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A Low Tube Voltage Technique Reduces the Radiation Dose at Retrospective ECG-gated Cardiac Computed Tomography for Anatomical and Functional Analyses

Posted Jul 31, 2011 Updated Apr 10, 2023
By Seitaro Oda MD , Daisuke Utsunomiya MD , Yoshinori Funama MD , Kazuo Awai MD , Kazuhiro Katahira MD , Takeshi Nakaura MD , Yumi Yanaga MD , Tomohiro Namimoto MD , Yasuyuki Yamashita MD
11 min read

Rationale and Objectives

To investigate the effect of low-tube-voltage technique on a cardiac computed tomography (CT) for coronary arterial and cardiac functional analyses and radiation dose in slim patients.

Materials and Methods

We enrolled 80 patients (52women, 28 men; mean age, 68.7 ± 8.9 years) undergoing retrospective electrocardiogram-gated 64-slice cardiac CT. Forty were subjected to the low (80-kV) and 40 to the standard (120-kV) tube-voltage protocol. Quantitative parameters of the coronary arteries (ie, CT attenuation, image noise, and the contrast-to-noise ratio [CNR]) were calculated, as were the effective radiation dose and the figure of merit (FOM). Each coronary artery segment was visually evaluated using a 5-point scale. Cardiac function calculated by using low-tube-voltage cardiac CT was compared with that on echocardiographs.

Results

CT attenuation and image noise were significantly higher at 80- than 120-kV ( P < .01). CNR of the left and right coronary artery was 18.4 ± 3.8 and 18.5 ± 3.3, respectively, at 80 kV; these values were 19.7 ± 2.7 and 19.8 ± 2.8 at 120 kV; the difference was not significant. The estimated effective radiation dose was significantly lower at 80 than 120 kV (6.3 ± 0.6 vs. 13.9 ± 1.1 mSv, P < .01) and FOM was significantly higher at 80 than 120 kV ( P < .01). At visual assessment, 99% of the coronary segments were diagnostic quality; the two protocols did not differ significantly. We observed a strong correlation and good agreement between low-tube-voltage cardiac CT and echocardiography for cardiac functional analyses.

Conclusion

Low-tube-voltage cardiac CT significantly reduced the radiation dose by approximately 55% in slim patients while maintaining anatomical image quality and accuracy of cardiac functional analysis.

Cardiac computed tomography (CT) using multidetector CT (MDCT) yields high diagnostic accuracy for the evaluation of coronary artery stenosis and cardiac function . However, the widespread use of cardiac CT raises concerns with respect to radiation exposure. At 64-slice MDCT, the radiation exposure is high; it ranges from 11–15 mSv to 21 mSv for the coronary artery and from 14 to 26 mSv for coronary bypass graft studies with an extended scan length. To minimize the patient dose, conventional electrocardiogram (ECG)-dependent tube current modulation is used; at 37–40%, the savings from this technique are moderate . Another recently-implemented low-dose CT technique is prospective “step-and-shoot” ECG triggering. This technique allows for a radiation dose reduction of 77–79% . However, it requires a regular, low heart rate, which may not be obtainable in all patients, and the evaluation of cardiac function is not possible because images are acquired during end-diastole only. Strategies for reducing the radiation dose for anatomical and functional analyses at cardiac CT involve lowering the tube voltage. The use of a low-tube-voltage technique drastically reduces the radiation dose and effectively increases contrast enhancement . However, with the low-tube-voltage technique, the transmitted radiation is decreased and image noise is increased; this may constitute a disadvantage in clinical practice where diagnostic image quality must be maintained. To our knowledge, the effects of the low-tube-voltage technique on anatomical and functional analyses at cardiac CT have not been adequately assessed.

We investigated the effects of low-tube-voltage retrospective ECG-gated cardiac CT on qualitative and quantitative image quality parameters and the radiation dose in slim patients by comparing low- (80 kV) and standard tube-voltage settings (120 kV).

Materials and methods

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Study Population

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Cardiac CT Examination Technique

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Qualitative Image Quality Analysis

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Quantitative Image Quality Analysis

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Evaluation of the CT Radiation Dose

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Evaluation of Cardiac Function

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Statistical Analysis

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Results

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Table 1

Demographic Data of Patients

80-kV Protocol 120-kV Protocol_P_ Value Number of patients 40 40 Age 68.3 ± 8.2 69.0 ± 9.6 .71 Sex (male/female) 13/27 15/25 .64 Body weight (kg) 50.5 ± 5.9 51.7 ± 7.9 .45 Body mass index (kg/m 2 ) 20.7 ± 2.2 21.1 ± 2.3 .39 Heart rate (beats/minute) 56.5 ± 7.4 59.4 ± 6.8 .08

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Qualitative Image Quality

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Table 2

Qualitative Assessment of Overall Image Quality

80-kV Protocol 120-kV Protocol_P_ Value Total number of evaluated segments 320 320 Mean score 4.83 4.82 .80 Diagnostic image 316 (98.8) 318 (99.4) .41 Nondiagnostic image 4 (1.3) 2 (0.6) .41

Numbers in parentheses are percentages.

Figure 1, (a) A 67-year-old woman with chest pain. Curved multiplanar reconstruction (CPR) of the left anterior descending artery and volume-rendered image using the 80-kV protocol. The image quality is excellent (score 5), computed tomography (CT) attenuation (left main coronary artery; LMA) = 608.4 HU, contrast enhancement (LMA) = 702.5 HU, image noise = 37.6 HU, contrast-to-noise ratio (CNR) (LMA) = 18.7, effective dose = 6.3 mSv, figure-of-merit (FOM) (LMA) = 55.5. (b) A 76-year-old woman with chest pain. CPR of the left anterior descending artery and volume-rendered image using the 120-kV protocol. The image quality is excellent (score 5), CT attenuation (LMA) = 376.1 HU, contrast enhancement (LMA) = 466.3 HU, image noise = 25.5 HU, CNR (LMA) = 19.6, effective dose = 12.9 mSv, FOM (LMA) = 29.8.

Figure 2, A 58-year-old man with chest pain. Curved multiplanar reconstruction of the left anterior descending artery and volume-rendered image using the 80-kV protocol. Observers judged the quality of the images as diagnostically unsatisfactory (score 2).

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Quantitative Image Quality

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Figure 3, Box plot showing the results of quantitative analysis of the quality of images obtained with the 80- and 120-kV protocols. (a) Mean computed tomography attenuation in the ascending aorta, proximal right coronary artery (RCA), and left main coronary artery (LMA) was significantly higher with the 80- than the 120-kV protocol ( P < .01). (b) Contrast enhancement in the proximal RCA and LMA was significantly higher under the 80- than the 120-kV protocol ( P < .01). (c) Image noise was significantly higher with the 80- than the standard 120-kV protocol ( P < .01). (d) Contrast-to-noise ratio of the RCA and LMA was 18.5 ± 3.3 and 18.4 ± 3.8, respectively, at 80 kV; these values were 19.8 ± 2.8 and 19.7 ± 2.7 at 120 kV. The difference was not statistically significant (RCA and LMA, P = .06).

Table 3

Quantitative Assessment of Image Quality

80-kV Protocol 120-kV Protocol_P_ Value CT attenuation (HU) Ascending aorta 624.9 ± 88.1 388.2 ± 51.8 <.01 Proximal RCA 613.8 ± 81.4 374.7 ± 41.1 <.01 LMA 612.4 ± 89.0 374.3 ± 50.8 <.01 Contrast enhancement (HU) Proximal RCA 715.2 ± 78.9 472.1 ± 45.7 <0.01 LMA 713.8 ± 88.0 471.7 ± 55.4 <0.01 Image noise (HU) 39.6 ± 6.3 24.3 ± 3.6 <0.01 CNR Proximal RCA 18.5 ± 3.3 19.8 ± 2.8 0.06 LMA 18.4 ± 3.8 19.7 ± 2.7 0.06

CNR, contrast-to-noise ratio; HU, Hounsfield units; LMA, left main coronary artery; RCA, right coronary artery.

Data are presented as the mean ± SD.

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CT Radiation Dose and FOM

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Figure 4, Box plot shows the dose length product (DLP) and the estimated radiation dose under the 80- and 120-kV protocol. Under the 120-kV protocol, the mean DLP and estimated radiation dose were 989.8 ± 78.6 mGy･cm and 13.9 ± 1.1 mSv, respectively. With the reduced tube-voltage (80-kV) protocol a significant reduction in the mean DLP (453.3 ± 40.3 mGy･cm) and effective dose (6.3 ± 0.6 mSv) was achieved ( P < .01). The radiation dose reduction was approximately 55%.

Figure 5, Box plot showing the figure of merit (FOM) of the right coronary artery (RCA) and left main coronary artery (LMA) under the 80- and 120-kV protocol. FOM of the RCA and LMA was significantly higher at 80- than 120-kV (RCA, 56.3 ± 22.7 vs. 28.8 ± 8.3, P < .01; LMA, 55.9 ± 22.4 vs. 28.7 ± 8.2, P < .01).

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Cardiac Function

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Figure 6, The (a) 80-kV and (b) 120-kV protocols showing Bland-Altman analyses comparing ejection fraction obtained with cardiac computed tomography (CT) and echocardiography. The x axis shows the average of cardiac CT and echocardiography. The point of intersection with the y axis indicates the bias of cardiac CT. Dotted lines show the 95% limit of bias. There was no evident systematic bias for EF values between cardiac CT and echocardiography under both tube-voltage protocols.

Figure 7, The (a) 80-kV and (b) 120-kV protocol showing the correlation between ejection fraction calculated with cardiac computed tomography (CT) and echocardiography. There was a strong correlation between cardiac CT and echocardiographic ejection fraction values at both tube-voltage settings (80 kV, r = 0.90, P < .01; 120-kV, r = 0.91, P < .01).

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Discussion

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Academic Radiology, Volume 18, Issue 8
Journals General Radiology
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