implants, such as sternal wires used after thoracic surgery can cause hypo- and hyperdense artifacts [1–3]. There are three phenomena that cause metal artifacts: (i) beamhardening caused by absorption of low-energetic photons [3,4], (ii) photon starvation resulting from complete absorption of low-energetic photons [3,5] and (iii) scatter artifacts [1]. These different artifact types can negatively impact image assessment and diagnostic interpretability, potentially

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affecting surrounding structures, such as surrounding soft tissue and bone [1–3,5].

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Dual-energy CT enables reconstruction of virtual monoenergetic images (VMI) which can be utilized

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as an effective method for metal artifact reduction [6–8]. Separate assessment of higher and lower energetic photons allow reconstruction of VMI that aim to simulate CT images acquired with a true

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monoenergetic x-ray. Resistance to beam-hardening should be increased in VMI at higher keV-values

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having the potential to reduce artifacts caused by implanted metal material [6–11]. There are different technological approaches to dual-energy CT, of which dual-source, dual-spin, split/twin

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beam, and kVp-switching, are the clinically most applied tube-based solutions whereas the spectral-

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detector CT (SDCT) is the only detector-based, clinically available variant. The latter uses a single xray source and a detector that separately detects low-energetic photons at the upper layer and highenergetic photons at the lower layer [6,9].

This study evaluates if reduction of metal artifacts from sternal wires is possible with VMI obtained from spectral-detector CT (SDCT) as compared to conventional CT images and how this possibly improves diagnostic assessment of the surrounding tissue.

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Journal Pre-proof 2. Material and methods

This prospective study was approved by the local institutional review board. Criteria for inclusion were: (i) age: ≥18 years and (ii) examination on a clinical spectral-detector CT (IQon, Philips, Cleveland, USA) between May 2017 and August 2018 using the protocol mentioned below. Informed written consent was waived under Code of Federal Regulations (title 45, §46.116d). Investigations were carried out in accordance with the Health Insurance Portability and Accountability Act. In this

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specific analysis 30 patients, fulfilling the following additional criteria were included: (i) contrast

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enhanced chest CT examinations in portal venous phase acquired in supine head-first position, (ii) acquisitions with hypo- and hyperattenuating artifacts due to sternal wires after thoracic surgery,

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were included. Scan indications were solely clinical, and no scan was conducted explicitly for the

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study purpose.

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2.1 Imaging protocol

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A body-weight adapted volume of iodinated contrast media (Optiray 350, Guerbet, Bloomington, IN, USA; 1.5ml/kg) was injected through an antecubital vein or port system. 70 seconds after injection,

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image acquisition was initiated. The following scan parameters were applied: matrix 512x512, collimation 64x0.625mm, pitch 1.08, rotation time 0.33s. 120kVp was set as tube voltage and automatic tube current modulation was used (DoseRight 3D-DOM, Philips Healthcare, Best, The Netherlands).

Conventional polychromatic CT images (CI) were reconstructed with a hybrid iterative reconstruction algorithm (iDose4, level 3; Philips Healthcare, Cleveland, USA). VMI were reconstructed using a dedicated spectral reconstruction algorithm (range 40-200 keV with an increment of 10 keV; Spectral B, denoising level 3, Philips Healthcare, Cleveland, USA). Slice thickness for reconstruction were 3 mm.

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Journal Pre-proof 2.2 Objective analysis Objective image assessment was carried out using regions of interest (ROI) with a defined size of 50 mm2. To avoid unrepresentative tissue, ROI size was reduced in few cases. ROI were placed on CI and then copied to the same location on VMI. ROI were drawn in most pronounced hypo/hyperdense artifacts and corresponding reference tissue without impairment by artifacts (e.g. when the hypodense artifact impairs muscle the corresponding reference tissue is contralateral normal appearing muscle without artifact impairment). The vendor proprietary image viewer was used for

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measurements (IntelliSpace Portal v9, Philips, Cleveland USA). Images were assessed in axial plane

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using soft tissue window settings (window level: 60 HU, window width: 350 HU). Attenuation (HU)

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image noise and indicative of artifact burden [12].

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and standard deviation (SD) were recorded. SD within artifacts were considered representative for

In VMI, changes in HU-values appear along different keV-values (e.g. at higher keV-values muscle

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and arteries show a decrease of attenuation whereas attenuation of fat increases) [9]. To account

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for these changes, we calculated the corrected attenuation which was defined as the difference between HU-values in a certain tissue (hypo- and hyperdense artifact) with visual presence of

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artifacts and corresponding artifact free reference tissue (e.g. muscle, fat, lymph node), as proposed earlier [13–15]. Correspondingly complete artifact reduction should be accomplished by reaching 0 HU. Further, we analogously calculated the corrected image noise to account for a general lower image noise in high-keV images, as applied before [15]. The corrected image noise was also calculated in similar fashion as the difference between image noise in a certain tissue (hypo/hyperdense artifacts) with visual presence of artifacts and corresponding artifact free reference tissue. The artifact size was measured as the largest artifact diameter, expressed in mm, in CI and VMI at 40, 60, 80, 100, 120, 130, 160 as well as 200 keV. 2.3 Visual analysis 6

Journal Pre-proof The visual assessment was carried out by two board-certified fellowship-trained radiologists. Extent of reduction of hypo- and hyperdense artifacts, diagnostic assessment of surrounding soft tissue (muscle, fat, lymph nodes and mediastinal organs) were rated on 5-point Likert-scales. A detailed description of the visual assessment can be seen in Table 1. Appearance of overcorrection or new artifacts were rated on a tertiary scale (Table 1). Visual assessment was conducted in CI and the following VMI reconstruction levels: 70; 100; 130; 160; 200 keV. The use of a larger increment was intended to allow for detection of relevant changes rather than to obscure differences by evaluating

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too similar images. Image parameters for visual assessment were: slice thickness 3 mm, axial plane,

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soft tissue window settings. Manual window setting adaption was deliberately allowed as keV-values

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and artifacts strongly impact on appropriate window settings and we therefore left this explicitly to the readers’ discretion [16]. The subjective readers also recorded the keV-values with best overall

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assessment balancing extent of artifact reduction versus loss of soft tissue contrast. For the visual

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2.4 Statistical analysis

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analysis the subjective readers were unaware of the results from the objective analysis.

JMP Software was used for statistical analyses (V12, SAS Institute, Cary, USA). Results are reported

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as mean (± standard deviation) for quantitative results and as median and 10/90-percentile for qualitative results. Shapiro-Wilk test was used to test for normal distribution. Wilcoxon-test with Steel adjustment for multiple comparisons was used for further testing. The statistical significance was set to p<0.05. Intraclass correlation coefficient (ICC) was determined and interpreted as earlier suggested [17]; agreement being poor <0.40; fair 0.40-0.59; good 0.60-0.75; and excellent

0.75-1.0.

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Journal Pre-proof 3. Results

30 patients affected by hypo- and hyperdense artifacts due to sternal wires after thoracic surgery were included in the final analysis. Average age of the patient cohort was 69.2±12.3 years (range: 30-84 years). The 30 patients consisted of 11 women and 19 men. Most patients received imaging for staging and restaging purposes. These patients were diseased with the following types of neoplasms: lung cancer n=9, breast cancer n=4, pancreatic cancer n=4, lymphoma n=3, prostate

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cancer n=3, renal cancer n=2, and rectal cancer n=1. Two patients received imaging to investigate

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unclear inflammation and two patients to investigate unclear weight loss.

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3.1 Objective assessment

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In the hypodense artifacts, corrected attenuation increased significantly with higher-keV VMI ranging from 110-200 keV, representing an approximation of their true values (Table 2, Figure 1-4).

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Corrected attenuation values reached 0 HU between 170 and 180 keV for hypodense artifacts. This

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indicates that the hypodense artifacts could be completely reduced as corrected attenuation is the difference between artifact impaired tissue and not artifact impaired reference tissue. Further, HU-

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values even turned positive within hypodense artifacts at keV-values of 180 keV or higher, most likely corresponding to a very slight overcorrection of the initial artifacts (Table 2, Figure 1). Further, VMI ≥130 keV reduced the hyperdense artifacts indicated by a decrease of corrected attenuation (Table 2, Figure 1-4). Corrected attenuation in the hyperdense artifacts did not reach 0 HU or turned negative at any energy level. Corrected image noise defined as the difference between the standard deviations of tissue with and reference tissue without artifact impairment was found significantly reduced at higher keV-levels for hypo- and hyperdense artifacts (≥160 keV/≥130 keV; Table 2, Figure 5) indicating a significant reduction of artifacts.

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Journal Pre-proof Artifact size was largest in VMI at 40 keV (7.4±2.5 mm) and significantly decreased with VMI ≥60 keV (p=0.036) and was smallest at 200 keV (1.1±0.6 mm, p<0.001, Table 3). In CI, the artifact size was 5.1±1.7 mm, which was further significantly decreased with VMI ≥80 keV (p=0.024, Table 3). More details on artifact size can be found in Table 3. 3.2 Visual assessment The visual analysis supplemented the objective results. Hypo- and hyperdense sternal artifacts could

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be significantly reduced using higher-keV VMI (≥100 keV) with best results observed in images at 200

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keV (Table 4, Figure 2-4). Hypodense artifacts were generally found to be more severe than hyperdense artifacts. Diagnostic assessment of surrounding soft tissue was significantly improved by

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VMI ≥100 keV, with best results between 160-200 keV (Table 4, Figure 2-4). Diagnostic assessment of

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the sternum could be significantly improved for VMI of 100 keV or higher, with best results between

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130-200 keV (Table 4, Figure 2-4). Overcorrection or introduction of new artifacts at high-keV values were not reported in the visual assessment. Overall interrater agreement was excellent (ICC = 0.83);

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more detailed results are given in Table 4. The subjective readers rated individually for each patient

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the keV-value with overall best diagnostic image assessment. They were ranging between 114-200 keV with an overall average of 147.1±15.2 keV.

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Journal Pre-proof 4. Discussion

Sternal wires used after thoracic surgery cause artifacts that can negatively affect image assessment and diagnostic interpretability in chest imaging. In our study, VMI derived from spectral-detector CT demonstrated an effective reduction of these artifacts demonstrated in the objective and subjective analysis. Visually, artifact reduction resulted in improved assessment of surrounding soft tissue and sternal bone.

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CT artifacts from implanted metal hardware can present differently in shape and severity depending

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on the type of implant and location, as it has been shown in dedicated studies, i.e. total hip

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replacements and dental implants [8,14,18]. Similarly, artifacts from sternal wires can impair image quality and assessment of the surrounding structures. This commonly affects sternal bone and soft

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tissue, including retrosternal mediastinal structures which can be crucial for answering clinical

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questions. Objective and subjective results in our study showed that effective reduction of these artifacts is possible by higher keV VMI, thereby improving image quality and assessment of

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surrounding soft tissue and bone. This could be valuable in staging and restaging examination to

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improve visualization and detection of potential soft tissue and lymph node metastases Further, patients that develop a fever and local inflammation signs after thoracic surgery could profit from improved depiction of the sternal bone and soft tissue. Also, high-keV VMI could improve assessment in patients that have suspicion of hardware loosening or failure after surgery. Although artifact reduction was strongest at 200 keV regarding both the objective and subjective analysis, mean optimal keV-value for image interpretation was 147.1±15.2 keV, ranging between 114-200 keV and diagnostic assessment was rated best between 130-200 keV. This was probably due to the incremental loss of soft tissue contrast caused by VMI at higher keV [9,15,19]. Therefore, we recommend keV-values between 130-200 keV in clinical use with need for individual adjustment for each patient. For this application spectral-detector CT appears ideal as adjustment of VMI keVvalues is possible in real time and adaption of the optimal keV-values is individually applicable.

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Journal Pre-proof Further, VMI are always created and retrospectively available without dependency on raw data or prior planning due to the detector-based acquisition [9,14]. The study from Secchi et al. investigated reduction of artifacts from highly attenuating material in cardiac examinations using higher keV VMI images from dual-source dual-energy CT [20]. Amongst other, they evaluated the reduction of artifacts from sternal wires. Similar to our results, artifact size was substantially reduced in 120 keV VMI compared to 40 keV VMI (8.4 vs. 2.6 mm). Our results

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showed a reduction of artifact size from 7.4 to 1.8 mm comparing 40 keV and 120 keV VMI. In our study, VMI higher than 120 keV were also assessed. VMI at 200 keV showed the strongest artifact

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reduction with the smallest artifact size of 1.1 mm. Given the comparable artifact reduction on the

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two studies, it can be inferred that the used dual-energy approach (dual-source versus dual-layer

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spectral detector) might have limited influence on the effectiveness of artifact reduction by VMI.

energy systems.

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Conclusions drawn by our study and Secchi et al. might therefore also be applied to other dual-

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Artifacts from different types of metal material is ubiquitous in all fields of CT imaging, negatively

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affecting diagnostic assessment and therapy planning in neuro, head and neck, chest, cardiac, abdominal as well as musculoskeletal imaging [21–23]. Adequate methods to reduce these artifacts and improve image quality are therefore warranted. VMI supplied by dual-energy CT have been proven to be effective for metal artifact reduction in different types of metal implants (e.g. dental implants/crowns, total hip replacements, implants/screws of the extremities, highly attenuating material in cardiac CT). Extent and shape as well as the effectiveness of VMI varies depending on the implant material composition, its size and location in the body [8,14,18,20,24–26]. The previously reported capabilities for artifact reduction are not specific to the applied technology or vendor [24,27–33]. Differences between the vendors rather matter regarding availability of VMI and radiation dose: SDCT detects low- and high-energetic photons on the level of the detector, therefore no interference occurs with the x-ray source and theoretically, neutrality of radiation dose should be

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Journal Pre-proof accomplished [9]. A recent study by Haneder et al. even reported lower radiation dose parameters in a SDCT scanner compared to a single-energy CT system from the same vendor [34]. Due to their design, tube-based dual-energy CT approaches should not allow for this dose neutrality and instead require higher radiation doses to offer dual-energy acquisitions. However, this is unclear given the mixed results on various studies so far, which showed different conclusions and reported doses as lower, comparable and also higher compared to single energy CT scans [35–37]. As every SDCT acquisition offers the possibility to retrospectively reconstruct high keV VMI it can be used for

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artifact reduction whenever needed for diagnostic assessment. In contrast, conventional single-

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energy CT require increased radiation dose (increased peak tube voltage - kVp and/or high tube

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current -mAs) for an effective artifact reduction which needs to be used with caution especially in young patients and patients undergoing multiple follow up examinations [3]. Further, with

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conventional CT, decision for artifact reduction needs to be taken before the performance of the

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examination and it cannot be applied retrospectively on demand as it is possible with SDCT.

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In SDCT dual-energy acquisitions are always enabled due to the detector-based separation of photons (which is not the case for all tube-based dual-energy CT systems), thereby a spectral image

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dataset is created for each scan and VMI can always be retrospectively reconstructed and made available on demand using the vendors proprietary spectral image viewer. The image viewer allows for reconstruction of CI and VMI at different keV levels. To achieve best artifact reduction, optimal VMI can be quickly selected in real time as a continuous variable between 40-200 keV. This workflow has very little influence on workflow and, if anything, only marginally increases reading time. Another workflow that has no/minimal impact on reading time is to reconstruct VMI at a certain keV levels whenever sternal device artifacts are present at the CT scanner. The VMI are then directly sent to the picture archiving and communication system (PACS) and available to the image reader as an addition to the conventional image reconstructions [9].

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Journal Pre-proof Overcorrection and the appearance of new artifacts in VMI at higher keV-values have been described in earlier studies, for example in presence of dental implants and vertebral screws[13–15]. On the other hand, larger implants, resulting from total hip arthroplasties rather do not suffer from introduction of new artifacts at higher keV VMI [18,38]. At higher keV images, overcorrection can be seen in initial artifacts which have the opposite characteristics regarding attenuation, but do not impair diagnostic assessment compared to the CI [14,15]. In our study, the readers did not perceive an overcorrection or introduction of new artifacts, most likely because sternal wires are usually

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rather thin [1,3–5,13,18]. Even though the objective analysis showed minor overcorrection of

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hypodense artifacts, as evidenced by change from negative to positive values in the corrected

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attenuation between 180 and 190 keV, the positive change was minimal (<3 HU), which is most likely

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not visually perceivable and should rather be seen just as a total reduction of the original artifact. CT artifacts can be evaluated or measured with different approaches. In radiological studies, the

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most common method is to conduct ROI-based measurements of attenuation and standard

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deviation to objectively assess artifact severity [18,22,39]. Further, artifact size can be measured as a quantitative parameter to assess artifact reduction, similar to prior studies [18,20]. Opposing to this,

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rather more complex methods have also been used, such as specialized algorithms within the image or projection domain that enable artifact quantification [27,28,40]. It needs to be considered that these algorithms may be more specialized and therefore might have a higher validity. To address general changes in attenuation and image noise that occur along with changing keV-levels of VMI not related to artifacts or their reduction, we applied corrected attenuation/image noise as an intraindividual comparison between artifact impaired tissue and correspondent not impaired reference tissue [9,14]. Besides artifacts, iterative reconstructions also impact image noise and thereby might influence our applied objective approach; however, any possible limitations that might occur by the applied objective analysis should be accounted by our dedicated and detailed visual analysis by two experienced and independent readers rating both artifact reduction and diagnostic assessment.

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Journal Pre-proof 5. Conclusion

Virtual monoenergetic images at high-keV values offered effective reduction of artifacts from sternal wires, thereby improving image quality and assessment of surrounding structures. Recommended keV-values for best diagnostic assessment range between 130-200 keV but need individual

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adjustments.

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Journal Pre-proof 7. Tables

Table 1 - Visual analysis Extent of hypo- and hyperdense artifacts

(5) Metal artifacts are absent/almost absent; (4) minor; (3) mild; (2) moderate (1) pronounced

Diagnostic assessment of surrounding sternal (5) full diagnostic quality; (4) mildly affected soft tissue diagnostic quality; (3) moderately hampered diagnostic quality; (2) restricted diagnostic quality;(1) insufficient diagnostic quality (5) full diagnostic quality; (4) mildly affected diagnostic quality; (3) moderately hampered diagnostic quality; (2) restricted diagnostic quality;(1) insufficient diagnostic quality

Presence of overcorrection or new artifacts as compared to CI

(3) no overcorrection or new artifacts; (2) overcorrection, new opposite artifact in the location of initial artifacts without additional impairment of diagnostic assessment compared to CI; (1) new artifacts in the same or different location of initial artifacts with additional impairment of diagnostic assessment compared to CI

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Diagnostic assessment of the sternal bone

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Journal Pre-proof Table 2 - Objective assessment of artifact reduction and surrounding tissue Corrected attenuation/HU-values Corrected image noise/SD

120 keV 130 keV 140 keV 160 keV 200 keV

p-values CI vs. VMI 80 keV CI vs. VMI 90 keV CI vs. VMI 100 keV CI vs. VMI 110 keV CI vs. VMI 120 keV CI vs. VMI 130 keV CI vs. VMI 140 keV CI vs. VMI 160 keV CI vs. VMI 200 keV

30.7±28.5

48.1±33.0

-78.5±73.1 -54.1±53.0 -38.1±41.1 -27.2±34.4 -19.6±30.9 -14.1±29.2 -9.9±28.3 -6.8±28.1 -2.3±28.3 2.4±29.2

191.9±99.5 157.3±76.8 134.5±63.5 119.2±55.8 108.3±51.4 100.4±48.9 94.6±47.4 90.1±46.4 83.8±45.7 76.9±45.4

33.6±31.7 25.6±22.9 20.5±17.4 17.0±13.8 14.8±11.5 13.1±10.1 11.9±9.1 11.0±8.5 9.8±7.7 8.6±7.3

49.3±38.6 39.2±28.7 32.7±22.6 28.2±18.8 25.2±16.4 22.9±14.9 21.3±14.0 20.1±13.3 18.4±12.6 16.7±11.9

p>0.05 p>0.05 p>0.05 p=0.016 p=0.002 p<0.001 p<0.001 p<0.001 p<0.001

p>0.05 p>0.05 p>0.05 p>0.05 p>0.05 p=0.033 p=0.012 p=0.002 p<0.001

p>0.05 p>0.05 p>0.05 p>0.05 p>0.05 p>0.05 p>0.05 p=0.017 p=0.007

p>0.05 p>0.05 p<0.05 p=0.033 p=0.012 p=0.007 p=0.003 p<0.001 p<0.001

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110 keV

156.7±70.8

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100 keV

-66.2±70.8

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90 keV

Hyperdense artifact

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80 keV

Hypodense artifact

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70 keV

Hyperdense artifact

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CI VMI

Hypodense artifact

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CI - conventional imaging, VMI - virtual monoenergetic images, significant changes in HU-values as compared to CI are marked in bold (p<0.05)

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Journal Pre-proof Table 3 - Objective assessment of artifact size Maximum artifact diameter (mm) 5.1±1.7

60 keV 80 keV 100 keV 120 keV 130 keV 160 keV 200 keV

7.4±2.5 6.2±2.5 4.1±1.4 2.8±0.9 1.8±0.6 1.5±0.5 1.2±0.6 1.1±0.6

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CI - conventional imaging, VMI - virtual monoenergetic images, significant changes in artifact size compared to CI are marked in bold (p<0.05)

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40 keV

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CI VMI

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Journal Pre-proof Table 4 - Subjective assessment of artifact reduction and surrounding structures Artifact extent Diagnostic assessment Hypodense Hyperdense Soft tissue and organs Sternal bone 2 (2-3) 2 (3-4) 3 (1-3) 4 (2-5)

CI VMI 70 keV 100 keV 130 keV 160 keV 200 keV ICC p-values CI vs. VMI 70 keV CI vs. VMI 100 keV CI vs. VMI 130 keV CI vs. VMI 160 keV CI vs. VMI 200 keV

2 (1-3) 3 (2-4) 4 (3-4) 4 (3-5) 4.5 (4-5) 0.77

4 (2-4) 4 (3-5) 5 (4-5) 5 (4-5) 5 (4-5) 0.75

3 (1-3) 3 (2-4) 4 (3-5) 4 (4-5) 5 (4-5) 0.81

p>0.05 p<0.001 p<0.001 p<0.001 p<0.001

p>0.05 p<0.001 p<0.001 p<0.001 p<0.001

p>0.05 p<0.001 p<0.001 p<0.001 p<0.001

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4 (2-5) 4 (3-5) 4 (3-5) 5 (4-5) 5 (4-5) 0.80

New artifacts

3 (3-3) 3 (3-3) 3 (3-3) 3 (3-3) 3 (3-3) 1.0

p>0.05 p=0.014 p<0.001 p<0.001 p<0.001

Soft tissue - Surrounding sternal soft tissue; New artifacts - Introduction of new artifacts or overcorrection; ICC - Intraclass correlation; CI - Conventional images; VMI - Virtual monoenergetic images; Data is reported as median and 10/90-percentile, significant changes in HU-values as compared to CI are marked in bold (p<0.05)

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Journal Pre-proof 8. Figure legends

Figure 1: Box-plot diagram displaying corrected attenuation values in hypo- and hyperdense artifacts surrounding the sternal wires in conventional CT images (CI) and virtual monoenergetic images (VMI, 40–200 keV). VMI significantly reduce hypodense artifacts at ≥100 keV and hyperdense artifacts at ≥130 keV. Rising keV-levels result in increasing corrected HU-values in hypodense artifacts and decreasing corrected HU-values in the hyperdense artifacts. Thus, both approximate their true

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values. Corrected attenuation values reached 0 HU between 170 and 180 keV for hypodense

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artifacts. This indicates that the hypodense artifacts could be eliminated as corrected attenuation is the difference between artifact impaired tissue and not artifact impaired reference tissue. Further,

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corrected HU-values even turned positive within hypodense artifacts at keV-values at ≥180 keV,

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most likely representing mild overcorrection of the initial artifacts.

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Figure 2: VMI allow for an almost complete reduction of hypodense artifacts from sternal wires and thereby improved assessment of the surrounding soft tissue. Axial CT images at the level of

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upper sternum in a patient with history of pancreatic cancer, were reconstructed as conventional

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(CI) and virtual monoenergetic images (VMI, 70 keV, 100 keV, 130 keV, 160 keV and 200 keV). Predominantly hypodense artifacts are seen emanating from sternal wires on the conventional image (red arrows, top left) which are incrementally improved by VMI at ≥130 keV and are almost completely resolved at 200 keV (red arrows, bottom right). A left anterior chest wall seroma/hematoma from pacemaker implantation is partially visualized. Figure 3: VMI allow for a complete reduction of hyperdense artifacts from sternal wires resulting in improved assessment of the sternal bone and surrounding soft tissues. Axial CT images at the level of upper sternum in an 80 year old female with history of renal cancer and prior cardiothoracic surgery, were reconstructed as conventional (CI) and virtual monoenergetic images (VMI, 70 keV, 100 keV, 130 keV, 160 keV and 200 keV). A complete reduction of hyperdense artifacts (blue arrows)

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Journal Pre-proof is achieved at high-keV images ≥160 keV improving assessment of the surrounding sternal bone and soft tissue. The hypodense artifacts (red arrows) are also significantly reduced on higher keV images. Figure 4: Effective reduction of artifacts from sternal wires improve characterization/diagnosis of presternal tumoral calcinosis and facilitate delineation as well as detection of a retrosternal ectopic parathyroid adenoma. A 41-year-old male patient with history from end stage renal and coronary artery disease as well as after cardiothoracic surgery presents with a growing presternal

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tumoral swelling. Axial CT images (5A and 5B) were reconstructed as conventional (CI), virtual monoenergetic images (VMI, 70 keV, 100 keV, 130 keV, 160 keV and 200 keV). In addition,

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Technetium-99m Sestamibi single photon emission computed tomography (SPECT)/ CT was

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performed (5C).

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4 A, Hyperdense artifacts from sternal wires (red arrows) impair assessment of the presternal

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multilobulated partly calcified tumoral lesion on CI (green arrow). Higher keV VMI significantly decrease metal artifacts and improved depiction of layering hyperattenuating material within the

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presternal lesion, i.e. sedimentation sign, which is typical for tumoral calcinosis. Furthermore, on the

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high-keV images assessment of the sternum is improved and calcification between two halves of the sternum is better seen (blue arrows), further consolidating the diagnosis. Optimal reduction of the mainly hyperdense artifacts (red arrows) is achieved at 200 keV images. 4 B&C, High-keV VMI images (≥160 keV) significantly reduced hypodense artifacts (red arrows), which let to better visualization of the anterior mediastinum and detection of a small retrosternal nodular lesion (green arrows), which was subsequently confirmed to be an ectopic parathyroid adenoma based by radiotracer uptake on SPECT/CT imaging and supporting laboratory data. Figure 5: Box-plot diagram displaying corrected image noise values in hypo- and hyperdense artifacts surrounding the sternal wires in conventional CT images (CI) and virtual monoenergetic images (VMI,

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Journal Pre-proof 40–200 keV). Image noise is regarded as an indicator of artifacts and could be significantly reduced

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in hypo- and hyperdense artifacts surrounding the sternal wires at higher keV-values.

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Journal Pre-proof Highlights:

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In chest CT imaging, sternal wires can cause metal artifacts that impair depiction of surrounding soft tissue and bone High-keV VMI effectively reduce artifacts from sternal wires and improve assessment of surrounding structures Recommended keV-values for best diagnostic assessment range between 130-200 keV but need individual adjustments

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Figure 1

Figure 2

Figure 3

Figure 4

Figure 5