Metal artifact reduction by dual-layer computed tomography using virtual monoenergetic images

Optimisation of monoenergetic images to reduce banding artifacts in the lower cervical spine using dual-layer spectral computed tomography: a retrospective study

AIM

To investigate the application value of dual-layer detector computed tomography (CT) single-energy spectral images for reducing artefacts in the lower cervical spine.

MATERIALS AND METHODS

Sixty-three patients who underwent neck examination using spectral CT between March 2022 and January 2023 were selected as the participants. Conventional mixed-energy images and spectral imaging data at 40-200 KeV were obtained from the spectral CT scans. The standard deviation (SD), signal-to-noise ratio (SNR), |CTdifference| (the difference in CT value between the C6-7 and C3-4 artefact regions), and contrast-to-noise ratio (CNR) of the C6-7 spinal canal were measured. The image quality of the artefact region in the conventional and single images were compared.

RESULTS

The SDintervertebral disc value was highest in the single-level 40 KeV images and lowest in the 120 KeV images (P<0.05). The SNR was lowest in the 40 KeV images (2.07 ± 2.74) and highest in the 120 KeV images (6.20 ± 5.26) (P<0.05). The CTdifference was highest in the 40 KeV images and lowest in the 120 KeV images (108.38 vs. 10.63 ± 8.58). The subjective image quality scores were lowest in the 40 KeV images and highest in the 120 KeV images.

CONCLUSION

Our findings suggest that single-energy 120 KeV images may offer benefits such as low SDintervertebral disc, elevated SNR, higher CNR, and improved image quality.

Iodine concentration, HU accuracy, and metal artifacts evaluation on second-generation dual-layer spectral detector CT images with metal implants: a phantom study

BACKGROUND

Metal implants may affect the image quality, iodine concentration (IC), and CT Hounsfield unit (HU) quantification accuracy.

PURPOSE

To investigate the quantitative accuracy of IC and HU from dual-layer spectral detector (DLCT) in the presence of metal artifacts.

MATERIAL AND METHODS

An experimental cylindrical phantom containing eight iodine inserts and two metal inserts was designed. The phantom underwent scanning at three radiation dose levels and two tube voltage settings. A set of conventional images (CIs), virtual monoenergetic images (VMIs), and iodine concentration maps (ICMs) were generated and measured for all the eight iodine inserts. Quantitative indicators of mean absolute percentage error (MAPE), artifact index (AI), contrast-to-noise ratio (CNR), signal-to-noise ratio (SNR), and standard deviation (SD) on CIs and VMIs were calculated for IC and HU. Subjective score evaluation was also conducted.

RESULTS

The MAPEiodine values of all regions of interest across different scanning configurations were all <5%. Almost all APEiodine values were <5%, indicating that metal artifacts had little impact on IC measurements. When the tube voltage was fixed, the SD value of attenuation decreased with the increase of the tube current; this is also true when the tube current was fixed. The middle energy reconstructions seemed to give a good balance between reducing artifacts and improving contrast.

CONCLUSION

VMIs from DLCT can reduce metal artifacts, the accuracy of IC quantification is not sensitive to imaging parameters. In summary, metal implants exhibit minimal impact on image quality and IC quantification accuracy in reconstructed images from DLCT.

Photon-Counting Detector CT: Clinical Utility of Virtual Monoenergetic Imaging Combined With Tin Prefiltration to Reduce Metal Artifacts in the Postoperative Ankle

OBJECTIVES

The aim of this study was to compare the effectiveness and clinical utility of virtual monoenergetic image (VMI) reconstructions in computed tomography (CT) scans with and without tin prefiltration on a photon-counting detector (PCD) CT system to reduce metal implant artifacts in the postoperative ankle.

MATERIALS AND METHODS

This retrospective study included patients with internal fixation of the ankle scanned with and without tin prefiltration (Sn) on a PCD CT scanner between March and October 2023. Virtual monoenergetic images between 60 and 190 keV were reconstructed with a 10-keV increment in a bone kernel for both acquisitions (VMI Sn and VMI Std , respectively). Noise measurements assessed artifact reduction in the most prominent near-metal image distortions and were compared between acquisitions modes as well as between polychromatic images and VMIs. Three readers assessed the visibility of osseous healing along with interpretability and artifact extent for 5 reconstruction levels.

RESULTS

A total of 48 patients (21 females, 27 males; mean age, 55.1 ± 19.4 years) were included in this study. Tin-prefiltered acquisitions (n = 30) had a lower artifact level for polychromatic images and VMIs compared with non-tin-prefiltered acquisitions (n = 18; P ≤ 0.043). A significant reduction of metal artifacts was observed for VMI Sn ≥120 keV compared with polychromatic images (hyperdense artifacts: 40.2 HU [interquartile range (IQR) 39.8] vs 14.0 HU [IQR 11.1]; P ≤ 0.01 and hypodense artifacts: 91.2 HU [IQR 82.4] vs 29.7 HU [IQR 39.6]; P ≤ 0.001). For VMI Std , this applied to reconstructions ≥100 keV (hyperdense artifacts: 57.7 HU [IQR 33.4] vs 19.4 HU [IQR 27.6]; P ≤ 0.001 and hypodense artifacts: 106.9 HU [IQR 76.1] vs 57.4 HU [IQR 55.7]; P ≤ 0.021). For visibility of osseous healing, VMI Sn at 120 keV yielded higher ratings compared with polychromatic images ( P ≤ 0.001), whereas image interpretability was rated better ( P = 0.023), and artifact extent was rated lower ( P ≤ 0.001) compared with polychromatic images.

CONCLUSIONS

Tin-prefiltered VMI at 120 keV showed a significant reduction in metal artifacts compared with polychromatic images, whereas visibility of osseous healing and image interpretability was improved. Therefore, tin-prefiltration PCD CT with VMI reconstructions may be a helpful complement to postsurgical CT imaging of the ankle in patients with metal implants.

Optimization of photon counting CT for cardiac imaging in patients with left ventricular assist devices: An in-depth assessment of metal artifacts

PURPOSE

Photon counting CT (PCCT) holds promise for mitigating metal artifacts and can produce virtual mono-energetic images (VMI), while maintaining temporal resolution, making it a valuable tool for characterizing the heart. This study aimed to evaluate and optimize PCCT for cardiac imaging in patients during left ventricular assistance device (LVAD) therapy by conducting an in-depth objective assessment of metal artifacts and visual grading.

METHODS

Various scan and reconstruction settings were tested on a phantom and further evaluated on a patient acquisition to identify the optimal protocol settings. The phantom comprised an empty thoracic cavity, supplemented with heart and lungs from a cadaveric lamb. The heart was implanted with an LVAD (HeartMate 3) and iodine contrast. Scans were performed on a PCCT (NAEOTOM Alpha, Siemens Healthcare). Metal artifacts were assessed by three objective methods: Hounsfield units (HU)/SD measurements (DiffHU and SDARTIFACT), Fourier analysis (AmplitudeLowFreq), and depicted LVAD volume in the images (BloomVol). Radiologists graded metal artifacts and the diagnostic interpretability in the LVAD lumen, cardiac tissue, lung tissue, and spinal cord using a 5-point rating scale. Regression and correlation analysis were conducted to determine the assessment method most closely associated with acquisition and reconstruction parameters, as well as the objective method demonstrating the highest correlation with visual grading.

RESULTS

Due to blooming artifacts, the LVAD volume fluctuated between 27.0 and 92.7 cm3. This variance was primarily influenced by kVp, kernel, keV, and iMAR (R2 = 0.989). Radiologists favored pacemaker iMAR, 3 mm slice thickness, and T3D keV and kernel Bv56f for minimal metal artifacts in cardiac tissue assessment, and 110 keV and Qr40f for lung tissue interpretation. The model adequacy for DiffHU SDARTIFACT, AmplitueLowFreq, and BloomVol was 0.28, 0.76, 0.29, and 0.99 respectively for phantom data, and 0.95, 0.98, 1.00, and 0.99 for in-vivo data. For in-vivo data, the correlation between visual grading (VGSUM) and DiffHU SDARTIFACT, AmplitueLowFreq, and BloomVol was -0.16, -0.01, -0.48, and -0.40 respectively.

CONCLUSION

We found that optimal scan settings for LVAD imaging involved using 120 kVp and IQ level 80. Employing T3D with pacemaker iMAR, the sharpest allowed vascular kernel (Bv56f), and VMI at 110 keV with kernel Qr40 yields images suitable for cardiac imaging during LVAD-therapy. Volumetric measurements of the LVAD for determination of the extent of blooming artifacts was shown to be the best objective method to assess metal artifacts.

Reduction of metal artefacts from bilateral hip prostheses during lower extremity computed tomography angiography: an experimental phantom study

INTRODUCTION

Image quality reduction due to metallic artefacts is a significant challenge during vascular computed tomography (CT) imaging of the lower extremities in patients with hip prostheses. This study aims to analyse various reconstruction algorithms' ability to reduce metal artefacts due to two types of hip prostheses during lower extremity CT angiography examinations.

METHODS

A pelvis phantom was fabricated with the insertion of a tube filled with contrast media to simulate the femoral artery, and the phantom was then CT scanned with and without hip prostheses. Multimodal images were acquired using different kilovoltage peak (kVp) settings and reconstructed with different algorithms, such as filtered back projection (FBP), iterative reconstruction (iDose4), iterative model-based reconstruction (IMR) and orthopaedic metal artefact reduction (O-MAR). Image quality was assessed based on image noise, signal-to-noise ratio (SNR) and Hounsfield unit (HU) deviation.

RESULTS

The IMR approach significantly improved image quality compared to iDose4 and FBP. For the vascular region, O-MAR improves SNR by 5 ± 1, 23 ± 5 and 42 ± 9 for FBP, iDose4 and IMR respectively, and improves HU precision towards the baseline values by 49% and 83% for FBP and IMR, respectively. The noise reduction was 71% and 89% for FBP and IMR, and 57% for iDose4. O-MAR greatly enhances SNR corrections among the most severe artefacts, with 29 ± 1 and 43 ± 4 for FBP and IMR, compared to iDose4 by 37 ± 7.

CONCLUSION

IMR combined with O-MAR could improve the CT angiography of the lower extremities of patients with a hip prosthesis.

The Impact of AI on Metal Artifacts in CBCT Oral Cavity Imaging

OBJECTIVE

This study aimed to assess the impact of artificial intelligence (AI)-driven noise reduction algorithms on metal artifacts and image quality parameters in cone-beam computed tomography (CBCT) images of the oral cavity.

MATERIALS AND METHODS

This retrospective study included 70 patients, 61 of whom were analyzed after excluding those with severe motion artifacts. CBCT scans, performed using a Hyperion X9 PRO 13 × 10 CBCT machine, included images with dental implants, amalgam fillings, orthodontic appliances, root canal fillings, and crowns. Images were processed with the ClariCT.AI deep learning model (DLM) for noise reduction. Objective image quality was assessed using metrics such as the differentiation between voxel values (ΔVVs), the artifact index (AIx), and the contrast-to-noise ratio (CNR). Subjective assessments were performed by two experienced readers, who rated overall image quality and artifact intensity on predefined scales.

RESULTS

Compared with native images, DLM reconstructions significantly reduced the AIx and increased the CNR (p < 0.001), indicating improved image clarity and artifact reduction. Subjective assessments also favored DLM images, with higher ratings for overall image quality and lower artifact intensity (p < 0.001). However, the ΔVV values were similar between the native and DLM images, indicating that while the DLM reduced noise, it maintained the overall density distribution. Orthodontic appliances produced the most pronounced artifacts, while implants generated the least.

CONCLUSIONS

AI-based noise reduction using ClariCT.AI significantly enhances CBCT image quality by reducing noise and metal artifacts, thereby improving diagnostic accuracy and treatment planning. Further research with larger, multicenter cohorts is recommended to validate these findings.

Variation in Hounsfield unit calculated using dual-energy computed tomography: comparison of dual-layer, dual-source, and fast kilovoltage switching technique

The purpose of the study is to investigate the variation in Hounsfield unit (HU) values calculated using dual-energy computed tomography (DECT) scanners. A tissue characterization phantom inserting 16 reference materials were scanned three times using DECT scanners [dual-layer CT (DLCT), dual-source CT (DSCT), and fast kilovoltage switching CT (FKSCT)] changing scanning conditions. The single-energy CT images (120 or 140 kVp), and virtual monochromatic images at 70 keV (VMI70) and 140 keV (VMI140) were reconstructed, and the HU values of each reference material were measured. The difference in HU values was larger when the phantom was scanned using the half dose with wrapping with rubber (strong beam-hardening effect) compared with the full dose without the rubber (reference condition), and the difference was larger as the electron density increased. For SECT, the difference in HU values against the reference condition measured by the DSCT (3.2 ± 5.0 HU) was significantly smaller (p < 0.05) than that using DLCT with 120 kVp (22.4 ± 23.8 HU), DLCT with 140 kVp (11.4 ± 12.8 HU), and FKSCT (13.4 ± 14.3 HU). The respective difference in HU values in the VMI70 and VMI140 measured using the DSCT (10.8 ± 17.1 and 3.5 ± 4.1 HU) and FKSCT (11.5 ± 21.8 and 5.5 ± 10.4 HU) were significantly smaller than those measured using the DLCT120 (23.1 ± 27.5 and 12.4 ± 9.4 HU) and DLCT140 (22.3 ± 28.6 and 13.1 ± 11.4 HU). The HU values and the susceptibility to beam-hardening effects varied widely depending on the DECT scanners.

Advanced Imaging of Total Knee Arthroplasty

The prevalence of total knee arthroplasty (TKA) is increasing with the aging population. Although long-term results are satisfactory, suspected postoperative complications often require imaging with the implant in place. Advancements in computed tomography (CT), such as tin prefiltration, metal artifact reduction algorithms, dual-energy CT with virtual monoenergetic imaging postprocessing, and the application of cone-beam CT and photon-counting detector CT, allow a better depiction of the tissues adjacent to the metal. For magnetic resonance imaging (MRI), high bandwidth (BW) optimization, the combination of view angle tilting and high BW, as well as multispectral imaging techniques with multiacquisition variable-resonance image combination or slice encoding metal artifact correction, have significantly improved imaging around metal implants, turning MRI into a useful clinical tool for patients with suspected TKA complications.

Coronary computed tomography angiography for clinical practice

Coronary artery disease (CAD) is a common condition caused by the accumulation of atherosclerotic plaques. It can be classified into stable CAD or acute coronary syndrome. Coronary computed tomography angiography (CCTA) has a high negative predictive value and is used as the first examination for diagnosing stable CAD, particularly in patients at intermediate-to-high risk. CCTA is also adopted for diagnosing acute coronary syndrome, particularly in patients at low-to-intermediate risk. Myocardial ischemia does not always co-exist with coronary artery stenosis, and the positive predictive value of CCTA for myocardial ischemia is limited. However, CCTA has overcome this limitation with recent technological advancements such as CT perfusion and CT-fractional flow reserve. In addition, CCTA can be used to assess coronary artery plaques. Thus, the indications for CCTA have expanded, leading to an increased demand for radiologists. The CAD reporting and data system (CAD-RADS) 2.0 was recently proposed for standardizing CCTA reporting. This RADS evaluates and categorizes patients based on coronary artery stenosis and the overall amount of coronary artery plaque and links this to patient management. In this review, we aimed to review the major trials and guidelines for CCTA to understand its clinical role. Furthermore, we aimed to introduce the CAD-RADS 2.0 including the assessment of coronary artery stenosis, plaque, and other key findings, and highlight the steps for CCTA reporting. Finally, we aimed to present recent research trends including the perivascular fat attenuation index, artificial intelligence, and the advancements in CT technology.

Potential of photon counting computed tomography derived spectral reconstructions to reduce beam-hardening artifacts in chest CT

PURPOSE

Aim of the recent study is to point out a method to optimize quality of CT scans in oncological patients with port systems. This study investigates the potential of photon counting computed tomography (PCCT) for reduction of beam hardening artifacts caused by port-implants in chest imaging by means of spectral reconstructions.

METHOD

In this retrospective single-center study, 8 ROIs for 19 spectral reconstructions (polyenergetic imaging, monoenergetic reconstructions from 40 to 190 keV as well as iodine maps and virtual non contrast (VNC)) of 49 patients with pectoral port systems undergoing PCCT of the chest for staging of oncologic disease were measured. Mean values and standard deviation (SD) Hounsfield unit measurements of port-chamber associated hypo- and hyperdense artifacts, bilateral muscles and vessels has been carried out. Also, a structured assessment of artifacts and imaging findings was performed by two radiologists.

RESULTS

A significant association of keV with iodine contrast as well as artifact intensity was noted (all p < 0.001). In qualitative assessment, utilization of 120 keV monoenergetic reconstructions could reduce severe and pronounced artifacts completely, as compared to lower keV reconstructions (p < 0.001). Regarding imaging findings, no significant difference between monoenergetic reconstructions was noted (all p > 0.05). In cases with very high iodine concentrations in the subclavian vein, image distortions were noted at 40 keV images (p < 0.01).

CONCLUSIONS

The present study demonstrates that PCCT derived spectral reconstructions can be used in oncological imaging of the thorax to reduce port-derived beam-hardening artefacts. When evaluating image data sets within a staging, it can be particularly helpful to consider the 120 keV VMIs, in which the artefacts are comparatively low.

Updates on the Applications of Spectral Computed Tomography for Musculoskeletal Imaging

Spectral CT represents a novel imaging approach that can noninvasively visualize, quantify, and characterize many musculoskeletal pathologies. This modality has revolutionized the field of radiology by capturing CT attenuation data across multiple energy levels and offering superior tissue characterization while potentially minimizing radiation exposure compared to traditional enhanced CT scans. Despite MRI being the preferred imaging method for many musculoskeletal conditions, it is not viable for some patients. Moreover, this technique is time-consuming, costly, and has limited availability in many healthcare settings. Thus, spectral CT has a considerable role in improving the diagnosis, characterization, and treatment of gout, inflammatory arthropathies, degenerative disc disease, osteoporosis, occult fractures, malignancies, ligamentous injuries, and other bone-marrow pathologies. This comprehensive review will delve into the diverse capabilities of dual-energy CT, a subset of spectral CT, in addressing these musculoskeletal conditions and explore potential future avenues for its integration into clinical practice.

Needle artifact reduction during interventional CT procedures using a silver filter

BACKGROUND

MAR algorithms have not been productized in interventional imaging because they are too time-consuming. Application of a beam hardening filter can mitigate metal artifacts and doesn't increase computational burden. We evaluate the ability to reduce metal artifacts of a 0.5 mm silver (Ag) additional filter in a Multidetector Computed Tomography (MDCT) scanner during CT-guided biopsy procedures.

METHODS

A biopsy needle was positioned inside the lung field of an anthropomorphic phantom (Lungman, Kyoto Kagaku, Kyoto, Japan). CT acquisitions were performed with beam energies of 100 kV, 120 kV, 135 kV, and 120 kV with the Ag filter and reconstructed using a filtered back projection algorithm. For each measurement, the CTDIvol was kept constant at 1 mGy. Quantitative profiles placed in three regions of the artifact (needle, needle tip, and trajectory artifacts) were used to obtain metrics (FWHM, FWTM, width at - 100 HU, and absolute error in HU) to evaluate the blooming artifact, artifact width, change in CT number, and artifact range. An image quality analysis was carried out through image noise measurement. A one-way analysis of variance (ANOVA) test was used to find significant differences between the conventional CT beam energies and the Ag filtered 120 kV beam.

RESULTS

The 120 kV-Ag is shown to have the shortest range of artifacts compared to the other beam energies. For needle tip and trajectory artifacts, a significant reduction of - 53.6% (p < 0.001) and - 48.7% (p < 0.001) in the drop of the CT number was found, respectively, in comparison with the reference beam of 120 kV as well as a significant decrease of up to - 34.7% in the artifact width (width at - 100 HU, p < 0.001). Also, a significant reduction in the blooming artifact of - 14.2% (FWHM, p < 0.001) and - 53.3% (FWTM, p < 0.001) was found in the needle artifact. No significant changes (p > 0.05) in image noise between the conventional energies and the 120 kV-Ag were found.

CONCLUSIONS

A 0.5 mm Ag additional MDCT filter demonstrated consistent metal artifact reduction generated by the biopsy needle. This reduction may lead to a better depiction of the target and surrounding structures while maintaining image quality.

Advances in metal artifact reduction in CT images: A review of traditional and novel metal artifact reduction techniques

Metal artifacts degrade CT image quality, hampering clinical assessment. Numerous metal artifact reduction methods are available to improve the image quality of CT images with metal implants. In this review, an overview of traditional methods is provided including the modification of acquisition and reconstruction parameters, projection-based metal artifact reduction techniques (MAR), dual energy CT (DECT) and the combination of these techniques. Furthermore, the additional value and challenges of novel metal artifact reduction techniques that have been introduced over the past years are discussed such as photon counting CT (PCCT) and deep learning based metal artifact reduction techniques.

Optimal combination periprosthetic vasculature visualization and metal artifact reduction by spectral computed tomography using virtual monoenergetic images in total hip arthroplasty

OBJECTIVES

To investigate the optimal parameters of spectral CT for preferably visualizing the periprosthetic vasculature and metal artifact reduction (MAR) in total hip arthroplasty (THA).

METHODS

A total of 34 THA of 30 patients were retrospectively included. Image reconstructions included conventional image (CI), CI combined with MAR (CIMAR), and virtual monoenergetic images (VMI) combined with MAR (VMIMAR) at 50-120 keV. The attenuation and standard deviation of the vessel and artifact, and the width of artifact were measured. Qualitative scoring was evaluated including the vascular contour, the extent of artifact, and overall diagnostic evaluation.

RESULTS

The attenuation, noise of the vessel and artifact, and the width of artifact decreased as the energy level increased (p < 0.001). The downtrend was relatively flat at 80-120 keV, and the vascular attenuation dropped to 200 HU at 90 keV. The qualitative rating of vascular contour was significantly higher at CIMAR (3.47) and VMIMAR 60-80 keV (2.82-3.65) compared with CI (2.03) (p ≤ 0.029), and the highest score occurred at 70 and 80 keV (3.65 and 3.56). The score of the extent of artifact was higher at VMIMAR 80 keV than CIMAR (3.53 VS 3.12, p = 0.003). The score of the overall diagnostic evaluation was higher at VMIMAR 70 and 80 keV (3.32 and 3.53, respectively) than CIMAR (3.12) (p ≤ 0.035).

CONCLUSION

Eighty kiloelectron volts on VMIMAR, providing satisfactorily reduced metal artifacts and improved vascular visualization, can be an optimal recommended parameter of spectrum CT for the assessment of periprosthetic vasculature in THA patients.

CRITICAL RELEVANCE STATEMENT

The metal artifact is gradually reducing with increasing energy level; however, the vascular visualization is worsening. The vascular visualization is terrible above 100 keV, while the vessel is disturbed by artifacts below 70 keV. The best performance is found at 80 keV.

KEY POINTS

• VMIMAR can provide both reduced metal artifacts and improved vascular visualization. • Eighty kiloelectron volts on VMIMAR performs best in vascular visualization of total hip arthroplasty patients. • Energy spectrum CT is recommended for routine use in patients with total hip arthroplasty.

Spectral metal artifact reduction after posterior spinal fixation in photon-counting detector CT datasets

PURPOSE

To investigate the usefulness of virtual monoenergetic image (VMI) reconstructions derived from scans on a novel photon-counting detector CT (PCD-CT) for artifact reduction in patients after posterior spinal fixation.

METHODS

This retrospective cohort study included 23 patients status post posterior spinal fixation. Subjects were scanned on a novel PCD-CT (NAEOTOM Alpha, Siemens Healthineers, Erlangen, Germany) as part of routine clinical care. 14 sets of VMI reconstructions were derived in 10 keV increments for the interval 60-190 keV. The mean and the standard deviation (SD) of CT-values in 12 defined locations around a pair of pedicle screws on one vertebral level and the SD of homogenous fat were measured and used to calculate an artifact index (AIx).

RESULTS

Averaged over all regions, the lowest AIx was observed at VMI levels of 110 keV (32.5 (27.8-37.9)) which was significantly different from those of VMIs ≤ 90 keV (p < 0.001) or ≥160 keV (p < 0.015), respectively. Overall AIx values increased in both lower- and higher-keV levels. Regarding individual locations, either a monotonous AIx-decrease for increasing keV values or an AIx-minimum in intermediate-keV levels (100-140 keV) was found. In locations adjacent to larger metal parts, the increase of AIx values at the high-end of the keV spectrum was mainly explained by a reappearance of streak artifacts.

CONCLUSION

Our findings suggest that 110 keV is the optimal VMI setting for overall artifact suppression. In specific anatomical regions, however, slight adjustments towards higher-keV levels may provide better results.

Combining iterative metal artifact reduction and virtual monoenergetic images severely reduces hip prosthesis-associated artifacts in photon-counting detector CT

Aim of this study was to assess the impact of virtual monoenergetic images (VMI) in combination and comparison with iterative metal artifact reduction (IMAR) on hip prosthesis-associated artifacts in photon-counting detector CT (PCD-CT). Retrospectively, 33 scans with hip prosthesis-associated artifacts acquired during clinical routine on a PCD-CT between 08/2022 and 09/2022 were analyzed. VMI were reconstructed for 100-190 keV with and without IMAR, and compared to polychromatic images. Qualitatively, artifact extent and assessment of adjacent soft tissue were rated by two radiologists using 5-point Likert items. Quantitative assessment was performed measuring attenuation and standard deviation in most pronounced hypodense and hyperdense artifacts, artifact-impaired bone, muscle, vessels, bladder and artifact-free corresponding tissue. To quantify artifacts, an adjusted attenuation was calculated as the difference between artifact-impaired tissue and corresponding tissue without artifacts. Qualitative assessment improved for all investigated image reconstructions compared to polychromatic images (PI). VMI_100keV in combination with IMAR achieved best results (e.g. diagnostic quality of the bladder: median PI: 1.5 (range 1-4); VMI_100keV+IMAR: 5 (3-5); p < 0.0001). In quantitative assessment VMI_100keV with IMAR provided best artifact reduction with an adjusted attenuation closest to 0 (e.g. bone: PI: 302.78; VMI_100keV+IMAR: 51.18; p < 0.0001). The combination of VMI and IMAR significantly reduces hip prosthesis-associated artifacts in PCD-CT and improves the diagnostic quality of surrounding tissue.

An Optimized Method for Microcomputed Tomography Analysis of Trabecular Parameters of Metal Scaffolds for Bone Ingrowth

Owing to its superior mechanical and biological properties, titanium metal is widely used in dental implants, orthopedic devices, and bone regenerative materials. Advances in 3D printing technology have led to more and more metal-based scaffolds being used in orthopedic applications. Microcomputed tomography (μCT) is commonly applied to evaluate the newly formed bone tissues and scaffold integration in animal studies. However, the presence of metal artifacts dramatically hinders the accuracy of μCT analysis of new bone formation. To acquire reliable and accurate μCT results that reflect new bone formation in vivo, it is crucial to lessen the impact of metal artifacts. Herein, an optimized procedure for calibrating μCT parameters using histological data was developed. In this study, the porous titanium scaffolds were fabricated by powder bed fusion based on computer-aided design. These scaffolds were implanted in femur defects created in New Zealand rabbits. After 8 weeks, tissue samples were collected to assess new bone formation using μCT analysis. Resin-embedded tissue sections were then used for further histological analysis. A series of deartifact two-dimensional (2D) μCT images were obtained by setting the erosion radius and the dilation radius in the μCT analysis software (CTan) separately. To get the μCT results closer to the real value, the 2D μCT images and corresponding parameters were subsequently selected by matching the histological images in the particular region. After applying the optimized parameters, more accurate 3D images and more realistic statistical data were obtained. The results demonstrate that the newly established method of adjusting μCT parameters can effectively reduce the influence of metal artifacts on data analysis to some extent. For further validation, other metal materials should be analyzed using the process established in this study.

Impact of CT Photon-Counting Virtual Monoenergetic Imaging on Visualization of Abdominal Arterial Vessels

PURPOSE

The novel photon-counting detector (PCD) technique acquires spectral data for virtual monoenergetic imaging (VMI) in every examination. The aim of this study was the evaluation of the impact of VMI of abdominal arterial vessels on quantitative and qualitative subjective image parameters.

METHODS

A total of 20 patients that underwent an arterial phase computed tomography (CT) scan of the abdomen with a novel PCD CT (Siemens NAEOTOM alpha) were analyzed regarding attenuation at different energy levels in virtual monoenergetic imaging. Contrast-to-noise ratio (CNR) and signal-to-noise ratio (SNR) were calculated and compared between the different virtual monoenergetic (VME) levels with correlation to vessel diameter. In addition, subjective image parameters (overall subjective image quality, subjective image noise and vessel contrast) were evaluated.

RESULTS

Our research showed decreasing attenuation levels with increasing energy levels in virtual monoenergetic imaging regardless of vessel diameter. CNR showed best overall results at 60 keV, and SNR at 70 keV with no significant difference to 60 keV (p = 0.294). Subjective image quality was rated best at 70 keV for overall image quality, vessel contrast and noise.

CONCLUSIONS

Our data suggest that VMI at 60-70 keV provides the best objective and subjective image quality concerning vessel contrast irrespective of vessel size.

Effects of different virtual monoenergetic CT image data on chest wall post-processing "unfolded ribs" and proposal of an algorithm improvement

PURPOSE

To find out if the use of different virtual monoenergetic data sets enabled by DECT technology might have a negative impact on post-processing applications, specifically in case of the "unfolded ribs" algorithm. Metal or beam hardening artifacts are suspected to generate image artifacts and thus reduce diagnostic accuracy. This paper tries to find out how the generation of "unfolded rib" CT image reformates is influenced by different virtual monoenergetic CT images and looks for possible improvement of the post-processing tool.

MATERIAL AND METHODS

Between March 2021 and April 2021, thin-slice dual-energy CT image data of the chest were used creating "unfolded rib" reformates. The same data sets were analyzed in three steps: first the gold standard with the original algorithm on mixed image data sets followed by the original algorithm on different keV levels (40-120 keV) and finally using a modified algorithm which in the first step used segmentation based on mixed image data sets, followed by segmentation based on different keV levels. Image quality (presence of artifacts), lesion and fracture detectability were assessed for all series.

RESULTS

Both, the original and the modified algorithm resulted in more artifact-free image data sets compared to the gold standard. The modified algorithm resulted in significantly more artifact-free image data sets at the keV-edges (40-120 keV) compared the original algorithm. Especially "black artifacts" and pseudo-lesions, potentially inducing false positive findings, could be reduced in all keV level with the modified algorithm. Detection of focal sclerotic, lytic or mixed (k = 0.990-1.000) lesions was very good for all keV levels. The Fleiss-kappa test for detection of fresh and old rib fractures was ≥ 0.997.

CONCLUSION

The use of different virtual monoenergetic keVs for the "unfolded rib" algorithm is generating different artifacts. Segmentation-based artifacts could be eliminated by the proposed new algorithm, showing the best results at 70-80 keV.

Metal implants on abdominal CT: does split-filter dual-energy CT provide additional value over iterative metal artifact reduction?

PURPOSE

To assess image quality and metal artifact reduction in split-filter dual-energy CT (sfDECT) of the abdomen with hip or spinal implants using virtual monoenergetic images (VMI) and iterative metal artifact reduction algorithm (iMAR).

METHODS

102 portal-venous abdominal sfDECTs of patients with hip (n = 71) or spinal implants (n = 31) were included in this study. Images were reconstructed as 120kVp-equivalent images (Mixed) and VMI (40-190 keV), with and without iMAR. Quantitative artifact and image noise was measured using 12 different ROIs. Subjective image quality was rated by two readers using a five-point Likert-scale in six categories, including overall image quality and vascular contrast.

RESULTS

Lowest quantitative artifact in both hip and spinal implants was measured in VMI_190keV-iMAR. However, it was not significantly lower than in Mixed_iMAR (for all ROIs, p = 1.00), which were rated best for overall image quality (hip: 1.00 [IQR: 1.00-2.00], spine: 3.00 [IQR:2.00-3.00]). VMI_50keV-iMAR was rated best for vascular contrast (hip: 1.00 [IQR: 1.00-2.00], spine: 2.00 [IQR: 1.00-2.00]), which was significantly better than Mixed (both, p < 0.001). VMI_50keV-iMAR provided superior overall image quality compared to Mixed for hip (1.00 vs 2.00, p < 0.001) and similar diagnostic image quality for spinal implants (2.00 vs 2.00, p = 0.51).

CONCLUSION

For abdominal sfDECT with hip or spinal implants Mixed_iMAR images should be used. High keV VMI do not further improve image quality. IMAR allows the use of low keV images (VMI_50keV) to improve vascular contrast, compared to Mixed images.

Dual-Energy Computed Tomography Applications to Reduce Metal Artifacts in Hip Prostheses: A Phantom Study

Metal components of hip prostheses cause severe artifacts in CT images, influencing diagnostic accuracy. Metal artifact reduction (MAR) software and virtual monoenergetic reconstructions on dual-energy CT (DECT) systems are possible solutions that should be considered. In this study, we created a customized adjustable phantom to quantify the severity of artifacts on periprosthetic tissues (cortical and spongious bone, soft tissues) for hip prostheses. The severity of artifacts was classified by different thresholds of deviation from the CT numbers for reference objects not affected by artifacts. The in vitro setup was applied on four unilateral and three bilateral configurations of hip prostheses (made of titanium, cobalt, and stainless steel alloys) with a DECT system, changing the energy of virtual monoenergetic reconstructions, with and without MAR. The impact of these tools on the severity of artifacts was scored, looking for the best scan conditions for the different configurations. For titanium prostheses, the reconstruction at 110 keV, without MAR, always minimized the artifacts. For cobalt and stainless-steel prostheses, MAR should always be applied, while monoenergetic reconstruction alone did not show clear advantages. The available tools for reducing metal artifacts must therefore be applied depending on the examined prosthetic configuration.

Comparison of Metal Artifact Reduction Algorithms in Patients with Hip Prostheses: Virtual Monoenergetic Images vs. Orthopedic Metal Artifact Reduction

Purpose

To assess the usefulness of various metal artifact reduction (MAR) methods in patients with hip prostheses.

Materials and Methods

This retrospective study included 47 consecutive patients who underwent hip arthroplasty and dual-energy CT. Conventional polyenergetic image (CI), orthopedic-MAR (O-MAR), and virtual monoenergetic image (VMI, 50-200 keV) were tested for MAR. Quantitative analysis was performed in seven regions around the prostheses. Qualitative assessments included evaluation of the degree of artifacts and the presence of secondary artifacts.

Results

The lowest amount of image noise was observed in the O-MAR, followed by the VMI. O-MAR also showed the lowest artifact index, followed by high-keV VMI in the range of 120-200 keV (soft tissue) or 200 keV (bone). O-MAR had the highest contrast-to-noise ratio (CNR) in regions with severe hypodense artifacts, while VMI had the highest CNR in other regions, including the periprosthetic bone. On assessment of the CI of pelvic soft tissues, VMI showed a higher structural similarity than O-MAR. Upon qualitative analysis, metal artifacts were significantly reduced in O-MAR, followed by that in VMI, while secondary artifacts were the most frequently found in the O-MAR (p < 0.001).

Conclusion

O-MAR is the best technique for severe MAR, but it can generate secondary artifacts. VMI at high keV can be advantageous for evaluating periprosthetic bone.

Metal Artifact Reduction Dual-Energy CT as an Accurate and Reliable Method for Measuring Total Knee Arthroplasty Femoral Component Rotation Compared to Conventional CT

This article determines the accuracy and reliability of dual-energy computed tomography (DECT) with metal artifact reduction (MAR) in the evaluation of femoral component rotation after total knee arthroplasty (TKA), in comparison with conventional CT images. A total of 49 patients (mean age, 69 years; 42 women) who underwent TKA between January 2019 and March 2020 were retrospectively enrolled. Femoral component rotation, including the anatomic and surgical transepicondylar axes, was evaluated with preoperative conventional CT and postoperative conventional CT and DECT with MAR. Surgical femoral component rotation was also assessed as a reference standard. Accuracy was assessed using paired t-test, and inter- and intraobserver reliability using intraclass correlation coefficients (ICCs) based on postoperative conventional CT and DECT with MAR. Clinical outcomes were evaluated using the Knee Society objective and functional scores. Accuracy of femoral component rotation was not significantly different from that of surgical rotation with both conventional CT and DECT with MAR. However, inter- and intraobserver reliability were better for DECT with MAR (ICC: 0.953-0.966) than for conventional CT (ICC: 0.641-0.749). The Knee Society objective and functional scores improved 1 year postoperatively. CONCLUSION:  DECT with MAR showed accurate and more reliable results than did conventional CT in the evaluation of femoral component rotation after TKA.

Combination of Iterative Metal Artifact Reduction and Virtual Monoenergetic Reconstruction Using Split-Filter Dual-Energy CT in Patients With Dental Artifact on Head and Neck CT

Background: Head and neck CT may be limited by dental hardware artifact. Both post-processing based iterative metal artifact reduction (iMAR) and virtual monoenergetic reconstruction (VMR) from dual-energy CT (DECT) can reduce metal artifact. Their combination is poorly described for single-source DECT systems. Objective: To compare metal artifact reduction between VMR, iMAR, and their combination, using split-filter single-source DECT in patients with severe dental hardware artifact. Methods: This retrospective study included 44 patients (9 female, 35 male; mean age 66.0±10.4 years) who underwent head and neck CT exhibiting severe dental hardware artifact. Standard images, VMR, iMAR, and VMRi-MAR were reconstructed; VMR and VMRiMAR used 40, 70, 100, 120, 150, and 190 keV. ROIs were placed to measure corrected attenuation in pronounced hyperattenuating and hypoattenuating artifacts and artifact-impaired soft tissue, as well as corrected artifact-impaired soft tissue noise. Two radiologists independently assessed soft tissue interpretability (1-5 scale); pooled ratings were analyzed. Readers selected the preferred reconstruction for each patient. Results: Mean hyperattenuating artifact corrected attenuation was 521.0 for standard, 496.4-892.2 for VMR, 48.2 for iMAR, and 32.8-91.0 for VMRiMAR. Mean hypoattenuating artifact corrected attenuation was -455.1 for standard, -408.5 to -679.9 for VMR, -37.3 for iMAR, and -17.8 to -36.9 for VMRiMAR. Mean artifact-impaired soft tissue corrected attenuation was 10.8 for standard, -0.6 to 24.9 for VMR, 4.3 for iMAR, and -2.0 to 7.8 for VMRiMAR. Mean artifact-impaired soft tissue corrected noise was 58.7 for standard, 38.2-129.7 for VMR, 11.0 for iMAR, and 5.8-45.6 for VMRiMAR. Median soft tissue interpretability was 1.2 for standard, 1.1-1.2 for VMR, 3.7 for iMAR, and 2.0-3.8 for VMRiMAR. Artifact-impaired soft tissue corrected attenuation and soft tissue interpretability were significantly improved (p<.05) for VMRiMAR versus iMAR only at 100 keV. Readers preferred VMRiMAR at 100 keV in 56.8% and 59.1% of examinations. Conclusion: For reducing severe dental material artifact, iMAR has greater impact than VMR. Though iMAR and VMRiMAR were overall similar, VMRiMAR exhibited small benefit at 100 keV. Clinical Impact: VMR and iMAR techniques, using split-filter DECT, may be combined for clinical head and neck imaging to reduce artifact from dental hardware and improve image quality.

Principles and Applications of Dual Energy Computed Tomography in Neuroradiology

Dual-energy computed tomography (DE CT) is a promising tool with many current and evolving applications. Available DE CT scanners usually consist of one or two tubes, or use layered detectors for spectral separation. Most DE CT scanners can be used in single energy or dual-energy mode, except for the layered detector scanners that always acquire data in dual-energy mode. However, the layered detector scanners can retrospectively integrate the data from two layers to obtain conventional single energy images. DE CT mode enables generation of virtual monochromatic images, blended images, iodine quantification, improving conspicuity of iodinated contrast enhancement, and material decomposition maps or more sophisticated quantitative analysis not possible with conventional SE CT acquisition with an acceptable or even lower dose than the SE CT. This article reviews the basic principles of dual-energy CT and highlights many of its clinical applications in the evaluation of neurological conditions.

Reduction of microwave ablation needle related metallic artifacts using virtual monoenergetic images from dual-layer detector spectral CT in a rabbit model with VX2 tumor

The purpose of the study was to investigate the application of virtual monoenergetic images (VMIs) in reducing metal artifacts in rabbit VX2 liver cancer models treated with microwave ablation (MWA) therapy. A total of 31 VX2 liver cancer models that accepted CT-guided percutaneous microwave ablation were analyzed. Conventional images (CIs) with the most severe metallic artifacts and their corresponding energy levels from 40 to 200 keV with 10 keV increment of VMIs were reconstructed for further analysis. Objective image analysis was assessed by recording the attenuation (HU) and standard deviation of the most severe hyper/hypodense artifacts as well as artifact-impaired liver parenchyma tissue. Two radiologists visually evaluated the extent of artifact reduction, assessed data obtained by a diagnostic evaluation of liver tissues, and appraised the appearance of new artifacts according to the grade score. Statistical analysis was performed to compare the difference between CIs and each energy level of VMIs. For subjective assessment, reductions in hyperdense and hypodense artifacts were observed at 170–200 keV and 160–200 keV, respectively. The outcomes of the diagnostic evaluation of adjacent liver tissue were statistically higher at 140–200 keV for VMIs than for CIs. In terms of objective evaluation results, VMIs at 90–200 keV reduced the corrected attenuation of hyperdense and of artifact-impaired liver parenchyma compared with CIs (P < 0.001). When VMIs at 80–200 keV decreased the hypodense artifacts (P < 0.001). Therefore, we concluded that VMIs at 170–200 keV can obviously decrease the microwave ablation needle-related metal artifacts objectively and subjectively in rabbit VX2 liver cancer models.

Low-Dose MDCT of Patients With Spinal Instrumentation Using Sparse Sampling: Impact on Metal Artifacts

OBJECTIVE. The purpose of our study was to evaluate simulated sparse-sampled MDCT combined with statistical iterative reconstruction (SIR) for low-dose imaging of patients with spinal instrumentation. MATERIALS AND METHODS. Thirty-eight patients with implanted hardware after spinal instrumentation (24 patients with short- or long-term instrumentation-related complications [i.e., adjacent segment disease, screw loosening or implant failure, or postoperative hematoma or seroma] and 14 control subjects with no complications) underwent MDCT. Scans were simulated as if they were performed with 50% (P50), 25% (P25), 10% (P10), and 5% (P5) of the projections of the original acquisition using an in-house-developed SIR algorithm for advanced image reconstructions. Two readers performed qualitative image evaluations of overall image quality and artifacts, image contrast, inspection of the spinal canal, and diagnostic confidence (1 = high, 2 = medium, and 3 = low confidence). RESULTS. Although overall image quality decreased and artifacts increased with reductions in the number of projections, all complications were detected by both readers when 100% of the projections of the original acquisition (P100), P50, and P25 imaging data were used. For P25 data, diagnostic confidence was still high (mean score ± SD: reader 1, 1.2 ± 0.4; reader 2, 1.3 ± 0.5), and interreader agreement was substantial to almost perfect (weighted Cohen κ = 0.787-0.855). The mean volumetric CT dose index was 3.2 mGy for P25 data in comparison with 12.6 mGy for the original acquisition (P100 data). CONCLUSION. The use of sparse sampling and SIR for low-dose MDCT in patients with spinal instrumentation facilitated considerable reductions in radiation exposure. The use of P25 data with SIR resulted in no missed complications related to spinal instrumentation and allowed high diagnostic confidence, so using only 25% of the projections is probably enough for accurate and confident diagnostic detection of major instrumentation-related complications.

Dual-Layer Detector CT With Virtual Noncalcium Imaging: Diagnostic Performance in Patients With Suspected Wrist Fractures

OBJECTIVE. The purpose of this study was to evaluate the diagnostic utility of dual-layer CT (DLCT) for evaluating wrist injuries and to compare it with MRI. MATERIALS AND METHODS. The cases of 62 patients with suspected wrist fractures who underwent imaging with both DLCT and MRI from January 2018 through February 2019 were retrospectively reviewed. By means of a calcium suppression algorithm, virtual noncalcium (VNCa) image reconstruction was performed, and the images were reviewed by two readers to identify fractures, bone contusions, and nontraumatic lesions in the radius, ulna, and carpal bones. Sensitivity, specificity, PPV, and NPV were calculated and compared between standard CT and VNCa images with a combination of standard CT and MRI as the reference standard. RESULTS. Use of DLCT with VNCa reconstruction increased the sensitivity of diagnosis of fractures in the radius and carpal bones over that of standard CT alone; occult fractures were detected that were not seen with standard CT. The sensitivity and specificity for detecting radius fracture were 98.1% and 93.8% for DLCT and 96.3% and 93.8% for standard CT. For detecting carpal bone fracture, sensitivity and specificity were 100% and 98.9% for DLCT and 93.8% and 100% for standard CT. VNCa reconstruction also had good diagnostic accuracy with regard to diagnosing nonfracture bone contusions in carpal bones. The accuracy was comparable to that of MRI with sensitivity of 92.9% and specificity of 94.5%. Interreader agreement in interpreting VNCa images was generally good to excellent. CONCLUSION. DLCT with VNCa reconstruction is a promising tool for identifying occult wrist fractures and nonfracture contusion injuries in patients with wrist trauma.

Dual energy imaging in cardiothoracic pathologies: A primer for radiologists and clinicians

Recent advances in dual-energy imaging techniques, dual-energy subtraction radiography (DESR) and dual-energy CT (DECT), offer new and useful additional information to conventional imaging, thus improving assessment of cardiothoracic abnormalities. DESR facilitates detection and characterization of pulmonary nodules. Other advantages of DESR include better depiction of pleural, lung parenchymal, airway and chest wall abnormalities, detection of foreign bodies and indwelling devices, improved visualization of cardiac and coronary artery calcifications helping in risk stratification of coronary artery disease, and diagnosing conditions like constrictive pericarditis and valvular stenosis. Commercially available DECT approaches are classified into emission based (dual rotation/spin, dual source, rapid kilovoltage switching and split beam) and detector-based (dual layer) systems. DECT provide several specialized image reconstructions. Virtual non-contrast images (VNC) allow for radiation dose reduction by obviating need for true non contrast images, low energy virtual mono-energetic images (VMI) boost contrast enhancement and help in salvaging otherwise non-diagnostic vascular studies, high energy VMI reduce beam hardening artifacts from metallic hardware or dense contrast material, and iodine density images allow quantitative and qualitative assessment of enhancement/iodine distribution. The large amount of data generated by DECT can affect interpreting physician efficiency but also limit clinical adoption of the technology. Optimization of the existing workflow and streamlining the integration between post-processing software and picture archiving and communication system (PACS) is therefore warranted.

Assessment of the image quality of virtual monochromatic spectral computed tomography images: a phantom study considering object contrast, radiation dose, and frequency characteristics

Fast kilovoltage (kVp)-switching technology cannot obtain conventional 120 kVp images; thus, 70 keV virtual monochromatic spectral computed tomography (CT) images (VMSI) are generally used. The contrast-to-noise ratio (CNR) is used to evaluate the image quality of VMSI; however, CNR does not include frequency characteristics. The present study aimed to investigate the evaluation methods of VMSI considering frequency characteristics by comparing the image quality of 70 keV VMSI with that of conventional 120 kVp images. The evaluated object contrasts were 70 and 300 Hounsfield units (HU). Scans used two radiation dose levels: low (LD) and standard (SD). The volume CT dose index of LD and SD was 4.8- and 12 mGy, respectively. Images were reconstructed by filtered back projection, evaluating CNR, noise power spectrum (NPS), task transfer function (TTF), and system performance (SP) function calculated as TTF²/ NPS. The total NPS values (spatial frequency range: 0.2 ~ 0.4 mm^-1) of 70 keV VMSI were higher than those of 120 kVp images. The spatial frequency TTF values that reached 10% (f_10%) of the 70 keV VMSI changed based on object contrast. For the low-contrast condition, a lower f_10% was observed with 70 keV VMSI. The CNR of 70 keV VMSI was comparable to that of 120 kVp images in low- and high-contrast conditions. However, for 70 keV VMSI, SP of low-contrast was low, and SP of high-contrast was high, compared with those of 120 kVp images. This study suggested that only CNR was not sufficient to evaluate the image quality of VMSI; thus, evaluation methods considering frequency characteristics should be used.

Metal artifacts reduction in computed tomography: A phantom study to compare the effectiveness of metal artifact reduction algorithm, model-based iterative reconstruction, and virtual monochromatic imaging

The purpose of this study was to compare the effectiveness of a metal artifact reduction algorithm (MAR), model-based iterative reconstruction (MBIR), and virtual monochromatic imaging (VMI) for reducing metal artifacts in CT imaging.A phantom study was performed for quantitatively evaluating the dark bands and fine streak artifacts generated by unilateral hip prostheses. Images were obtained by conventional scanning at 120 kilovolt peak, and reconstructed using filtered back projection, MAR, and MBIR. Furthermore, virtual monochromatic images (VMIs) at 70 kilo-electron volts (keV) and 140 keV with/without use of MAR were obtained by dual-energy CT. The extents and mean CT values of the dark bands and the differences in the standard deviations and location parameters of the fine streak artifacts evaluated by the Gumbel method in the images obtained by each of the methods were statistically compared by analyses of variance.Significant reduction of the extent of the dark bands was observed in the images reconstructed using MAR than in those not reconstructed using MAR (all, P < .01). Images obtained by VMI at 70 keV and 140 keV with use of MAR showed significantly increased mean CT values of the dark bands as compared to those obtained by reconstructions without use of MAR (all, <.01). Significant reduction of the difference in the standard deviations used to evaluate fine streak artifacts was observed in each of the image sets obtained with VMI at 140 keV with/without MAR and conventional CT with MBIR as compared to the images obtained using other methods (all, P < .05), except between VMI at 140 keV without MAR and conventional CT with MAR. The location parameter to evaluate fine streak artifacts was significantly reduced in CT images obtained using MBIR and in images obtained by VMI at 140 keV with/without MAR as compared to those obtained using other reconstruction methods (all, P < .01).In our present study, MAR appeared to be the most effective reconstruction method for reducing dark bands in CT images, and MBIR and VMI at 140 keV appeared to the most effective for reducing streak artifacts.

Quantitative positron emission tomography imaging in the presence of iodinated contrast media using electron density quantifications from dual-energy computed tomography

PURPOSE

As preparation for future positron emission tomography (PET)/dual-energy computed tomography (DECT)T imaging modality and new possible clinical applications, the study aimed to evaluate the utility of clinically available spectral results from a DECT system for improving attenuation corrections of PET acquisitions in the presence of iodinated contrast media. The dependence of the accuracy of PET quantification values, reconstructed with conventional and spectral-based attenuation corrections, was examined as a function of the amount of iodine content and x-ray radiation exposure.

METHODS

Measurements were performed on commercial PET/CT and DECT systems, using a semi-anthropomorphic phantom with seven centrifuge tubes in its bore. Five different configurations of tube contents were scanned by both PET/CT and DECT. With the aim of mimicking clinically observed concentrations, in all phantom configurations the center tube contained a high concentration of radionuclide while the peripheral tubes contained a lower concentration of radionuclide. Iodine content was incrementally increased between phantom configurations by replacing iodine-free tubes with tubes that contained the original radionuclide concentration within a 10 mg/ml iodine dilution. DECT-based attenuation correction maps were generated by scaling electron density spectral results into corresponding 511 keV photon linear attenuation coefficients.

RESULTS

Mean SUV values obtained from the nominal PET reconstruction, using conventional CT images as input for the attenuation correction, demonstrate a monotonic increase of 8.6% when the water and radionuclide mixtures were replaced by iodine, water, and radionuclide (same level of activity) mixture. Mean SUV values obtained from the DECT-based reconstruction, in which the attenuation correction utilizes electron density values as input, demonstrate different, more stable behavior across all iodine insert configurations, with a standard deviation to mean ratio of less than 1%. This observed behavior was independent of the area size used for measurement. A minor radiation dose dependency of the electron density values (below 0.5%) was observed. This resulted in consistent (iodine independent) PET quantification behavior, which persisted even at the lowest radiation dose levels tested in our experiment, that is, 25% of the radiation dose utilized for CT acquisition in the clinical PET/CT protocol.

CONCLUSIONS

Utilization of DECT-generated electron density estimations for attenuation correction benefit PET quantification consistency in the presence of iodine and at nominal and low DECT radiation exposure levels. The ability to correctly account for iodinated contrast media in PET acquisitions will allow the development of new clinical applications that rely on the quantitative capabilities of spectral CT technologies and modern PET systems.

Improved diagnostic confidence in evaluating bone non-union using virtual monochromatic dual-energy CT

PURPOSE

The aim of this study was to determine whether virtual monochromatic dual-energy CT imaging improves the evaluation of suspected non-union of the appendicular skeleton treated with titanium or stainless steel intramedullary nails and plates.

METHOD

Forty-one patients with a clinical suspected non-union with hardware in place were included and scanned on a dual-source CT-scanner using 100/Sn150kVp. Images including titanium hardware were extracted at 130 keV. Images including stainless steel hardware were extracted at 150 keV. Monochromatic 70 keV images served as reference. Non-union confirmed during revision surgery was used as gold standard. A musculoskeletal radiologist and orthopedic trauma surgeon evaluated images on image quality, degree and location of consolidation, non-union type and diagnostic confidence.

RESULTS

Likert scores with respect to image quality improved from 0.88 to 1.83 (p < 0.001) in high (130 and 150) keV images. High keV images reduced the number of false negative non-unions based on consolidation grade with 5% (p = 0.283). Agreement between observers regarding location of consolidation and non-union type did not improve in 130 and 150 keV images. Diagnostic confidence improved from 1.43 to 2.37 in high keV images compared to 70 keV images (p < 0.001). Overall diagnostic confidence was higher in intramedullary nails than plates (p < 0.05).

CONCLUSIONS

Use of virtual monochromatic 130 and 150 keV dual-energy CT compared to 70 keV images improves the evaluation of suspected non-union of the appendicular skeleton treated with titanium or stainless steel intramedullary nails and plates.

The Optimal Combination of Monochromatic and Metal Artifact Reconstruction Dual-energy CT to Evaluate Total Knee Replacement Arthroplasty

PURPOSE

To evaluate the effects of gemstone spectral imaging-metal artifact reduction (GSI-MAR) on different dual-energy CT monochromatic images for patients with total knee replacement arthroplasty (TKRA) and to identify an appropriate protocol for clinical practice.

METHOD

We enrolled 34 patients with TKRA. CT images were iteratively reconstructed with or without GSI-MAR at different energy levels (70, 95, 115, and 140 keV). Two radiologists evaluated the objective and subjective image qualities and MAR-related new artifacts at the femoral and tibial levels. For objective analysis, the mean CT number and image noise of the selected regions of interest in the bone and muscle were recorded. To quantitatively evaluate the performance of GSI-MAR, a structural similarity index (SSIM) was used. For subjective analysis, streak artifacts and diagnostic confidence in detecting periprosthetic complications were assessed. Objective and subjective indicators were compared among the image combinations.

RESULTS

In the femoral component, 140 keV monochromatic energy images with GSI-MAR showed the lowest mean CT number, image noise, SSIM value, and streak artifacts, and the best diagnostic confidence. In the tibial component, the image noise differed significantly, but the SSIM and subjective indicators were similar among the image combinations. MAR-related new artifacts were noted in 14.7% of images, and all of them were observed in only the femoral component.

CONCLUSION

GSI-MAR with higher-energy monochromatic images showed fewer metal artifacts and better visualization. We recommend 140 keV with GSI-MAR for improving image quality and 140 keV without GSI-MAR for identifying MAR-related new artifacts when evaluating TKRA.

Benefit of dual-layer spectral CT in emergency imaging of different organ systems

Computed tomography (CT) has been the first choice of imaging technique in the emergency department and has a crucial role in many acute conditions. Since its implementation, spectral CT has gained widespread application with the potential to improve diagnostic performance and impact patient care. In spectral CT, images are acquired at two different energy levels allowing this technique to differentiate tissues by exploiting their energy-dependent attenuation properties. Dual-layer spectral CT provides additional information with its material decomposition applications that include virtual non-contrast imaging, iodine density, and effective atomic number (Zeff) maps along with virtual monoenergetic images without the need for preselection of a protocol. This review aims to demonstrate its added value in the emergency department in different organ systems enabling better evaluation of inflammatory and ischaemic conditions, assessment of organ perfusion, tissue/lesion characterisation and mass detection, iodine quantification, and the use of lower volumes of contrast medium. With improved diagnostic performance, spectral CT could also aid in rapid decision-making to determine the treatment method in many acute conditions without increased radiation dose to the patient.

Quantification of metal artifacts in computed tomography: methodological considerations

Numerous methods for artifact quantification in computed tomography (CT) imaging have been suggested. This study evaluated their utility with regards to correspondence with visual artifact perception and reproducibility. Two titanium rods (5 and 10 mm) were examined with 25 different scanning- and image-reconstruction parameters resulting in different types and extents of artifacts. Four radiologists evaluated every image against each other using an in-house developed software. Rating was repeated two times (2,400 comparisons = 2 times × 4 readers × 300 comparisons). Rankings were combined to obtain a reference ranking. Proposed approaches for artifact quantification include manual measurement of attenuation, standard deviation and noise and sophisticated algorithm-based approaches within the image- and frequency-domain. Two radiologists conducted manual measurements twice while the aforementioned algorithms were implemented within the Matlab-Environment allowing for automated image analysis. The reference ranking was compared to all aforementioned methods for artifact quantification to identify suited approaches. Besides visual analysis, Kappa-statistics and intraclass correlation coefficients (ICC) were used. Intra- and Inter-reader agreements of visual artifact perception were excellent (ICC 0.85-0.92). No quantitative method was able to represent the exact ranking of visually perceived artifacts; however, ICC for manual measurements were low (ICC 0.25-0.97). The method that showed best correspondence and reproducibility used a Fourier-transformed linear ROI and lower-end frequency bins. Automated measurements of artifact extent should be preferred over manual measurements as the latter show a limited reproducibility. One method that allows for automated quantification of such artefacts is made available as an electronic supplement.

Interdisciplinary consensus of virtual monochromatic dual-energy CT images: is there discrepancy in preferred photon energy between surgeons and radiologists for the assessment of non-unions?

AIM

To investigate possible differences between surgeons and radiologists in selecting optimal photon energy settings from a set of virtual monochromatic dual-energy computed tomography (CT) images for the assessment of bone union in patients with a suspected non-union of the appendicular skeleton.

MATERIALS AND METHODS

Fifty patients suspected of having bone non-union after operative fracture treatment with a variety of fixation implants were included. Patients were scanned on a dual-source CT machine using 150/100-kVp. Monochromatic images were extracted at 70, 90, 110, 130, 150, and 190 keV. Images were reviewed by 159 orthopaedic trauma surgeons and 12 musculoskeletal radiologists in order to select the best and worst energy setting to assess bone union. Furthermore, a confidence score (1-4) was given in selecting the best and worst setting to assess bone union.

RESULTS

Monochromatic 190 keV images were selected most frequently as the optimal energy in titanium (34.8%), stainless steel (40%), and combined implants of stainless steel and titanium (40.5%). Confidence scores and average optimal energies were higher and average worst energies were lower for radiologists compared to surgeons in all hardware (p<0.05). Differences in optimal energy were not statistically significant for different alloys or type of fixation implant in both groups.

CONCLUSIONS

In both observer groups, 190 keV images were selected most frequently as the optimal energy to assess bone union in patients with a suspected non-union of the appendicular skeleton with hardware in situ. On average, musculoskeletal radiologists selected higher optimal and lower worst energy settings and were more confident in selecting both energy settings than orthopaedic trauma surgeons.

Dual-layer spectral computerized tomography for metal artifact reduction: small versus large orthopedic devices

INTRODUCTION

Metal artifacts limit the diagnostic utility of computerized tomography (CT) for implant-related complications. Dual-layer spectral detector CT imaging has been suggested for artifact reduction. Our objective was to evaluate the utility of spectral CT in artifact reduction in patients with small and large metal implants.

METHODS

In this prospective study, patients with metallic orthopedic implants underwent CT imaging using a prototype spectral detector CT scanner. Conventional images were generated with iterative reconstruction at 120 kVp, and virtual monochromatic images were generated at 20-keV intervals between 40 to 200 keV. Conventional and monochromatic images were compared quantitatively using signal-to-noise ratio (SNR) and artifact improvement. Qualitative analysis was performed independently by two musculoskeletal radiologists and included six image quality indicators.

RESULTS

A total of 12 patients were scanned. In monochromatic images, as the energy level increased, the artifact size decreased progressively (p < 0.01). When conventional and monochromatic images were compared, maximum reduction was seen at 200 keV. Using qualitative assessments, 160 and 180 keV levels had the best overall diagnostic image quality. With increased energy level, there was improvement in qualitative ratings of bone-metal interface conspicuity (p = 0.002), degree of streak artifact (p = 0.010) and trabecular bone definition at 1 cm from implant (p = 0.023), and a trend towards significance for bone definition at 5 cm, soft tissue detail and overall diagnostic quality. Subgroup analysis revealed superior artifact reduction in small implants compared to large hardware.

DISCUSSION

Our results support the utility of dual-layer spectral CT in metal artifact reduction. Virtual monochromatic images were diagnostically superior, especially for smaller implants. Virtual monoenergetic images at 160-180 keV are ideal for reducing artifacts.

Dual energy computed tomography virtual monoenergetic imaging: technique and clinical applications

Dual energy CT (DECT) has evolved into a commonly applied imaging technique in clinical routine due to its unique post-processing opportunities for improved evaluation of all body areas. Reconstruction of virtual monoenergetic imaging (VMI) series has shown beneficial effects for both non-contrast and contrast-enhanced DECT due to the flexibility to calculate low-keV VMI reconstructions to increase contrast and iodine attenuation, or to compute high-keV VMI reconstructions to reduce beam-hardening artefacts. The goal of this review article is to explain the technical background of VMI and noise-optimized VMI+ algorithms and to give an overview of useful clinical applications of the VMI technique in DECT of various body regions.

Utility of virtual monoenergetic images from spectral detector computed tomography in improving image segmentation for purposes of 3D printing and modeling

BACKGROUND

One of the key steps in generating three-dimensional (3D) printed models in medicine is segmentation of radiologic imaging. The software tools used for segmentation may be automated, semi-automated, or manual which rely on differences in material density, attenuation characteristics, and/or advanced software algorithms. Spectral Detector Computed Tomography (SDCT) is a form of dual energy computed tomography that works at the detector level to generate virtual monoenergetic images (VMI) at different energies/ kilo-electron volts (keV). These VMI have varying contrast and attenuation characteristics relative to material density. The purpose of this pilot project is to explore the use of VMI in segmentation for medical 3D printing in four separate clinical scenarios. Cases were retrospectively selected based on varying complexity, value of spectral data, and across multiple clinical disciplines (Vascular, Cardiology, Oncology, and Orthopedic).

RESULTS

In all four clinical cases presented, the segmentation process was qualitatively reported as easier, faster, and increased the operator's confidence in obtaining accurate anatomy. All cases demonstrated a significant difference in the calculated Hounsfield Units between conventional and VMI data at the level of targeted segmentation anatomy. Two cases would not have been feasible for segmentation and 3D printing using conventional images only. VMI data significantly reduced conventional CT artifacts in one of the cases.

CONCLUSION

Utilization of VMI from SDCT can improve and assist the segmentation of target anatomy for medical 3D printing by enhancing material contrast and decreasing CT artifact.

Metal artifacts in patients with large dental implants and bridges: combination of metal artifact reduction algorithms and virtual monoenergetic images provides an approach to handle even strongest artifacts

OBJECTIVES

This study compares reduction of strong metal artifacts from large dental implants/bridges using spectral detector CT-derived virtual monoenergetic images (VMI), metal artifact reduction algorithms/reconstructions (MAR), and a combination of both methods (VMI_MAR) to conventional CT images (CI).

METHODS

Forty-one spectral detector CT (SDCT) datasets of patients that obtained additional MAR reconstructions due to strongest artifacts from large oral implants were included. CI, VMI, MAR, and VMI_MAR ranging from 70 to 200 keV (10 keV increment) were reconstructed. Objective image analyses were performed ROI-based by measurement of attenuation (HU) and standard deviation in most pronounced hypo-/hyperdense artifacts as well as artifact impaired soft tissue (mouth floor/soft palate). Extent of artifact reduction, diagnostic assessment of soft tissue, and appearance of new artifacts were rated visually by two radiologists.

RESULTS

The hypo-/hyperattenuating artifacts showed an increase and decrease of HU values in MAR and VMI_MAR (CI/MAR/VMI_MAR-200keV: - 369.8 ± 239.6/- 37.3 ± 109.6/- 46.2 ± 71.0 HU, p < 0.001 and 274.8 ± 170.2/51.3 ± 150.8/36.6 ± 56.0, p < 0.001, respectively). Higher keV values in hyperdense artifacts allowed for additional artifact reduction; however, this trend was not significant. Artifacts in soft tissue were reduced significantly by MAR and VMI_MAR. Visually, high-keV VMI, MAR, and VMI_MAR reduced artifacts and improved diagnostic assessment of soft tissue. Overcorrection/new artifacts were reported that mostly did not hamper diagnostic assessment. Overall interrater agreement was excellent (ICC = 0.85).

CONCLUSIONS

In the presence of strong artifacts due to large oral implants, MAR is a powerful mean for artifact reduction. For hyperdense artifacts, MAR should be supplemented by VMI ranging from 140 to 200 keV. This combination yields optimal artifact reduction and improves the diagnostic image assessment in imaging of the head and neck.

KEY POINTS

• Large oral implants can cause strong artifacts. • MAR is a powerful tool for artifact reduction considering such strong artifacts. • Hyperdense artifact reduction is supplemented by VMI of 140-200 keV from SDCT.

Accuracy of iodine density thresholds for the separation of vertebral bone metastases from healthy-appearing trabecular bone in spectral detector computed tomography

PURPOSE

To evaluate quantitative iodine density mapping (IDM) with spectral detector computed tomography (SDCT) as a quantitative biomarker for separation of vertebral trabecular bone metastases (BM) from healthy-appearing trabecular bone (HTB).

MATERIALS AND METHODS

IRB-approved retrospective single-center-study of portal venous SDCT datasets acquired between June 2016 and March 2017. Inclusion of 43 consecutive cancer patients with BM and 40 without. Target lesions and non-affected control vertebrae were defined using follow-up imaging, MRI, and/or bone scintigraphy. ID and standard deviation were determined with ROI measures by two readers in (a) bone metastases, (b) HTB of BM patients and controls, and (c) ID of various vessels. Volumetric bone mineral density (vBMD) of the lumbar spine and age were recorded. Multivariate ROC analyses und Wilcoxon test were used to determine thresholds for separation of BM and HTB. p < 0.05 was considered significant.

RESULTS

ID measurements of 40 target lesions and 83 reference measurements of HTB were acquired. Age (p < 0.0001) and vBMD (p < 0.05) affected ID measurements independently in multivariate models. There were significant differences of ID between metastases (n = 43) and HTB ID (n = 124; mean 5.5 ± 0.9 vs. 3.5 ± 0.9; p < 0.0001), however, with considerable overlap. In univariate analysis, increased ID discriminated bone lesions (AUC 0.90) with a maximum combined specificity/sensitivity of 77.5%/90.7% when applying a threshold of 4.5 mg/ml. Multivariate regression models improved significantly when considering vBMD, the noise of ID, and vertebral venous ID (AUC 0.98).

CONCLUSION

IDM of SDCT yielded a statistical separation of vertebral bone lesions and HTB. Adjustment for confounders such as age and lumbar vBMD as well as for vertebral venous ID and lesion heterogeneity improved discrimination of trabecular lesions.

KEY POINTS

• SDCT iodine density mapping provides the possibility for quantitative analysis of iodine uptake in tissue, which allows to differentiate bone lesions from healthy bone marrow. • Age and vBMD have a significant impact on iodine density measurements. • Iodine density measured in SDCT yielded highest sensitivity and specificity for the statistical differentiation of vertebral trabecular metastases and healthy trabecular bone using an iodine density threshold of 4.5 mg/ml (most performant)-5.0 mg/ml (optimized for specificity).

Clinical Assessment of Metal Artifact Reduction Methods in Dual-Energy CT Examinations of Instrumented Spines

OBJECTIVE

The purpose of this study was to evaluate the performance of three metal artifact reduction methods in dual-energy CT (DECT) examinations of instrumented spines.

MATERIALS AND METHODS

Twenty patients with instrumented spines who underwent spine DECT were retrospectively identified. All scans were obtained on a dual-source 128-MDCT scanner. In addition to the original DE mixed images, DECT images were reconstructed using an iterative metal artifact reconstruction algorithm (DE iMAR), virtual monochromatic imaging (VMI) algorithm (DE Mono+), and a combination of the two algorithms DE iMAR and DE Mono+, which we refer to here as "DE iMAR Mono+." The four image series were anonymized and randomized for a reader study. Four experienced neuroradiologists rated the images in terms of artifact scores of four anatomic regions and overall image quality scores in both bone and soft-tissue display window settings. In addition, a quantitative analysis was performed to assess the performance of the three metal artifact reduction methods.

RESULTS

There were statistically significant differences in the artifact scores and overall image quality scores among the four methods (both, p < 0.001). DE iMAR Mono+ showed the best artifact scores and quality scores (all, p < 0.001). The intraclass correlation coefficient for the overall image quality score was 0.779 using the bone display window and 0.892 using the soft-tissue display window (both, p < 0.001). In addition, DE iMAR Mono+ reduced the artifacts by the greatest amount in the quantitative analysis.

CONCLUSION

The method that used DE iMAR Mono+ showed the best performance of spine metal artifact reduction using DECT data. These results may be specific to this CT vendor and implant type.