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 Table of Contents  
ORIGINAL ARTICLE
Year : 2015  |  Volume : 1  |  Issue : 1  |  Page : 28-33

Application study of 640-slice computed tomography low dose coronary angiography


Department of Radiology, Union Hospital, Tongji Medical College, Huazhong University of Science and Technology, Wuhan 430022, Hubei Province, China

Date of Web Publication30-Sep-2015

Correspondence Address:
Ziqiao Lei
Department of Radiology, Union Hospital, Tongji Medical College, Huazhong University of Science and Technology, Wuhan 430022, Hubei Province
China
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Source of Support: This project was supported by the Natural Science Foundation of Hubei Province, China (No. 2014CFB986), Conflict of Interest: None declared.


DOI: 10.4103/2226-8561.143948

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  Abstract 

Objective: The aim was to explore image quality and radiation dose in patients with different heart rates in 640-slice volume computed tomography (CT) coronary angiography by using tube voltage of 100 kV. Materials and Methods: The 220 consecutive patients clinically suspected or confirmed of coronary artery disease were divided into three groups: 67 cases in 1 beat group (heart rate <65 bpm); 134 cases in 2 beats group (65 bpm ≤heart rate <80 bpm); 19 cases in 3 beats group (heart rate >80 bpm). When scanning was completed, the best phase for coronary arteries would be chosen. Various postprocessing reconstructions of coronary arteries and branches, such as volume reconstruction (VR), maximum density projection, multiplanar reconstruction, curved surface reconstruction, were used. We classified image quality and made statistical analysis according to 4-grades method. We also compared scalability of coronary arterial segments and radiation doses between the groups. Results: There were no significant differences in the scalability of coronary arterial segments between the groups. Effective radiation doses of the three groups were (2.5±0.8) mSv, (8.4±3.1) mSv and (11.2±3.8)mSv. The radiation doses between the groups showed statistical difference (P < 0.05). Conclusion: 640-slice volume CT can be adapted to changes during heart rate, and ensure the image quality under the condition of 100 kV, and radiation doses were significantly reduced in patients with heart rate <65 bpm.

Keywords: Coronary artery, coronary artery disease, radiation dose, tomography, X-ray computed


How to cite this article:
Lei Z, Han P, Xu H, Yu J. Application study of 640-slice computed tomography low dose coronary angiography. Digit Med 2015;1:28-33

How to cite this URL:
Lei Z, Han P, Xu H, Yu J. Application study of 640-slice computed tomography low dose coronary angiography. Digit Med [serial online] 2015 [cited 2019 Sep 15];1:28-33. Available from: http://www.digitmedicine.com/text.asp?2015/1/1/28/143948


  Introduction Top


With the rapid development of MSCT technology and improvement of medical image reconstruction algorithm of mathematics, computed tomography coronary angiography (CCTA) has become a noninvasive method to diagnose coronary artery disease.[1],[2] However, due to biological effect caused by X-ray ionizing radiation, together with image quality affected by changes in heart rate significantly, CCTA has some deficiencies.[3],[4],[5] In recent years, 640-slice volume computed tomography (CT) has been gradually applied in clinical practice, and its maximum Z-axis width of detector could reach 160 mm, making it possible to cover the entire heart and finish volume data of heart within a single heart beat in a single rotation of gangtry.[6] The present study has collected 220 consecutive cases, which had undergone 640-slice volume CCTA in our hospital. We retrospectively analyzed its influence of heart rate changes on image quality and radiation dose.


  Materials and Methods Top


General information

A total of 220 consecutive patients scanned with 640-slice CCTA were chosen from June 2013 to September 2013 in our hospital, among whom 120 cases were male, and 100 cases were female aged 27-83 years (mean: 61.2 ± 11.1 years), with a body mass index (BMI) of 17.42-34.16 kg/m 2 (mean: 25.01 ± 2.56 kg/m 2). All cases were suspected or confirmed of coronary artery disease, including 9 cases of coronary arterial stent implantation, 11 cases of arrhythmia, and 35 cases with beta blockers. Exclusion criteria included severe renal insufficiency, iodine allergy history, cardiac pacemaker implantation, poor breathless, and coronary artery bypass surgery history.

According to heart rates scanned in real time, we divided the patients into three groups: 67 cases in 1 beat group (heart rate <65 bpm); 134 cases in 2 beats group (65 bpm ≤heart rate <80 bpm); 19 cases in 3 beats group (heart rate >80 bpm).

Preparation before computed tomography scanning

Patients were requested to keep fasting more than 4 h. For patients with resting heart rate more than 80 bpm, they were demanded to take 25-50 mg beta blocker under condition of no contraindications and administered 0.5 mg nitroglycerin sublingually 3 min before scanning. All the patients signed informed consent and were informed with the scanning process. Patients were trained to breathe effectively, and were asked to keep chest and abdomen still when breathless.

Scanning machine and parameters

Toshiba aquilion one 640-slice volume CT was used, and prospective heart switch control calcium score scanning was regularly scanned. Scanning tube voltage was 100 kV, and tube current modulation was used according to BMI (BMI ≤18 kg/m 2, 350-400 mA; BMI 19-24 kg/m 2, 350-400 mA; BMI 25-29 kg/m 2, 400-500 mA; BMI >30 kg/m 2, 500-580 mA). Scanning ranged from 1 cm below tracheal subcarinal to diaphragmatic surface, acquisition time window was 70~80% between R-R period when heart rate was <65 bpm, and it was 30~80% R-R interphase when heart rate was more than 65 bpm, scanning FOV was 320 × 320, and display matrix was 512 × 512. Double cylinder of high pressure syringe was used to inject contrast medium at a rate of 4.0-5.0 mL/s with a volume of 45-55 mL, and then 30 mL saline was injected at the same rate. Set region of interest (ROI) in the descending aorta to monitor CT values, and trigger threshold was 250 HU. When the value of ROI was more than 250 HU, volume scanning would be started after breathing instructions.

Image reconstruction

Volume data contained 75% of automatic reorganization period and optimal period phase, slice thickness of reconstructed transversal images was 0.5 mm, with a 0.25 mm thickness interval, and soft tissue reconstruction algorithm was used. If preset period phase couldn't meet the needs of diagnosis, coronary arterial segments with poor display will be reconstructed to get clear transversal images according to electrocardiogram (ECG) editing option, and then images will be sent to application software in Vitrea FX postprocessing workstation to get volume reconstruction, maximum density projection, multiplanar reconstruction and curved surface reconstruction image coronary arteries.

Evaluation of radiation dose

The present study only recorded radiation dose of coronary CTA scanning, without radiation dose of tomography, calcium score and contrast agent tracking monitoring. We recorded volume CT dose indexes, the product dose length (DLP), and the effective radiation dose (ED). ED will be calculated with DLP multiplied by the k conversion, and k was conversion coefficient (0.014 mSv· mGy −1 cm −1)[7],[8] according to quality standards of the European commission on CT guidance standard.

Evaluation of image quality

According to the American Heart Association of coronary artery segmentation method, the coronary arteries are divided into 15 segments,[9] and image quality was evaluated by four grades: 1- (optimal) continuous localized, no motion artifact, margin of blood vessels was clear; 2- (good) localized continuously, there were minor artifacts, blood vessels had mild interference; 3- (medium) moderate artifact, but blood vessels localized continuously; 4- (poor) severely disturbed by artifacts, blood vessels presented double-line sign, vascular contours cannot be distinguished.[10] 1, 2, 3 points were considered to be evaluable images, and 4 point was considered to be unavailable for diagnosis. Two experienced radiologists assessed image quality of coronary angiography using double-blinded method independently. If evaluations were not consistent, they would negotiate to obtain a consistent conclusion. For severe calcification, stent implantation and diameter of the vessel segment <1.5 mm did not make any evaluation.

Statistical analysis

SPSS 13.0 software was used to make statistical analysis. Cases of general information, scanning parameters, heart rate and radiation dose were tested with variance test, coronary overall image quality assessment with Kruskal–Wallis and Median test, image quality of 15 segmental coronary arteries and 3 main branches were evaluated with variance test.


  Results Top


General information and scanning parameters

General information and the scanning parameters of patients in three groups showed no statistical difference (P>0.05) [Table 1].
Table 1: General information and scanning parameters of patients in three groups

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Evaluation of coronary arteries

Except anatomical variations, stents, calcification and diameter of vessel segment were <1.5 mm, a total evaluation of coronary arterial segments were 2587, and 8 segments (0.3%) were not evaluable for diagnosis caused by motion artifacts, including 2 segments for RCA2, 1 for RCA3, 1 for PLA, 1 for OM and 2 for LAD3. Ratio of excellent and good grades image quality between the three groups of coronary arteries showed no statistically significant difference (P > 0.05). The evaluable segment ratio between groups had no statistical difference (P > 0.05) [Figure 1],[Figure 2],[Figure 3] and [Table 2].
Figure 1: (a-d) 1 beat group. A is volume reconstruction images, LAD and LCX clear display, artifact free; (b-d) respectively maximum density projection images of RCA, LAD, LCX angiography, the coronary artery artifact free, no obvious plaque and stenosis

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Figure 2: (a-d) Two beat group. A is volume reconstruction images, LAD and LCX clear display, artifact free, compared with the Figure 1a image, the naked eye observation of no difference; (b-d) respectively maximum density projection images of RCA, LAD, LCX angiography, the coronary artery wall smooth, artifact free, compared with the Figure 1b-d image, the naked eye observation of no difference

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Figure 3: (a-d) 3 beat group. A is volume reconstruction images, LAD and LCX clear display, artifact free, compared with Figures 1a and 2a image, the naked eye observation of no difference; (b-d) respectively maximum density projection images of RCA, LAD, LCX angiography, each vessel coronary artery artifact free, noncalcified plaque formation of proximal LAD stenosis is about 5-10%, only a little, compared with Figure 1b-d, Figure 2b-d image, the naked eye observation of no difference

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Table 2: Evaluation of coronary arteries of three groups

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Evaluation of radiation dose

The relationship between radiation dose and heart rate had statistical difference (P < 0.05), and radiation dose of high heart rate group was significantly higher than that of low heart rate group [Table 3]. Effective radiation dose of three groups were (2.5±0.8) mSv, (8.4±3.1) mSv, (11.2±3.8) mSv, respectively, and the difference was statistically significant.
Table 3: Radiation dose of three groups

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  Discussion Top


Relationship of low tube voltage with image quality and radiation dose

Most previous studies about CCTA scanning mainly focused on using lower tube current to reduce radiation dose, but there are some limitations of this method to some extent, and SNR of image quality will be effected by lowering tube current.[11],[12]

X-ray absorption of iodine increases when a lower tube voltage is used, as long as the mean effective energy of the polychromatic X-ray is closer to the k-edge of the iodine (33.2 keV). Therefore, the use of a lower tube voltage protocol for CT increases the photoelectric effect and decreases Compton scattering, leading to a higher mean attenuation value of the iodine. In addition, X-ray intensity affects low-density resolution instead of high-density resolution, because it has no effect on image density and contrast. Therefore, body parts with natural high contrast such as lung and sinus should be applied with lower tube voltage. Use of contrast material increased density contrast between coronary arteries and the surrounding tissues.[13],[14] Moreover, radiation dose is proportional to the square of tube voltage in the same filter conditions and constant current conditions.[15],[16] According to the principle above, using lower tube voltage has little influence on imaging quality of CCTA, and can reduce radiation dose at the same time.

Abada et al.[17] found an 80% reduction in radiation dose by applying 80 kV in coronary CTA compared with 120 kV. Pflederer et al.[18] got a 39% reduction in radiation dose by applying 100 kV in dual-source CT coronary CTA compared with 120 kV. In the present study, there is no statistical difference in patients' age, BMI, scanning scope and tube current between the three groups. The coronary arterial enhancements were significantly increased using 100 kV compared with previous studies using 120 kV, making contrast between coronary vessels and surrounding soft tissues increased obviously. There is no statistical difference in image quality of coronary arterial segments, and radiation dose was decreased obviously compared with 120 kV scanning protocol. Therefore, lower tube voltage could be applied on the basis of meeting diagnosis requirements.[19],[20],[21]

Relation between heart rate and image quality

Heart is a motive organ, and image quality of coronary angiography is mainly influenced by time resolution. Time resolution of 640-slice CT is only 175 ms compared with 82.5 ms of dual-source CT, but 160 mm wide detector of 640-slice CT makes it possible to cover the whole heart in a single gangtry rotation, and diagnose coronary arterial disease with high accuracy. It was reported that diagnosis ratio of coronary segments in patients with atrial fibrillation reached 96%.[22],[23] The present research also confirmed that 640-slice CT can keep good image quality in a wide range of heart rate. Our results showed that 640-slice CT has a good adaptability with high heart rate because heart rate was negatively related to image quality, meaning that image quality tends to decrease slightly as the heart rate increases. Due to the influence of heart rate on CCTA, image quality mainly depends on CT time resolution.[24] Although 175 ms time resolution of 640-slice CT reduces the coronary artery display capability in patients with high heart rate in theory, it avoids helical interpolation algorithm of registration and deviation problem caused by different cardiac cycles, with coronary artery in different phase space. Hence, the author get to the conclusion that coverage width is the main reason for good image quality in patients with high heart rate. Moreover, 640-slice CT automatically skips abnormal heart rate to wait for the next normal heart beat exposure, ensuring great image quality in patients with arrhythmia.

In the unevaluable coronary arterial segments in the present study, two segments occurred in the middle of right coronary artery, and the others occurred in the distant segments of coronary arteries and tiny branches; eight segments were caused by motion artifacts. Therefore, lumen size of coronary arteries is an important factor affecting image quality.

Relation between heart rate and radiation dose

Computed tomography coronary angiography has advantage of noninvasive and high diagnosis accuracy rate,[25],[26] and has been applied in clinical practice in several aspects. However, X-ray ionizing radiation of CCTA will increase the risk of cancers. Hence, it has an important practical significance in CCTA on how to reduce radiation dose without loss in image quality.

Prospective heart switch control scanning only exposed in phase of presented cardiac cycle, with the remaining period without X-ray exposing, which significantly reduces radiation dose of patients. This technology has been widely applied in clinical practice in recent years.[27],[28],[29] According to R-R phase change of synchronous ECG in 640-slice CT, the machine will decide whether to increase the acquisition of heart rate. Once phase variation between adjacent R-R is larger, the software will automatically increase the acquisition of heart rate to get more volume data to ensure great image quality (a beat for heart rate <65 bpm, 2 beats for 65 bpm ≤heart rate <80 bpm, 3 beats for heart rate bpm). Average radiation dose of present study was (4.45 ± 2.7) mSv, slightly <7.2 mSv as reported by Rybicki et al.[30] The related cause of this result was different ratio of different tube voltage and different center composition proportion between the two studies. The present study used lower tube voltage of 100 kV, and was mainly composed of two or more heart beats, but previous literature report [30] studied prospective heart switch control cases mainly for a single heartbeat acquisition. But z axis width of 640-slice CT was 160 mm, which can cover the whole heart. There's no overlapping or spiral scan as scanning bed remained motionless, and average radiation dose was significantly lower than the same technique of 12.3-14.7 mSv.[31],[32] In the present study, in terms of radiation dose, single heartbeat data was collected if heart rate was below 65 bpm, and the proportion was 30.5% (67/220) between 70% and 80% R-R exposure period; two heartbeats data was collected if 65 bpm pmlecte rate <80 bpm, and the proportion was 60.9% (134/220) between 30% and 80% R-R exposure time window; three heartbeats data was collected if heart rate was >80 bpm, and the proportion was 8.6% (19/220) between 30% and 80% of R-R exposure time window. The faster the heart rate is, the wider acquisition time window will be. Then the number of acquisition heartbeat will be increased, and the patients have to be exposed to increased radiation dose. Effective radiation doses of three groups above were (2.5 vs. 8) mSv, (8.4 vs. 1) mSv, and (11.2 vs. 8) mSv, respectively, and the difference had statistically significant meaning. So rapid heart rate obtained with high image quality is at the expense of high radiation doses.


  Conclusion Top


640-slice volume CT can adapt to different heart rates, which ensures image quality, and radiation dose in patients with heart rate <65 bpm is significantly reduced.

 
  References Top

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