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The Influence of Iterative Reconstruction on Coronary Artery Calcium Scoring—Phantom and Clinical Studies

Posted Feb 28, 2017 Updated Apr 10, 2023
By Seitaro Oda MD PhD , Daisuke Utsunomiya MD PhD , Takeshi Nakaura MD PhD , Yoshinori Funama PhD , Hideaki Yuki MD PhD , Masafumi Kidoh MD PhD , Kenichiro Hirata MD , Narumi Taguchi MD , Keiichi Honda RT , Hiroko Takaoka MD , Yuji Iyama MD , Kazuhiro Katahira MD PhD , Katsuo Noda MD , Shuichi Oshima MD PhD , Shinichi Tokuyasu AD , Yasuyuki Yamashita MD PhD
9 min read

Rationale and Objectives

We compared the effect of iterative model reconstruction (IMR), filtered back projection (FBP), and hybrid iterative reconstruction (HIR) on coronary artery calcium (CAC) scoring.

Materials and Methods

CAC scans of 30 consecutive patients (18 men and 12 women, age 70.1 ± 12.2 years) were reconstructed with FBP, HIR, and IMR, and the image noise was measured on all images. Two radiologists independently measured the CAC scores using semiautomated software, and interobserver agreement was evaluated. Statistical analysis included the Spearman correlation coefficient and Bland-Altman analysis.

Results

The mean image noise on FBP, HIR, and IMR images was 48.0 ± 7.9, 29.6 ± 4.8, and 9.3 ± 1.3 Hounsfield units, respectively. The difference among all reconstruction combinations was significant ( P < .01). The CAC score on HIR and IMR scans was 4.2% and 8.9% lower, respectively, than the CAC score on FBP images. There was no significant difference in the mean CAC score among the three reconstructions. The interobserver correlation was excellent for all three reconstructions (r 2 = 0.96 FBP, 0.99 HIR, 0.99 IMR); the best Bland-Altman measure of agreement was with IMR, followed by HIR and FBP.

Conclusion

For CAC scoring, IMR can reduce the image noise and blooming artifacts, and consequently lowers the measured CAC score. IMR can lessen measurement variability and yield stable, reproducible measurements.

Introduction

Coronary artery calcium (CAC) is a strong predictor of future cardiovascular events . Multidetector computed tomography (MDCT) is commonly used to assess CAC as part of individual risk evaluations , and CAC scores are obtained at CT screening . As more patients undergo CAC scoring and repeat scanning for treatment monitoring , CAC measurements must yield robust results with low variability to allow meaningful comparisons. A disadvantage of the most commonly used quantification method by Agatston scoring is its limited reproducibility at repeated examinations owing to factors such as the image noise, motion artifacts, and the partial volume effect, etc. . This led to the introduction of two new algorithms, the calcium volume and the calcium mass score , to complement and possibly replace the Agatston score. However, the Agatston score remains the most widely used scoring method because there is strong evidence that it is highly useful in individuals with atherosclerotic heart disease.

As iterative reconstruction (IR) helps to reduce the quantum noise associated with standard convolution-filtered back projection (FBP) reconstruction, it is increasingly integrated in clinical CT studies . Earlier studies indicated that the use of a hybrid IR (HIR) algorithm for cardiac CT, compared to the use of FBP, improves the image quality, allows for a reduction in radiation exposure, and improves the image quality . HIR comprises two denoising components, a sinogram restoration phase that reduces correlated noise and bias artifacts in the projection space, and an iterative denoising process in the image space that reduces the uncorrelated quantum-mottle noise. However, some image noise persists and artifacts may be introduced because of the non-global model of noise reduction. Iterative model reconstruction (IMR), a knowledge-based IR algorithm, is the latest advance in the field of reconstruction techniques. Compared to prior-generation IR, IMR is mathematically more complex, but also more accurate and can provide significantly better image quality than FBP and HIR at cardiac CT . However, the effects of IMR on CAC scoring are still unclear. Renker et al. reported that IR significantly reduced blooming artifacts and calcium volumes on cardiac CT images. If blooming artifacts are reduced by IR, the effects on the detection of small lesions, the assessment of the CAC score, and subsequent risk classification may be possible. We performed phantom and clinical studies to compare the influence of IR on CAC scoring with the effect of standard FBP.

Materials and Methods

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CT Data Acquisition

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CT Image Reconstruction

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CAC Score Measurements

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Phantom Study

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Figure 1, (a) Photograph of the anthropomorphic thorax phantom used to quantify coronary artery calcium (QRM-Cardio-Phantom; QRM GmbH, Moehrendorf, Germany). (b) Diagram of the frontal view of the calibration insert containing three sets of calcified cylinders measuring 1, 3, and 5 mm in diameter; their calcium hydroxyapatite density equals 200, 400, and 800 mg HA/cm 3 , respectively. (c) Transverse computed tomography image of the anthropomorphic thorax phantom with nine different calcifications.

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Clinical Study

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TABLE 1

Patient Characteristics

Number of patients 30 Age (y) 70.1 ± 12.2 Female/male 12/18 Body weight (kg) 57.5 ± 10.6 Body mass index (kg/m 2 ) 22.7 ± 2.9 Average heart rate (beats/min) 59.0 ± 8.1

Note: Data are mean ± SD.

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Statistical Analysis

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Results

Phantom Study

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Figure 2, Graph showing the relative measurement error of the coronary artery calcium (CAC) volume (a) , mean CAC attenuation (b) , and the Agatston score (c) . The CAC volume and the Agatston score were lower, and mean CAC attenuation was higher with iterative model reconstruction (IMR) than with filtered back projection and hybrid iterative reconstruction. IMR yielded stable measurements irrespective of the tube current setting.

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Clinical Study

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TABLE 2

Results of the Clinical Study

FBP HIR IMR_P_ Value Image noise (HU) 47.3 ± 8.5 27.9 ± 5.1 8.9 ± 1.4 <.01 \* Agatston score Observer 1 495.2 ± 535.6 474.5 ± 529.1 451.6 ± 510.4 .72 Observer 2 503.1 ± 555.5 479.9 ± 542.1 455.2 ± 518.3 .84

FBP, filtered back projection; HIR, hybrid iterative reconstruction; HU, Hounsfield units; IMR, iterative model reconstruction.

Note: Data are mean ± SD.

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Figure 3, Bland-Altman analysis of the Agatston score for filtered back projection (FBP) (a) , hybrid iterative reconstruction (HIR) (b) , and iterative model reconstruction (IMR) (c) . The lowest Bland-Altman limit of agreement was with IMR, followed by HIR and FBP.

Figure 4, Coronary artery calcium scoring images of a 72-year-old man. Axial cardiac computed tomography images reconstructed with filtered back projection (FBP) (a) , hybrid iterative reconstruction (HIR) (b) , and iterative model reconstruction (IMR) (c) at the level of the distal segment of the right coronary artery. Highlighted dots are candidates for coronary artery calcium, automatically identified by the scoring software. There are many scattered false-positive dots on the FBP image (a) , and a few false-positive dots on the HIR image (b) . The fewest false-positive dots are seen on the IMR image (c) . The total Agatston scores were 1512 for FBP, 1438 for HIR, and 1321 for IMR.

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Discussion

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Figure 5, Graph of the computed tomography (CT) attenuation profile on a cross section of a 5-mm-diameter calcified lesion (800 mg HA/cm 3 ). Iterative model reconstruction reduced blooming artifacts and improved spatial resolution. As the improved spatial resolution changed the CT attenuation profile into a more rectangular shape, CT attenuation was increased.

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Academic Radiology, Volume 24, Issue 3
Journals General Radiology
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