1. Novel single-source high-pitch protocol for CT angiography of the aorta: comparison to high-pitch dual-source protocol in the context of TAVI planning Int J Cardiovasc Imaging (2013) Ludwig-Maximilians-University Hospital Munich

2. Introduction Aortic valve stenosis is a very common disorder. Recently, a new procedure of trans-catheter aortic valve implantation (TAVI) has been introduced for high-risk patients that are not eligible for open cardiac surgery. For an optimal preparation of a TAVI procedure, It is importance to know the anatomic configuration of the aortic valve annulus.

3. Aside from trans-esophageal echocardiography, computed tomography angiography (CTA) is mainly being used for planning of TAVI.

4. The aim of this study It has been shown that CTA for TAVI planning may be performed with very little contrast agent and low radiation exposure when using a high-pitch scan mode in 2nd generation dual-source CT (DSCT). To evaluate the image quality of a comparable high- pitch scan mode in a modern single-source CT (SSCT) system.

5. Methods Patients Retrospectively evaluated 40 patients referred for CT-angiography for planning purposes prior to TAVI procedure. 20 patients an ECG-triggered high-pitch (3.4) scan protocol in 2nd generation DSCT. 20 scanned in a SSCT system using a similar low-dose scan protocol.

6. DSCT scan protocol Prior to the examination heart rate during free breathing. An ECG-electrodes were placed on the patients’ chest. From upper thoracic aperture to the proximal femoral arteries. 60 ml contrast medium (Imeron 350), rate at 4 ml/s, 100 ml saline chaser. Using bolus tracking, ROI in the ascending aorta,the scan was initiated 10 s after contrast enhancement exceeded 100 HU.

7. CT acquisition parameters: slice collimation 2 *128 *0.6 mm. gantry rotation time 280 ms; pitch 3.4; tube voltage 100 kV. Automated tube current modulation was enabled with a references mAs of 444. Cranio-caudal scan direction. Initiation of data acquisition was planned at 60% of the RR- interval.

8. SSCT scan protocol The high-pitch scan mode with a pitch value of 1.7 in the used CT system does not offer ECG- synchronization. Automated tube current modulation was enabled with a references mAs of 85 mAs. Others were same as the DSCT protocol.

9. Evaluation of image quality Image quality (IQ) was rated by two readers who were blinded regarding the used CT system using 4-point grading-scales.

10. Image quality was rated separately regarding the anatomical landmarks. Fig. 1 Five image quality scores that have been graded in a blinded fashion by two experienced readers.

11. For rating of image quality reconstructions with 0.6 mm slice thickness were analyzed using a dedicated 3D software tool referred to as ‘‘TAVI planning’’. Defined as the mean intravascular density (HU) in a standardized ROI and image noise as the corresponding standard deviation in the ROI. (2 mm image reconstructions).

12. Radiation dose CT dose index (CTDIvol) and dose–length product (DLP) were obtained from the CT scanning protocol of each CTA study. The effective dose, DLP and an organ weighting (k = 0.017 mSv /mGy cm) was calculated.

13. Statistical analysis Variables were compared between both CT systems. Continuous variables were expressed as mean ± standard deviation and were compared using the Wilcoxon’s rank sum test. Categoric variables as frequencies and percentages. To evaluate if there is a relation between heart rate and image quality, patients were divided into two groups (1:heart rate 70 bpm) A p value of < 0.05 was considered statistically significant.

14. Results There was a significant difference in mean heart rate during free breathing. Radiation exposure was moderate in both patient groups with even lower dose- length-product in SSCT.

17. In patients with a heart rate of 70 bpm or below mean overall image quality score was on a comparable high level in both CT systems (SSCT: 3.7 ± 0.5; DSCT: 3.8 ± 0.4). When >70 bpm overall image quality score dropped to 3.4 ± 0.6 in SSCT ;while in DSCT (IQ score 3.9 ± 0.4). No significant correlation was observed between overall image quality and heart rate (SSCT: r = -0.43, p = 0.859; DSCT: r = -0.22, p = 0.347).

18. Regarding objective image quality parameters there was no significant difference in signal intensity and image noise between patients examined using SSCT and DSCT.

19. Discussion Our study demonstrates a high-pitch protocol for SSCT systems can be performed with a comparable level of radiation and contrast agent exposure and without substantial compromises in image quality compared to DSCT for TAVI planning examinations.

20. Discussion Our study demonstrates a novel high-pitch protocol for SSCT systems. Compared to DSCT, this planning examinations can be performed with. a)a comparable level of radiation. b)contrast agent exposure. c)without substantial compromises in image quality.

21. Discussion Overall image quality was slightly but significantly lower in the single-source high-pitch protocol when compared to dual-source high-pitch image acquisition (p = 0.037). Reasons as follows The 2nd generation DSCT offers a temporal resolution of 75 ms while it is 142.5 ms in the used SSCT system. Maximum achievable table speed in the used SSCT system is 230 mm/s (pitch value 1.7) while it is 458 mm/s in DSCT (pitch value 3.4).

22. In previous studies The high-pitch scan mode in DSCT has been shown to provide diagnostic image quality of the aortic root and the ascending aorta even without ECG- synchronisation. It may be assumed that after implementation of ECG- synchronization in the high-pitch scan mode in SSCT image quality will increase.

23. Bolen et al. retrospectively compared image quality of aortic CTA between ECG-triggered high-pitch and non-ECG-triggered standard- pitch scan mode in DSCT in 101 patients. The difference in image quality between both scan modes was more pronounced compared to the actual analysis. In the current analysis The difference in image quality between the high- pitch mode in SSCT and DSCT is just moderate (which may be explained by the use of a higher pitch value of 1.7 in SSCT).

24.  Compared to other TAVI CT protocols with radiation exposure up to 23.7 mSv,this protocol giving an effective radiation dose of 3.3 mSv is extremely low.  Compared to DSCT effective radiation dose in SSCT was even lower (4.3 vs. 3.3 mSv p = 0.01). This difference may be explained by an application of lower tube current in SSCT.

25.  Image noise did not differ significantly between both study groups. That may be due to a later detector technology in the used SSCT system.

26.  We observed lower signal intensity in overweight and especially in obese patients resulting in a reduced but still diagnostic image quality. Therefore, it might be reasonable to increase the iodine flux in these patients.

27. limitations a)A main limitation of this study is the small patient cohort. b)CT measurements of the aortic valve anulus have not been compared to an external reference standard (e.g. echocardiography). c)The scan modes used in the current study are vendor-specific. So the results may not be transferred to other CT-scanners from other vendors.

28. Conclusion This study presents a novel high-pitch protocol for modern SSCT scanners. It allows CT angiography for TAVI planning with a similar radiation dose and contrast agent exposition and only small compromises in image quality compared to a high-pitch protocol on a DSCT scanner.