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DOI: 10.1148/radiol.2363040039
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(Radiology 2005;236:810-814.)
© RSNA, 2005

Cardiac Imaging

Coronary Artery Calcium Measurement with Multi–Detector Row CT and Low Radiation Dose: Comparison between 55 and 165 mAs1

Joseph Shemesh, MD, Rami Evron, PhD, Nira Koren-Morag, PhD, Sara Apter, MD, Judith Rozenman, MD, Dorith Shaham, MD, Yacov Itzchak, MD and Michael Motro, MD

1 From the Grace Ballas Research Unit of the Cardiac Rehabilitation Institute, Chaim Sheba Medical Center, Sackler School of Medicine (J.S., M.M.) and Division of Epidemiology and Preventive Medicine (N.K.), Tel-Aviv University, Tel-Hashomer 52621, Israel; CT Engineering, Philips Medical Systems, Haifa, Israel (R.E.); Department of Diagnostic Imaging, Chaim Sheba Medical Center, Tel-Hashomer, Israel (S.A., J.R., Y.I.); and Department of Radiology, Hadassah Medical Center, Jerusalem, Israel (D.S.). From the 2003 RSNA Annual Meeting. Received January 8, 2004; revision requested March 9; final revision received November 16; accepted December 10. Supported by a grant from Philips Medical Systems. Address correspondence to J.S. (e-mail: dshemesh{at}netvision.net.il).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
PURPOSE: To prospectively compare the results of coronary artery calcium (CAC) measurements obtained with 55- and 165-mAs electrocardiographically gated multi–detector row computed tomography (CT).

MATERIALS AND METHODS: Institutional clinical study review board approval and written informed consent were obtained. Fifty-one consecutive subjects (mean age, 59 years ± 10) were scanned consecutively by using 165 and 55 mAs. For each examination, the number of lesions, total calcium score (TCS) calculated with Agatston algorithm (130-HU threshold), and calcium mass (in milligrams) were measured. Noise was measured by averaging 1 standard deviation of the CT attenuation values in five consecutive transverse sections of the ascending aorta. Paired t test and Pearson correlation were used to compare measurements between the examinations.

RESULTS: By using 55 mAs, CAC was detected (TCS > 0) in all 33 subjects in whom CAC was initially detected with 165 mAs. The mean values of CAC measures with 165 and 55 mAs, respectively, were as follows: number of lesions, 6.2 ± 9.6 and 6.1 ± 9.4; TCS, 123 ± 223 and 126 ± 225; and calcium mass, 23.25 mg ± 43 and 24.25 mg ± 44 (P value was not significant for all parameters). Significant high correlation was found between the two methods for all measures (r > 0.90, P < .01). Similar results were obtained with analysis by coronary vessel. Image noise was 9.3 HU ± 2.1 with 165 mAs and 14.7 HU ± 3.9 with 55 mAs (P < .001), with a parallel decrease in the volume CT dose index from 12 to 4 mGy.

CONCLUSION: Radiation dose can be reduced (eg, 55 mAs) for CAC detection and measurement at multi–detector row CT and provides results comparable to those obtained with 165 mAs.

© RSNA, 2005


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
Coronary artery calcium (CAC) is an unequivocal marker of coronary atheroclerosis (1,2). It can be detected and measured with fast computed tomographic (CT) techniques such as electron beam CT and multi–detector row CT. Repeat multi–detector row CT had been successfully performed to track the progression of CAC in different clinical settings (3–7). Screening of the asymptomatic population for detection of early-stage coronary artery disease is one of the most promising applications of CAC measurements, which may also require serial scans. However, the high image quality of multi–detector row CT is achieved at the expense of a substantial amount of ionizing radiation. Exposure to radiation should be a major concern in the screening population and in others who are candidates for repeat examinations. Furthermore, the long-term effect of this irradiation is not well defined. Thus, it is essential to reduce the CT radiation dose to the minimal level that will allow an adequate quantification of CAC. Only a few studies have been published in which low doses were used for CAC quantification: in vitro with a phantom (8) and in vivo by using multi–detector row CT (9–12). Thus, the purpose of our study was to prospectively compare the results of CAC measurements obtained with 55- and 165-mAs electrocardiographically gated multi–detector row CT.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 References
 
The study was approved by our institutional clinical study review board, and written informed consent was obtained from all participants. Subjects were informed of the radiation dose associated with each examination.

Subjects
Between January and April 2003, 51 consecutive subjects (37 men and 14 women; mean age, 59 years ± 9; age range, 33–80 years) underwent coronary CT for coronary risk factors (n = 34) or chest pain evaluation (n = 17). Subjects with a history of cardiac surgery, stents, or a pacemaker were excluded. The sample size was calculated on the basis of a paired t test with power of 80%.

Image Acquisition
CT scanning of the heart was performed by using a four-detector row scanner (MX8000; Philips Medical Systems, Cleveland, Ohio). Each subject underwent two consecutive CT examinations. The first examination was performed with 165 mAs, and the second examination was performed with 55 mAs. Subjects remained stationary on the table during both examinations, without changing position. Other CT parameters were a prospectively electrocardiographically triggered sequential scan mode, 4 x 2.5-mm collimation (ie, four detector rows with 2.5-mm section thickness), and 120 kVp, and these parameters were identical for both examinations. At each trigger, images were acquired with a gantry rotation time of 500 msec and a temporal resolution of 330 msec (a 180° scan plus additional scan coverage for fan-beam width). The triggering time point was defined as that which ensured data acquisition would occur completely or mostly within the diastolic phase. The scanning duration, which was dependent on heart rate, lasted 15–25 seconds for 120 mm of coverage. The images were reconstructed in a field of view (200–280 mm) that encompassed the entire heart. This field of view resulted in a 0.4–0.5 mm/pixel resolution in a 512 x 512 matrix CT image.

Data Measurement and Analysis
CT images were transferred to a workstation (MxView 5.0; Philips Medical Systems) with dedicated cardiac analysis software. The total calcium score (TCS) was calculated with the Agatston algorithm by a reader (J.S.) with 10 years of experience in coronary calcium reading. With this algorithm, a calcified lesion was defined as one or more contiguous pixels with CT attenuation equal to or greater than 130 HU in each section. An attenuation factor was determined on the basis of the maximal CT attenuation of the lesion as follows: factor 1 = 130–199 HU, factor 2 = 200–299 HU, factor 3 = 300–399 HU, and factor 4 = 400 HU or greater. The calcium score was calculated by multiplying the area of each calcified plaque by the corresponding attenuation factor. The TCS was determined by summing the lesion scores for all sections and normalizing the result to the standard 3-mm section thickness with use of the Agatston method. The calcium score for each coronary vessel (left main, left anterior descending, left circumflex, and right coronary arteries) was also calculated. Calcium mass was calculated by means of a calibration factor, which was determined from a calibration phantom (IMP, Erlangen, Germany) (13). Low- and high-dose scans were randomly evaluated while the reader was blinded to the results of the other examination. The interval between the first and second reading was 1 week.

To calculate intraobserver variation, a second reading of each scan was performed after the enrollment of the last subject (2 weeks to 4 months from the first study), with the reader blinded to the previous results.

Image Noise Measurement
We measured image noise levels by placing a region of interest of approximately 100 mm2 within the ascending aorta. Noise was defined on each scan as the average of 1 standard deviation of the CT attenuation values in five consecutive transverse sections of the ascending aorta. Noise measurement was performed by the same reader (J.S.) for each scan at the time of calcium scoring.

Statistical Analysis
Data were analyzed by using SPSS software (version 11.0; SPSS, Chicago, Ill). The TCS, the calcium score per each coronary vessel, and the number of lesions obtained in the two examinations were compared by using a two-tailed paired-samples t test. Correlation of the calcium scores between the two examinations was measured with the Pearson correlation coefficient. P value of .05 or less was considered to indicate a statistically significant difference.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 References
 
Total Calcium Score
No significant differences were found in the TCS and the score of each vessel between the 55- and 165-mAs dose examinations (Table 1).


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  		TABLE 1. Calcium Score and Vessel Score

 
With a power of 80%, the sample size of 51 subjects enabled us to detect a significant difference in the TCS (≥14 score units, which is a clinically insignificant difference) between 55- and 165-mAs dose scans. In our study, we observed a difference of 6 score units, which suggests similar results with use of both doses.

There were no significant differences in the measures of the calcium mass between the 165- and the 55-mAs dose (23 mg ± 43 vs 24 mg ± 44, respectively; P = .24). The TCS between the high- and low-dose scans was highly correlated with respect to the Agatston method and calcium mass (r = 0.97, P < .001 and r = 0.99, P < .001, respectively). This high correlation was also found for each vessel (Table 2). The absolute difference in the TCS as calculated with the Agatston algorithm between high- and low-dose scans ranged from 0 to 28 score units. Fourteen subjects had a higher score and 17 had a lower score when measured with 55 mAs compared with 165 mAs.


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  		TABLE 2. Pearson Correlation between 165- and 55-mAs Studies for Calcium Score

 
Intraobserver Variation
The results of the second reading were highly correlated with those of the first reading (r = 0.99, Agatston method; r = 1.0, calcium mass).

Lesion Detection
By using 55 mAs, CAC was detected (TCS > 0) in all 33 subjects in whom CAC was detected on the initial 165-mAs scan. A total of 328 lesions were detected with 165 mAs, compared with 314 lesions detected with 55 mAs. No significant differences were found between the two examinations in the total number of lesions and the number of lesions per vessel (Table 3). With a power of 80%, our sample size enabled us to detect a clinically important difference between the number of lesions on the 55-mAs scans and the number of lesions on the 165-mAs scans (0.9 and higher, which is not a clinically important difference). In our study, we observed a difference of 0.2 lesions, which suggests similar results with use of both doses.


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  		TABLE 3. Total Number of Lesions and Lesions per Vessel

 
The same total number of lesions was detected with each dose in 29 (57%) of the 51 subjects. An absolute difference of one to three lesions was found between the examinations in 19 subjects (37%), and in only three subjects, we found a difference of more than three lesions between the examinations. In Table 4 we present the absolute differences per vessel by counting both negative and positive differences.


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  		TABLE 4. Absolute Differences in Number of Lesions per Vessel

 
Image Noise
Image noise was 9.3 HU ± 2.1 (range, 5–15 HU) with 165 mAs and 14.7 HU ± 3.9 (range, 8–28 HU) with 55 mAs (P < .001); the volume CT dose index decreased from 12 to 4 mGy, respectively. Correlation between the noise in the two examinations yielded an r value of 0.89 (P < .001). The Figure is an example of CT images obtained in the same subject at both 165 and 55 mAs.



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  		A, Transverse 165-mAs CT scan shows a small calcific lesion (arrow) in the proximal left anterior descending coronary artery. The lesion score was 67. The average image noise at the level of the ascending aorta is 12.2 HU. B, Transverse 55-mAs CT scan of the same patient. The same calcific lesion (arrow) could be clearly detected, with a calcium score of 61 and image noise of 15.8 HU. Automatic highlighting of the calcification was used, which displayed the calcification as gray on both A and B.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
References
 
The main finding of our study is that applying a low milliampere-second of 55 mAs for CAC detection and measurement at multi–detector row CT produces results very close to those obtained with the currently recommended 165-mAs dose (14) for all the parameters. This is mainly owing to the finding that, despite the increase in image noise, when 55 mAs was applied the noise remained below 30 HU (range, 8–28 HU) in all subjects. Therefore, the 130-HU threshold for calcium is 3 to 10 standard deviations higher than the average attenuation level of blood (40 HU). Thus, calcified lesions can be well identified with a good discrimination from noise artifacts.

Results of our study strengthen those of previous studies in which a low milliampere-second was used (8–12). Takahashi and Bae (10) assessed the interscan variability of CAC as measured with two low milliampere-second levels (40 and 80 mAs) by using a comparable technique of prospective electrocardiographically gated multi–detector row CT. They calculated the calcium volume and calcium score in 33 asymptomatic subjects by performing two consecutive CT examinations, the first with a dose of either 40 (n = 18) or 80 (n = 15) mAs and the second with a dose of 150 mAs. In 27 of the 33 subjects, they found CAC deposits on CT scans of both examinations. Five subjects had no calcium deposit at either examination. One subject had a calcium deposit at only one examination. The TCS between the high and low-dose scans was highly and significantly correlated (r = 0.98) and was not significantly different (P = .58).

We used 120 kVp in our study, compared with 140 kVp used by Takahashi and Bae, which further decreases patient exposure. Furthermore, we could confirm in vivo the findings of Hong et al (8), who assessed in vitro the effect of radiation dose on CAC quantification at multi–detector row CT, by using a cardiac phantom at various milliamperes from 20 to 160 mA. They found a clear tendency for image noise to decrease as the milliampere-second level increased, while no tendency was found for the Agatston score or calcium mass errors to vary with the milliampere-second. In our study, the TCS calculated with the Agatston algorithm and the calcium mass were not significantly changed despite the increase in image noise (5–15 HU for 165 mAs and 8–28 HU for 55 mAs). We found only mild differences in the TCS (range, 0–28) between the two doses. Fourteen subjects had a higher score and 17 had a lower score when measured with 55 mAs compared with 165 mAs, which reflects the interscan variability rather than a tendency of 55-mAs scores to be lower or higher than the scores obtained with 165 mAs. This suggests that the databases of score distributions according to age and sex (15,16), which were compiled from scans obtained with the 165-mAs protocol, may be used to determine the score percentiles of patients scanned with the lower dose scan protocol.

Image Noise
In our study, we maintained a constant tube voltage of 120 kVp and decreased the milliampere-second from 165 mAs at the first examination to 55 mAs at the second examination. This resulted in an increase of noise from 9.3 HU ± 2.1 to 14.7 HU ± 3.9 but a parallel decrease of the radiation dose to one-third as compared with that at 165 mAs: The volume CT dose index decreased from 12 to 4 mGy. According to the recommendation of the multiscanner, multimanufacturer, international standard for the quantification of CAC at cardiac CT (17), the image noise should be maintained at a target of 20 HU for all body sizes. We achieved this target in 88% (45 of 51) of the subjects with 55-mAs scans, while in the remaining six subjects the noise level was above 20 HU.

Study Limitations
The main goal of the present study was to compare the results of two irradiation doses. To minimize the effects of factors that may potentially affect the results, such as another reader, the same reader interpreted the scans. Interobserver variation was not evaluated in the present study.

In summary, strong evidence was found that 55 mAs is an adequate dose that provides results of calcium measures comparable to those obtained at 165 mAs. Radiation dose can thus be reduced for CAC detection and measurement at multi-detector row CT. However, the appropriate use of low-dose irradiation for calcium scoring, particularly for large-size patients, should be further studied. The methods should be further refined to individualize an adequate radiation dose for each subject.


   FOOTNOTES
 

Abbreviations: CAC = coronary artery calcium • TCS = total calcium score

Author contributions: Guarantor of integrity of entire study, J.S.; study concepts, M.M., J.S., R.E., D.S.; study design, J.S., N.K., R.E.; literature research, S.A., D.S.; clinical studies, J.S., M.M.; data acquisition, N.K.; data analysis/interpretation, N.K., J.S.; statistical analysis, N.K.; manuscript preparation, J.S., J.R.; manuscript definition of intellectual content, Y.I., M.M., R.E.; manuscript editing, Y.I., S.A.; manuscript revision/review, M.M.; manuscript final version approval, J.S.


   References
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 

  1. Wexler L, Brundage B, Crouse J, et al. Coronary artery calcification: pathophysiology, epidemiology, imaging methods, and clinical implications—a statement for health professionals from the American Heart Association. Circulation 1996;94:1175–1192.[Free Full Text]
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  8. Hong C, Bae KT, Pilgram TK, Suh J, Bradley D. Coronary artery calcium measurement with multi-detector row CT: in vitro assessment of effect of radiation dose. Radiology 2002;225:901–906.[Abstract/Free Full Text]
  9. Mahnken AH, Wildberger JE, Simon J, et al. Detection of coronary calcifications: feasibility of dose reduction with a body weight-adapted examination protocol. AJR Am J Roentgenol 2003;181:533–538.[Abstract/Free Full Text]
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  16. Silber S, Shemesh J, Weg N, Yoon HC. Multicenter age and gender distribution of coronary artery calcification as measured by four-slice computed tomography in 5345 people (abstr). Eur Heart J 2003; 24(suppl):570.
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