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Comparison of Patient Radiation Dose for 128-Row Multidetector Coronary Computed Tomography Angiography by Prospective Versus Retrospective ECG-Triggering Techniques

การเปรียบเทียบปริมาณรังสีที่ผู้ป่วยได้รับจากการตรวจหลอดเลือดหัวใจโดยเครื่องเอกซเรย์คอมพิวเตอร์ชนิด 128 สไลซ์โดยวิธี prospective และ retrospective ECG-triggering

Narumol Chaosuwannakit (นฤมล เชาว์สุวรรณกิจ) 1, Pattarapong Makarawate (ภัทรพงษ์ มกรเวส) 2

หลักการและวัตถุประสงค์: เพื่อศึกษาถึงปริมาณรังสีที่ได้รับทั้งหมดในอวัยวะของผู้ป่วยที่ได้รับการตรวจหลอดเลือดหัวใจโดยเครื่องเอกซเรย์คอมพิวเตอร์โดยวิธี prospective และ retrospective ECG-triggering

วิธีการศึกษา: ศึกษาข้อมูลย้อนหลังของผู้ป่วย 233 ราย ที่ได้รับการส่งตรวจหลอดเลือดหัวใจโดยเครื่องเอกซเรย์คอมพิวเตอร์ในด้านปริมาณรังสีที่ได้รับทั้งหมดในอวัยวะของผู้ป่วยโดยผู้ป่วยได้รับการตรวจโดยวิธีที่แตกต่างกันซึ่งขึ้นอยู่กับอัตราการเต้นของหัวใจผู้ป่วย

ผลการศึกษา: ปริมาณรังสีที่ได้รับทั้งหมดในอวัยวะของผู้ป่วยที่ได้รับการตรวจหลอดเลือดหัวใจโดยเครื่องเอกซเรย์คอมพิวเตอร์มีค่าเฉลี่ย 2.8 ถึง 11.5 mSv วิธีการตรวจโดย Prospective ECG-triggering (PT) สามารถทำให้ผู้ป่วยได้รับปริมาณรังสีลดลงถึงร้อยละ64 เมื่อเทียบกับการตรวจโดย Retrospective ECG-triggering (RT)

สรุป: เนื่องจากปริมาณรังสีที่ได้รับทั้งหมดในอวัยวะของผู้ป่วยที่ได้รับการตรวจหลอดเลือดหัวใจโดยเครื่องเอกซเรย์คอมพิวเตอร์มีค่าแตกต่างกันมากจากวิธีการตรวจที่ต่างกันดังนั้นรังสีแพทย์และนักรังสีเทคนิคต้องตระหนักถึงความสำคัญและเลือกใช้วิธีการตรวจที่เหมาะสมกับผู้ป่วยแต่ละราย

คำสำคัญ: ปริมาณรังสีที่ผู้ป่วยได้รับ, การตรวจหลอดเลือดหัวใจโดยเครื่องเอกซเรย์คอมพิวเตอร์, การตรวจหลอดเลือดหัวใจ, เครื่องเอกซเรย์คอมพิวเตอร์,

Background and objective: To patient radiation dose in a group of patients who underwent 128-row multidetector coronary computed tomography angiography (CCTA) performed with prospective electrocardiographic (ECG) triggering with radiation dose in a group of patients who underwent CCTA performed with retrospective ECG-triggering.

Method: We performed a retrospective review of 233 consecutive patients referred for CCTA. CCTAs was performed using different scanning protocols depend on patient’s heart rate. The effective radiation dose was calculated for each patient.

Results: Depending on different dose saving techniques and heart rate, the effective whole-body dose of a cardiac scan ranged from 2.8 to 11.5mSv. Prospective ECG-triggering (PT) has the greatest potential to reduce the effective dose to 64 %, compared to a comparable scan protocol with retrospective ECG-triggering (RT).

Conclusion: Due to this broad variability in radiation exposure of coronary CTA, the radiologist and technician should be aware of the different dose reduction strategies.

Introduction

Coronarycomputed tomography angiography (CCTA) has increasingly gained importance as anoninvasive, fast and accurate study for diagnosing coronary artery disease(CAD).^1-7 The relatively highradiation exposure in CCTA compared with invasive conventional coronaryangiography still remains a challenge.^8,9 Hence, not only careful selection of patientssuitable for CCTA examination, but reduction of the patient radiation exposurein CCTA without compromising diagnostic accuracy should also be aimed for. CCTAperformed with 64-detector CT without the use of ECG pulsing typically resultin radiation doses ranging from 15 to 21 mSv ⁵ and 9 mSv with theuse of ECG pulsing.^10,11

It is well known that the radiationdose in cardiac imaging can substantially be reduced by the use of prospectiveECG-triggering data acquisitions.¹² The method of prospective ECG-triggering (PT) has been evaluated forimage quality of the coronary arteries and diagnostic accuracy as well aseffective radiation dose in several studies.^13-19 Prospective ECG-triggering(PT) is a technique used with CCTA that uses forward-looking prediction of Rwave timing, step-and-shoot non spiral acquisition during imaging and uniquecone beam reconstruction.²⁰ By contrast, CCTA with standardretrospective ECG-triggering (RT) uses backward-looking measurement of R wavetiming, spiral scanning during table motion and more traditional cone beamreconstruction. In addition, with PT,the x-ray beam is turned on for only a short portion of diastole, and it isturned off during the rest of the R-R cycle, whereas with RT, the x-ray beam isturned on throughout the R-R interval (Fig 1). Initialresults showed that PT may be able to provide sufficient image quality with lowradiation dose.^13-16 Hence, thepurpose of our study was to directly compare a prospective ECG-triggering (PT)technique with retrospectively ECG-triggering (RT) CT techniques in CCTA withrespect to effective radiation dose.

Figure 1 Figure showed retrospective ECG-triggering (RT) and prospective ECG-triggering (PT). With RT technique, the x-ray beam is turned on throughout the R-R cycle. The PT technique is characterised by applying full tube current only at predefined phases of the RR interval and complete pausing of the tube current during the rest of the cardiac cycle, while the table moves to the next imaging position (step-by-step acquisition instead of helical acquisition)

Methods

Patients

This retrospective studyincluded 233 patients (149 males and 84 females) referred for CCTA, betweenDecember 2010-December 2011. The indications for CCTA were an abnormal,equivocal or non-diagnostic stress test, chest pain, evaluation of cardiomegalyand congestive heart failure, as well as the evaluation of cardiac etiology ofsyncope. Patients with an intermediate probability of coronary artery disease(CAD) were also referred for a CCTA as a first test. The above are consideredappropriate indications for CCTA, based on the criteria of the American College of Cardiology (ACC) ²¹andthe recent American Heart Association Scientific Statement on Cardiac CT.²¹ Exclusion criteria for CCTAincluded the presence of multiple ectopic beats, atrial fibrillation, renalfailure, pregnancy and a history of allergic reaction to iodine-containingcontrast agents. The present study was approved by the Ethics Committee of the Faculty ofMedicine, Khon KaenUniversity, Khon Kaen, Thailand,and informed consent was obtained from allpatients.

Cardiac CT AngiographyProtocol

CCTA examinations wereperformed on a 128-slice MDCT (Brilliance 128, Philips Healthcare, Netherland)using prospective (PT) or retrospective (RT) ECG-triggering with the followingparameters: 128 × 0.6 collimation, 0.3 sec rotation time, pitch of0.32, 120 kV tube voltage and 185 reference mAs. Image acquisition was performed duringinspiratory breath-hold. To familiarize the patient with the protocol,breath-holding was practiced before the examination. A contrast agent bolus of80-100 ml was injected with a mean flow rate of 5 mL/s followed by a 50 ml salineflush. For timing purposes, an automated bolus-tracking software was used,starting the scan automatically 6 seconds after contrast agent density in thedescending aorta reached a predefined threshold of 130 HU. The entire volume ofthe heart was covered during one breath-hold in approximately 5 seconds withsimultaneous recording of the ECG trace. Patients were scanned in the supine position twice, first withoutcontrast medium to calculate the calcium score and secondly after contrastmedium injection. Studies were acquired in the cranio-caudal direction from thelevel of the carina to just below the diaphragm. For optimal motion-free image quality, datasets were reconstructed in mid diastole (mean interval, 614 ± 175 ms after theR wave). The patients presenting with stable sinusrhythm and a heart rate below 70 beat/min (bpm) we performed CCTA with PTtechnique and performed CCTA with RT technique for the patients who hadminimal cardiac ectopic beats or higher heart rate (more than 70 bpm). Imageswere first constructed at 75% of the R-R interval; images were thenreconstructed at 0%–90% of the R-R interval in 10% increments. Additionalreconstruction windows were constructed after examination of initial datasetsif motion or noise artifacts were present.

Cardiac CT AngiographyImage Reconstruction

All CT datasets weretransferred to a dedicated workstation. To evaluate the coronary arteries, theimages were reconstructed with a small FOV (120-190 mm), which wasrestricted to the heart region. The images were reviewed in the axial, coronal,and sagittal planes, using a mediastinal window (width: 450, level: 35), lungwindow (width: 1,500, level: -700), and bone window(width: 1,500, level: 450) for all examinations.

Measurementof the radiation exposure

The dose-length product(DLP) displayed by the CT unit was recorded for each CCTA. The effective dose of CCTA was estimated by a methodproposed by the Fleischner Society.²² The effective dose is derived from the productof the DLP and a conversion coefficient for the anatomical region examinedwhich is 0.017 mSv mGy^-1 cm^-1 for the chest. However, itshould be noted that a uniform conversion coefficient for all images is notentirely accurate as it does not account for the different conditions in theexaminations.

Statisticalanalysis

Statisticalanalyses were performed using SPSS software version 16 (SPSS, Inc., Chicago, IL, USA).Continuousdata were expressed as mean±SD and categorical variables were given aspercentages.

Comparisonsof the patient characteristics, the CT parameters and the dose estimates wereperformed using the t-test for normally distributed data and using theChi-squared test if not. A significance level of

p <0.05 was considered a statistically significant resultand all reported p-values were two-sided.

Results

Clinicalcharacteristics of the patients are summarized in Table 1. Patientsin the prospective ECG-triggering (PT) group and those in theretrospective ECG-triggering (RT) group did not differ significantly in age, sex or mean body mass index.However, there was a significant difference between the groups for mean heartrate (59±9 beats/min for the PT group, 75±11 for the RT group; p<0.03)(Table 1). Dependingon different dose saving techniques and heart rate, the effective whole-bodydose of a cardiac scan ranged from 2.8 to 11.5@@START_COMMENT[if !vml]@@END_COMMENT@@START_COMMENT[endif]@@END_COMMENTmSv. In PT group, themean DLP was 184±66 mGy cm, resulting in an effective radiation dose perexamination of 3.1±1.1 mSv. In the RG group, mean DLP was 501± 198 mGy cm,resulting in an effective radiation dose per examination of 8.5±3.4 mSv.PT group showed a significantly lower DLP andconsequently a significantly lower mean effective radiation dose compared withRG group (p<0.001) (Fig 2). The mean patient radiation dose was 60% lower forprospective gating than for retrospective gating

Table 1 Patient characteristicsfor the two groups (n=233)

	Prospective gating (n=84)	Retrospective gating (n=149)	p-value
Age (years), mean±SD	59.2±10.5	61.5±9.8	0.85
Male sex^π	50 (60)	99 (66)	0.56
Body mass index(kg/m²), mean±SD	22.1±6	23.6±5	0.76
Heart rate (beats/min), mean±SD	59±9	75±11	0.03

^πData are numbers ofpatients and data in parentheses are percentages.

Figure 2 Prospective ECG-triggering (PT) group showed a significantly lower mean effective radiation dose compared with retrospective ECG-triggering (RT) group, p<0.001.

Discussion

The use of CCTA withretrospective gating results in good image quality and few nonevaluablecoronary artery segments; however, the radiation dose to patients is relativelyhigh.^23,24 The aim of our study was todirectly compare a prospective triggering(PT) CT technique with retrospectively gated (RT)helical CT techniques in CCTA with respect to effective radiation dose. Thex-ray beam is turned on throughout the R-R cycle using RT technique. To reduce radiation exposure to the patient we use tubecurrent modulation technique which is uses a stronger tube currentduring the key imaging portions of diastole and a weaker tube current duringthe rest of the R-R interval. Still, patients typically receive a radiationdose of 8–19 mSv with intensity-modulated retrospective gating.^24,25

Thetechnique of prospective triggering itself is characterised byapplying full tube current only at predefined phases of the RR interval andcomplete pausing of the tube current duringthe rest of the cardiac cycle, while the table moves to the next imagingposition (step-by-step acquisition instead of helical acquisition). It hascommonly been used for calcium-scoring techniques and has recently beenintroduced into CCTA protocols: recent studies using single-source 64-slice CTsystems have shown that the prospective triggering method offers sufficientimage quality of the coronary arteries with substantially reduced effectiveradiation doses of only 2–4 mSv.^15-20 The major disadvantage of theprospectively ECG-triggered protocol in CCTA lies in the limited predefinedinterval for data acquisition, which is placed in the mid-diastole phase. As aconsequence only images reconstructed from a singlephase of the cardiac cycle are available for diagnostic interpretation of theentire coronary artery tree.^26,27At higher heart rates ( >70 bpm)reconstruction ofadditional data in the systole may be required fordiagnostic image quality. Therefore, the prospectivetriggeringmode is typically performed in patients withregular heart beatsbelow 70 bpm. A further drawback to be mentioned when using prospectivetriggering is that no information on the valvular function ^28,29or global and regional ventricularfunction ³⁰ can be obtained.

In this study, we compareda group of patients who underwent prospectively gated CCTA with a group ofpatients who underwent retrospectively gated cardiac CT and who were matchedfor age, sex, and body mass index. The result of our study showed that mean effectiveradiation dose in group PT was as low as3.1±1.1 mSv. In RT group the radiation dosewas significantly higher as 7.8±3.4 mSv. Wealso found that prospective gating resulted in a radiation dose that was 60%lower than that with retrospective gating.Consequently, in patients with a heart rate over 70 bpm the physician has tojudge whether retrospectively gated CCTA is an option in the individual patientregarding the relatively high radiation exposure.

Instead,after calculating the risks, administration of beta-blockers before theexamination to reduce the heart rate for prospective triggering, or,alternatively, invasive angiography should be considered. Taking into accountthe advantages and disadvantages of the different techniques, the followingguidelines for the selection of different CCTA protocols should be considered.In patients with slow and regular heart rate a protocol with prospectivetriggering should be chosen, whereas in patients with faster or irregular heartrate retrospective gating should be considered. Tube voltage should be adaptedto the patients’ body weight.

Conclusion

In thepresent study, we compared CCTA examinations performed on a 128-slice systemusing a prospective triggering (PT) technique with retrospective gating (RT)technique, with respect to radiation dose and image quality. The patientspresenting with stable sinus rhythm and a heart rate below 70 bpm, CCTA withthe PT technique offers a significantly lower effective radiation dose comparedwith CCTA using RT techniques. However, benefits from prospectively gated cardiac CT mustbe weighed against two current limitations which are imaging at heart rateshigher than 70 beats per minute is not recommended and functional cardiac information isnot obtained.

References

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