Generative models for reproducible coronary calcium scoring

Purpose: Coronary artery calcium (CAC) score, i.e., the amount of CAC quantified in CT, is a strong and independent predictor of coronary heart disease (CHD) events. However, CAC scoring suffers from limited interscan reproducibility, which is mainly due to the clinical definition requiring application of a fixed intensity level threshold for segmentation of calcifications. This limitation is especially pronounced in non-electrocardiogram-synchronized computed tomography (CT) where lesions are more impacted by cardiac motion and partial volume effects. Therefore, we propose a CAC quantification method that does not require a threshold for segmentation of CAC. Approach: Our method utilizes a generative adversarial network (GAN) where a CT with CAC is decomposed into an image without CAC and an image showing only CAC. The method, using a cycle-consistent GAN, was trained using 626 low-dose chest CTs and 514 radiotherapy treatment planning (RTP) CTs. Interscan reproducibility was compared to clinical calcium scoring in RTP CTs of 1662 patients, each having two scans. Results: A lower relative interscan difference in CAC mass was achieved by the proposed method: 47% compared to 89% manual clinical calcium scoring. The intraclass correlation coefficient of Agatston scores was 0.96 for the proposed method compared to 0.91 for automatic clinical calcium scoring. Conclusions: The increased interscan reproducibility achieved by our method may lead to increased reliability of CHD risk categorization and improved accuracy of CHD event prediction.

Dose reduction for CT coronary calcium scoring with a calcium-aware image reconstruction technique: a phantom study

Objective

To assess the dose reduction potential of a calcium-aware reconstruction technique, which aims at tube voltage-independent computed tomography (CT) numbers for calcium.

Methods and materials

A cardiothoracic phantom, mimicking three different patient sizes, was scanned with two calcium inserts (named D100 and CCI), containing calcifications varying in size and density. Tube voltage was varied both manually (range 70–150 and Sn100 kVp) and automatically. Tube current was automatically adapted to maintain reference image quality defined at 120 kVp. Data was reconstructed with the standard reconstruction technique (kernel Qr36) and the calcium-aware reconstruction technique (kernel Sa36). We assessed the radiation dose reduction potential (volumetric CT dose index values (CTDIvol)), noise (standard deviation (SD)), mean CT number (HU) of each calcification, and Agatston scores for varying kVp. Results were compared with the reference acquired at 120 kVp and reconstructed with Qr36.

Results

Automatic selection of the optimal tube voltage resulted in a CTDIvol reduction of 22%, 15%, and 12% compared with the reference for the small, medium, and large phantom, respectively. CT numbers differed up to 64% for the standard reconstruction and 11% for the calcium-aware reconstruction. Similarly, Agatston scores deviated up to 40% and 8% for the standard and calcium-aware reconstruction technique, respectively.

Conclusion

CT numbers remained consistent with comparable calcium scores when the calcium-aware image reconstruction technique was applied with varying tube voltage. Less consistency was observed in small calcifications with low density. Automatic reduction of tube voltage resulted in a dose reduction of up to 22%.

Key Points

• The calcium-aware image reconstruction technique allows for consistent CT numbers when varying the tube voltage.

• Automatic reduction of tube voltage results in a reduced radiation exposure of up to 22%.

• This study stresses the known limitations of the current Agatston score technique.

Electronic supplementary material

The online version of this article (10.1007/s00330-020-06709-9) contains supplementary material, which is available to authorized users.

Coronary calcium scoring with partial volume correction in anthropomorphic thorax phantom and screening chest CT images

INTRODUCTION

The amount of coronary artery calcium determined in CT scans is a well established predictor of cardiovascular events. However, high interscan variability of coronary calcium quantification may lead to incorrect cardiovascular risk assignment. Partial volume effect contributes to high interscan variability. Hence, we propose a method for coronary calcium quantification employing partial volume correction.

METHODS

Two phantoms containing artificial coronary artery calcifications and 293 subject chest CT scans were used. The first and second phantom contained nine calcifications and the second phantom contained three artificial arteries with three calcifications of different volumes, shapes and densities. The first phantom was scanned five times with and without extension rings. The second phantom was scanned three times without and with simulated cardiac motion (10 and 30 mm/s). Chest CT scans were acquired without ECG-synchronization and reconstructed using sharp and soft kernels. Coronary calcifications were annotated employing the clinically used intensity value thresholding (130 HU). Thereafter, a threshold separating each calcification from its background was determined using an Expectation-Maximization algorithm. Finally, for each lesion the partial content of calcification in each voxel was determined depending on its intensity and the determined threshold.

RESULTS

Clinical calcium scoring resulted in overestimation of calcium volume for medium and high density calcifications in the first phantom, and overestimation of calcium volume for high density and underestimation for low density calcifications in the second phantom. With induced motion these effects were further emphasized. The proposed quantification resulted in better accuracy and substantially lower over- and underestimation of calcium volume even in presence of motion. In chest CT, the agreement between calcium scores from the two reconstructions improved when proposed method was used.

CONCLUSION

Compared with clinical calcium scoring, proposed quantification provides a better estimate of the true calcium volume in phantoms and better agreement in calcium scores between different subject scan reconstructions.

Influence of dose reduction and iterative reconstruction on CT calcium scores: a multi-manufacturer dynamic phantom study

To evaluate the influence of dose reduction in combination with iterative reconstruction (IR) on coronary calcium scores (CCS) in a dynamic phantom on state-of-the-art CT systems from different manufacturers. Calcified inserts in an anthropomorphic chest phantom were translated at 20 mm/s corresponding to heart rates between 60 and 75 bpm. The inserts were scanned five times with routinely used CCS protocols at reference dose and 40 and 80% dose reduction on four high-end CT systems. Filtered back projection (FBP) and increasing levels of IR were applied. Noise levels were determined. CCS, quantified as Agatston and mass scores, were compared to physical mass and scores at FBP reference dose. For the reference dose in combination with FBP, noise level variation between CT systems was less than 18%. Decreasing dose almost always resulted in increased CCS, while at increased levels of IR, CCS decreased again. The influence of IR on CCS was smaller than the influence of dose reduction. At reference dose, physical mass was underestimated 3–30%. All CT systems showed similar CCS at 40% dose reduction in combinations with specific reconstructions. For some CT systems CCS was not affected at 80% dose reduction, in combination with IR. This multivendor study showed that radiation dose reductions of 40% did not influence CCS in a dynamic phantom using state-of-the-art CT systems in combination with specific reconstruction settings. Dose reduction resulted in increased noise and consequently increased CCS, whereas increased IR resulted in decreased CCS.

CT attenuation values of blood and myocardium: rationale for accurate coronary artery calcifications detection with multi-detector CT

Objectives

To determine inter-session and intra/inter-individual variations of the attenuations of aortic blood/myocardium with MDCT in the context of calcium scoring. To evaluate whether these variations are dependent on patients’ characteristics.

Methods

Fifty-four volunteers were evaluated with calcium scoring non-enhanced CT. We measured attenuations (inter-individual variation) and standard deviations (SD, intra-individual variation) of the blood in the ascending aorta and of the myocardium of left ventricle. Every volunteer was examined twice to study the inter-session variation. The fat pad thickness at the sternum and noise (SD of air) were measured too. These values were correlated with the measured aortic/ventricular attenuations and their SDs (Pearson). Historically fixed thresholds (90 and 130 HU) were tested against different models based on attenuations of blood/ventricle.

Results

The mean attenuation was 46HU (range, 17-84HU) with mean SD 23HU for the blood, and 39HU (10-82HU) with mean SD 18 HU for the myocardium. The attenuation/SD of the blood were significantly higher than those of the myocardium (p<0.01). The inter-session variation was not significant. There was a poor correlation between SD of aortic blood/ventricle with fat thickness/noise. Based on existing models, 90 HU threshold offers a confidence interval of approximately 95% and 130 HU more than 99%.

Conclusions

Historical thresholds offer high confidence intervals for exclusion of aortic blood/myocardium and by the way for detecting calcifications. Nevertheless, considering the large variations of blood/myocardium CT values and the influence of patient’s characteristics, a better approach might be an adaptive threshold.

Can nontriggered thoracic CT be used for coronary artery calcium scoring? A phantom study

PURPOSE

Coronary artery calcium score, traditionally based on electrocardiography (ECG)-triggered computed tomography (CT), predicts cardiovascular risk. However, nontriggered CT is extensively utilized. The study-purpose is to evaluate the in vitro agreement in coronary calcium score between nontriggered thoracic CT and ECG-triggered cardiac CT.

METHODS

Three artificial coronary arteries containing calcifications of different densities (high, medium, and low), and sizes (large, medium, and small), were studied in a moving cardiac phantom. Two 64-detector CT systems were used. The phantom moved at 0-90 mm∕s in nontriggered low-dose CT as index test, and at 0-30 mm∕s in ECG-triggered CT as reference. Differences in calcium scores between nontriggered and ECG-triggered CT were analyzed by t-test and 95% confidence interval. The sensitivity to detect calcification was calculated as the percentage of positive calcium scores.

RESULTS

Overall, calcium scores in nontriggered CT were not significantly different to those in ECG-triggered CT (p>0.05). Calcium scores in nontriggered CT were within the 95% confidence interval of calcium scores in ECG-triggered CT, except predominantly at higher velocities (≥50 mm∕s) for the high-density and large-size calcifications. The sensitivity for a nonzero calcium score was 100% for large calcifications, but 46%±11% for small calcifications in nontriggered CT.

CONCLUSIONS

When performing multiple measurements, good agreement in positive calcium scores is found between nontriggered thoracic and ECG-triggered cardiac CT. Agreement decreases with increasing coronary velocity. From this phantom study, it can be concluded that a high calcium score can be detected by nontriggered CT, and thus, that nontriggered CT likely can identify individuals at high risk of cardiovascular disease. On the other hand, a zero calcium score in nontriggered CT does not reliably exclude coronary calcification.

A phantom study of the effect of heart rate, coronary artery displacement and vessel trajectory on coronary artery calcium score: potential for risk misclassification

BACKGROUND

Accurate coronary artery calcium scoring improves risk stratification in some strata of the population.

OBJECTIVE

We evaluated individual and combined effects of reader experience, heart rate, vessel displacement, and trajectory on computed tomography (CT) Agatston score, calcium volume, and calcium mass in a cardiac phantom model.

METHODS

A cardiac motion phantom was scanned with a 64-slice CT scanner with artificial electrocardiogram gating with combinations of the following: heart rates 60, 80, and 100 beat/min; vessel displacement of 1.25 and 2.5 cm; and multiple vessel trajectories of craniocaudal, right-left, anteroposterior, right coronary artery (RCA), left anterior descending, and left circumflex (LCX). Calcium quantification was done by 2 different readers with the use of 3 methods: Agatston, calcium volume, and calcium mass.

RESULTS

Heart rate, coronary displacement, and trajectory had significant effects on all 3 techniques, with a general decrease in score as the heart rate increased. A vessel displacement of 2.5 cm decreased the Agatston score by 16% (P < 0.0001) and LCX motion decreased the score by 17% (P < 0.0001). Combined effects often resulted in larger differences; for example, a heart rate of 60 beat/min, vessel displacement of 1.25 cm, and RCA motion resulted in an Agatston score of 907, whereas with a heart rate of 100 beat/min, vessel displacement of 2.5 cm, and LCX motion the score was 604.

CONCLUSION

The calcium score is affected by heart rate, vessel displacement, and trajectory.

Comparison of calcium scoring with 4-multidetector computed tomography (4-MDCT) and 64-MDCT: a phantom study

OBJECTIVE

To determine differences in coronary artery calcium (CAC) measurement performed with the use of 2 generations of multidetector computed tomography (CT) scanners of the same manufacturer.

METHODS

Agatston Score (AS) and calcium mass (CM) were measured with a 4-row scanner (AS4 and CM4) and a 64-row scanner (AS64 and CM64) using a cardiac phantom with calcium inserts.

RESULTS

The results of the AS measurements (mean ± SD) varied significantly between the equipment: 880.6 ± 30.1 (AS4) vs 586.5 ± 24.0 (AS64; P < 0.0001). The AS interscanner variability was 31.6% for the phantom and from 25.5% to 110.1% for particular inserts. Mean ± SD CM values were different as well: 192.8 ± 5.0 mg (CM4) vs 152.4 ± 2.6 mg (CM64; P < 0.0001). Determination of CM with 64-row CT was more accurate than that with an older scanner; the mean relative error was -9.1% and 15.0%, respectively (P < 0.0001). The CM interscanner variability was 23.3% for the phantom and from 19.0% to 122.8% for particular inserts. The interexamination variability ranged from 1.7% (CM64) to 5.6% (AS4).

CONCLUSIONS

Coronary artery calcium scoring with the 64-row CT scanner is more accurate than with the 4-row device The difference between the results of AS and CM measurements carried out with both scanners is statistically significant.

Coronary calcium mass scores measured by identical 64-slice MDCT scanners are comparable: a cardiac phantom study

To assess whether absolute mass scores are comparable or differ between identical 64-slice MDCT scanners of the same manufacturer and to compare absolute mass scores to the physical mass and between scan modes using a calcified phantom. A non-moving anthropomorphic phantom with nine calcifications of three sizes and three densities was scanned 30 times on three 64-slice MDCT scanners of manufacturer A and on three 64-slice MDCT scanners of manufacturer B in both sequential and spiral scan mode. The mean mass scores and mass score variabilities of seven calcifications were determined for all scanners; two non-detectable calcifications were omitted. It was analyzed whether identical scanners yielded similar or significantly different mass scores. Furthermore mass scores were compared to the physical mass and mass scores were compared between scan modes. The mass score calibration factor was determined for all scanners. Mass scores obtained on identical scanners were similar for almost all calcifications. Overall, mass score differences between the scanners were small ranging from 1.5 to 3.4% for the total mass scores, and most differences between scanners were observed for high density calcifications. Mass scores were significantly different from the physical mass for almost all calcifications and all scanners. In sequential mode the total physical mass (167.8 mg) was significantly overestimated (+2.3%) for 4 out of 6 scanners. In spiral mode a significant overestimation (+2.5%) was found for system B and a significant underestimation (-1.8%) for two scanners of system A. Mass scores were dependent on the scan mode, for manufacturer A scores were higher in sequential mode and for manufacturer B in spiral mode. For system A using spiral scan mode no differences were found between identical scanners, whereas a few differences were found using sequential mode. For system B the scan mode did not affect the number of different mass scores between identical scanners. Mass scores obtained in the same scan mode are comparable between identical 64-slice CT scanners and identical 64-slice CT scanners on different sites can be used in follow-up studies. Furthermore, for all systems significant differences were found between mass scores and the physical calcium mass; however, the differences were relatively small and consistent.