Evaluate of the effect of low tube voltage on the radiation dosage using 640-slice coronary CT angiography

Abstract

BACKGROUND:

640-slice coronary CT angiography is becoming an accurate and reliable method of diagnosing coronary heart disease. However, how to reduce the radiation dosage while ensuring the clinically acceptable image quality remains a quite challenging issue.

OBJECTIVE:

To evaluate the effect of low tube voltage on radiation dosage under 640-slice coronary CT angiography (CCTA).

METHODS:

Four hundred patients (236 males, 164 females) with coronary heart disease and underwent CCTA using DCVT were classified into A1 (tube voltage: 120 kV; exposure phase window: 30–80%), B1 (120 kV; 70–80%), A2 (100 kV; 30–80%) and B2 group (100 kV; 70–80%), respectively. Image qualities and effective dose (ED) were assessed and compared.

RESULTS:

No significant differences were observed among the groups in terms of age, height, weight and body mass index (BMI) (P > 0.05). ED were significantly lower in 100 kV group (P < 0.05). CT values of coronary artery in 100 kV groups were 13.5% and 17.3% higher than 120 kV group. ED in B1 group were 64.5% and 67.0% lower than A1 group. ED in B2 group were 65.4% and 65.2% lower than A2 group.

CONCLUSION:

When using a 640-slice CCTA prospective ECG-gating scanning mode, it is preferable to use a 100 kV tube voltage setting because compared to 120 kV tube voltage protocol, it seems to significantly decrease the mean effective radiation dose, without significantly lowering both the subjective and objective image quality.

Keywords

640-slices CT coronary CT angiography radiation dosage tube voltage image quality

1 Introduction

Coronary artery disease (CAD) is the leading cause of death in developed countries [1 –3] and, although coronary catheterisation remains the diagnostic gold standard in the evaluation of coronary arteries, several studies demonstrated that CCTA (coronary computed tomography angiography), thanks to its high sensitivity and negative predictive value, represents an effective alternative in the assessment of suspected CAD [3 –5].

With the multi-slice spiral CT (MSCT) technology improving, CT coronary artery imaging (CTA) is becoming a reliable method during coronary heart disease diagnosis. How to reduce the radiation dosage as well as ensure the image quality is a hot topic [6 –10]. Concerns over radiation risks of CCTA prompted the CT scanner manufacturers to develop several techniques to lower the radiation exposure, such as ECG- based tube current modulation, prospective ECG gating, noise reduction filters, automatic exposure control systems, low tube voltage protocols and iterative methods of image reconstruction (ASIR, MBIR, IRIS, AIDR) [11]. Some studies showed that the radiation dosage is associated with the scanning method, tube voltage and tube current [12, 13]. Prospective electrocardiograph (ECG)-gating is an advantaged scanning technology applied in clinical practice [14]. It is reported that compared to retrospective ECG-gating scanning mode, prospective ECG-gating scanning mode produced lower radiation dosage [15 –17].

Studies conducted over the past years using 64-slices, DS 64-slices and 320-row CT scanners have demonstrated that the use of low tube voltage is an important tool to lower radiation exposure because the latter is proportional to the square of tube voltage [18 –20]. Nevertheless, low tube voltage is inevitably accompanied by an increase in image noise and, thus, its use is advisable only for patients with a BMI (Body Mass Index) lower than 25–30 [20 –22]. 640-slice coronary CT angiography is becoming an accurate and reliable method of diagnosing coronary heart disease. However, the question regarding how to reduce the radiation dosage companied with ensuring the image quality remains unclear.

In this regard, the main purpose of our study is to evaluate the impact of a 100 kV tube voltage protocol on image quality and radiation dose under 640-slice coronary CT angiography (CCTA).

2 Materials and methods

2.1 Patients

From December 2012 to April 2013, patients diagnosed with coronary heart disease and underwent CTA using DCVT were enrolled into the study. Informed consent was obtained from each patient and this study was approved by the Ethics Committee of local hospital. The inclusion criteria were chest pain, chest tightness, precordial discomfort, palpitations, and flustered. The exclusion criteria were: poor kidney function (serum creatinine >1.5 mg/dl), prior coronary artery surgery, coronary stents, heart rate (HRs) >65 beats per minute (bpm) after beta-blocker treatment.

All subjects were randomly assigned to groups A1/A2 & B1/B2. The patients (heart rate ≥65 bmp or fluctuation of heart rate ≥5 BPM) that underwent CCTA at tube voltage of 120 kV and 30–80% exposure phase window were classified into A1 group (112 patients). The patients (heart rate <65 bmp or fluctuation of heart rate <5 BPM) that underwent CCTA at tube voltage of 120 kV and 70–80% exposure phase window were classified into B1 group (88 patients). The patients (heart rate ≥65 bmp or fluctuation of heart rate ≥5 BPM) that underwent CCTA at tube voltage of 100 kV and 30–80% exposure phase window were classified into A2 group (109 patients). The patients (heart rate <65 bmp or fluctuation of heart rate <5 BPM) that underwent CCTA at tube voltage of 100 kV and 70–80% exposure phase window were classified into B2 group (91 patients).

2.2 Methods

One hour prior the CT acquisition, blood pressure and heart rate of all subjects were measured and registered. Respiratory training was performed to ensure that the breath-holding was ideal during the examination. Patients were maintained at supine position with arms on the top of the head. A 20G catheter needle was placed in the elbow vein. Nonionic contrast medium iohexol (45–55 ml, 350 mg I/ml, Schering, Germany) was injected into the elbow vein with the flow rate of 4.5–5.5 ml/s using high pressure syringe with dual channel. And then 30 ml normal saline was injected. CT scan was performed in inspiratory apnea under the perspective ECG-gating scan model using 640-slice CCTA (Toshiba, Aquilion one, Toshiba Medical Systems, Japan). The scanning range was from 1 cm below the carina of trachea to the cardiac septum. Contrast agent tracing technology was used under the automatic mode. The trigger threshold value of descending aorta was set at 180 Hu (Hounsfeld units), the revolving speed was 0.35 s per circle, and the collecting range of volume data was 12–16 cm. The tube current range was set by self-regulation.

2.3 Objective image quality assessment

Objective Image Quality assessment of the proximal coronary arteries was performed by a cardiovascular radiologist with 10 years of experience in the interpretation of CCTA. The following areas were assessed: the ascending aorta, coronary sinus, left anterior descending artery, circumflex artery, right coronary artery (10 mm from the opening) and the thoracic wall muscle, and included in left front, right front of thoracic wall deep level muscles. Background noise was defined as the average noise of the air before the thoracic wall muscle. The signal to noise ratio (SNR) was the average CT value of coronary artery/background noise. The contrast to noise ratio (CNR) was the ratio of (the average CT value of coronary artery minus the average CT value of thoracic wall muscle)/background noise.

2.4 Subjective image quality analysis

Subjective assessment of images quality was performed by two experienced cardiac radiologists on a workstation (Vitrea FX 2.1, Toshiba), who were blinded to details of CT datasets. Each reviewer evaluated the images independently and in a random fashion. Discordant grades were discussed between the radiologists until a consensus rating was reached. The images were evaluated using the 15- segment method put forward by American Heart Association [23]: RCA, segments 1–4; LM coronary artery, segment 5; LAD coronary artery, segments 6–10; LCX coronary artery, segments 11–15. Image quality was assessed on the segments with a diameter >2.0 mm and those distal to an occluded vessel were excluded. We used 5-point scale to evaluate the vessel segments [15]: 5 (excellent) = excellent wall delineation and opacification of the artery lumen without movement artifacts and noise-associated blurring; 4 (very good) = very good wall delineation and opacification of the artery lumen, with minimal movement artifacts and image noise; 3 (good) = good wall delineation and opacification of the artery lumen, with moderate movement artifacts and image noise; 2 (adequate) = severely impaired wall delineation and opacification of the artery lumen, because of severe movement artifacts and/or image noise; 1 (non-diagnostic) = poor artery wall delineation (because of severe movement artifact and/or marked image noise-associated blurring) and lack of vessel attenuation.

2.5 Estimation of radiation dose

Volume CT dose index (CTDIvol) and dose length product (DLP) were recorded from the console display of the CT scanner. The radiation dose was calculated. The effective dose (ED, mSv) = computed tomographic dose index volume (CTDIvol)×dose length×k (k = 0.017 mSv/(mGy×cm) [15].

2.6 Statistical analysis

Data were expressed as mean±SD and analyzed with statistical software SPSS 18.0 (SPSS, Chicago, IL, USA). Variance analysis and post hoc tests were used. P < 0.05 were considered statistically significant. The baseline data, image qualities, CTDIvol and ED were compared between the groups.

3 Results

3.1 Baseline data of the patients

Four hundred patients (236 males, 164 females) with a mean age of 57.8±10.7 years were enrolled into the study. There were 112 patients in A1 group, 88 patients in B1 group, 109 patients in A2 group and 91 patients in B2 group. The radiation dose and image quality were assessed in all patients. The patients’ baseline data and relevant CT results were summarized in Table 1. There was no statistical significance (P > 0.05) (including age, height, weight and body mass index) among the groups.

Table 1
Baseline data of patients in the 4 groups (mean±SD)

Baseline 120 kV A1 120 kV B1 100 kV A2 100 kV B2 P-value

Age(years) 54.3±15.8 51.8±17.1 51.8±17.1 53.1±17.3 >0.05

Height(cm) 168.5±7.9 168.0±8.8 167.3±8.6 167.9±8.2 >0.05

Weight(kg) 69.0±11.61 70.2±11.1 69.4±10.6 68.3±11.2 >0.05

Body mass index 24.1±3.5 24.9±3.2 24.8±3.1 24.2±3.2 >0.05

Baseline	120 kV A1	120 kV B1	100 kV A2	100 kV B2	P-value
Age(years)	54.3±15.8	51.8±17.1	51.8±17.1	53.1±17.3	>0.05
Height(cm)	168.5±7.9	168.0±8.8	167.3±8.6	167.9±8.2	>0.05
Weight(kg)	69.0±11.61	70.2±11.1	69.4±10.6	68.3±11.2	>0.05
Body mass index	24.1±3.5	24.9±3.2	24.8±3.1	24.2±3.2	>0.05

3.2 Image quality

The whole coronary artery segments with a diameter >2.0 mm were evaluated. The objective evaluation of the image quality was consistent to the subjective ones. It showed that image quality was excellent without artifacts (Figs. 1 and 2). As shown in Table 3, the number of segments with subjective scores above 4 (images without artifacts) accounted for 97.93% (1374/1403), 97.85% (1366/1396), 97.91% (1361/1390), and 97.38% (1382/1413) in each group and there was no significant difference among the four groups (P > 0.05). The average subjective scores of the images in the four groups were 4.9±0.4, 4.8±0.5, 4.7±0.5 and 4.7±0.6, respectively (P > 0.05). Among the 1500 potentially analyzable segments, 93.53% (1403/1500), 93.07% (1396/1500), 92.67% (1390/1500) and 94.20% (1413/1500) segments were considered to be assessable in the four groups. The percentages of non-diagnostic segments were not significant different among the four groups (P > 0.05).

Fig.1

The images of a female patient (age: 46 years old, heart rate: 65–71 bpm, tube voltage: 120 kV, tube current: 280 mA, exposure time phase: 30–80%). A: The axial image showed the opening of left coronary artery; B: The CPR image showed left anterior descending coronary artery (image quality score: 5).

Fig.2

The images of a male patient (age: 38 years old, heart rate: 57–61 bpm, tube voltage: 120 kV, tube current: 350 mA, exposure time phase: 70–80%). A: The axial image showed the opening of left coronary artery; B: The VR image showed left anterior descending coronary artery (image quality score: 5).

Table 2

CT image data of patients in the 4 groups (mean±SD)

Item	120 kV A1	120 kV B1	100 kV A2	100 kV B2	P-value
Background noise	10.0±3.9	10.3±4.1	14.8±6.2	15.1±5.5
Mean CT value of muscle (Hu)	50.9±14.7	54.7±15.4	60.1±14.8	61.3±15.3
Mean CT value of coronary artery (Hu)	496.6±104.5	489.5±94.5	560.7±105.2	546.9±107.1
Coronary scoring	4.9±0.4	4.8±0.5	4.7±0.5	4.7±0.6
DLP (mGy.cm)	571.6±140.5	190.3±48.7	398.0±68.3	142.3±38.4	<0.05
ED (mSy)	9.7±2.4	3.2±0.8	6.8±1.2	2.4±0.7	<0.05
SNR	59.4±32.6	58.9±38.8	47.3±31.1	43.3±25.9	>0.05
CNR	53.3±29.9	52.4±34.6	42.1±27.2	38.6±24.0	>0.05

Table 3

Subjective image quality scores among the for groups

Score	120 kV A1 (N = 1403)	120 kV B1 (N = 1396)	100 kV A2 (N = 1390)	100 kV B2 (N = 1413)
5	1220	1169	1012	1044
4	154	197	349	332
3	28	27	24	31
2	1	3	5	6
1	0	0	0	0

Pearson’s Chi-square test with Yates’ continuity correction. Fisher’s exact test was used when data frequences expected in the contingency tables were less than five.

3.3 Radiation dosage

ED were significantly lower in the 30–80% exposure phase window group (B1 and B2 groups) compared with those in the 30–80% exposure phase window group (A1 and A2 groups) (Table 2). Compared with A1 group, ED in A2 group decreased 25.9% and 26.6% respectively with statistical significance (p < 0.01) (Fig. 1). Compared with B1 group, ED in B2 group decreased 21.4% and 22.6% respectively with statistical significance (p = 0.012, p = 0.001) (Fig. 2). CT values of coronary artery in 100 kV groups (A2 and B2 groups) were 13.5% and 17.3% respectively higher than those in 120 kV groups (A1 and B1 groups).

With the same tube voltage and different exposure time phase, radiation dosage showed significant difference. ED in V B1 group were 64.5% and 67.0% respectively lower compared with those in A1 group with statistical significance (p < 0.01). ED in B2 group were 65.4% and 65.2% respectively lower compared with those in A2 group with statistical significance (p < 0.01). SNR, CNR and subjective image scores in A2 and B2 groups were lower than A1 and B1 groups (p > 0.05).

4 Discussion

The results showed that, when using same exposure phase window setting of 640-slice CCTA, low tube voltage could reduce the radiation dosage. Compared with 120 kV tube voltage, 100 kV tube voltage produced less radiation dosage without significantly lowering subjective and objective image quality.

Limited by the width of the detector, the previous multi-slice spiral CT adopted retrospective ECG gating mode, using multiple cardiac cycle and small pitch. Thus, the scanning time was long and the radiation dosage was high [24, 25]. We used Toshiba 320 row 640-slice CT (DVCT) which employed wide detector of 16 cm. The whole heart could be covered at the direction of Z axis and the prospective ECG gating mode is available, which reduces the radiation dosage. Besides, the spatial resolution of 640-slice CT can be improved to 0.31 mm and the scan bed doesn’t need to move during the process of scanning, which avoids ladder artifact and leads to good image quality [26]. The images in the present study showed good quality without artifact.

Previous studies found that low tube current could reduce the radiation dosage, but the image noise increased, CNR was decreased, the reduction of radiation dosage was limited [27, 28]. Form formula of X-ray intensity [29]: I = KiZU² (I: X-ray intensity; i: tube current; K: proportionality factor; Z: The anode atomic number of the target material; U: tube voltage). X-ray intensity had a positive relationship with tube current and the square of tube voltage, so compared with decreasing the tube current, decreasing the tube voltage could obviously decrease the radiation dosage [30, 31]. In the present study, with the same exposure time phase and different tube voltages, the CTDIvol and ED decreased in A2 group compared with A1 group; CTDIvol and ED decreased in B2 group compared with B1 group. It suggested that low tube voltage could reduce the radiation dosage. With the same tube voltage and different exposure time phase, the results showed the reduced the exposure time phase or cardiac cycle contributed to lowered radiation dosage.

Image noise was derived from the density values within a large region of interest in the left ventricle [32]. Previous studies reported that SNR was defined as the ratio of the mean density of the contrast-filled left ventricular chamber which divided by image noise [16, 33], CNR was defined as the difference between the mean density of the contrast-filled left ventricular chamber and the left ventricular wall, which was divided by image noise [34, 35]. After reducing the tube voltage, photo electric effect of X-ray and body tissue would increase, CT value of the blood vessels with iodinated contrast agent was increased, but the change was not obvious in the peripheral soft tissue. Thus it increased the contrast between blood vessel and peripheral soft tissue and then improved the CNR [36]. In this study, SNR, CNR and subjective image scores in A2 and B2 groups were lower than A1 and B1 groups, but there was no statistical difference.

The goal of reducing the CT radiation dosage is to reduce radiant harm to human as well as achieves good image quality. The reduction range of radiation dosage in the present study was less than the report of Yan et al. [37]. This might be due to that automatic regulating tube current was adopted in this study, thus the compensatory tube current increased slightly after reducing the tube voltage. But it needs further investigation.

There are some limitations in the present study. First, only two values of tube voltage were investigated in this study. Second, the image quality scoring system was largely subjective and thus potentially biased. We also did not test the diagnostic accuracy with coronary catheterization correlation. Thirdly, the objective image quality assessment done by one radiologist alone is likely to cause bias in the experimental results, therefore ROI was taken several times in the specific location of the image and then the mean value of ROI was taken in our study, which ensures the accuracy and objectivity of the results to a certain extent.

5 Conclusions

Our study shows that, using a 640-slice CCTA prospective ECG-gating scanning mode, it is preferable to use a 100 kV tube voltage setting because compared to 120 kV tube voltage protocol, it seems to significantly decrease the mean effective radiation dose, without significantly lowering subjective and objective image quality.

Funding

This work was supported by the <Key Development Projects of Shandong Province >under Grant <number: 2011GGB14011>.

Conflict of interest

The authors declare that there is no conflict of interest.

Footnotes

Acknowledgments

None.

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