Improved Image Quality in Head and Neck CT Using a 3D Iterative Approach to Reduce Metal Artifact

A New Iterative Metal Artifact Reduction Algorithm for Both Energy-Integrating and Photon-Counting CT Systems

OBJECTIVES

The aim of this study was to introduce and evaluate a new metal artifact reduction framework (iMARv2) that addresses the drawbacks (residual artifacts after correction and user preferences for image quality) associated with the current clinically applied iMAR.

MATERIALS AND METHODS

A new iMARv2 has been introduced, combining the current iMAR with new modular components to remove residual metal artifacts after image correction. The postcorrection image impression is adjustable with user-selectable strength settings. Phantom scans from an energy-integrating and a photon-counting detector CT were used to assess image quality, including a Gammex phantom and anthropomorphic phantoms. In addition, 36 clinical cases (with metallic implants such as dental fillings, hip replacements, and spinal screws) were reconstructed and evaluated in a blinded and randomized reader study.

RESULTS

The Gammex phantom showed lower HU errors compared with the uncorrected image at almost all iMAR and iMARv2 settings evaluated, with only minor differences between iMAR and the different iMARv2 settings. In addition, the anthropomorphic phantoms showed a trend toward lower errors with higher iMARv2 strength settings. On average, the iMARv2 strength 3 performed best of all the clinical reconstructions evaluated, with a significant increase in diagnostic confidence and decrease in artifacts. All hip and dental cases showed a significant increase in diagnostic confidence and decrease in artifact strength, and the improvements from iMARv2 in the dental cases were significant compared with iMAR. There were no significant improvements in the spine.

CONCLUSIONS

This work has introduced and evaluated a new method for metal artifact reduction and demonstrated its utility in routine clinical datasets. The greatest improvements were seen in dental fillings, where iMARv2 significantly improved image quality compared with conventional iMAR.

Iterative Metal Artifact Reduction in Head and Neck CT Facilitates Tumor Visualization of Oral and Oropharyngeal Cancer Obscured by Artifacts From Dental Hardware

RATIONALE AND OBJECTIVES

The purpose of this study was to evaluate the diagnostic utility of iterative metal artifact reduction (iMAR) in computed tomography (CT)-imaging of oral and oropharyngeal cancers when obscured by dental hardware artifacts and to determine the most appropriate iMAR settings for this purpose.

MATERIALS AND METHODS

The study retrospectively enrolled 27 patients (8 female, 19 male; mean age 64±12.7years) with histologically confirmed oral or oropharyngeal cancer obscured by dental artifacts in contrast-enhanced CT. Raw CT data were reconstructed with ascending iMAR strengths (levels 1/2/3/4/5) and one reconstruction without iMAR (level 0). For subjective analysis, two blinded radiologists rated tumor visualization and artifact severity on a five-point Likert scale. For objective analysis, signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), and artifact index (AI) were determined.

RESULTS

iMAR reconstructions improved the subjective image quality of tumor edge and contrast, and the objective parameters of tumor SNR and CNR, reaching their optimum at iMAR levels 4 and 5 (P<.001). AI decreased with iMAR reconstructions reaching its minimum at iMAR level 5 (P<.001). Tumor detection rates increased 2.4-fold with iMAR 5, 2.1-fold with iMAR 4, and 1.9-fold with iMAR 3 compared to reconstructions without iMAR. Disadvantages such as algorithm-induced artifacts increased significantly with higher iMAR strengths (P<.05), reaching a maximum with iMAR 5.

CONCLUSION

iMAR significantly improves CT imaging of oral and oropharyngeal cancers, as confirmed by both subjective and objective measures, with best results at highest iMAR strengths.

Personalization of thoracoabdominal CT examinations using scanner integrated clinical decision support systems - Impact on the acquisition technique, scan range, and reconstruction type

OBJECTIVES

This study evaluates the impact of a scanner-integrated, customized clinical decision support system (CDSS) on the acquisition technique, scan range, and reconstruction in thoracoabdominal CT.

MATERIALS AND METHODS

We applied CDSS in contrast-enhanced examinations of the trunk with various clinical indications on a recent scanner with the capability of dual-energy CT (DECT), anatomic landmark detection (ALD), and iterative metal-artifact reduction (MAR). Simple and comprehensive questions about the patient's breath hold capability, the anatomical region of interest, and metal implants can be answered after the localizer. The acquisition technique (single energy, SECT, or dual energy), scan range (chest-abdomen-pelvis or chest-abdomen), and reconstruction technique (with or without MAR) were then automatically adapted in the examination protocols in coherence with these selections. Retrospectively, we compared the usage rates for these techniques in 624 examinations on the study scanner with 740 examinations on a comparable scanner without CDSS. Subgroup analysis of effective dose (ED), scan duration, and image quality (IQ) was performed in the study group.

RESULTS

CDSS leads to an increased usage rate of DECT (64.4% vs. 2.8%) and MAR (75.4% vs. 44.0%). All scan range adaptations by ALD were successful. The resulting subjective IQ between single energy and DECT acquisitions was comparable (all p > 0.05). Scan duration was significantly longer in DECT than in SECT (16.9 s vs. 6.5 s; p < 0.001). However, the objective IQ was significantly higher in DECT (CNRD 2.1 vs. 1.8; p < 0.01), and the ED significantly lower (6.7 mSv vs. 7.6 mSv; p = 0.004).

CONCLUSION

CDSS for thoracoabdominal CT leads to a substantially increased usage rate of innovative techniques during acquisition and reconstruction. Patients with adapted protocols benefit from improved image quality and increased post-processing options at lower radiation doses.

Feasibility of In Vivo Metal Artifact Reduction in Contrast-Enhanced Dedicated Spiral Breast Computed Tomography

BACKGROUND

Radiopaque breast markers cause artifacts in dedicated spiral breast-computed tomography (SBCT). This study investigates the extent of artifacts in different marker types and the feasibility of reducing artifacts through a metal artifact reduction (MAR) algorithm.

METHODS

The pilot study included 18 women who underwent contrast-enhanced SBCT. In total, 20 markers of 4 different types were analyzed for artifacts. The extent of artifacts with and without MAR was measured via the consensus of two readers. Image noise was quantitatively evaluated, and the effect of MAR on the detectability of breast lesions was evaluated on a 3-point Likert scale.

RESULTS

Breast markers caused significant artifacts that impaired image quality and the detectability of lesions. MAR decreased artifact size in all analyzed cases, even in cases with multiple markers in a single slice. The median length of in-plain artifacts significantly decreased from 31 mm (range 11-51 mm) in uncorrected to 2 mm (range 1-5 mm) in corrected images (p ≤ 0.05). Artifact size was dependent on marker size. Image noise in slices affected by artifacts was significantly lower in corrected (13.6 ± 2.2 HU) than in uncorrected images (19.2 ± 6.8 HU, p ≤ 0.05). MAR improved the detectability of lesions affected by artifacts in 5 out of 11 cases.

CONCLUSION

MAR is feasible in SBCT and improves the image quality and detectability of lesions.

A nonlinear scaling-based normalized metal artifact reduction to reduce low-frequency artifacts in energy-integrating and photon-counting CT

BACKGROUND

Metal within the scan plane can cause severe artifacts when reconstructing X-ray computed tomography (CT) scans. Both in clinical use and recent research, normalized metal artifact reduction (NMAR) has established as the reference method for correcting metal artifacts, but NMAR introduces inconsistencies within the sinogram, which can cause additional low-frequency artifacts after image reconstruction.

PURPOSE

This paper introduces an extension to NMAR by applying a nonlinear scaling function (NLS-NMAR) to reduce low-frequency artifacts, which get introduced by the reconstruction of interpolation-edge-related sinogram inconsistencies in the normalized sinogram domain.

METHODS

After linear interpolation of the metal trace, an NLS function is applied in the prior-normalized sinogram domain to reduce the impact of the interpolation edges during filtered backprojection. After sinogram denormalization and image reconstruction, the low frequencies of the NLS image are combined with different high frequencies to restore anatomic details. An anthropomorphic dental phantom with removable metal inserts was utilized on two different CT systems to quantitatively assess the artifact reduction performance in terms of HU deviations and the root-mean-square-error within relevant regions of interest. Clinical dental examples were assessed to qualitatively demonstrate the problem of the interpolation-related blooming as well as to demonstrate the performance of the NLS function to reduce respective artifacts. To quantitatively prove HU consistency, HU values were assessed in central ROIs in the clinical cases. In addition, single clinical cases of a hip replacement and pedicle screws in the spine are shown to demonstrate the method's results in other body regions.

RESULTS

The NLS-NMAR can minimize the effect of interpolation-related sinogram inconsistencies and thus reduce resulting hyperdense blooming artifacts. In the phantom results, the reconstructions with the NLS-NMAR-corrected low frequencies demonstrate the lowest error. In the qualitative assessment of the clinical data, the NLS-NMAR shows a tremendous enhancement in image quality, also performing best within all assessed images series.

CONCLUSION

The NLS-NMAR provides a small yet effective extension to conventional NMAR by reducing low-frequency hyperdense metal trace-interpolation-related artifacts in computed tomography.

Non-linearly scaled (NLS) prior image-controlled frequency split for high-frequency metal artifact reduction in Computed Tomography

PURPOSE

This paper introduces a new approach for the dedicated reduction of high-frequency metal artifacts, which applies a non-linear scaling transfer function (NLS) on the high-frequency projection domain to reduce artifacts, while preserving edge information and anatomic detail by incorporating prior image information.

METHODS

A non-linear scaling function is applied to suppress high-frequency streak artifacts, but to restrict the correction to metal projections only, scaling is performed in the sinogram domain. Anatomic information should be preserved and is excluded from scaling by incorporating a prior image from tissue-classification. The corrected high-frequency sinogram is reconstructed and combined with the low-frequency component of an NMAR image. Scans of different anthropomorphic phantoms were acquired (unilateral hip, bilateral hip, dental implants, and embolization coil). Multiple ROIs were drawn around the metal implants and HU deviations were analyzed. Clinical datasets including single image slices of dental fillings, a bilateral hip implant, spinal fixation screws, and an aneurysm coil were reconstructed and assessed.

RESULTS

The prior image-controlled non-linear scaling function can remove streak artifacts while preserving anatomic detail within the bone and soft tissue. The qualitative analysis of clinical cases showed a tremendous enhancement within dental fillings and neuro coils, and a significant enhancement within spinal screws or hip implants. The phantom scan measurements support this observation. In all phantom setups, the NLS-corrected result showed lowest HU derivation and the best visualization of the data.

CONCLUSIONS

The prior image-controlled NLS provides a method to reduce high-frequency streaks in metal-corrupted CT data. This article is protected by copyright. All rights reserved.

Iterative metal artifact reduction on a clinical photon counting system—technical possibilities and reconstruction selection for optimal results dependent on the metal scenario

Objective. To give an overview about technical possibilities for metal artifact reduction of the first clinical photon-counting CT system and assess optimal reconstruction settings in a phantom study, assessing monoenergetic imaging (VMI) and iterative metal artifact reduction (iMAR). Approach. Scans were performed with 120 kV and Sn140 kV on the first clinical photon-counting detector CT scanner. To quantify artifact reduction, anthropomorphic phantoms (hip, dental, spine, neuro) were assessed, in addition to a tissue characterization phantom (Gammex) to quantify the HU restoration accuracy, all with removable metal inserts. Each setup was reconstructed with and without dedicated iMAR, and VMIs were computed in 10 keV steps from 40 keV (60 keV at Sn140 kV) to 190 keV for all setups (ground truth and metal with and without iMAR). To find the optimal energy, pixel-wise errors were computed in relevant ROIs in water-equivalent tissue around the metal in each phantom setup. To assess HU restoration potential, measurements were performed in the Gammex phantom’s inserts. Main results. Large metal objects (hip head) or metal with high atomic numbers (dental and neuro) do not benefit from higher-energetic reconstructions. The hip shaft (large, low atomic number) comprises a lower base artifact level than the head, still without an energetic optimum. Within the spine (short penetration length, low atomic number) an energy optimum could be identified for both spectra (100 keV for 120 kV and 120 keV for Sn140 kV). The Gammex showed best HU restoration at 100 keV for 120 kV and at 110 keV for Sn140 kV. In all cases, additional iMAR reduced the base artifact level. Significance. This study shows that a novel photon-counting CT system has the capability to reduce metal artifacts in metal types with low atomic number and low penetration length by applying VMI. For all other metal types, additional iMAR is required to reduce artifacts.

Subtraction iodine imaging with area detector CT to improve tumor delineation and measurability of tumor size and depth of invasion in tongue squamous cell carcinoma

Purpose

Tumor size and depth of invasion (DOI) are mandatory assessments for tumor classification in tongue cancer but are often non-assessable on CT due to dental artifacts. This study investigated whether subtraction iodine imaging (SII) would improve tumor delineation and measurability.

Materials and methods

Fifty-seven consecutive patients with tongue cancer, who underwent scanning with a 320-row area detector CT with contrast administration and were treated with surgical resection, were retrospectively evaluated. CT was reconstructed with single-energy projection-based metallic artifact reduction (sCT). SII was generated by subtracting the pre-contrast volume scans from the post-contrast volume scans using a high-resolution deformable registration algorithm. MRI scans were also evaluated for comparing the ability of measurements. Two radiologists visually graded the tumor delineation using a 5-point scale. Tumor size and DOI were measured wherever possible. The tumor delineation score was compared using the Wilcoxon signed-rank method. Spearman’s correlations between imaging and pathological measurements were calculated. Intraclass correlation coefficients of measurements between readers were estimated.

Results

The tumor delineation score was greater on sCT-plus-SII than on sCT alone (medians: 3 and 1, respectively; p < 0.001), with higher number of detectable cases observed with sCT-plus-SII (36/57 [63.2%]) than sCT alone (21/57 [36.8%]). Tumor size and DOI measurability were higher with sCT-plus-SII (29/57 [50.9%]) than with sCT alone (17/57 [29.8%]). MRI had the highest detectability (52/57 [91.2%]) and measurability (46/57 [80.7%]). Correlation coefficients between radiological and pathological tumor size and DOI were similar for sCT (0.83–0.88), sCT-plus-SII (0.78–0.84), and MRI (0.78–0.90). Intraclass correlation coefficients were higher than 0.95 for each modality.

Conclusions

SII improves detectability and measurability of tumor size and DOI in patients with oral tongue squamous cell carcinoma, thus increasing the diagnostic potential. SII may also be beneficial for cases unevaluable on MRI due to artifacts or for patients with contraindications to MRI.

Imaging for Treated Aneurysms (Including Clipping, Coiling, Stents, Flow Diverters)

Intracranial aneurysms are common in the adult population and carry a risk of rupture leading to catastrophic subarachnoid hemorrhage. Treatment of aneurysms has evolved significantly, with the introduction of new techniques and devices for minimally invasive and endovascular approaches. Follow-up imaging after aneurysm treatment is standard of care to monitor for recurrence or other complications, and the preferred imaging modality and schedule for follow-up are areas of active research. The modality and follow-up schedule should be tailored to treatment technique, aneurysm characteristics, and patient factors.

Comprehensive assessments of the open mouth dynamic maneuver and metal artifact reduction algorithm on computed tomography images of the oral cavity and oropharynx

Objectives

To determine the optimal utility of the open mouth maneuver and Metal Artifact Reduction for the Orthopedic Implants (O-MAR) technique for CT of the oral cavity and oropharynx.

Methods

Between July 2017 and May 2019, 59 subjects who underwent both conventional and open mouth head and neck CT scans were included in this retrospective study. All images were reconstructed using the O-MAR algorithm. With conventional CT with/without the O-MAR (CTc_O/CTc) and open mouth CT with/without O-MAR (CTo_O/CTo), one reader measured the noise level in multiple anatomic regions of the oral cavity and oropharynx. Visual scores for the streak artifact and overall subjective image quality were assessed by two independent readers.

Results

For the mobile tongue, retromolar trigone, and palatine tonsil, the mean noise was significantly lower, and the mean visual scores were significantly higher, with CTo than with CTc or CTc_O (all, P < 0.001). The mean visual scores were higher with CTo_O than with CTo for the mobile tongue and palatine tonsil (all, P < 0.001). Contrarily, for the mouth floor and tongue base, the mean noise was significantly higher with CTo_O than with CTc or CTc_O, and the mean visual scores were significantly higher with CTc than with CTo or CTo_O (all, P < 0.001).

Conclusions

The open mouth maneuver and O-MAR technique can have different influences on the CT image quality according to the anatomical subsites of the oral cavity and oropharynx.

3D X-ray micro-computed tomography imaging for the microarchitecture evaluation of porous metallic implants and scaffolds

As an advanced microscopy technology with strong sample adaptability and non-destructive three-dimensional (3D) characteristics, X-ray micro-computed tomography (Micro-CT) can establish the overall connection between various microarchitecture parameters and accelerate the research process of porous metallic implants and scaffolds. In this review, the Micro-CT based quantitative evaluation methods of microarchitecture and bone formation are investigated. To ensure reliability of the results, the Micro-CT setup is discussed briefly and the essential image processing algorithms are introduced in detail. The significance and limitations of Micro-CT are analyzed in the context of research on porous metallic implants. We also discuss the future development of Micro-CT technology in the field of biological tissue engineering.

Effect of dental metal artifact conversion volume on dose distribution in head-and-neck volumetric-modulated arc therapy

Purpose

During treatment planning for head‐and‐neck volumetric‐modulated arc therapy (VMAT), manual contouring of the metal artifact area of artificial teeth is done, and the area is replaced with water computed tomography (CT) values for dose calculation. This contouring of the metal artifact areas, which is performed manually, is subject to human variability. The purpose of this study is to evaluate and analyze the effect of inter‐observer variation on dose distribution.

Methods

The subjects were 25 cases of cancer of the oropharynx for which VMAT was performed. Six radiation oncologists (ROs) performed metal artifact contouring for all of the cases. Gross tumor volume, clinical target volume, planning target volume (PTV), and oral cavity were evaluated. The contouring of the six ROs was divided into two groups, small and large groups. A reference RO was determined for each group and the dose distribution was compared with those of the other radiation oncologists by gamma analysis (GA).

As an additional experiment, we changed the contouring of each dental metal artifact area, creating enlarged contours (L), reduced contours (S), and undrawn contours (N) based on the contouring by the six ROs and compared these structure sets.

Results

The evaluation of inter‐observer variation showed no significant difference between the large and small groups, and the GA pass rate was 100%. Similar results were obtained comparing structure sets L and S, but in the comparison of structure sets L and N, there were cases with pass rates below 70%.

Conclusions

The results show that the artificial variability of manual artificial tooth metal artifact contouring has little effect on the dose distribution of VMAT. However, it should be noted that the dose distribution may change depending on the contouring method in cases where the overlap between PTV and metal artifact areas is large.

Iterative metal artifact reduction in aortic CTA after Onyx®-embolization

•

The iMAR algorithms can reduce the severe metal artifacts from Onyx® glue-casts in CTA.

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The iMAR algorithms restores non-diagnostic examinations to acceptable diagnostic quality in most cases.

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It is beneficial to use several iMAR algorithms to ensure an optimal result.

Purpose

Onyx® embolization causes severe artifacts on subsequent CT-examinations, thereby seriously limiting the diagnostic quality.

The purpose of this work was to compare the diagnostic quality of the tailored metal artifact reducing algorithms iMAR to standard reconstructions of CTA in patients treated with Onyx® embolization.

Method

Twelve consecutive patients examined with Dual Energy CTA after Onyx® embolization were included. One standard image dataset without iMAR, and eight image datasets with different iMAR algorithms were reconstructed. Mean attenuation and noise were measured in the aorta or iliac arteries close to the Onyx® glue-cast and compared to the reference level in the diaphragmatic aorta. Mean attenuation and noise were also measured in the psoas muscle close to the Onyx®-glue and compared to the reference level in the psoas muscle at the level of the diaphragm.

Subjective image quality and severity of artifacts was assessed by two experienced interventional radiologists blinded to reconstruction details.

Results

All iMAR reconstructions had less distortion of the attenuation than the standard reconstructions and were also rated significantly better than the standard reconstructions by both interventional radiologists.

Conclusion

The iMAR algorithms can significantly reduce metal artifacts and improve the diagnostic quality in CTA in patients treated with Onyx® embolization, in many cases restoring non-diagnostic examinations to acceptable diagnostic quality.

What scans we will read: imaging instrumentation trends in clinical oncology

Oncological diseases account for a significant portion of the burden on public healthcare systems with associated costs driven primarily by complex and long-lasting therapies. Through the visualization of patient-specific morphology and functional-molecular pathways, cancerous tissue can be detected and characterized non-invasively, so as to provide referring oncologists with essential information to support therapy management decisions. Following the onset of stand-alone anatomical and functional imaging, we witness a push towards integrating molecular image information through various methods, including anato-metabolic imaging (e.g., PET/CT), advanced MRI, optical or ultrasound imaging.

This perspective paper highlights a number of key technological and methodological advances in imaging instrumentation related to anatomical, functional, molecular medicine and hybrid imaging, that is understood as the hardware-based combination of complementary anatomical and molecular imaging. These include novel detector technologies for ionizing radiation used in CT and nuclear medicine imaging, and novel system developments in MRI and optical as well as opto-acoustic imaging. We will also highlight new data processing methods for improved non-invasive tissue characterization. Following a general introduction to the role of imaging in oncology patient management we introduce imaging methods with well-defined clinical applications and potential for clinical translation. For each modality, we report first on the status quo and, then point to perceived technological and methodological advances in a subsequent status go section. Considering the breadth and dynamics of these developments, this perspective ends with a critical reflection on where the authors, with the majority of them being imaging experts with a background in physics and engineering, believe imaging methods will be in a few years from now.

Overall, methodological and technological medical imaging advances are geared towards increased image contrast, the derivation of reproducible quantitative parameters, an increase in volume sensitivity and a reduction in overall examination time. To ensure full translation to the clinic, this progress in technologies and instrumentation is complemented by advances in relevant acquisition and image-processing protocols and improved data analysis. To this end, we should accept diagnostic images as “data”, and – through the wider adoption of advanced analysis, including machine learning approaches and a “big data” concept – move to the next stage of non-invasive tumour phenotyping. The scans we will be reading in 10 years from now will likely be composed of highly diverse multi-dimensional data from multiple sources, which mandate the use of advanced and interactive visualization and analysis platforms powered by Artificial Intelligence (AI) for real-time data handling by cross-specialty clinical experts with a domain knowledge that will need to go beyond that of plain imaging.

Efficiency of Iterative Metal Artifact Reduction Algorithm (iMAR) Applied to Brain Volume Perfusion CT in the Follow-up of Patients after Coiling or Clipping of Ruptured Brain Aneurysms

Metal artifacts resulting from coiling or clipping of a brain aneurysm degrade image quality and reduce diagnostic usefulness of computed tomography perfusion CTP. Our aim was to assess the diagnostic value of the iterative metal artifact reduction algorithm (iMAR) in CTP studies after coiling or clipping of ruptured intracranial aneurysms. Fifty-eight CTP exams performed in 32 patients were analysed. iMAR was applied to the source images from the CT scanner. Perfusion maps were generated from datasets both with and without iMAR, and both datasets were compared qualitatively and quantitatively. Qualitative analysis included evaluation of intensity of artifacts, image quality, presence of new artifacts, and the reader’s confidence in their diagnosis as well as diagnostic impression. Quantitative analysis included evaluation of tissue attenuation curves, evaluation of region of interest (ROI)-based measurement of perfusion values at levels that do and do not contain metal, compared to previously published reference ranges of perfusion values. Our results showed that application of iMAR reduced artifacts and significantly improved image quality. New artifacts were observed adjacent to metallic implants, but did not limit the evaluation of other regions. After correction for artifact readers’ confidence in their diagnosis increased from 41.3% to 87.9%, and the diagnostic impression changed in 31% of the exams. No difference between tissue attenuation curves was found. For slices without metal, no difference was noted between values measured before and after iMAR, and the total number of ROIs in the reference range of perfusion values was unchanged. At the level of the metal implant, 89.85% of ROIs obtained before using iMAR showed calculation errors. After using iMAR, only 1.7% showed errors. Before iMAR 3.1% of values were in the reference range, whereas after iMAR this increased to 33.1%. In conclusion, our results show that iMAR is an excellent tool for reducing artifacts in CTP. It is therefore recommended for use in clinical practice, particularly when severe artifacts are present, or when hypoperfusion is suspected at the level of the coil or clip. After the application of iMAR, the perfusion values at the level of the metal can be better calculated, but may not lie within the reference range; therefore, quantitative analysis at the level of artifacts is not advisable.

Metallic dental artifact reduction in computed tomography (Smart MAR): Improvement of image quality and diagnostic confidence in patients with suspected head and neck pathology and oral implants

PURPOSE

We determined whether the Smart MAR metal artifact reduction tool - a three-stage, projection-based, post processing algorithm - improves subjective and objective image quality and diagnostic confidence in patients with dental artifacts and suspected head and neck pathology compared to standard adaptive statistical iterative reconstructions (ASIR V) alone.

METHOD

The study included 100 consecutive patients with nonremovable oral implants or dental fillings and suspected oropharyngeal cancer or abscess. CT raw data of a single-source multislice CT scanner were postprocessed using ASIR V alone and with additional Smart MAR reconstruction. Image quality of baseline ASIR V and Smart MAR-based reconstruction series was compared both quantitatively (5 regions of interest, ROIs) and qualitatively (two independent raters).

RESULTS

Additional Smart MAR reconstruction significantly seems to improve both attenuation and noise adjacent to implants and in more distant areas (all p < 0.001) compared to standard ASIR V reconstructions alone. Signal-to-noise ratio (SNR; p = 0.001) and contrast-to-noise ratio were improved significantly (CNR; p = 0.001). Smart MAR improved visualization of tumor/abscess (detected in 36 of 100 patients, 36%) and representative oropharyngeal tissue (p < 0.001). In 8 of 36 patients (22%), tumor was only detected in Smart MAR series. Mean total DLP was 506.8mGy*cm; average CTDIvol was 5.5 mGy.

CONCLUSIONS

The supplementary use of the Smart MAR post-processing tool seems to significantly improve both subjective and objective image quality as well as diagnostic confidence and lesion detection in CT of the head and neck. In 22% of cases, the tumor was detected only in Smart MAR reconstructed images.

Artifact Reduction in the Diagnosis of Vasospasm in Computed Tomographic Perfusion: Potential of Iterative Metal Artifact Reduction

OBJECTIVE

This study aimed to analyze the possibility of artifact reduction using a new iterative metal artifact reduction algorithm (iMAR) in the diagnosis of perfusion deficits due to vasospasms and to evaluate its clinical relevance.

METHODS

Sixty-one volume perfusion computed tomographies of 24 patients after coiling or aneurysm clipping were reconstructed using standard-filtered back-projection and iMAR retrospectively. The degree of artifacts was evaluated as well as the size of the nonevaluable area. Diagnostic performance was evaluated compared with digital subtraction angiography.

RESULTS

Artifacts were present in 39 of 61 volume perfusion computed tomography examinations. Image quality (score, 1.0 vs 1.6; P < 0.01) was higher and the size of the signal loss was reduced significantly by iMAR (intracranial metal artifacts, 887 mm vs 359 mm [P < 0.01]; cranial bolt, 3008 mm vs 837 mm [P < 0.01]). Digital subtraction angiography confirmed vasospasms in 11 (92%) of 12 patients.

CONCLUSION

The iMAR yields higher image quality by reducing artifacts compared with filtered back-projection.

Radiation dose and image quality in intraoperative CT (iCT) angiography of the brain with stereotactic head frames

OBJECTIVES

Intraoperative CT (iCT) angiography of the brain with stereotactic frames is an integral part of navigated neurosurgery. Validated data regarding radiation dose and image quality in these special examinations are not available. We therefore investigated two iCT protocols in this IRB-approved study.

METHODS

Retrospective analysis of patients, who received a cerebral stereotactic iCT angiography on a 128 slice CT scanner between February 2016 and December 2017. In group A, automated tube current modulation (ATCM; reference value 410 mAs) and automated tube voltage selection (reference value 120 kV) were enabled, and only examinations with a selected voltage of 120 kV were included. In group B, fixed parameters were applied (300 mAs, 120 kV). Radiation dose was measured by assessing the volumetric CT dose index (CTDI_vol), dose length product (DLP) and effective dose (ED). Signal-to-noise ratio (SNR) and image noise were assessed for objective image quality, visibility of arteries and grey-white differentiation for subjective image quality.

RESULTS

Two hundred patients (n = 100 in each group) were included. In group A, median selected tube current was 643 mAs (group B, 300 mAs; p < 0.001). Median values of CTDI_vol, DLP and ED were 91.54 mGy, 1561 mGy cm and 2.97 mSv in group A, and 43.15 mGy, 769 mGy cm and 1.46 mSv in group B (p < 0.001). Image quality did not significantly differ between groups (p > 0.05).

CONCLUSIONS

ATCM yielded disproportionally high radiation dose due to substantial tube current increase at the frame level, while image quality did not improve. Thus, ATCM should preferentially be disabled.

KEY POINTS

• Automated tube current modulation (ATCM) yields disproportionally high radiation dose in intraoperative CT angiography of the brain with stereotactic head frames. • ATCM does not improve overall image quality in these special examinations. • ATCM is not yet optimised for CT angiography of the brain with major extracorporeal foreign materials within the scan range.

Beam hardening artifacts of liquid embolic agents: comparison between Squid and Onyx

BACKGROUND

Initial clinical experience with Squid shows subjectively reduced artifacts on post-embolization CT scans compared with Onyx. To further investigate these observations, we aimed to perform a comparison of artifacts between Squid and Onyx in a controlled in vitro model.

MATERIALS AND METHODS

Onyx 18 and all four variants of Squid (Squid 18, Squid 18 low density (LD), Squid 12, Squid 12 LD) were each injected in dimethylsulfoxide (DMSO) compatible test tubes. The tubes containing precipitated embolic material were inserted in a CT phantom for conventional and flat panel CT acquisitions. Beam hardening artifacts were quantified using objective and subjective measurements.

RESULTS

Objective evaluation of artifacts within regions of interest (ROIs) placed around the embolic material on CT and flat panel CT images demonstrated significantly lower noise and Hounsfield unit (HU) range values for all four Squid products compared with Onyx 18. On both CT and flat panel CT, LD variants of Squid 18 and Squid 12 had significantly lower noise and HU range values than their normal density counterparts on longitudinal ROIs. When using subjective measures for diagnostic value within ROIs placed around the embolic material on both CT and flat panel CT images, the number of non-diagnostic ROIs was significantly higher for Onyx 18 than for all four Squid variants.

CONCLUSION

All four variants of Squid induced fewer beam hardening artifacts than Onyx 18 on CT and flat panel CT acquisitions. LD variants of Squid induced fewer artifacts than their normal density counterparts.

Comparison of dual- and single-source dual-energy CT in head and neck imaging

OBJECTIVES

The aim of this study was to compare image quality of single-source dual-energy CT (SS-DECT) with third-generation dual-source dual-energy CT (DS-DECT) in head and neck cancer.

MATERIALS AND METHODS

One hundred two patients with histologically proven head and neck cancer were prospectively randomized to undergo radiation dose-matched SS-DECT (n = 51, 120 kV, split-filter technique, 384 ref. mAs) or DS-DECT (n = 51, 80/Sn150 kV, tube A 100/tube B 67 ref. mAs). Inline default images (DI) and virtual monoenergetic images (VMI) for two different low energies (40 and 60 keV) were reconstructed. Objective image quality was evaluated as dose-normalized contrast to noise ratio (CNRD), and subjective image quality was rated on a 5-point Likert scale.

RESULTS

In both groups, highest CNRD values for vessel and tumor attenuation were obtained at 40 keV. DS-DECT was significantly better than SS-DECT regarding vessel and tumor attenuation. Overall subjective image quality in the SS-DECT group was highest on the DI followed by 40 keV and 60 keV. In the DS-DECT group, subjective image quality was highest at 40 keV followed by 60 keV and the DI. Forty kiloelectron volts and 60 keV were significantly better in the DS-DECT compared to the SS-DECT group (both p < 0.01).

CONCLUSIONS

In split-filter SS-DECT as well as in DS-DECT, highest overall image quality in head and neck imaging can be obtained with a combination of DI and low keV reconstructions. DS-DECT is superior to split-filter SS-DECT in terms of subjective image quality and vessel and tumor attenuation.

KEY POINTS

• Image quality was diagnostic with both dual-energy techniques; however, the dual-source technique delivered significantly better results. • Highest overall image quality in head and neck imaging can be obtained with a combination of default images and low keV reconstructions with both dual-energy techniques. • The results of this study may have relevance for the decision-making process regarding replacement of CT scanners and focused patient examination considering image quality and subsequent therapeutic decision-making.

CT Dental Artifact: Comparison of an Iterative Metal Artifact Reduction Technique with Weighted Filtered Back-Projection

Background

Dental hardware produces streak artifacts on computed tomography (CT) images reconstructed with the standard weighted filtered back projection (wFBP) method.

Purpose

To perform a preliminary evaluation of an iterative metal artifact reduction (IMAR) technique to assess its ability to improve anatomic visualization over wFBP in patients with dental amalgam or other hardware.

Material and Methods

CT images from patients with dental hardware were reconstructed using wFBP and IMAR software and soft-tissue or bone window/level settings. The anatomy most affected by metal artifacts was identified. Two neuroradiologists determined subjective and objective imaging features, including overall metal artifact score (1 = severe artifacts, 5 = no artifacts), soft-tissue visualization score of the most-compromised structure, and artifact length along the skin surface. CT numbers were used to quantify artifact severity.

Results

Twenty-four patients were included. IMAR improved overall metal artifact score in 18/24 cases (median =2 ± 0.9 vs. 1 ± 0.6, P < 0.001). Mean CT number in the most-affected anatomical structure significantly improved with IMAR (94.6 vs. 219 HU, P = 0.002) and length of affected skin surface decreased (40.4 mm vs. 118.7 mm, P < 0.001). However, osseous/dental artifactual defects were found in 22/24 cases with IMAR vs. 11/24 with wFBP.

Conclusion

IMAR software reduced metal artifact both subjectively and objectively and improved visualization of adjacent soft tissues. However, it produced a higher rate of artifactual defects in the teeth and bones than wFBP. Our findings support the use of IMAR as a valuable complement to, but not a replacement for, standard wFBP image reconstruction.

Iterative metal artefact reduction (MAR) in postsurgical chest CT: comparison of three iMAR-algorithms

OBJECTIVES

The purpose of this study was to evaluate the impact of three novel iterative metal artefact (iMAR) algorithms on image quality and artefact degree in chest CT of patients with a variety of thoracic metallic implants.

METHODS

27 postsurgical patients with thoracic implants who underwent clinical chest CT between March and May 2015 in clinical routine were retrospectively included. Images were retrospectively reconstructed with standard weighted filtered back projection (WFBP) and with three iMAR algorithms (iMAR-Algo1 = Cardiac algorithm, iMAR-Algo2 = Pacemaker algorithm and iMAR-Algo3 = ThoracicCoils algorithm). The subjective and objective image quality was assessed.

RESULTS

Averaged over all artefacts, artefact degree was significantly lower for the iMAR-Algo1 (58.9 ± 48.5 HU), iMAR-Algo2 (52.7 ± 46.8 HU) and the iMAR-Algo3 (51.9 ± 46.1 HU) compared with WFBP (91.6 ± 81.6 HU, p < 0.01 for all). All iMAR reconstructed images showed significantly lower artefacts (p < 0.01) compared with the WFPB while there was no significant difference between the iMAR algorithms, respectively. iMAR-Algo2 and iMAR-Algo3 reconstructions decreased mild and moderate artefacts compared with WFBP and iMAR-Algo1 (p < 0.01).

CONCLUSION

All three iMAR algorithms led to a significant reduction of metal artefacts and increase in overall image quality compared with WFBP in chest CT of patients with metallic implants in subjective and objective analysis. The iMARAlgo2 and iMARAlgo3 were best for mild artefacts. IMARAlgo1 was superior for severe artefacts. Advances in knowledge: Iterative MAR led to significant artefact reduction and increase image-quality compared with WFBP in CT after implementation of thoracic devices. Adjusting iMAR-algorithms to patients' metallic implants can help to improve image quality in CT.

Contouring and dose calculation in head and neck cancer radiotherapy after reduction of metal artifacts in CT images

BACKGROUND

Delineation accuracy of the gross tumor volume (GTV) in radiotherapy planning for head and neck (H&N) cancer is affected by computed tomography (CT) artifacts from metal implants which obscure identification of tumor as well as organs at risk (OAR). This study investigates the impact of metal artifact reduction (MAR) in H&N patients in terms of delineation consistency and dose calculation precision in radiation treatment planning.

MATERIAL AND METHODS

Tumor and OAR delineations were evaluated in planning CT scans of eleven oropharynx patients with streaking artifacts in the tumor region preceding curative radiotherapy (RT). The GTV-tumor (GTV-T), GTV-node and parotid glands were contoured by four independent observers on standard CT images and MAR images. Dose calculation was evaluated on thirty H&N patients with dental implants near the treated volume. For each patient, the dose derived from the clinical treatment plan using the standard image set was compared with the recalculated dose on the MAR image dataset.

RESULTS

Reduction of metal artifacts resulted in larger volumes of all delineated structures compared to standard reconstruction. The GTV-T and the parotids were on average 22% (p < 0.06) and 7% larger (p = 0.005), respectively, in the MAR image plan compared to the standard image plan. Dice index showed reduced inter-observer variations after reduction of metal artifacts for all structures. The average surface distance between contours of different observers improved using the MAR images for GTV and parotids (p = 0.04 and p = 0.01). The median volume receiving a dose difference larger than ±3% was 2.3 cm³ (range 0-32 cm³).

CONCLUSIONS

Delineation of structures in the head and neck were affected by metal artifacts and volumes were generally larger and more consistent after reduction of metal artifacts, however, only small changes were observed in the dose calculations.

Dual energy CT angiography: pros and cons of dual-energy metal artifact reduction algorithm in patients after endovascular aortic repair

PURPOSE

To evaluate the value of metal artifact reduction (MAR) post-processing and iodine MD images in fast kV-switching dual-energy computed tomography (DECT) in patients after endovascular aortic repair (EVAR).

MATERIALS AND METHODS

Twenty-four consecutive EVAR patients (age 76 ± 9 years, 7/24 (29%) with coils, 9/24 (37.5%) with 10 endoleaks) who underwent DECT angiography were included in this HIPAA-compliant, IRB-approved retrospective study. Monochromatic reconstructions included 55, 60, 65, 70, and 75 keV with and without MAR and iodine MD images. Near field, far field, and vessel artifacts were assessed subjectively (1 = none; 5 = severe) and objectively by measuring noise and contrast-to-noise ratio. Visibility of endoleak was evaluated (1 = optimal; 5 = not visible).

RESULTS

MAR objectively decreased artifacts from EVAR stents in the near field (60.7 ± 25.4 HU vs. 70.1 ± 34.2; p = .002) and subjectively increased near field (3.2 ± 0.9 vs. 2.8 ± 0.6; p < .001), far field (2.2 ± 0.6 vs. 1.6 ± 0.6; p < .001), and vessel (3.1 ± 1.1 vs. 2.5 ± 0.9; p < .001) artifacts. Near-field artifacts from coils were reduced by the MAR objectively (72.4 ± 24.8 vs. 182.7 ± 57.3 HU; p < .001) and subjectively (4.5 ± 0.5 vs. 4.9 ± 0.4; p = .02). CNR of standard reconstructions was optimal at 60 keV (38.3 ± 16.8). Reconstructions without MAR and iodine MD images provided improved endoleak visualization in 6/10 (60%) of cases (median 1 for both) compared to MAR (median 3) (p < 0.001). However, MAR improved visualization in 1/10 (10%) cases due to endoleak location adjacent to a coil.

CONCLUSION

DECT with MAR reduced artifacts from coils and improved endoleak visualization in 1/10 (10%) cases due to location adjacent to a coil. However, MAR impaired endoleak visualization in 6/10 (60%) cases and should be reviewed combined with 60 keV standard reconstructions and iodine MD images.

Metal Artifact Reduction in Computed Tomography After Deep Brain Stimulation Electrode Placement Using Iterative Reconstructions

OBJECTIVES

Diagnostic accuracy of intraoperative computed tomography (CT) after deep brain stimulation (DBS) electrode placement is limited due to artifacts induced by the metallic hardware, which can potentially mask intracranial postoperative complications. Different metal artifact reduction (MAR) techniques have been introduced to reduce artifacts from metal hardware in CT. The purpose of this study was to assess the impact of a novel iterative MAR technique on image quality and diagnostic performance in the follow-up of patients with DBS electrode implementation surgery.

MATERIALS AND METHODS

Seventeen patients who had received routine intraoperative CT of the head after implantation of DBS electrodes between March 2015 and June 2015 were retrospectively included. Raw data of all patients were reconstructed with standard weighted filtered back projection (WFBP) and additionally with a novel iterative MAR algorithm. We quantified frequencies of density changes to assess quantitative artifact reduction. For evaluation of qualitative image quality, the visibility of numerous cerebral anatomic landmarks and the detectability of intracranial electrodes were scored according to a 4-point scale. Furthermore, artifact strength overall and adjacent to the electrodes was rated.

RESULTS

Our results of quantitative artifact reduction showed that images reconstructed with iterative MAR (iMAR) contained significantly lower metal artifacts (overall low frequency values, 1608.6 ± 545.5; range, 375.5-3417.2) compared with the WFBP (overall low frequency values, 4487.3 ± 875.4; range, 2218.3-5783.5) reconstructed images (P < 0.004). Qualitative image analysis showed a significantly improved image quality for iMAR (overall anatomical landmarks, 2.49 ± 0.15; median, 3; range, 0-3; overall electrode characteristics, 2.35 ± 0.16; median, 2; range, 0-3; artifact characteristics, 2.16 ± 0.08; median, 2.5; range, 0-3) compared with WFBP (overall anatomical landmarks, 1.21 ± 0.64; median, 1; range, 0-3; overall electrode characteristics, 0.74 ± 0.37; median, 1; range, 0-2; artifact characteristics, 0.51 ± 0.15; median, 0.5; range, 0-2; P < 0.002).

CONCLUSIONS

Reconstructions of cranial CT images with the novel iMAR algorithm in patients after DBS implantation allows an efficient reduction of metal artifacts near DBS electrodes compared with WFBP reconstructions. We demonstrated an improvement of quantitative and qualitative image quality of iMAR compared with WFBP in patients with DBS electrodes.

Clinical evaluation of the iterative metal artefact reduction algorithm for post-operative CT examination after maxillofacial surgery

OBJECTIVES

Metal artefacts present challenges to both radiologists and clinicians during post-operative imaging. Such artefacts reduce the diagnostic effectiveness of CT scans and mask findings that could be vital for patient management. Thus, a powerful artefact reduction tool is necessary when imaging patients with metal implants. Our aim was to test the recently introduced iterative metal artefact reduction (iMAR) algorithm in patients with maxillofacial implants.

METHODS

Images from 17 patients with diverse maxillofacial metal implants who had undergone CT scans were qualitatively and quantitatively analyzed before and after metal artefact reduction with iMAR.

RESULTS

After iMAR application, images exhibited decreased artefacts and improved image quality, leading to detection of lesions that were previously masked by artefacts. The application of iMAR did not affect image quality in regions distant from the metal implants.

CONCLUSIONS

The application of iMAR to CT examinations of patients with maxillofacial metal implants leads to artefact reduction, improvement of image quality and increased diagnostic utility. Routine implementation of iMAR during imaging of patients with metal hardware implants could add diagnostic value to their CT examinations.

Low-Dose CT of the Paranasal Sinuses: Minimizing X-Ray Exposure with Spectral Shaping

OBJECTIVES

Shaping the energy spectrum of the X-ray beam has been shown to be beneficial in low-dose CT. This study's aim was to investigate dose and image quality of tin filtration at 100 kV for pre-operative planning in low-dose paranasal CT imaging in a large patient cohort.

METHODS

In a prospective trial, 129 patients were included. 64 patients were randomly assigned to the study protocol (100 kV with additional tin filtration, 150mAs, 192x0.6-mm slice collimation) and 65 patients to the standard low-dose protocol (100 kV, 50mAs, 128 × 0.6-mm slice collimation). To assess the image quality, subjective parameters were evaluated using a five-point scale. This scale was applied on overall image quality and contour delineation of critical anatomical structures.

RESULTS

All scans were of diagnostic image quality. Bony structures were of good diagnostic image quality in both groups, soft tissues were of sufficient diagnostic image quality in the study group because of a high level of noise. Radiation exposure was very low in both groups, but significantly lower in the study group (CTDI_vol 1.2 mGy vs. 4.4 mGy, p < 0.001).

CONCLUSIONS

Spectral optimization (tin filtration at 100 kV) allows for visualization of the paranasal sinus with sufficient image quality at a very low radiation exposure.

KEY POINTS

• Spectral optimization (tin filtration) is beneficial to low-dose parasinus CT • Tin filtration at 100 kV yields sufficient image quality for pre-operative planning • Diagnostic parasinus CT can be performed with an effective dose <0.05 mSv.