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Coronary Artery Calcification: Effect of Size of Field of View on Multi–Detector Row CT Measurements¹

¹ From the Mallinckrodt Institute of Radiology, Washington University School of Medicine, Campus Box 8131, 510 S Kingshighway Blvd, St Louis, MO 63110. Received September 10, 2003; revision requested November 20; revision received December 18; accepted January 30, 2004. Address correspondence to K.T.B. (e-mail: baet@mir.wustl.edu).

	ABSTRACT

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The effect of the size of the field of view (FOV) on coronary artery calcium measurements at multi–detector row computed tomography (CT) was assessed. Coronary multi–detector row CT was performed with an identical protocol in 100 consecutive subjects. CT images were reconstructed at different FOV sizes (210, 260, and 310 mm). Calcified coronary lesions were detected in all three image sets in 52 subjects. The FOV sizes tested for multi–detector row CT coronary screening had a negligible effect on coronary artery calcium measurements (P .06). However, risk stratification decreased by one level in seven of 52 subjects when the FOV increased from 210 or 260 to 310 mm.

© RSNA, 2004

Index terms: Coronary vessels, calcification, 548.81 • Coronary vessels, CT, 548.1211 • Computed tomography (CT), image processing, 548.1211 • Computed tomography (CT), image quality, 548.1211

	INTRODUCTION

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Coronary artery calcification measured at electron-beam computed tomography (CT) has been used as a surrogate marker of coronary artery disease (1,2). A scoring system proposed by Agatston et al (1) has been widely adopted for quantifying coronary artery calcium on electron-beam CT scans. To improve the accuracy and reproducibility of coronary artery calcium measurements, researchers have investigated the effects of various CT scanning parameters (3–5) and calcium score algorithms (6–8). In recent years, multi–detector row CT has been increasingly applied in clinical coronary screening. The calcium quantity measured with multi–detector row CT is closely correlated with that measured with electron-beam CT (9). Indeed, the knowledge gained from the electron-beam CT coronary artery calcium measurement serves as a useful reference for multi–detector row CT.

Scanning techniques at multi–detector row CT have evolved rapidly and offer a more flexible choice of scanning parameters than do scanning techniques at electron-beam CT. Several recent studies have addressed the effect of various scanning and image parameters on multi–detector row CT coronary artery calcium measurements (10,11). To our knowledge, results of no systematic study on the effect of the size of the field of view (FOV) have been reported. The size of the FOV in published studies of electron-beam CT coronary artery calcium measurement was quite variable, ranging from 260 to 350 mm (3–8). The matrix size of the CT image is typically fixed at 512 x 512. We postulate that variations in FOV would affect coronary artery calcium measurements. For instance, with a larger FOV, a larger region of interest is covered within the FOV. As a result, each pixel contains more tissues that are subject to increased partial volume averaging. This increase in partial volume averaging may reduce the sensitivity of CT for detecting small calcified plaques and may alter CT pixel attenuation values.

The purpose of this study, therefore, was to assess what effect the size of the FOV has on coronary artery calcium measurements obtained at multi–detector row CT.

	Materials and Methods

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Subjects
This retrospective study was approved by the institutional clinical study review board, and informed consent was not required. We retrospectively reconstructed CT images of 100 consecutive subjects who had undergone a coronary screening examination at our institution between February 2003 and August 2003. The subjects were asymptomatic and self-referred to the study. There were 73 men and 27 women. The age range of the men was 38–81 years (mean age, 56.9 years ± 9.4 [standard deviation]), and the age range of the women was 40–76 years (mean age, 53.0 years ± 11.5). The age range of all subjects was 38–81 years (mean age, 55.9 years ± 10.1). The exclusion criteria for this examination were previous coronary stent placement, the presence of a cardiac pacemaker, and a history of cardiac surgery.

Data Acquisition
All examinations had been performed with the same multi–detector row CT scanner (Somatom Plus 4 VolumeZoom; Siemens, Forchheim, Germany) and no contrast enhancement. Scanning started at the level approximately 1 cm below the carina and ended at the inferior margin of the heart. The scanning parameters were kept constant for all subjects and consisted of the use of a sequential mode, 4 x 2.5-mm collimation, 120 kVp, 100 mA, a 500-msec rotation time, and prospective electrocardiographic triggering at 50% of the R-R interval. With the same protocol, CT scanning of a coronary CT calibration phantom (QRM, Moehrendorf, Germany) was performed for coronary calcium mass calculation.

Three sets of CT images were reconstructed from the raw CT projection data by using three FOV sizes (210, 260, and 310 mm). The smallest FOV size (210 mm) was chosen to cover the entire heart consistently and liberally without excluding the cardiac border in all patients. The 260-mm FOV is the FOV most commonly used for coronary calcium scoring at electron-beam CT. It covers most of the anterior and medial portions of the chest. Finally, the largest FOV size (310 mm) was selected to provide the same increase in size—that is, 50 mm—relative to the middle FOV size that occurred between the smallest and the middle FOV sizes. The largest FOV usually encompasses the entire thorax. Example CT images obtained at the three FOVs are shown in Figure 1. All CT images had a 2.5-mm section thickness and a 512 x 512 matrix size, resulting in a nominal voxel size of 0.41 x 0.41 x 2.5 mm³ for the 210-mm FOV, 0.51 x 0.51 x 2.5 mm³ for the 260-mm FOV, and 0.61 x 0.61 x 2.5 mm³ for the 310-mm FOV.

View larger version (91K): [in this window] [in a new window] [Download PPT slide]   Figure 1a. Transverse multi-detector row CT images for coronary artery calcium screening reconstructed with (a) 210-mm, (b) 260-mm, and (c) 310-mm FOV in 54-year-old man show the same level of the heart and calcified lesions (arrowheads) in the coronary arteries. Although the 210-mm FOV appears to include too much of the lung region and could have been reduced to cover only the heart at this level, the heart occupies a larger cross-sectional area of the CT images at more inferior levels. The use of smaller FOVs may not completely cover the heart at inferior levels.

View larger version (88K): [in this window] [in a new window] [Download PPT slide]   Figure 1b. Transverse multi-detector row CT images for coronary artery calcium screening reconstructed with (a) 210-mm, (b) 260-mm, and (c) 310-mm FOV in 54-year-old man show the same level of the heart and calcified lesions (arrowheads) in the coronary arteries. Although the 210-mm FOV appears to include too much of the lung region and could have been reduced to cover only the heart at this level, the heart occupies a larger cross-sectional area of the CT images at more inferior levels. The use of smaller FOVs may not completely cover the heart at inferior levels.

View larger version (100K): [in this window] [in a new window] [Download PPT slide]   Figure 1c. Transverse multi-detector row CT images for coronary artery calcium screening reconstructed with (a) 210-mm, (b) 260-mm, and (c) 310-mm FOV in 54-year-old man show the same level of the heart and calcified lesions (arrowheads) in the coronary arteries. Although the 210-mm FOV appears to include too much of the lung region and could have been reduced to cover only the heart at this level, the heart occupies a larger cross-sectional area of the CT images at more inferior levels. The use of smaller FOVs may not completely cover the heart at inferior levels.

The calcium quantity was expressed in terms of score, volume, and mass. The score and volume were determined according to the Agatston scoring system (1) and the volume algorithm proposed by Callister et al (12), respectively. Mass was determined by multiplying the calcium CT attenuation value and volume by a calibration factor that was derived from calibration phantom scanning, as described in a previous report (13).

On the basis of the calcium score derived from review of each of the three CT image sets, the same radiologist assigned to each subject a level of risk stratification for future cardiac events according to the guidelines described by Rumberger et al (14). Subjects were at very low risk if the calcium score was 0, at low risk if the calcium score was 1 or greater but less than or equal to 10, at moderate risk if the calcium score was greater than 10 but less than or equal to 100, at moderately high risk if the calcium score was greater than 100 but less than or equal to 400, and at high risk if the calcium score was greater than 400.

The same radiologist measured the CT attenuation within the aorta root lumen on each of the three CT image sets of different FOV sizes by placing a circular region of interest. The standard deviation of the CT attenuation value was taken as a measure of image noise. The region of interest was maintained at the same section level between the three image sets. The size of the region of interest was adjusted to encircle the same anatomic area (100 mm²) at different FOV sizes to minimize the effect of region of interest size variation on CT attenuation measurements.

Statistical Analysis
Significant differences in age between the male and female subject groups were tested by using the independent samples t test. The calcium quantities for the lesions and subjects and image noise were compared between the three CT image sets. Continuous data were compared as means ± standard deviations. Patterns were tested for statistical significance with paired t tests and repeated-measures analysis of variance. A P value of less than .05 was considered to indicate a statistically significant difference. All statistical analyses were performed by using JMP software (SAS Institute, Cary, NC).

	Results

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Lesions
For the 100 evaluated subjects, there was no significant difference in age between the male and the female groups (P = .12, independent samples t test). Forty-five subjects had no detectable coronary artery calcium in any of three CT image sets reconstructed with different FOV sizes. Calcified lesions were detected in at least one of the three image sets in the remaining 55 subjects. Among these 55 subjects, three had a single lesion (calcium score, <1), which was detected only on images reconstructed with 210- and 260-mm FOVs. The other 52 subjects (39 men and 13 women; mean age, 58.8 years ± 9.2) had lesions that were detected in all three image sets: 348, 340, and 326 lesions were found on images reconstructed with 210-, 260-, and 310-mm FOVs, respectively.

The lesions that were not detected at the 260-mm (n = 8) and 310-mm (n = 22) FOVs had calcium scores of less than 2 and did not contribute to a significant change in the total calcium score, volume, or mass for the subjects (P .48, paired t test). Thus, the statistical analysis for evaluating the effect of FOV size on calcium measurements was performed with the 326 lesions (in 52 subjects) that were detected in all three CT image sets. The anatomic distribution of these lesions was as follows: There were 11 lesions in the left main artery, 130 lesions in the left anterior descending artery, 65 lesions in the circumflex artery, and 120 lesions in the right coronary artery. The calcium score range was 0.5–748.8 for the lesions and 0.5–3110.5 for the subjects. The median calcium score was 11.9 for the lesions and 83.4 for the subjects.

Risk Stratification
The calcium scores calculated by using the 210- and 260-mm FOV image sets resulted in identical risk stratification of the subjects. There were seven subjects in the low-risk category, 22 subjects in the moderate-risk category, 11 subjects in the moderately high–risk category, and 12 subjects in the high-risk category. However, the risk stratification decreased by one level in seven of the 52 subjects when the FOV increased from 210 or 260 to 310 mm: The risk stratification decreased from moderate to low in four subjects, from moderately high to moderate in two subjects, and from high to moderately high in one subject.

Image Noise and Calcium Measurements
Table 1 lists the mean CT attenuation and its standard deviation, image noise, as measured in the ascending aorta, for each FOV. No statistical difference in image noise was observed between the CT image sets of different FOV sizes (P = .23, analysis of variance). The calcium scores for individual subjects were not statistically different between the CT image sets of different FOV sizes (P = .35, analysis of variance) (Fig 2). Tables 2 and 3 summarize the means and standard deviations of calcium score, volume, and mass as measured on the CT image sets of different FOV sizes for the individual lesions and for the subjects. None of the differences between measurements at different FOV sizes were statistically significant (P .06, analysis of variance).

View larger version (21K): [in this window] [in a new window] [Download PPT slide]   Figure 2. Graph shows difference in calcium scores of individual subjects between different FOVs (in millimeters). Note that the calcium score at the second FOV is subtracted from the calcium score at the first.

	Discussion

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Our results suggest that a standardization of FOV may be desirable for clinical coronary CT screening when the Agatston scoring system is used. The Agatston score is sensitive to both partial volume averaging and changes in spatial resolution because its calculation depends on the step function of the weighting factor and the peak Hounsfield unit value in the lesion. A change in image FOV (and thus pixel size) affects the degree of partial volume averaging within pixels and thus affects coronary artery calcium scores.

The use of a consistent FOV size for calcium scoring is particularly important for subjects who have scores close to the cutoff thresholds of risk stratification. Results of a previous study by Arad el al (15) suggest that patients with coronary artery calcium scores greater than 160 are at increased risk for a cardiac event during short-term follow-up. Rumberger et al (14) proposed that primary and secondary prevention and risk factor modification be implemented on the basis of score thresholds of 0, 1, 10, 100, and 400. For a score near the thresholds, a small change may result in placement in different risk categories that are subject to different management regimens. However, three-dimensional algorithms, such as calcium volume and mass measurements, are less susceptible to changes in FOV size than the calcium scoring system.

Our results demonstrate that calcium score and risk category may be underestimated with use of a large FOV. The partial volume averaging and measurement error would be reduced with use of a smaller FOV. However, a substantial reduction in FOV may increase image noise and thus affect the detection of calcified lesions—particularly at electron-beam CT because of the low signal-to-noise ratio associated with this type of CT. This increase in image noise due to use of a smaller FOV is less critical at multi–detector row CT. The multi–detector row CT image noise measured in this study (about 13.4 HU) is about half the image noise that has been reported for electron-beam CT (26–28 HU) (8).

According to our measured data, a threshold of 130 HU corresponds to over six times the image noise plus the mean CT attenuation in the ascending aorta (about 42.8 HU). The use of 130 HU at multi–detector row CT may effectively eliminate the influence of noise on calcium detection and measurement. Our data indicated that no significant difference in image noise was found between the three CT image sets reconstructed at different FOV sizes. It appears that the changes in the calcium quantities were attributable solely to the change in partial volume effects associated with FOV size. Thus, a small FOV can be used for coronary artery calcium measurements at multi–detector row CT with no penalty of a substantial increase in noise.

In addition, the results of this study demonstrate that the sensitivity for detecting small calcified lesions is improved with reduced FOV size. Although these lesions may not substantially contribute to the total calcium burden in the majority of subjects, the improved sensitivity would benefit subjects with low amounts of coronary artery calcium because the absence of coronary artery calcium is considered a strong indicator of the absence of obstructive coronary artery disease. One potential advantage of using a larger FOV for the purpose of screening CT is that CT images may include more incidental noncardiac findings (16).

Our study had several limitations. First, the inclusion of a relatively small number of subjects and the evaluation of only three FOV sizes may have reduced the general applicability of the study. Second, we assigned risk stratification levels to the subjects on the basis of only the absolute values of their coronary calcium score. Results of a previous study (17) have suggested that age- and sex-adjusted score values may be more appropriate than absolute score values for assessing individual risk. This topic requires further study. Last, our study did not address the effect of the size of FOV on interscan or interobserver variability in coronary artery calcium measurements at multi–detector row CT.

In conclusion, the results of this study demonstrate that the FOV sizes we tested for multi–detector row CT coronary screening had a negligible effect on coronary artery calcium score, volume, and mass measurements but changed the score-based risk stratification in a few subjects. A standardization of FOV size in clinical coronary artery calcium screening is desirable for measuring the calcium score. Because of the improved sensitivity for detecting small calcified lesions that we observed, the use of a small, consistent FOV is recommended for multi–detector row CT coronary artery calcium measurements.

	FOOTNOTES

 
Abbreviation: FOV = field of view

Authors stated no financial relationship to disclose.

Author contributions: Guarantors of integrity of entire study, C.H., K.T.B.; study concepts and design, C.H., K.T.B.; literature research, C.H., F.Z.; clinical studies, C.H., F.Z., K.T.B.; data acquisition and analysis/interpretation, all authors; statistical analysis, C.H., T.K.P.; manuscript preparation, all authors; manuscript definition of intellectual content, revision/review, and final version approval, C.H., K.T.B.; manuscript editing, C.H., F.Z., K.T.B.

	REFERENCES

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