Radiation dose reduction at coronary artery calcium scoring by using a low tube current technique and hybrid iterative reconstruction

Computed Tomography-Based Coronary Artery Calcium Score Calculation at a Reduced Tube Voltage Utilizing Iterative Reconstruction and Threshold Modification Techniques: A Feasibility Study

BACKGROUND

The coronary artery calcium score (CACS) indicates cardiovascular health. A concern in this regard is the ionizing radiation from computed tomography (CT). Recent studies have tried to introduce low-dose CT techniques to assess CACS. We aimed to investigate the accuracy of iterative reconstruction (IR) and threshold modification while applying low tube voltage in coronary artery calcium imaging.

METHODS

The study population consisted of 107 patients. Each subject underwent an electrocardiogram-gated CT twice, once with a standard voltage of 120 kVp and then a reduced voltage of 80 kVp. The standard filtered back projection (FBP) reconstruction was applied in both voltages. Considering Hounsfield unit (HU) thresholds other than 130 (150, 170, and 190), CACS was calculated using the FBP-reconstructed 80 kVp images. Moreover, the 80 kVp images were reconstructed utilizing IR at different strength levels. CACS was measured in each set of images. The intraclass correlation coefficient (ICC) was used to compare the CACSs.

RESULTS

A 64% reduction in the effective dose was observed in the 80 kVp protocol compared to the 120 kVp protocol. Excellent agreement existed between CACS at high-level (strength level = 5) IR in low-kVp images and the standard CACS protocol in scores ≥ 11 (ICC > 0.9 and p < 0.05). Increasing the threshold density to 190 HU in FBP-reconstructed low-kVp images yielded excellent agreement with the standard protocol in scores ≥ 11 (ICC > 0.9 and p < 0.05) and good agreement in score zero (ICC = 0.84 and p = 0.02).

CONCLUSIONS

The modification of the density threshold and IR provides an accurate calculation of CACS in low-voltage CT with the potential to decrease patient radiation exposure.

Coronary Artery Calcium Data and Reporting System (CAC-DRS): A Primer

The Coronary Artery Calcium Data and Reporting System (CAC-DRS) is a standardized reporting method for calcium scoring on computed tomography. CAC-DRS is applied on a per-patient basis and represents the total calcium score with the number of vessels involved. There are 4 risk categories ranging from CAC-DRS 0 to CAC-DRS 3. CAC-DRS also provides risk prediction and treatment recommendations for each category. The main strengths of CAC-DRS include a detailed and meaningful representation of CAC, improved communication between physicians, risk stratification, appropriate treatment recommendations, and uniform data collection, which provides a framework for education and research. The major limitations of CAC-DRS include a few missing components, an overly simple visual approach without any standard reference, and treatment recommendations lacking a basis in clinical trials. This consistent yet straightforward method has the potential to systemize CAC scoring in both gated and non-gated scans.

Impact of Adaptive Statistical Iterative Reconstruction-V on Coronary Artery Calcium Scores Obtained From Low-Tube-Voltage Computed Tomography - A Patient Study

OBJECTIVE

To evaluate the impact of adaptive statistical iterative reconstruction-V (ASIR-V) on the accuracy of ultra-low-dose coronary artery calcium (CAC) scoring.

MATERIALS AND METHOD

One-hundred-and-three patients who underwent computed tomography (CT) for CAC scoring were prospectively included. All underwent standard scanning with 120-kilovolt-peak (kVp) and with 80- and 70-kVp tube voltage. ASiR-V was applied to the 80- and 70-kVp scans at different levels. The 120-kVp scans reconstructed with filtered back projection served as the standard of reference. Recently published novel kVp-adapted thresholds were used for calculation of CAC scores from 80- and 70-kVp scans and the resulting CAC scores were compared against the standard of reference. Patients were stratified into six CAC score risk categories: 0, 1-10, 11-100, 101-400, 401-1000, and >1000.

RESULTS

Increasing levels of ASIR-V led to an increasing underestimation of CAC scores with bias ranging from -128 to -118 and from -205 to -198 for the 80- and 70-kVp scans, respectively, when compared with the standard of reference. Reconstruction with 20% and 40% ASIR-V for the 80- and 70-kVp scans, respectively, yielded noise levels comparable to the standard of reference. Nevertheless, a change in risk-class was observed in 29 (28.6%) and 46 (44.7%) patients, exclusively to a lower risk-class, when CAC scores were derived from these reconstructions.

CONCLUSION

ASIR-V leads to noise reduction in CT scans acquired with low tube-voltages. However, ASIR-V introduces substantial inaccuracies and marked underestimation of ultra-low-dose CAC scoring as compared with standard-dose CAC scoring despite normalization of noise.

Comparison of Nongated Chest CT and Dedicated Calcium Scoring CT for Coronary Calcium Quantification Using a 256-Dector Row CT Scanner

BACKGROUND

Coronary artery calcification (CAC) is a marker of atherosclerosis and an independent risk factor for cardiac-related mortality and frequently detected on noncontrast chest CT. We aimed to investigate the reliability and accuracy of determining CAC using noncontrast, nongated chest CT with 256-detector row.

MATERIALS AND METHODS

A total of 1318 patients for chest examination were enrolled to undergo both nongated chest CT and dedicated calcium-scoring CT (CSCT) on a 256-detector row CT scanner. The chest CT was scanned in fast-helical mode with 8 cm collimation, 0.28 second rotation speed and pitch 0.992:1 to cover entire chest. CSCT used single prospective ECG-triggered cardiac axial mode with 0.28 second rotation speed covering only the heart. CAC scores (Agatston, mass, and volume) were determined using both image sets and were statistically compared.

RESULTS

Sensitivity and specificity of nongated chest CT for determining positive CAC was 94.8% (182/192) and 100%, respectively. The agreement in assessing the quantitative Agatston, volume, and mass scores between the nongated chest CT and CSCT was almost perfect, with the intraclass correlation coefficient values of 0.998, 0.999, and 0.999, respectively. Additionally, there was a good agreement in CAC quantification between the nongated chest CT and dedicated CSCT with small coefficient of variation: mass score (9.0%), volume score (9.5%), and Agatston score (12.6%).

CONCLUSION

Nongated chest CT with 256-detector row is a reliable imaging mode for detecting and quantifying calcifications in coronary arteries compared with dedicated calcium-scoring CT.

Ultra-low-dose coronary artery calcium scoring using novel scoring thresholds for low tube voltage protocols-a pilot study

Aims

To determine if tube-adapted thresholds for coronary artery calcium (CAC) scoring by computed tomography at 80 kilovolt-peak (kVp) tube voltage and 70-kVp yield comparable results to the standard 120-kVp protocol.

Methods and results

We prospectively included 103 patients who underwent standard scanning with 120-kVp tube voltage and additional scans with 80 kVp and 70 kVp. Mean body mass index (BMI) was 27.9 ± 5.1 kg/m2. For the lowered tube voltages, we applied novel kVp-adapted thresholds for calculation of CAC scores and compared them with standard 120-kVp scans using intraclass correlation and Bland-Altman (BA) analysis. Furthermore, risk-class (CAC score 0/1-10/11-100/101-400/>400) changes were assessed. Median CAC score from 120-kVp scans was 212 (interquartile range 25-901). Thirteen (12.6%) patients had zero CAC. Using the novel kVp-adapted thresholds, CAC scores derived from 80-kVp scans showed excellent correlation (r = 0.994, P < 0.001) with standard 120-kVp scans with BA limits of agreement of -235 (-39.5%) to 172 (28.9%). Similarly, for 70-kVp scans, correlation was excellent (r = 0.972, P < 0.001) but with broader limits of agreement of -476 (-85.0%) to 270 (48.2%). Only 2 (2.8%) reclassifications were observed for the 80-kVp scans in patients with a BMI <30 kg/m2 (n = 71), and 2 (6.1%) for the 70-kVp scans in patients with a BMI <25 kg/m2 (n = 33). Mean effective radiation dose was 0.60 ± 0.07 millisieverts (mSv), 0.19 ± 0.02 mSv, and 0.12 ± 0.01 mSv for the 120-kVp, 80-kVp, and 70-kVp scans, respectively.

Conclusion

The present study suggests that CAC scoring with reduced peak tube voltage is accurate if kVp-adapted thresholds for calculation of CAC scores are applied while offering a substantial further radiation dose reduction.

Adaptive Iterative Dose Reduction 3D Integrated with Automatic Tube Current Modulation for CT Coronary Artery Calcium Quantification: Comparison to Traditional Filtered Back Projection in an Anthropomorphic Phantom and Patients

RATIONALE AND OBJECTIVES

We aimed to evaluate integrated adaptive iterative dose reduction 3D (AIDR 3D) algorithm in automatic tube current modulation (ATCM) for the quantification of coronary artery calcium score (CACS) and cardiac risk stratification.

MATERIALS AND METHODS

A thoracic phantom with calcium inserts of known densities was scanned with filtered back projection (FBP) and AIDR 3D algorithms in small- and medium-sized phantoms. Twenty-four patients underwent two consecutive scans of CACS with FBP and AIDR 3D algorithms. The absolute Agatston score, Agatston score risk, volume score, and Agatston score percentile-based risk were compared, and concordance coefficients and agreement plots were made.

RESULTS

Agatston and volume scores were significantly different between the phantom sizes (P < .01). There were no significant differences in the Agatston scores between FBP and AIDR 3D for the medium phantoms (P = .25). In the patients, there were no significant differences in Agatston and volume scores between FBP and AIDR 3D (P = .06 and P = .09, respectively). The correlation coefficients of Agatston and volume scores with AIDR 3D were excellent compared to those of FBP. There were no significant differences in Agatston score risk and Agatston score percentile-based risk between FBP and AIDR 3D (P = .74 and P = 1, respectively). There was mean dose reduction of 57.8% ± 18.6% for AIDR 3D.

CONCLUSION

The absolute Agatston score differed between FBP and AIDR 3D reconstructions. However, the cardiac risk categorizations of the two methods were comparable. An integrated AIDR 3D algorithm with automatic tube current modulation enables radiation dose savings at a consistent noise level without sacrificing CACS.

Accuracy of coronary artery calcium scoring with tube current reduction by 75%, using an adaptive iterative reconstruction algorithm

OBJECTIVE

To assess the accuracy of an iterative reconstruction (IR) technique for coronary artery calcium scoring with reduced radiation dose.

METHODS

163 consecutive patients underwent twofold scanning by 320-row detector CT at 120 kVp. A low-dose scan at 25% tube current but with standard scan length (14 cm) was followed by a standard dose scan with routine tube current but reduced scan length (10 cm). Reduced dose images were constructed using filtered back-projection (FBP) and IR (adaptive iterative dose reduction in three dimensions). The standard dose scan reconstructed with FBP served as the gold standard for comparisons. Image noise and Agatston coronary calcium scores were determined and compared between the groups.

RESULTS

Compared with FBP at standard dose, noise at reduced dose increased markedly with FBP but remained low with IR. Mean Agatston score with FBP at reduced dose showed a significant increase as compared with FBP at standard dose. No significant difference was observed when applying IR at reduced dose. At reduced dose, 38 (23.3%) patients were reassigned to a different cardiovascular risk category with FBP but only 8 (4.9%) with IR. Out of 47 patients with a zero Agatston score, 15 patients (31.9%) were false-positive with FBP at reduced dose, but no false positives were found with IR.

CONCLUSION

IR allows accurate coronary artery calcium scoring with a radiation dose reduced by 75%. Advances in knowledge: The application of adaptive iterative dose reduction in three dimensions allows the maintenance of accurate Agatston scores and risk stratification at significantly reduced tube current, thus reducing the patient's exposure to ionizing radiation.

Coronary Artery Calcium Imaging in the ROBINSCA Trial: Rationale, Design, and Technical Background

RATIONALE AND OBJECTIVES

To describe the rationale, design, and technical background of coronary artery calcium (CAC) imaging in the large-scale population-based cardiovascular disease screening trial (Risk Or Benefit IN Screening for CArdiovascular Diseases [ROBINSCA]).

MATERIALS AND METHODS

First, literature search was performed to review the logistics, setup, and settings of previously performed CAC imaging studies, and current clinical CAC imaging protocols of participating centers in the ROBINSCA trial were evaluated. A second literature search was performed to evaluate the impact of computed tomography parameter settings on CAC score.

RESULTS

Based on literature reviews and experts opinion an imaging protocol accompanied by data management protocol was created for ROBINSCA. The imaging protocol should consist of a fixed tube voltage, individually tailored tube current setting, mid-diastolic electrocardiography-triggering, fixed field-of-view, fixed reconstruction kernel, fixed slice thickness, overlapping reconstruction and without iterative reconstruction. The analysis of scans is performed with one type and version of CAC scoring software, by two dedicated and experienced researchers. The data management protocol describes the organization of data handling between the coordinating center, participating centers, and core analysis center.

CONCLUSION

In this paper we describe the rationale and technical considerations to be taken in developing CAC imaging protocol, and we present a detailed protocol that can be implemented for CAC screening purposes.

DIAGNOSTIC REFERENCE LEVELS IN CARDIAC COMPUTED TOMOGRAPHY ANGIOGRAPHY: A SYSTEMATIC REVIEW

Cardiac computed tomography angiography (CCTA) is a commonly used diagnostic tool for cardiovascular disease. Despite constant improvements to imaging technologies, the radiation dose to patients from CCTA remains a concern when using this procedure. There remains a need for optimisation of CCTA procedures and accurate dose monitoring to reduce the potential risk of cancer. Establishing diagnostic reference levels (DRLs) allows for the assessment of radiation dose variations, enabling strategies aimed at standardising doses across radiological centres. This systematic review explores the literature on CCTA methodologies that have been used to establish DRLs. A search was carried out using the Web of Science, SCOPUS, Medline, CINAHL and EMBASE databases. Reference lists of published articles were also assessed to identify further articles. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) methodology was employed to evaluate articles for relevance. Articles were included if they assessed DRLs in CCTA. The search resulted in 448 articles, of which, six were included after a thorough screening process. The literature demonstrates a wide dose variation in reported CCTA DRLs ranging from 671 to 1510 mGy cm in DLP. Where reported, CTDIvol DRLs ranged from 26 to 70 mGy. Differences were found in the methodologies used for establishing CCTA DRLs, including the sampling methodology used for identifying suitable patients and scanning protocols. This current review emphasises the need for an international standardisation for DRLs establishment methods, to provide a more comparable global measurement of dose variations across CT sites.

Dose reduction techniques in coronary calcium scoring: The effect of iterative reconstruction combined with low tube voltage on calcium scores in a thoracic phantom

OBJECTIVES

To define a dose-reduced coronary calcium CT protocol that provides similar calcium score values as a conventional 120kVp protocol.

METHODS

A thorax phantom containing 100 calcifications was scanned with the reference protocol (120kVp, 90 ref mAs, FBP) and 30 dose-reduced protocols (70-110kVp, 90 ref mAs, FBP and iterative reconstruction (IR) levels 1-5) with 3rd generation dual-source CT. For protocols that yielded similar detectability and calcium scores as the reference protocol, additional scans were acquired at reduced ref mAs. Kendall's τ_b and independent-samples-median test were used to determine trends and differences in contrast/signal-to-noise ratio (CNR and SNR), dose and calcium scores.

RESULTS

The detectability and calcium scores decreased at increasing IR levels (τb<-0.825, p<0.001) and increasing tube voltage (τ_b<-0.679, p<0.001). For 90kVp-IR3 and 100kVp-IR1, similar detectability and calcium score was found compared to the reference protocol (p>0.206). For these protocols, lower tube currents did not affect the detectability and Agatston score (p>0.206), while CNR and SNR were similar/higher compared to the reference protocol (0.008<p<0.206). Dose reduction was 60.6% (90kVp-IR3) and 43.6% (100kVp-IR1).

CONCLUSIONS

The protocol of 90kVp-IR3 and 100kVp-IR1 yielded similar calcium detectability, Agatston score and image quality as the reference protocol, with dose reduction up to 60.6%.

In vitro evaluation of a new iterative reconstruction algorithm for dose reduction in coronary artery calcium scoring

Background

Coronary artery calcium (CAC) scoring is a widespread tool for cardiac risk assessment in asymptomatic patients and accompanying possible adverse effects, i.e. radiation exposure, should be as low as reasonably achievable.

Purpose

To evaluate a new iterative reconstruction (IR) algorithm for dose reduction of in vitro coronary artery calcium scoring at different tube currents.

Material and Methods

An anthropomorphic calcium scoring phantom was scanned in different configurations simulating slim, average-sized, and large patients. A standard calcium scoring protocol was performed on a third-generation dual-source CT at 120 kVp tube voltage. Reference tube current was 80 mAs as standard and stepwise reduced to 60, 40, 20, and 10 mAs. Images were reconstructed with weighted filtered back projection (wFBP) and a new version of an established IR kernel at different strength levels. Calcifications were quantified calculating Agatston and volume scores. Subjective image quality was visualized with scans of an ex vivo human heart.

Results

In general, Agatston and volume scores remained relatively stable between 80 and 40 mAs and increased at lower tube currents, particularly in the medium and large phantom. IR reduced this effect, as both Agatston and volume scores decreased with increasing levels of IR compared to wFBP (P < 0.001). Depending on selected parameters, radiation dose could be lowered by up to 86% in the large size phantom when selecting a reference tube current of 10 mAs with resulting Agatston levels close to the reference settings.

Conclusion

New iterative reconstruction kernels may allow for reduction in tube current for established Agatston scoring protocols and consequently for substantial reduction in radiation exposure.

Risk Stratification for Primary Prevention of Coronary Artery Disease: Roles of C-Reactive Protein and Coronary Artery Calcium

Risk stratification of individuals at risk for atherosclerotic cardiovascular disease (ASCVD) plays an important role in primary prevention of cardiovascular disease. In addition to risk scores derived from conventional cardiovascular risk factors, high sensitivity C-reactive protein (hs-CRP) and coronary artery calcium (CAC) have emerged as two of the widely accepted non traditional risk factors for atherosclerotic disease that have shown incremental prognostic value in predicting cardiovascular events. This review systematically assesses the role of hs-CRP and CAC in various studies and demonstrates meta-analyses of the incremental prognostic value of hs-CRP and CAC in identifying patients at risk of future CVD events. Compared with this, CAC showed better incremental prognostic value and might be a better indicator of ASCVD risk in asymptomatic adults.

Prospective evaluation of the influence of iterative reconstruction on the reproducibility of coronary calcium quantification in reduced radiation dose 320 detector row CT

BACKGROUND

Coronary artery calcium (CAC) predicts coronary heart disease events and is important for individualized cardiac risk assessment. This report assesses the interscan variability of CT for coronary calcium quantification using image acquisition with standard and reduced radiation dose protocols and whether the use of reduced radiation dose acquisition with iterative reconstruction (IR; "reduced-dose/IR ") allows for similar image quality and reproducibility when compared to standard radiation dose acquisition with filtered back projection (FBP; "standard-dose/FBP") on 320-detector row computed tomography (320-CT).

METHODS

200 consecutive patients (60 ± 9 years, 59% male) prospectively underwent two standard- and two reduced-dose acquisitions (800 total scans, 1600 reconstructions) using 320 slice CT and 120 kV tube voltage. Automated tube current modulation was used and for reduced-dose scans, prescribed tube current was lowered by 70%. Image noise and Agatston scores were determined and compared.

RESULTS

Regarding stratification by Agatston score categories (0, 1-10, 11-100, 101-400, >400), reduced-dose/IR versus standard-dose/FBP had excellent agreement at 89% (95% CI: 86-92%) with kappa 0.86 (95% CI: 0.81-0.90). Standard-dose/FBP rescan agreement was 93% (95% CI: 89-96%) with kappa = 0.91 (95% CI: 0.86-0.95) while reduced-dose/IR rescan agreement was similar at 91% (95% CI: 87-94%) with kappa 0.88 (95% CI: 0.83-0.93). Image noise was significantly higher but clinically acceptable for reduced-dose/IR (18 Hounsfield Unit [HU] mean) compared to standard-dose/FBP (16 HU; p < 0.0001). Median radiation exposure was 74% lower for reduced- (0.37 mSv) versus standard-dose (1.4 mSv) acquisitions.

CONCLUSION

Rescan agreement was excellent for reduced-dose image acquisition with iterative reconstruction and standard-dose acquisition with filtered back projection for the quantification of coronary calcium by CT. These methods make it possible to reduce radiation exposure by 74%.

CLINICAL TRIAL REGISTRATION

URL: https://clinicaltrials.gov/ct2/show/NCT01621594.

UNIQUE IDENTIFIER

NCT01621594.

Iterative reconstruction can permit the use of lower X-ray tube current in CT coronary artery calcium scoring

OBJECTIVE:

CT coronary artery calcium scoring (CACS) is additive to traditional risk factors for predicting future cardiac events but is associated with relatively high radiation doses. We assessed the feasibility of CACS radiation dose reduction using a lower tube current and iterative reconstruction (IR).

METHODS:

Artificial noise was added to the raw data from 27 CACS studies from patients who were symptomatic to simulate lower tube current scanning (75, 50 and 25% original current). All studies were performed on the same CT scanner at 120 kVp. Data were reconstructed using filtered back projection [Quantum Denoising Software (QDS+)] and IR [adaptive iterative dose reduction three dimensional mild, standard and strong]. Agatston scores were independently measured by two readers. CACS percentile risk scores were calculated.

RESULTS:

At 75, 50 and 25% tube currents, all adaptive iterative dose reduction (AIDR) reconstructions decreased image noise relative to QDS+ (p < 0.05). All AIDR reconstructions resulted in small reductions in Agatston score relative to QDS+ at the standard tube current (p < 0.05). Agatston scores increased with QDS+ at 75, 50 and 25% tube current (p < 0.05), whereas no significant change was observed with AIDR mild at any tested tube current. No difference in the percentile risk score with AIDR mild at any tube current occurred compared with QDS+ at standard tube current (p > 0.05). Interobserver agreement for AIDR mild remained excellent even at 25% tube current (intraclass correlation coefficient 0.997).

CONCLUSION:

Up to 75% reduction in CACS tube current is feasible using AIDR mild.

ADVANCES IN KNOWLEDGE:

AIDR mild IR permits low tube current CACS whilst maintaining excellent intraobserver and interobserver variability and without altering risk classification.

Low Radiation Dose Calcium Scoring: Evidence and Techniques

Coronary computed tomography (CT) allows for the acquisition of thin slices of the heart and coronary arteries, which can be used to detect and quantify coronary artery calcium (CAC), a marker of atherosclerotic cardiovascular disease. Despite the proven clinical value in cardiac risk prognostication, there remain concerns regarding radiation exposure from CAC CT scans. There have been several recent technical advancements that allow for significant radiation dose reduction in CAC scoring. This paper reviews the clinical utility and recent literature in low radiation dose CAC scoring.

Quantifying the impact of using Coronary Artery Calcium Score for risk categorization instead of Framingham Score or European Heart SCORE in lipid lowering algorithms in a Middle Eastern population

Background

The use of the Coronary Artery Calcium Score (CACS) for risk categorization instead of the Framingham Risk Score (FRS) or European Heart SCORE (EHS) to improve classification of individuals is well documented. However, the impact of reclassifying individuals using CACS on initiating lipid lowering therapy is not well understood. We aimed to determine the percentage of individuals not requiring lipid lowering therapy as per the FRS and EHS models but are found to require it using CACS and vice versa; and to determine the level of agreement between CACS, FRS and EHS based models.

Methods

Data was collected for 500 consecutive patients who had already undergone CACS. However, only 242 patients met the inclusion criteria and were included in the analysis. Risk stratification comparisons were conducted according to CACS, FRS, and EHS, and the agreement (Kappa) between them was calculated.

Results

In accordance with the models, 79.7% to 81.5% of high-risk individuals were down-classified by CACS, while 6.8% to 7.6% of individuals at intermediate risk were up-classified to high risk by CACS, with slight to moderate agreement. Moreover, CACS recommended treatment to 5.7% and 5.8% of subjects untreated according to European and Canadian guidelines, respectively; whereas 75.2% to 81.2% of those treated in line with the guidelines would not be treated based on CACS.

Conclusion

In this simulation, using CACS for risk categorization warrants lipid lowering treatment for 5–6% and spares 70–80% from treatment in accordance with the guidelines. Current strong evidence from double randomized clinical trials is in support of guideline recommendations. Our results call for a prospective trial to explore the benefits/risks of a CACS-based approach before any recommendations can be made.