Kai Sun ＊,Guo-rong Liu ,Yue-chun Li ,Rui-juan Han ,Li-fang Cui ,Li-jun Ma ,Li-gang Li,and Chang-yong Li

1Department of Radiology,2Department of Cardiology,Baotou Central Hospital,Baotou 014040,China

3CT BM Clinic Marketing,Siemens Healthcare,Beijing 100102,China

4Department of Physiology,School of Basic Medical Sciences,Wuhan University,Wuhan 430071,China

CORONARY computed tomography angiography(CCTA) is a non-invasive tool with high diagnostic accuracy for the detection of coronary arterial stenoses.1-5Higher intracoronary attenuation in CCTA results in greater diagnostic accuracy for the detection of coronary artery stenoses.6In general,the test bolus or bolus-tracking method has been commonly used to determine the contrast material arrival time in CCTA.7,8Previous studies have shown that the bolustracking technique has been an effective tool for better synchronizing scanning with contrast enhancement.9-11The bolus-tracking technique resulted in a lower volume of contrast material,as the initial small contrast material bolus for the test bolus is not needed in bolus-tracking technique.7,9-11

Since the introduction of the second generation of dual-source CT (DSCT) systems,a new era in CT acquisition speed has started.Compared with older CT systems,the second generation of DSCT has two detector systems,each acquiring 128 slices,allowing CCTA examinations to be performed at a high-pitch,up to 3.4.12A gantry rotation time of 0.28 second enables a temporal resolution of 75 ms.This allows for the acquisition of all axial planes that comprise the heart in approximately 280 ms,and the CCTA acquisition time is reduced to a quarter of a second,allowing depiction of the entire heart within a single heart beat.Some studies have indicated that prospectively electrocardiogram (ECG)-gated high-pitch spiral acquisition mode (Flash mode) CCTA provides a high diagnostic accuracy for the assessment of coronary stenoses with an effective radiation dose below 1 mSv.13,14Nevertheless,for Flash mode,the scan delay time of checking up and respiratory instructions (occupied 4.9 seconds spoken in Chinese) in the bolus-tracking technique are relatively longer (total 12 seconds) and test bolus technique is commonly used to ensure the synchronization of contrast material administration.12,15

To our knowledge,it has not yet been investigated whether bolus-tracking techniques in Flash mode CCTA can acquire a better homogeneous enhancement.In this study,we aimed to investigate the feasibility of acquiring the similar homogeneous enhancement using bolus-tracking techniques with shortened time of respiratory instructions in Flash mode CCTA in comparison to the test bolus technique.

PATIENTS AND METHODS

Patients

One hundred and eighty-four consecutive patients who underwent CCTA with mean heart rate ≤65 beats per minute (bpm) were prospectively included in this study.The patients were randomly divided into two groups,92 in each group.Patients in the group A (37 women,55 men;mean age 56±11 years,range 35-78 years) received Flash mode CCTA using bolus-tracking technique with shortened instructional time of respiration,while those in the group B(44 women,48 men;mean age 55±10 years,range 37-76 years) using test bolus technique.The study protocol was approved by Institutional Review Board of Baotou Central Hospital,and all patients gave informed consents to participate in the study.Patients with previous coronary artery interventions,i.e.stenting and/or coronary artery bypass grafts,as well as the body mass index (BMI) ≥30 kg/m2were not included in this study.Patients with renal failure or any known allergy to contrast agent were not considered for CCTA.Patients with heart rate ＞65 bpm and irregular heart rates such as atrial fibrillation and premature ventricular contraction were also excluded from this study.

Scanning protocol and image reconstruction

All examinations were performed with a second-generation DSCT system (Somatom Definition Flash,Siemens Healthcare,Forchheim,Germany).Before the examination,the heart rate was measured.Patients with a heart rate of 65 bpm or higher were given 25-50 mg of metoprolol (Selokeen;AstraZeneca Pharmaceutics,London,United Kingdom) orally 1 hour before the examination to lower the heart rate,so that the diagnostic image quality was achieved in all coronary segments in single diastolic reconstruction.12,15,16All patients received a single dose of 0.8 mg nitroglycerin aerosol (Shandong Jingwei Pharmaceutical Co.Ltd.,Shangdong,China) 3 minutes prior to scanning.

In the group A,the bolus-tracking technique was performed by using dedicated software (CARE bolus,Siemens Healthcare).A total of 60 mL of iohexol (350 mgI/mL,Omnipaque;GE Healthcare,USA) was injected at a flow rate of 5 mL/s followed by administration of 60 mL saline chasing.Dynamic monitoring scanning using a series of dynamic low-dose monitoring scans (100 kV,20 mAs)began 10 seconds after the beginning of the intravenous contrast material injection.The delay between each monitoring scan was set to 2 seconds.The region of interest (ROI) was placed within the ascending aorta.The trigger threshold inside the ROI was set at 100 HU.We recorded a new short respiratory automatic patients instruction (“breathe in,hold breath”) which spent 1.5 seconds to replace the default respiratory automatic patients instruction (“Please breathe in and hold your breath.”) which occupied 4.9 seconds.Thus,the delay time of the bolus-tracking technique in Flash mode CCTA was shortened from 12 to 8 seconds.The data acquisition was initiated with a mean delay of 10 seconds after threshold in the ROI was reached.

In the group B,a test bolus of 15 mL iohexol was injected at a flow rate of 5 mL/s followed by a 15 mL saline chaser bolus for the evaluation of the scan delay during the acquisition of a series of dynamic low-dose monitoring scans (100 kV,20 mAs) at the level of the ascending aorta ROI to calculate the enhancement/time curve.Dynamic monitoring scans were positioned at the level of the ascending aorta.The delay between each monitoring scan acquisition was 2 seconds.Acquisition of the dynamic monitoring scans started 10 seconds after the beginning of the injection of intravenous contrast material.A ROI as large as the aortic root was drawn inside the lumen by one operator to generate an enhancement curve (generated by using DynEVA software,Siemens Healthcare),which showed time needed to reach the peak of maximum enhancement for the test bolus.Contrast material volume was 60 mL and injection rate was 5 mL/s followed by 60 mL saline chasing for the main angiographic bolus.

CT parameter settings were as follows∶detector collimation 2×64×0.6 mm,slice acquisition 2×128×0.6 mm by means of a z-flying focal spot,gantry rotation time 280 ms,pitch 3.4,tube current 370 mAs per rotation,and tube voltage 100 kV.17The scan range was from 2 cm below the tracheal carina to the diaphragm.The image acquisition phase started at 55% of the R-R interval for the patients.Images were reconstructed with a slice thickness of 0.6 mm,a reconstruction increment of 0.4 mm,and a softtissue convolution kernel (B26f) was applied.For the case with vessel wall calcifications,additional images were reconstructed using a sharp-tissue convolution kernel (B46)to compensate for blooming artifacts.

CCTA data analysis

Coronary segments were defined according to American Heart Association (AHA) standard.18The right coronary artery (RCA) was defined to include segments 1-4,segment 5 was counted in the left main artery (LM),segments 6-10 were included in the left anterior descending artery(LAD),and left circumflex artery (LCX) included segments 11-15.The intermediary artery was designated as segment 16.

The attenuation values and image noise were measured and contrast-to-noise ratio (CNR) was calculated by one independent radiologist,who was blinded to the results of the qualitative image analysis.The attenuation values(in Hounsfield units,HU) were assessed on transverse images in a ROI placed in the ascending aorta.Image noise was defined as the standard deviation of the attenuation value in ROI.19Vessel contrast was defined as the difference in mean attenuation (in HU) between the contrast-enhanced lumen of the vessel and the perivascular tissue.The CNR was defined as the ratio of vessel contrast and image noise.

Estimation of radiation dose

The effective radiation dose was calculated by applying a method from the European Guidelines on Quality Criteria for CT using the dose-length product (DLP) and a conversion coefficient of 0.014 mSv/(mGy·cm).20The DLP were taken from the patient protocol provided by the CT system.

Statistical analysis

All statistical analyses were performed using commercially available software SPSS (release 17.0,USA).Continuous variables (age,BMI,heart rate,scan time,the frequency of dynamic monitoring scans,dose,contrast material volume,attenuation values,image noise,CNR) were expressed as means±standard deviations and differences between two groups were assessed with the Student’sttest.APvalue below 0.05 was considered statistically significant.Categorical variables were expressed as frequencies or percentages.

RESULTS

CCTA were successfully performed in all patients without any side effects.There were no significant differences in age,BMI,heart rate,scan length and scan time between the two groups (allP＞0.05).An overview of the results for groups A and B can be found in Table 1.

Attenuation values,image noise,and CNR

There were no significant differences in the average attenuation values (P=0.183),image noise (P=0.414) or CNR (P=0.31) between the two groups (Table 1).Figure 1 and Figure 2 display the results of patient undergoing CCTA using bolus-tracking technique and test bolus technique,respectively.

Radiation dose and total contrast material volume

There were no significant differences in the frequency of dynamic monitoring scans (P=0.484),average radiation dose of dynamic monitoring scans (P=0.068) and estimated radiation doses of angiography (P=0.926) between the group A and group B (Table 1).Moreover,compared with test bolus technique,15 mL less contrast material volume was administered with the bolus-tracking technique.

DISCUSSION

The introduction of multi-detector row CT technology with acquisition times pushed the development of optimized shorter contrast material protocols in order to obtain consistent vascular enhancement in CCTA.A few techniques—namely,the fixed delay technique (on the basis of experience and related to angiographic data),the test bolus technique,and the bolus-tracking technique—enable the synchronization of contrast material administration and the start of data acquisition in clinical practice.7The synchronization of contrast material administration is becoming increasingly important owing to the introduction of faster spiral CT scanners.In general,the test bolus or bolus-tracking method has been commonly used in CCTA.7,8The test bolus technique is based on the intravenous injection of a small amount of contrast material (15 mL,generally 15%-20% of the main bolus) during the acquisition of a series of dynamic low-dose monitoring scans at the level of the vessel of interest.7The bolus-tracking technique is based on real-time monitoring of the main bolus during injection with the acquisition of a series of dynamic monitoring scans at the level of the vessel of interest.It is possible to start main scanning manually or automatically with a trigger threshold.7Several studies have compared test bolus and bolus-tracking in various applications,with mixed results.Cademartiret al7reported that bolus-tracking yields a more homogeneous enhancement compared to the test bolus technique.Johnsonet al21found no significant differences between test bolus and bolus-tracking for CCTA using single-source CT.In our clinical routine,the bolus-tracking technique results in a lower volume of contrast material,as the initial small contrast material bolus for the test bolus is not needed in bolus-tracking technique.7This phenomenon has been reported earlier and is one major advantage of the bolus-tracking technique.9-11

Table 1.Patients’ characteristics in the groups A and B§ (n=92)

Figure 1.A 74-year-old man underwent bolus-tracking technique CCTA.The attenuation value in the ascending aorta was 555 HU (A).The three-dimensional volume rendering image of dual-source spiral CT data (B).The electrocardiogram signal demonstrated that the Flash mode had scanned the entire heart within a single heart beat (C).Volume rendering (B),curved reformation of the right coronary artery (D),left anterior descending artery (E) and left circumflex artery (F) demonstrated that the image quality was classified as excellent in all coronary segments.

The recent development of new-generation DSCT systems allowed the introduction of low-dose high-pitch spiral acquisition protocols with a temporal resolution of 75 ms,12,13which provides a high diagnostic accuracy for the assessment of coronary stenoses with doses below 1 mSv in patients with low heart rate.17However,with high-pitch acquisition mode minimized contrast material volumes,sophisticated scan timing techniques has come to a new era as the contrast material needs to guarantee an enhancement in CCTA.In bolus-tracking technique of the Flash mode,time of default respiratory instructions spoken in different language is different.Spoken in Chinese,time of default respiratory instructions occupied 6.7 seconds,while it occupied 4.3 seconds spoken in English.And the delay time of default respiratory instructions spoken in English is 9 seconds,while the corresponding time is 12 seconds spoken in Chinese and 10 seconds spoken in German.Using default respiratory instructions spoken in English,Leschkaet al16reported data acquisition initiated with a mean delay of 9 seconds after reaching the threshold controlled by bolus-tracking technique for 35 patients undergoing dual-source CCTA.However,the scan delay time of default respiratory instructions spoken in Chinese with the bolus-tracking technique are relatively long (total 12 seconds) and test bolus technique is commonly used to ensure better synchronization of contrast material administration.22-24

Figure 2.A 53-year-old man received test bolus technique CCTA.The attenuation value in the ascending aorta was 519 HU (A).The three-dimensional volume rendering image of dual-source spiral CT data (B).The electrocardiogram signal demonstrated that the Flash mode had scanned the entire heart within a single heart beat (C).Volume rendering (B),curved reformation of the right coronary artery (D),left anterior descending artery (E) and left circumflex artery (F) demonstrated that the image quality was classified as excellent in all coronary segments.

In the present study,we recorded a new short respiratory instruction (“breathe in,hold breath”) to replace the default respiratory instruction (“Please breathe in and hold your breath…”).The delay time of this mode was shortened to insure synchronization of contrast material administration.Enhancement was measured in the ascending aorta which represents the contrast material that drains into the coronary arteries.The attenuation achieved at this level plays a major role in the optimal enhancement of the coronary arteries.We found that there were no significant differences in the average attenuation values,image noise or CNR in the ascending aorta between two groups.This result demonstrated that a homogeneous vascular enhancement of the coronary arteries as well as a sufficient image quality can be achieved with both the test bolus and the bolus-tracking technique.However,the adoption of bolus-tracking injection protocol for high-pitch DSCT coronary angiography preserves the desired attenuation of the coronary arteries with less contrast material in comparison to the test bolus technique.

In our study,the average effective radiation dose of dynamic monitoring scans of bolus-tracking and test bolus technique protocol had no significant differences.It is reasonable to state that because the similarity of the frequency of dynamic monitoring scans in the two groups resulted in the similar dose of dynamic monitoring scans.Because of the short CCTA acquisition time and high-pitch,the effective radiation doses of Flash mode CCTA were below 1 mSv in this study,which are similar to the doses reported in other recent trials that evaluated high-pitch DSCT.12,17,25

In conclusion,bolus-tracking technique with shortened time of respiratory instruction in Flash mode of DSCT yields the similar homogeneous enhancement with less contrast material in comparison to the test bolus technique.

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