Quality assurance phantoms for cone beam computed tomography: a systematic literature review

Objective image quality assurance in cone-beam CT: Test methods, analysis, and workflow in longitudinal studies

BACKGROUND

Standards for image quality evaluation in multi-detector CT (MDCT) and cone-beam CT (CBCT) are evolving to keep pace with technological advances. A clear need is emerging for methods that facilitate rigorous quality assurance (QA) with up-to-date metrology and streamlined workflow suitable to a range of MDCT and CBCT systems.

PURPOSE

To evaluate the feasibility and workflow associated with image quality (IQ) assessment in longitudinal studies for MDCT and CBCT with a single test phantom and semiautomated analysis of objective, quantitative IQ metrology.

METHODS

A test phantom (CorgiTM Phantom, The Phantom Lab, Greenwich, New York, USA) was used in monthly IQ testing over the course of 1 year for three MDCT scanners (one of which presented helical and volumetric scan modes) and four CBCT scanners. Semiautomated software analyzed image uniformity, linearity, contrast, noise, contrast-to-noise ratio (CNR), 3D noise-power spectrum (NPS), modulation transfer function (MTF) in axial and oblique directions, and cone-beam artifact magnitude. The workflow was evaluated using methods adapted from systems/industrial engineering, including value stream process modeling (VSPM), standard work layout (SWL), and standard work control charts (SWCT) to quantify and optimize test methodology in routine practice. The completeness and consistency of DICOM data from each system was also evaluated.

RESULTS

Quantitative IQ metrology provided valuable insight in longitudinal quality assurance (QA), with metrics such as NPS and MTF providing insight on root cause for various forms of system failure-for example, detector calibration and geometric calibration. Monthly constancy testing showed variations in IQ test metrics owing to system performance as well as phantom setup and provided initial estimates of upper and lower control limits appropriate to QA action levels. Rigorous evaluation of QA workflow identified methods to reduce total cycle time to ∼10 min for each system-viz., use of a single phantom configuration appropriate to all scanners and Head or Body scan protocols. Numerous gaps in the completeness and consistency of DICOM data were observed for CBCT systems.

CONCLUSION

An IQ phantom and test methodology was found to be suitable to QA of MDCT and CBCT systems with streamlined workflow appropriate to busy clinical settings.

A simplified low-cost phantom for image quality assessment of dental cone beam computed tomography unit

INTRODUCTION

A standardised testing protocol for evaluation of a wide range of dental cone beam computed tomography (CBCT) performance and image quality (IQ) parameters is still limited and commercially available testing tool is unaffordable by some centres. This study aims to assess the performance of a low-cost fabricated phantom for image quality assessment (IQA) of digital CBCT unit.

METHODS

A customised polymethyl methacrylate (PMMA) cylindrical phantom was developed for performance evaluation of Planmeca ProMax 3D Mid digital dental CBCT unit. The fabricated phantom consists of four different layers for testing specific IQ parameters such as CT number accuracy and uniformity, noise and CT number linearity. The phantom was scanned using common scanning protocols in clinical routine (90.0 kV, 8.0 mA and 13.6 s). In region-of-interest (ROI) analysis, the mean CT numbers (in Hounsfield unit, HU) and noise for water and air were determined and compared with the reference values (0 HU for water and -1000 HU for air). For linearity test, the correlation between the measured HU of different inserts with their density was studied.

RESULTS

The average CT number were -994.1 HU and -2.4 HU, for air and water, respectively and the differences were within the recommended acceptable limit. The linearity test showed a strong positive correlation (R2  = 0.9693) between the measured HU and their densities.

CONCLUSION

The fabricated IQ phantom serves as a simple and affordable testing tool for digital dental CBCT imaging.

An AI-based universal phantom analysis method based on XML-SVG wireframes with novel functional object identifiers

Objective.Quality assurance (QA) testing must be performed at regular intervals to ensure that medical devices are operating within designed specifications. Numerous QA phantoms and software packages have been developed to facilitate measurements of machine performance. However, due to the hard-coded nature of geometric phantom definition in analysis software, users are typically limited to the use of a small subset of compatible QA phantoms. In this work, we present a novel AI-based universal Phantom (UniPhan) algorithm that is not phantom specific and can be easily adapted to any pre-existing image-based QA phantom.Approach.Extensible Markup Language Scalable Vector Graphics (XML-SVG) was modified to include several new tags describing the function of embedded phantom objects for use in QA analysis. Functional tags include contrast and density plugs, spatial linearity markers, resolution bars and edges, uniformity regions, and light-radiation field coincidence areas. Machine learning was used to develop an image classification model for automatic phantom type detection. After AI phantom identification, UniPhan imported the corresponding XML-SVG wireframe, registered it to the image taken during the QA process, performed analysis on the functional tags, and exported results for comparison to expected device specifications. Analysis results were compared to those generated by manual image analysis.Main results.XML-SVG wireframes were generated for several commercial phantoms including ones specific to CT, CBCT, kV planar imaging, and MV imaging. Several functional objects were developed and assigned to the graphical elements of the phantoms. The AI classification model was tested for training and validation accuracy and loss, along with phantom type prediction accuracy and speed. The results reported training and validation accuracies of 99%, phantom type prediction confidence scores of around 100%, and prediction speeds of around 0.1 s. Compared to manual image analysis, Uniphan results were consistent across all metrics including contrast-to-noise ratio, modulation-transfer function, HU accuracy, and uniformity.Significance.The UniPhan method can identify phantom type and use its corresponding wireframe to perform QA analysis. As these wireframes can be generated in a variety of ways this represents an accessible automated method of analyzing image-based QA phantoms that is flexible in scope and implementation.

The importance of routine quality control for reproducible pulmonary measurements by in vivo micro-CT

Micro-computed tomography (CT) imaging provides densitometric and functional assessment of lung diseases in animal models, playing a key role either in understanding disease progression or in drug discovery studies. The generation of reliable and reproducible experimental data is strictly dependent on a system's stability. Quality controls (QC) are essential to monitor micro-CT performance but, although QC procedures are standardized and routinely employed in clinical practice, detailed guidelines for preclinical imaging are lacking. In this work, we propose a routine QC protocol for in vivo micro-CT, based on three commercial phantoms. To investigate the impact of a detected scanner drift on image post-processing, a retrospective analysis using twenty-two healthy mice was performed and lung density histograms used to compare the area under curve (AUC), the skewness and the kurtosis before and after the drift. As expected, statistically significant differences were found for all the selected parameters [AUC 532 ± 31 vs. 420 ± 38 (p < 0.001); skewness 2.3 ± 0.1 vs. 2.5 ± 0.1 (p < 0.001) and kurtosis 4.2 ± 0.3 vs. 5.1 ± 0.5 (p < 0.001)], confirming the importance of the designed QC procedure to obtain a reliable longitudinal quantification of disease progression and drug efficacy evaluation.

Impact of metal artefacts on subjective perception of image quality of 13 CBCT devices

OBJECTIVES

The overall objective of this study was to assess how metal artefacts impact image quality of 13 CBCT devices. As a secondary objective, the influence of scanning protocols and field of view on CBCT image quality with and without metal artefacts was also assessed.

MATERIALS AND METHODS

CBCT images were acquired of a dry human skull phantom considering three clinical simulated conditions: one without metal and two with metallic materials (metallic pin and implant). An industrial micro-CT was used as a reference to register the CBCT images. Afterwards, four observers evaluated 306 representative image slices from 13 devices, ranking them from best to worst. Furthermore, within each device, medium FOV and small FOV standard images were compared. General linear mixed models were used to assess subjective perception of examiners on overall image quality in the absence and presence of metal-related artefacts (p < 0.05).

RESULTS

Image quality perception significantly differed amongst CBCT devices (p < 0.05). Some devices performed significantly better, independently of scanning protocol and clinical condition. In the presence of metal artefacts, medium FOV standard scanning protocols scored significantly better, while in the absence of metal, small FOV standard yielded the highest performance.

CONCLUSIONS

Subjective image quality differs significantly amongst CBCT devices and scanning protocols. Metal-related artefacts may highly impact image quality, with a significant device-dependent variability and only few scanners being more robust against metal artefacts. Often, metal artefact expression may be somewhat reduced by proper protocol selection.

CLINICAL RELEVANCE

Metallic objects may severely impact image quality in several CBCT devices.

Two decades of research on CBCT imaging in DMFR - an appraisal of scientific evidence

OBJECTIVE

This article aims to appraise how scientific evidence related to CBCT has changed over the years, based on levels of evidence and diagnostic efficacy.

METHODS

A general search strategy was used in different databases (Pubmed, Embase, and Web of Science) to identify systematic reviews (SRs) on CBCT until November of 2020. The SRs included were divided according to different specialties of dentistry. A critical review of the articles was made, describing the level of evidence and efficacy.

RESULTS

In total, 75 articles were selected. There was an increase in the number of SRs on CBCT from 2014 onwards, as 83% of the SRs on this topic were published after 2013, and 72% between 2016 and to date. Twenty SRs (27%) performed meta-analysis. Only 28% of the SRs provided a detailed description of CBCT protocols. According to SR evidence, almost all specialties of dentistry have advanced concomitantly with the introduction of CBCT. The majority of SRs were related to clinical applications (level 2 of efficacy), followed by technical parameters (level 1 of efficacy). Only some CBCT models were mentioned in the SRs selected.

CONCLUSION

Over the course of 20 years, SRs related to CBCT applications for a broad range of dental specialties have been published, with the vast majority of studies at levels 1 and 2 of diagnostic efficacy. Not all CBCT models available on the market have been scientifically validated. At all times, one should remain cautious as such not to simply extrapolate in vitro results to the clinical setting. Also, considering the wide variety of CBCT devices and protocols, reported results should not be overstated or generalized, as outcomes often refer to specific CBCT devices and protocols.

Evaluation of cone-beam computed tomography over a small field of view in a water bath based on the modulation transfer function with repeating-edge oversampling

PURPOSE

The spatial resolution of cone-beam computed tomography (CBCT) in small fields of view (FOVs) is important for clinical applications. However, it is difficult to measure spatial resolution reliably due to error factors such as noise. The aim of this study was to obtain a modulation transfer function (MTF) more accurately.

METHODS

A CBCT apparatus was used with small FOV. An aluminum pipe slightly tilted at an inclination ratio of 77/3 (25.7) was used as the measurement phantom. The MTF was calculated from the edge image of the phantom. The actual oversampling ratio was determined by regression analysis. The experiment was repeated 16 times and the edge-spread function (ESF) was approximated by the least-square method. Furthermore, a low-pass filter (LPF) was applied to eliminate the component at frequencies above the Nyquist frequency. Finally, the MTF was calculated from the pre-processed ESF.

RESULTS

Results showed that pre-processing reduced the noise of the ESF. The MTFs at frequencies of 1.0 and 2.0 LP/mm were 0.59 and 0.18, respectively, in air and 0.52 and 0.16, respectively, in water.

CONCLUSION

The repeating-edge oversampling method combined with ESF pre-processing improved the accuracy of the MTF under clinically relevant conditions with a phantom.

A phantom for testing Cone Beam CTs

Cone Beam Computed Tomography (CBCT) scanners are becoming more common for dental and maxillofacial/head scanning, but performing image quality tests on these systems is difficult. There are quality assurance (QA) phantoms commercially available but they can be expensive, bulky and not optimised for CBCT imaging limits. Smaller phantoms often lack features that are recommended for testing CBCT systems. A custom made phantom can provide more useful test objects in a more convenient size and at a lower cost. The proposed phantom is called the "Karu" Cone Beam CT Phantom and is constructed with a 3D printed poly lactic acid (PLA) shell, with 3D printed inserts for holding the test details in place. Tests included are geometric accuracy (in three dimensions), Hounsfield Unit (HU) accuracy, low contrast detectability, spatial resolution (using line pairs), and uniformity/artifacts/noise. The phantom was scanned on a number of scanners and was clearly able to differentiate scanners producing poorer quality images from better quality ones. The phantom could be produced for under NZ $2000.

Evaluation of metal artefacts for two CBCT devices with a new dental arch phantom

OBJECTIVES

To create a new phantom design to evaluate the real impact of artefacts caused by titanium on bone structures in cone beam CT images considering different positions and quantity of metals in the dental arch, with and without metal artefact reduction (MAR).

METHODS

A three cylindrical polymethyl methacrylate (PMMA) plate phantom was designed containing eight perforations arranged to simulate the lower dental arch in the intermediate plate. Three titanium cylinders were positioned in different locations and quantities to test different clinical conditions and to quantify the impact of the metal artefact around five bone cylinders. Scans were carried out in seven different protocols (Control, A-F) in two cone beam CT devices (OP300 Maxio and Picasso Trio). Eight regions of interest around each cortical and trabecular bone were used to measure the grey value standard deviation corresponding the artefact expression in the Image J software. Both the artefact expression and the MAR effect were assessed using the Wilcoxon, Friedman (Dunn) and Kruskal-Wallis tests (significance level of 5%).

RESULTS

For both devices, MAR was statistically efficient only for the protocols E, and F. Protocol F (three metals on the adjacent area of the analysis region) showed higher artefact expression when compared to the others.

CONCLUSION

In conclusion, the new phantom design allowed the quantification of the metal artefact expression caused by titanium. The metal artefact expression is higher when more metal objects are positioned in the adjacent bone structures. MAR may not be effective to reduce artefact expression on the adjacencies of those objects for the devices studied.

Accuracy and Reproducibility of Linear and Angular Measurements in Virtual Reality: a Validation Study

The purpose of this experimental study is to validate linear and angular measurements acquired in a virtual reality (VR) environment via a comparison with the physical measurements. The hypotheses tested are as follows: VR linear and angular measurements (1) are equivalent to the corresponding physical measurements and (2) achieve a high degree of reproducibility. Both virtual and physical measurements were performed by two raters in four different sessions. A total of 40 linear and 15 angular measurements were acquired from three physical objects (an L-block, a hand model, and a dry skull) via the use of fiducial markers on selected locations. After both intra- and inter-rater reliability were evaluated using inter-class coefficient (ICC), equivalence between virtual and physical measurements was analyzed via paired t test and Bland-Altman plots. The accuracy of the virtual measurements was further estimated using two one-sided tests (TOST) procedure. The reproducibility of virtual measurements was evaluated via ICC as well as the repeatability coefficient. Virtual reality measurements were equivalent to physical measurements as evidenced by a paired t test with p values of 0.413 for linear and 0.533 for angular measurements and Bland-Altman plots in all three objects. The accuracy of virtual measurements was estimated to be 0.5 mm for linear and 0.7° for angular measurements, respectively. Reproducibility in VR measurements was high as evidenced by ICC of 1.00 for linear and 0.99 for angular measurements, respectively. Both linear and angular measurements in the VR environment are equivalent to the physical measurements with high accuracy and reproducibility.

Factors affecting modulation transfer function measurements in cone-beam computed tomographic images

Purpose

This study was designed to investigate the effects of voxel size, the oversampling technique, and the direction and area of measurement on modulation transfer function (MTF) values to identify the optimal method of MTF measurement.

Materials and Methods

Images of the wire inserts of the SedentexCT IQ phantom were acquired, and MTF values were calculated under different conditions (voxel size of 0.1, 0.2, and 0.3 mm; 5 oversampling techniques; simulated pixel location errors; and different directions and areas of measurement). The differences in the MTF values across various conditions were evaluated.

Results

The MTF 10 values showed smaller standard deviations than the MTF 50 values. Stable and accurate MTF values were obtained in the 0.1-mm voxel images. In the 0.3-mm voxel images, oversampling techniques of 11 lines or more did not show significant differences in MTF values depending on the presence of simulated location errors. MTF 10 values showed significant differences according to the direction and area of the measurement.

Conclusion

To measure more accurate and stable MTF values, it is better to measure MTF 10 values in small-voxel images. In large-voxel images, the proper oversampling technique is required. MTF values from the radial and tangential directions may be different, and MTF values vary depending on the measured area.

Optimization of exposure parameters and relationship between subjective and technical image quality in cone-beam computed tomography

Purpose

This study was performed to investigate the effect of exposure parameters on image quality obtained using a cone-beam computed tomography (CBCT) scanner and the relationship between physical factors and clinical image quality depending on the diagnostic task.

Materials and Methods

CBCT images of a SedentexCT IQ phantom and a real skull phantom were obtained under different combinations of tube voltage and tube current (Alphard 3030 CBCT scanner, 78–90 kVp and 2–8 mA). The images obtained using a SedentexCT IQ phantom were analyzed technically, and the physical factors of image noise, contrast resolution, spatial resolution, and metal artifacts were measured. The images obtained using a real skull phantom were evaluated for each diagnostic task by 6 oral and maxillofacial radiologists, and each setting was classified as acceptable or unacceptable based on those evaluations. A statistical analysis of the relationships of exposure parameters and physical factors with observer scores was conducted.

Results

For periapical diagnosis and implant planning, the tube current of the acceptable images was significantly higher than that of the unacceptable images. Image noise, the contrast-to-noise ratio (CNR), the line pair chart on the Z axis, and modulation transfer function (MTF) values showed statistically significant differences between the acceptable and unacceptable image groups. The cut-off values obtained using receiver operating characteristic curves for CNR and MTF 10 were useful for determining acceptability.

Conclusion

Tube current had a major influence on clinical image quality. CNR and MTF 10 were useful physical factors that showed significantly associations with clinical image quality.

Spatial resolution measurements by Radia diagnostic software with SEDENTEXCT image quality phantom in cone beam CT for dental use

OBJECTIVES

We aimed to employ the Radia diagnostic software with the safety and efficacy of a new emerging dental X-ray modality (SEDENTEXCT) image quality (IQ) phantom in CT, and to evaluate its validity.

METHODS

The SEDENTEXCT IQ phantom and Radia diagnostic software were employed. The phantom was scanned using one medical full-body CT and two dentomaxillofacial cone beam CTs. The obtained images were imported to the Radia software, and the spatial resolution outputs were evaluated. The oversampling method was employed using our original wire phantom as a reference. The resultant modulation transfer function (MTF) curves were compared. The null hypothesis was that MTF curves generated using both methods would be in agreement. One-way analysis of variance tests were applied to the f50 and f10 values from the MTF curves. The f10 values were subjectively confirmed by observing the line pair modules.

RESULTS

The Radia software reported the MTF curves on the xy-plane of the CT scans, but could not return f50 and f10 values on the z-axis. The null hypothesis concerning the reported MTF curves on the xy-plane was rejected. There were significant differences between the results of the Radia software and our reference method, except for f10 values in CS9300. These findings were consistent with our line pair observations.

CONCLUSIONS

We evaluated the validity of the Radia software with the SEDENTEXCT IQ phantom. The data provided were semi-automatic, albeit with problems and statistically different from our reference. We hope the manufacturer will overcome these limitations.