128-slice dual-source CT coronary angiography with prospectively electrocardiography-triggered high-pitch spiral mode: radiation dose,image quality,and diagnostic acceptability

Abstract

Background

Dual-source computed tomography (CT) enables CT coronary angiography (CTCA) with a prospectively electrocardiography (ECG)-triggered high-pitch spiral (HPS) mode.

Purpose

To evaluate the radiation dose, image quality, and diagnostic acceptability of the HPS mode in CTCA and to compare HPS with the step-and-shoot (SAS) and low-pitch spiral (LPS) modes.

Material and Methods

One hundred and thirty-eight patients who underwent CTCA with a 128-slice dual-source CT scanner were retrospectively included in this study. Seventeen patients (average heart rate of ≤65 beats per minute [bpm] prior to acquisition) were evaluated in the HPS mode, 88 (average heart rate of >65 and ≤80 bpm prior to acquisition) in the SAS mode, and 33 (average heart rate of >80 bpm prior to acquisition or patients with an unstable heart rhythm) in the LPS mode. Radiation dose and image noise were recorded for each patient. Diagnostic acceptability was graded using a four-point scale (1, unacceptable; 2, suboptimal; 3, acceptable; 4, fully acceptable).

Results

The effective dose in the HPS mode was 1.5 ± 0.2 mSv, which was lower than that in SAS (8.9 ± 2.7 mSv) and LPS (21.5 ± 4.3 mSv) modes. There were no significant differences in the image noise levels in the descending aorta and left atrium. The average per-patient diagnostic acceptability was 3.2, 3.6, and 3.7 in HPS, SAS, and LPS modes, respectively.

Conclusion

The radiation dose is lower with HPS than with other modes, and the HPS mode-acquired images of patients with heart rates of ≤65 bpm are nearly acceptable for diagnostic image interpretation.

Keywords

CT angiography cardiac adults comparative studies dosimetry

Introduction

With the recent evolution of the computed tomography (CT) scanner, non-invasive CT coronary angiography (CTCA) can be performed with sufficient image quality and diagnostic accuracy. Therefore, CTCA is generally used to diagnose coronary heart disease (CHD) (1). However, CTCA requires a higher radiation dose to obtain images that are appropriate for CHD diagnosis (2).

In retrospectively ECG-gated CTCA or the low-pitch spiral (LPS) mode, the full cardiac cycle must be imaged, and the heart images are retrospectively reconstructed at the required cardiac phases. As a result, the radiation dose is relatively high. On the other hand, in a prospective electrocardiography (ECG)-triggered mode or the step-and-shoot (SAS) mode, the prospective triggering allows data acquisition in a predefined cardiac phase, thus omitting X-ray exposure outside of this phase. Consequently, the radiation dose can be greatly reduced (3). The SAS mode results in low radiation exposure and provides high diagnostic accuracy during CHD assessments; however, this mode can only be applied in patients with stable sinus rhythms and low heart rates (4). A previous phantom study (5) also showed that the SAS mode requires a lower radiation dose than the LPS mode. The dual-source (DS)-SAS mode also yields diagnostic image quality for most coronary segments at a low radiation dose (6).

Furthermore, DSCT increases the temporal resolution compared with a single-source CT (7 –11). The higher temporal resolution of DSCT enables the use of CTCA with a prospectively ECG-triggered high-pitch spiral (HPS) mode (10 –13). Achenbach et al. (11) reported that HPS-CTCA provides excellent image quality at a consistent dose of <1.0 mSv in nonobese patients with low and stable heart rates. Neefjes et al. (12) also reported that HPS-CTCA can be applied to patients with regular and low (<55 beats per minute [bpm]) heart rates. On the other hand, a previous phantom study (13) revealed that HPS-CTCA provides an advantageously low radiation dose but also low image quality. Because most previous studies have compared only the image qualities and effective doses from HPS, SAS, and LPS modes in a patient population, the diagnostic acceptability of HPS-CTCA needs to be evaluated and compared with that of SAS and LPS modes in a patient population.

The aim of this study was to compare the radiation dose, image quality, and diagnostic acceptability of CTCA using three different (HPS, SAS, and LPS) modes when it is performed in a patient population.

Material and Methods

128-section DSCT scanner

A 128-section DSCT SOMATOM Definition Flash scanner (Siemens Healthcare, Erlangen, Germany) was used in this study. This CT scanner contained two X-ray tubes and two detector arrays with an angular offset of 95° that offer a spiral pitch up to 3.4. The minimum gantry rotation time is 280 ms, which provides a temporal resolution of 75 ms.

Using the 128-section DSCT, CTCA can be performed using three acquisition modes: prospectively ECG-triggered HPS, prospectively ECG-triggered SAS, and retrospectively ECG-gated LPS modes.

Patients

The 138 patients (70 men, 68 women) who underwent CTCA with the 128-slice DSCT scanner for CHD diagnosis were retrospectively included in this study. Patients who had received coronary artery bypass grafts were not included because a different contrast agent injection protocol was required for those patients. The retrospective data collection and review were approved by the Medical Ethics Committee of Kanazawa University (Reference Number 298).

All patients received 5 mg of nitroglycerin (Nitorol; Eizai, Tokyo, Japan) sublingually prior to obtaining localized radiographs. Patients with average heart rates of >80 bpm prior to acquisition received an intravenous beta-blocker (Inderal; AstraZeneca, Osaka, Japan) prior to acquisition. The acquisition mode was determined for each patient after consultation among cardiologists, radiologists, and radiologic technologists, and thus, 17 patients (average heart rates of ≤65 bpm prior to acquisition or patients without heavily calcified plaques, metallic stents, or metallic valves that would cause beam-hardening artifacts (13)) were studied using the HPS mode, 88 (average heart rate of >65 and ≤80 bpm prior to acquisition) using the SAS mode, and 33 (average heart rates of >80 bpm after receiving intravenous beta-blocker or patients with an unstable heart rhythm) using the LPS mode. The patient characteristics in each acquisition mode are shown in Table 1.

Table 1.

Comparison of patient characteristics among the three acquisition modes.

Characteristics	Acquisition mode			P value
Characteristics	HPS	SAS	LPS	P value
Patients (m/f) (n)	8/9	42/46	20/13	0.428
Age (years)	57 ± 21	66 ± 14	72 ± 9	0.003
Height (cm)	156.0 ± 6.9	157.7 ± 9.6	159.0 ± 10.5	0.718
Weight (kg)	52.8 ± 6.2	57.5 ± 11.0	58.9 ± 10.6	0.140
BMI	21.0 ± 2.0	23.8 ± 4.3	23.4 ± 3.8	0.156

BMI, body mass index; HPS, high-pitch spiral; LPS, low-pitch spiral; SAS, step-and-shoot.

Acquisition methods

All CT data were acquired during breath holding in the end-expiration phase. A contrast agent (Iopamidol, 370 mg iodine/mL; Bayer Pharma, Osaka, Japan) was injected at a volume of (Body weight [kg] × 0.5 + 20) ml through a power injector (Auto Enhance A-300; Nemoto Kyorindo, Tokyo, Japan) with an injection time of 12 s. This was followed by a saline solution flush with an injection time of 5 s.

Prior to acquisition, a test bolus protocol was performed as follows: the contrast agent was injected at the same injection rate as that of the acquisition with an injection time of 2.5 s. This was followed by a saline solution flush with an injection time of 5 s. The time of peak enhancement in the ascending aorta at the origin of the left main tract was measured using a series of images obtained from the test bolus protocol. Image acquisition began with a delay corresponding to the time of peak enhancement plus 6 s (in HPS mode) or 4 s (in SAS and LPS modes). The acquisition parameters in the three acquisition modes are shown in Table 2.

Table 2.

Acquisition parameters in the three acquisition modes.

Parameters	Acquisition mode
Parameters	HPS	SAS	LPS
Collimation (mm)	128 × 0.6	128 × 0.6	128 × 0.6
kV	120	120	120
mAs	340	340	340
Spiral pitch	3.4: 1	N/A	0.17: 1
Rotation time (s)	0.28	0.28	0.28
Padding window (%)	N/A	35–85 or 55–85	N/A
Slice thickness (mm)	0.75	0.75	0.75
Reconstruction kernel	B36f*	B36f*	B36f*

HeartView medium with advanced smoothing algorithm (Siemens Healthcare, Erlangen, Germany).

HPS, high-pitch spiral; LPS, low-pitch spiral; kV, kilovolt; mAs, milliampere-second; N/A, not applicable; SAS, step-and-shoot.

Data analysis

The average heart rate during acquisition; radiation dose (volume CT dose index [CTDIvol] and dose-length product [DLP] displayed on the CT console and effective dose [ED] estimated from DLP); and image noise in the ascending aorta, descending aorta, and left atrium (Fig. 1) were recorded for each patient. ED was estimated as DLP multiplied by a “k” conversion factor (0.014 mSv/mGycm) (14).

Fig. 1.

Regions in which image noise was measured. Black circles indicate the measured regions.

In addition, the per-patient and per-segment diagnostic acceptability (according to a 17-segment modified American Heart Association classification (15)) were graded using a four-point scale (1, unacceptable; 2, suboptimal; 3, acceptable; 4, fully acceptable) by two radiologists with 10 and 15 years of experience who specialized in cardiac radiology and were blinded to all clinical data. The diagnostic acceptability scores were based on the degree of image degradation caused by motion artifacts. Axial views, maximum intensity projections, and curved planar reconstruction images were used to identify coronary motion artifacts. Score discrepancies between the radiologists were resolved by consensus after an independent reading.

All statistical analyses were performed using commercially available software (SPSS statistics 19; IBM Corp., Armonk, NY USA). A one-way analysis of variance test was used to compare differences in continuous data (age, height, weight, body mass index [BMI], radiation dose, and image noise) among the three acquisition modes; the Kruskal–Wallis test was used to compare differences in qualitative data (the per-patient and per-segment diagnostic acceptability) among the three acquisition modes. Comparisons among three groups with respect to sex were calculated using the χ² test. Inter-observer variability for evaluating diagnostic acceptability was determined using κ statistics.

Results

Heart rate

The average heart rates during acquisition were 59.1 ± 6.0 bpm (range, 41–67 bpm) in the HPS mode, 68.1 ± 10.5 bpm (range, 36–90 bpm) in the SAS mode, and 76.6 ± 21.6 bpm (range, 41–145 bpm) in the LPS mode (P < 0.001).

Radiation dose

The radiation dose for each group is shown in Table 3. The average CTDIvol, DLP, and ED was lower in the HPS mode than in the SAS mode, whereas the average CTDIvol, DLP, and ED was lower in the SAS mode than in the LPS mode. There were significant differences among the three acquisition modes (P < 0.001).

Table 3.

Comparison of radiation doses among the three acquisition modes.

Characteristics	Acquisition mode			P value
Characteristics	HPS (n = 17)	SAS (n = 88)	LPS (n = 33)	P value
CTDIvol (mGy)	5.84 ± 0.56	45.14 ± 13.46	102.35 ± 19.36	<0.001
DLP (mGycm)	107 ± 12	633 ± 190	1534 ± 310	<0.001
ED (mSv)	1.5 ± 0.2	8.9 ± 2.7	21.5 ± 4.3	<0.001

CTDIvol, volume computed tomography dose index; DLP, dose-length product; ED, effective dose; HPS, high-pitch spiral; LPS, low-pitch spiral; SAS, step-and-shoot.

The average CTDIvol in the HPS mode was 5.84 ± 0.56 mGy (range, 5.49–6.80 mGy), which was approximately eight-fold lower than that in the SAS mode (average, 45.14 ± 13.46 mGy; range, 13.08–62.84 mGy) and approximately 18-fold lower than that in the LPS mode (average, 102.35 ± 19.36 mGy; range, 38.53–119.2 mGy). In the HPS mode, ED was 1.5 ± 0.2 mSv (range, 1.3–1.8 mSv), which was approximately six-fold lower than that in the SAS mode (average, 8.9 ± 2.7 mSv; range, 2.5–13.1 mSv) and approximately 14-fold lower than that in the LPS mode (average, 21.5 ± 4.3 mSv; range, 7.7–27.2 mSv).

Image noise

Image noise for each acquisition mode is shown in Table 4. There were significant differences among the three acquisition modes in terms of the image noise level in the ascending aorta (P < 0.001); the SAS mode had the highest level (average, 15.65 HU), followed by the HPS mode (14.86 HU), and the LPS mode had the lowest level (average, 12.55 HU). However, there were no significant differences in the image noise levels in the descending aorta and left atrium.

Table 4.

Comparison of image noise levels among the three acquisition modes.

Regions	Acquisition mode			P value
Regions	HPS (n = 17)	SAS (n = 88)	LPS (n = 33)	P value
Ascending aorta (HU)	14.86 ± 2.58	15.65 ± 3.80	12.55 ± 3.32	<0.001
Left atrium (HU)	18.82 ± 3.37	18.24 ± 4.45	16.69 ± 4.56	0.153
Descending aorta (HU)	17.46 ± 3.29	17.55 ± 5.11	15.46 ± 3.72	0.083

HPS, high-pitch spiral; HU, Hounsfield unit; LPS, low-pitch spiral; SAS, step-and-shoot.

Diagnostic acceptability

The per-patient and per-segment diagnostic acceptability scores for all three acquisition modes are shown in Tables 5 and 6, respectively. A total of 2050 coronary artery segments were present (average, 15 coronary artery segments per patient). The inter-observer agreement κ statistics for the per-patient and per-segment diagnostic acceptability evaluation were 0.980 and 0.949, respectively, indicating an “almost perfect agreement” (16). The average per-patient diagnostic acceptability were 3.2, 3.6, and 3.7 in HPS, SAS, and LPS modes, respectively, and these differences were significant (P = 0.001). The average per-segment diagnostic acceptability were 3.8, 4.0, and 3.9 in HPS, SAS, and LPS modes, respectively, and these differences were also significant (P < 0.001).

Table 5.

Comparison of the per-patient diagnostic acceptability among the three acquisition modes.

Score	Acquisition mode
Score	HPS (n = 17)	SAS (n = 88)	LPS (n = 33)
4 (fully acceptable)	5 (29.4%)	68 (77.3%)	21 (63.6%)
3 (acceptable)	11 (64.7%)	16 (18.2%)	10 (30.3%)
2 (suboptimal)	1 (5.9%)	4 (4.5%)	2 (6.1%)
1 (unacceptable)	0 (0.0%)	0 (0.0%)	0 (0.0%)

HPS, high-pitch spiral; LPS, low-pitch spiral; SAS, step-and-shoot.

Table 6.

Comparison of the per-segment diagnostic acceptability among the three acquisition modes.

Score	Acquisition mode
Score	HPS (n = 251)	SAS (n = 1306)	LPS (n = 493)
4 (fully acceptable)	202 (80.5%)	1240 (94.9%)	425 (86.2%)
3 (acceptable)	38 (15.1%)	62 (4.8%)	64 (13.0%)
2 (suboptimal)	11 (4.4%)	4 (0.3%)	4 (0.8%)
1 (unacceptable)	0 (0.0%)	0 (0.0%)	0 (0.0%)

HPS, high-pitch spiral; LPS, low-pitch spiral; SAS, step-and-shoot.

Differences in the right coronary artery curved planar reconstruction images obtained with the three acquisition modes are shown in Fig. 2. These images were obtained from three similar patients; all were men and approximately 60 years of age with average heart rates of approximately 60 bpm, heights of approximately 165 cm, and body weights of approximately 60 kg. The per-patient and per-segment diagnostic acceptability received scores of 4 (fully acceptable) in all three cases. Although the vessel attenuation differed among these three patients, this difference was patient-dependent and was not affected by the acquisition mode. Given these patient characteristics, no image degradation from motion artifacts was observed, and similar images could be obtained with either mode of operation.

Fig. 2.

Curved planar reconstruction images of the right coronary artery obtained from similar patients using three acquisition modes: (a) the high-pitch spiral (HPS) mode; (b) step-and-shoot (SAS) mode; (c) low-pitch spiral (LPS) mode.

With regard to the HPS mode, representative images obtained with the HPS mode are shown in Figs. 3 –5. The per-patient diagnostic acceptability was fully acceptable in five patients (29.4%; Fig. 3), acceptable (score 3) in 11 patients (64.7%; Fig. 4), and suboptimal (score 2) in one patient (5.8%; Fig. 5).

Fig. 3.

Computed tomography (CT) coronary angiography images acquired in the high-pitch spiral (HPS) mode of an 81-year-old man with a body mass index (BMI) of 23.1 kg/m². The average heart rate during acquisition was 53 beats per minute, volume CT dose index (CTDIvol) was 6.8 mGy, dose-length product (DLP) was 119 mGycm, and estimated effective dose (ED) was 1.7 mSv. The per-patient diagnostic acceptability was graded as 4 (fully acceptable). The images reveal severe stenoses caused by soft plaques at segment 6 (90% of the diameter) and segment 7 (75% of the diameter), according to a 17-segment modified American Heart Association classification: (a) angiographic view; (b) curved planar reconstruction image.

Fig. 4.

Images acquired from a 56-year-old woman with a body mass index (BMI) of 21.1 kg/m² in the high-pitch spiral (HPS) mode. The average heart rate during acquisition was 56 beats per minute, volume computed tomography dose index (CTDIvol) was 5.50 mGy, dose-length product (DLP) was 95 mGycm, and estimated effective dose (ED) was 1.3 mSv. The per-patient diagnostic acceptability was graded as 3 (acceptable). The images reveal mild stenosis with a pinpoint calcified lesion (<50% of the diameter) at segment 6 according to American Heart Association classification: (a) angiographic view; (b) curved planar reconstruction image.

Fig. 5.

Images acquired from a 71-year-old woman with a body mass index (BMI) of 22.4 kg/m² in the high-pitch spiral (HPS) mode. The average heart rate during acquisition was 65 beats per minute, volume computed tomography dose index (CTDIvol) was 6.78 mGy, dose-length product (DLP) was 127 mGycm, and estimated effective dose (ED) was 1.8 mSv. The per-patient diagnostic acceptability was graded as 2 (suboptimal). The left circumflex artery (segment 11 according to American Heart Association classification) in this image is blurred. Some soft plaques are apparent, but it is highly possible that these low-density areas are motion artifacts: (a) angiographic view; (b) curved planar reconstruction image.

Discussion

In this study, we not only compared the radiation doses and image qualities but also the diagnostic acceptability of HPS, SAS, and LPS modes when CTCA was performed in a patient population. Although the radiation dose with the HPS mode was lower than those of other modes, the diagnostic acceptability of this mode was lower than those of the SAS and LPS modes. The average ED in the HPS mode was 1.5 ± 0.2 mSv (range, 1.3–1.8 mSv), which was approximately six-fold lower than that in the SAS mode and approximately 14-fold lower than that in the LPS mode. Kröpil et al. (17) previously reported an average ED of 1.4 ± 0.7 mSv (range, 0.4–3.1 mSv) in the HPS mode and Wang et al. (18) reported an average ED of 1.30 ± 0.39 mSv (range, 0.64–1.97 mSv) in the HPS mode. In this study, we obtained similar ED values. The average CTDIvol in the HPS mode was 5.84 ± 0.56 mGy (range, 5.49–6.08 mGy), which was approximately eight-fold lower than that in the SAS mode and 18-fold lower than that in the LPS mode. Because CTDIvol serves as a starting value for dose estimates that are better tailored to the patient’s local dose (18), our results suggest that the patient’s local dose could be reduced over ED when the HPS mode is applied.

There were no significant differences in the image noise levels among the three modes except at the ascending aorta, and the image noise levels were relatively lower when the LPS mode was applied. We did not use tube current modulation (TCM) in the LPS mode group; we believe that this is effective for radiation dose optimization because the average patient’s ED in the LPS mode with TCM was only 2.3-fold higher than that in the SAS mode, with no deterioration in the image quality (19).

With regard to the per-patient diagnostic acceptability, most of patients had scores of 4 (fully acceptable) in the SAS and LPS modes, whereas the highest number of patients with scores of 3 (acceptable) was in the HPS mode. We believe this result occurred because the HPS mode incorporated a high pitch factor that caused image deterioration. However, 16 of the 17 cases had scores of 3 or 4 in the HPS mode, and images acquired in the HPS mode were at a near-acceptable level for diagnostic image interpretation. With regard to the per-segment diagnostic acceptability, a number of segments had scores of 4 (fully acceptable) in all modes of operation, indicating that nearly all segments were fully acceptable for CHD diagnosis with all modes of operation. Different tendencies were observed between the per-patient and per-segment diagnostic acceptability in the HPS mode. We believe this occurred because the per-patient diagnostic acceptability had a score of <4 even if image degradation was observed in only a few segments.

Based on our results, we recommend selecting the SAS mode as a first-line protocol because it maintains a balance between the radiation dose and image quality. Our recommendation is in agreement with that of some previous studies (13,20). Neefjes et al. (12) demonstrated a lower image quality with the HPS mode than with the SAS mode in patients with heart rates of ≥55 bpm, and aggressive beta-blocker administration before scanning may be considered in young patients to induce a heart rate of <55 bpm, thus permitting the use of the HPS mode. The optimal timing for CTCA acquisition for motion artifact minimization, particularly in the mid- to distal right coronary artery, is the end-systolic phase at a time point between 35% and 50% of one RR interval in patients with heart rates of >65 bpm (21). Therefore, an appropriate reconstruction phase or padding window width should be considered when HPS or SAS is applied to patients with heart rates of >65 bpm.

Our study has several limitations. First, it was performed at a single-center, and the retrospectively analyzed patients were all of Asian ethnicity with relatively low BMIs. Second, we did not use an iterative image reconstruction technique. Previous studies have demonstrated that HPS-CTCA provides high diagnostic accuracy when combined with an iterative image reconstruction technique (22,23), and it is possible that the diagnostic acceptability could be improved using an iterative image reconstruction technique. Finally, the diagnostic acceptability of the HPS mode was assessed in selected patients with presenting heart rates of ≤65 bpm before performing CTCA. A much cleaner approach would have been the randomized allocation of patients to the acquisition modes. However, the retrospective nature of this study precluded a randomized allocation of patients. Therefore, further studies that focus on heart rate selection will be required.

In conclusion, although the radiation dose in the HPS mode is low compared with that in other modes, the diagnostic acceptability of this mode is low compared with that of SAS and LPS modes. However, in patients with heart rates of ≤65 bpm, images acquired in the HPS mode are at a near-acceptable level for diagnostic image interpretation.

Footnotes

Acknowledgments

We thank Dr. Keiichi Kawai of Kanazawa Medical Center for his advice, comments, and suggestions.

Conflict of interest

None declared.

Funding

This work was supported by MEXT KAKENHI Grant Number 21791175.

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