Acta Radiol OnlineFirst, published on June 11, 2014 as doi:10.1177/0284185114535391 Original Article Image quality and radiation dose of lower extremity CTangiography at 70 kvp on an integrated circuit detector dual-source computed tomography Acta Radiologica 0(0) 1 7! The Foundation Acta Radiologica 2014 Reprints and permissions: sagepub.co.uk/journalspermissions.nav DOI: 10.1177/0284185114535391 acr.sagepub.com Li Qi 1,2, Yan E Zhao 1,2, Chang Sheng Zhou 1,2, James V Spearman 3, Matthias Renker 3,4, U Joseph Schoepf 1,2,3, Long Jiang Zhang 1,2 and Guang Ming Lu 1,2 Abstract Background: Despite the well-established requirement for radiation dose reduction there are few studies examining the potential for lower extremity CT angiography (CTA) at 70 kvp. Purpose: To compare the image quality and radiation dose of lower extremity CTA at 70 kvp using a dual-source CT system with an integrated circuit detector to similar studies at 120 kvp. Material and Methods: A total of 62 patients underwent lower extremity CTA. Thirty-one patients were examined at 70 kvp using a second generation dual-source CT with an integrated circuit detector (70 kvp group) and 31 patients were evaluated at 120 kvp using a first generation dual-source CT (120 kvp group). The attenuation and image noise were measured and signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) were calculated. Two radiologists assessed image quality. Radiation dose was compared. Results: The mean attenuation of the 70 kvp group was higher than the 120 kvp group (575 149 Hounsfield units [HU] vs. 258 38 HU, respectively, P < 0.001) as was SNR (44.0 22.0 vs 32.7 13.3, respectively, P ¼ 0.017), CNR (39.7 20.6 vs 26.6 11.7, respectively, P ¼ 0.003) and the mean image quality score (3.7 0.1 vs. 3.2 0.3, respectively, P < 0.001). The inter-observer agreement was good for the 70 kvp group and moderate for the 120 kvp group. The dose-length product was lower in the 70 kvp group (264.5 63.1 mgy cm vs. 412.4 81.5 mgy cm, P < 0.001). Conclusion: Lower extremity CTA at 70 kvp allows for lower radiation dose with higher SNR, CNR, and image quality when compared with standard 120 kvp. Keywords Angiography, low tube voltage, radiation dosage, X-ray computed tomography (CT) Date received: 6 January 2014; accepted: 21 April 2014 Introduction Digital subtraction angiography (DSA) has been considered the reference standard for diagnosing peripheral arterial disease (PAD). It is, however, invasive and carries certain limitations and risks (1 3). Computed tomography angiography (CTA), as a less invasive and safer examination, is an alternative to DSA which has gained widespread clinical acceptance for diagnosing disease of the peripheral arteries (4 6). It has been reported that CTA of the lower extremities has high sensitivity, specificity and accuracy in the assessment of the location and extent of peripheral arterial stenosis 1 Department of Medical Imaging, Jinling Hospital, Clinical School of South Medical University, Nanjing, Jiangsu Province, PR China 2 Department of Medical Imaging, Jinling Hospital, Clinical School of Nanjing University, Nanjing, Jiangsu Province, PR China 3 Department of Radiology and Radiological Science, Medical University of South Carolina, Charleston, SC, USA 4 Department of Cardiology, Kerckhoff Heart and Thorax Center, Bad Nauheim, Germany Corresponding author: Long Jiang Zhang, Department of Medical Imaging, Jinling Hospital, Medical School of Nanjing University, Nanjing, Jiangsu, 210002, PR China. Email: kevinzhlj@163.com; cjr.luguangming@vip.163.com
2 Acta Radiologica 0(0) when compared with DSA (1 3,7). Lower extremity CTA is not only applicable to the assessment of PAD, it is also a simple and robust method for the diagnosis and localization of vascular trauma in an acute lower-extremity injury (8). Catalano et al. showed high sensitivity (96%), accuracy (93%), and specificity (94%) of lower extremity CTA to detect peripheral arterial stenosis (7). The protocol used in the Catalano study had a weighted CT dose index as high as 12.2 mgy highlighting the need to investigate the potential for lower radiation dose protocols. Fraioli et al. investigated reduced radiation dose of lower extremity CTA by modifying milliamperage second (mas) and showed that reduction of the tube current to 50 mas resulted in a decrease of the weighted CT dose index to 9.4 mgy and the effective dose (ED) to 3.7 msv without image quality degradation (9). Lezzi et al. demonstrated that radiation dose reduction of 48% was achievable at lower extremity CTA without compromising image quality and diagnostic performance when the tube voltage was reduced from 120 kvp to 80 kvp (10). Recently, Dual et al. performed lower extremity CTA at 70 kvp in patients with known or suspected peripheral arterial occlusion diseases on a second generation dual-source CT scanner and reported an effective dose of 1.94 0.21 msv for CTA (11). However, no study has been performed to compare the image quality of 70 kvp lower extremity CTA with standard 120 kvp. The aim of our study, therefore, was to investigate the feasibility, image quality, and radiation dose of lower extremity CTA at 70 kvp using dual-source CT with an integrated circuit detector. Material and Methods Study subjects This retrospective study was approved by our institutional ethics board. A total of 62 patients (40 men, 22 women; mean age, 57.4 17.6 years; age range, 14 87 years) who had undergone lower extremity CTA because of known tumors (n ¼ 5), injury (n ¼ 7), or suspected arterial occlusive disease (n ¼ 50) were included in the study. Among these patients, 31 consecutive patients (19 men, 12 women; mean age, 55.3 20.2 years; age range, 14 87 years) had undergone lower extremity CTA at 70 kvp on a second generation dual-source CT system with an integrated circuit detector (70 kvp group). These studies were performed from May 2013 to July 2013 because of known tumors (n ¼ 3), injury (n ¼ 4), and suspected arterial occlusive disease (n ¼ 24). Another 31 consecutive patients (21 men, 10 women; mean age, 59.6 14.7 years; age range, 16 84 years) who had undergone CTA of the lower extremities at 120 kvp on a first generation dual-source CT system from March 2013 to May 2013 were selected as the control (120 kvp group). Of these 31 patients, two patients had soft tissue tumors, three patients had injuries, and 26 patients had arterial occlusive disease. CT protocol and image processing All CTA examinations of the lower extremities at 70 kvp were performed on a second generation dualsource CT system with an integrated circuit detector (Definition Flash, Siemens Medical Solutions, Forchheim, Germany). Integrated circuit detectors combine the photodiode and the analog-to-digital converter to an application-specific integrated circuit unit which is directly attached to the ceramic scintillators to reduce electronic noise, power consumption, and heat dissipation. Use of the application-specific integrated circuit detector potentially improves spatial resolution compared with previous detector technology by means of reduced cross-talk between the detector channels (12). For the 120 kvp lower extremity CTA, a first generation dual-source CT (Somatom Definition; Siemens Medical Solutions, Erlangen, Germany) was used. The acquisition parameters of the 70 kvp group were: tube voltage, 70 kvp; pitch, 0.6; collimation, 128 0.6 mm; slice thickness, 0.75 mm, reconstruction increment, 0.5 mm. The acquisition parameters of the 120 kvp group were: tube voltage, 120 kvp; pitch, 0.85; collimation, 64 0.6 mm; slice thickness, 1 mm, reconstruction increment, 0.7 mm. Automatic tube current modulation (CARE Dose 4D; Siemens Medical Solutions) was used in both acquisitions. Each acquisition was performed in cranio-caudal direction from the abdominal aortic bifurcation to the feet. An 18-gauge intravenous cannula was placed into a superficial vein positioned in the antecubital fossa. In all patients, contrast enhancement was achieved by injecting 80 ml of contrast material (iopromide 300 mg I/ml, Bayer Schering Pharma AG, Berlin, Germany) at 3.5 ml/s and 40 ml maintenance dose at 2 ml/s, followed by a 30 ml saline flush at 2 ml/s. Image acquisition started with a 7-s delay in the 70 kvp group and a 14-s delay in the 120 kvp group based on the general principle of venous injection of contrast medium at CTA (13), after the signal attenuation of the aortic bifurcation had reached the predefined threshold of 100 Hounsfield units (HU). Image analysis All datasets were transferred to a dedicated workstation (Syngo; Siemens Medical Solutions, Forchheim, Germany). The antero-posterior and transverse
Qi et al. 3 diameter of each patient was measured at the level of the aortic bifurcation on axial CT images as a metric of body habitus. Maximum intensity projection (MIP) and curved planar reformation (CPR) of the lower extremity vasculature were reconstructed for each patient. To evaluate objective image quality, the signal attenuations of four locations (aortic bifurcation, iliac bifurcation, proximal femoral artery, and proximal popliteal artery) were measured in transverse images and averaged to obtain a mean value. Moreover, the attenuation of the psoas muscle and the background of noise within subcutaneous fat (standard deviation) for the same section were also measured to calculate the signal-to-noise ratio (SNR) and the contrast-to-noise ratio (CNR) of each vascular segment. The attenuation was measured within regions of interest (ROI) within the vessels which were defined to be as large as possible while avoiding calcifications, plaques, and stenosis. The SNR and CNR were calculated as follows (14): SNR ¼ attenuation of the vessel lumen=image noise CNR ¼ ðattenuation of the vessel lumen attenuationofthepsoasmuscleþ=imagenoise In order to evaluate subjective image quality, all CT images (axial images, MIP and CPR images) were independently reviewed by two radiologists (with 5 and 2 years of reading experience, respectively) blinded to image acquisition parameters. Before image evaluation, the observers were instructed to divide vascular structures into four segments (aorto-iliac area, ilio-femoral region, femoro-popliteal district, and distal vessels) and then to score these segments using a 4-point scale according to the degree of noise artifacts, the presence of streak artifacts, and graininess of the images (9). The scoring criteria were as follows: poor (grade 1), graininess or streak artifacts were significant and did not provide sufficient information for the diagnosis; adequate (grade 2), the examination provided acceptable information but unsatisfactory image quality; good (grade 3), image quality was satisfactory enough to provide the information necessary to make an adequate radiological diagnosis; excellent (grade 4), image quality provided optimal information for a radiological diagnosis. In the event of observer disagreement, another reading session was convoked to reach a consensus. Radiation dose estimation For an estimation of the CT radiation dose, the CT volume dose index (CTDIvol) and the dose length product (DLP) of each patient were recorded. In addition, the acquisition range was also calculated by dividing the DLP by the CTDIvol. Statistical analysis Statistical analyses were performed using the SPSS software version 16.0 (SPSS Inc., Chicago, IL, USA). Quantitative variables were expressed as mean standard deviation, while categorical variables were expressed as frequency or percentage. The Mann- Whitney test was used to compare categorical characteristics and the independent-sample t test was used to compare continuous variables. Kappa analysis was used to assess the inter-observer agreement of the subjective image quality rating. A k value of less than 0.20 indicated poor agreement; a k value of 0.21 0.40, fair agreement; a k value of 0.41 0.60, moderate agreement; a k value of 0.61 0.80, good agreement; and a k value of 0.81 1.00, very good agreement. P < 0.05 was considered to indicate a statistically significant difference. Results Comparability of the 70 kvp and the 120 kvp group There were no statistically significant differences in age (55.3 20.2 years vs. 59.6 14.7 years, P ¼ 0.34), gender (men, 61% vs. 68%, P ¼ 0.60), antero-posterior body diameter (29.3 3.1 cm vs. 30.4 6.3 cm, P ¼ 0.37), transverse diameter (18.4 2.7 cm vs. 19.8 5.1 cm, P ¼ 0.20) and acquisition range (111.4 7.0 cm vs. 109.9 6.6 cm, P ¼ 0.38) between the 70 kvp group and the 120 kvp group. Objective and subjective image quality of lower extremity CTA The attenuations measured in all above-mentioned locations are shown in Table 1. The mean attenuation of the aortic-runoff was 575 149 HU for the 70 kvp group and 258 38 HU for the 120 kvp group (P < 0.001), indicating a 123% increase in attenuation. The attenuation of the psoas muscle in the 70 kvp group was significantly higher than in the 120 kvp group (58 13 HU, 49 12 HU; P ¼ 0.01). Furthermore, the image noise in the 70 kvp group was higher than in the 120 kvp group (14 4HUvs. 9 1 HU, respectively, P < 0.001), as were the SNR (44.0 22.0 vs. 32.7 13.3, respectively, P < 0.05) and CNR (39.7 20.6 vs. 26.6 11.7, respectively, P < 0.05) as shown in Table 1 and sample images are provided in Fig. 1. A total of 468 segments in 62 patients were assessed for subjective image quality, while 27 segments were excluded because of vascular occlusion (n ¼ 17), metal artifacts (n ¼ 3), and amputation (n ¼ 7). The mean image quality scores of the aorto-iliac area, the iliofemoral region, the femoro-popliteal district, and the runoff vessels for each group are displayed in Table 2,
4 Acta Radiologica 0(0) Table 1. Objective image quality of lower extremity CTA. Parameter showing relatively higher scores in the 70 kvp group compared with the 120 kvp group except for the iliofemoral region. The mean image quality score of the aortic-runoff was higher for the 70 kvp group (3.7 0.1) than that of the 120 kvp group (3.2 0.3, P < 0.001). The inter-observer agreement was good for the 70 kvp group (k ¼ 0.721, P < 0.001) and moderate for the 120 kvp group (k ¼ 0.565, P < 0.001). Radiation exposure estimation The DLP in 70 kvp group was significantly lower than in the 120 kvp group (70 kvp group vs. 120 kvp group, 264.5 63.1 mgy cm vs. 412.4 81.5 mgy cm, P < 0.001), as well as the CTDIvol (2.4 0.5 mgy vs. 3.8 0.7 mgy, P < 0.001). Discussion 70 kvp group 120 kvp group P value Aortic bifurcation Mean CT value (HU) 589 152 279 45 <0.001 SNR 45.5 23.5 35.4 14.6 0.046 CNR 41.2 22.0 29.2 13.1 0.012 Iliac bifurcation Mean CT value (HU) 582 159 261 40 <0.001 SNR 43.4 21.3 33.1 13.0 0.028 CNR 39.2 20.1 26.9 11.4 0.005 Proximal femoral artery Mean CT value (HU) 587 165 249 45 <0.001 SNR 44.8 22.6 31.7 13.6 0.008 CNR 40.5 21.3 25.5 12.1 0.001 Proximal popliteal artery Mean CT value (HU) 537 140 253 38 <0.001 SNR 41.5 21.5 32.1 12.9 0.04 CNR 37.2 20.1 25.9 11.4 0.01 Mean Mean CT value (HU) 575 149 258 38 <0.001 SNR 44.0 22.0 32.7 13.3 0.017 CNR 39.7 20.6 26.6 11.7 0.003 Psoas muscle (HU) 58 13 49 12 0.01 Noise (HU) 14 4 9 1 <0.001 CTA, CT angiography; CNR, contrast-to-noise ratio; SNR, signal-to-noise ratio. The present study demonstrates the feasibility of lower extremity CTA at 70 kvp using a second generation dual-source CT system with an integrated circuit detector. Although image noise was found to be significantly higher, the SNR, CNR, and image quality were improved in the 70 kvp group when compared with the 120 kvp group. Most importantly, the 70 kvp protocol resulted in a 36% DLP reduction compared to the protocol with 120 kvp settings on a first generation dual-source CT. Low-voltage technology has its unique advantages in CTA. A higher attenuation of the iodinated contrast agent can be achieved when a lower tube voltage protocol is used. The results of our investigation show that the use of 70 kvp leads to a significantly higher attenuation than 120 kvp with an increase of 123%. Our results are consistent with the experimental results of similar investigation, which showed a significantly higher attenuation in the low tube voltage group (100 kvp) compared to the standard tube voltage group (120 kvp) in aorto-iliac CTA (14). Importantly, the high attenuation in the 70 kvp group in our study also indicates the possibility of lowering the contrast material volume in this CTA protocol as has been demonstrated in other areas of the body (15). Lowering tube voltage inevitably increases image noise, which negatively affects image quality. SNR and CNR are more comprehensive measures of radiologic image quality for the assessment of noise on image information than signal intensity and contrast (14,16 19). To a certain extent increased vessel opacification can compensate for higher image noise of image sets with lower kvp which may result in a slightly lower or equivalent SNR at maintained diagnostic image quality when compared with standard kvp image sets. Prior studies have shown higher noise and slightly lower or equivalent SNR, CNR and diagnostic image quality in the low kvp group when compared against standard kvp settings (14,20,21). Despite higher image noise, however, our study shows improved SNR and CNR when comparing the 70 kvp group to the 120 kvp group. The difference between the present investigation and prior studies may be attributed to the difference in anatomical region or the lower tube voltage. In either case the present study demonstrates that low kvp acquisition (even as low as 70 kvp) is feasible in CTA of the lower extremities. This could be attributable to the reduced antero-posterior diameter of the anatomical region or the superficial location of the vessels of the lower extremities. In the present study, larger increases in attenuation paired with lesser increases in image noise within the 70 kvp group lead to higher SNR and CNR when maintaining the same volume of iodinated contrast agent. Importantly, subjective image quality as rated by two radiologists was superior in the 70 kvp group compared to the 120 kvp group except for the femoropopliteal region. The more obvious streak artifact from the femur with 70 kvp may account for the
Qi et al. 5 Fig. 1. Lower extremity CTA at 70 kvp in a 60-year-old woman (a d) and at 120 kvp in a 57-year-old woman (e h). (a, e) Curved multiplanar reconstructions with region of interest placement and measurements of the four segments. Higher attenuation and noise can be observed in lower extremities CTA using 70 kvp. (b d, f h) Maximum intensity projections. Higher contrast and sharpness are demonstrated in MIP reconstructions of lower extremity CTA at 70 kvp (b d) than at 120 kvp (f h). Table 2. Subjective image quality scores of the four arterial segments of the lower extremities. Score 70 kvp group 120 kvp group P value Aorto-iliac Reader 1 3.9 0.3 3.5 0.5 <0.001 Reader 2 4.0 0.1 3.2 0.4 <0.001 Both readers 4.0 0.2 3.3 0.5 <0.001 Ilio-femoral Reader 1 3.9 0.2 3.2 0.4 <0.001 Reader 2 3.9 0.3 3.2 0.4 <0.001 Both readers 3.9 0.2 3.2 0.4 <0.001 Femoro-popliteal Reader 1 3.2 0.4 3.2 0.4 0.673 Reader 2 3.0 0.1 3.2 0.4 0.252 Both readers 3.1 0.3 3.2 0.4 0.385 Distal vessels Reader 1 3.9 0.2 3.1 0.4 <0.001 Reader 2 3.9 0.3 3.1 0.3 <0.001 Both readers 3.9 0.3 3.0 0.2 < 0.001 Mean scores Reader 1 3.8 0.2 3.3 0.3 <0.001 Reader 2 3.7 0.1 3.2 0.4 <0.001 Both readers 3.7 0.1 3.2 0.3 <0.001 lower image quality score in the femoro-popliteal segment. Nevertheless, the mean subjective image quality score for the femoro-popliteal region was 3.1 0.3 in the 70 kvp group, which was still considered diagnostic. Compared with the 120 kvp group, the 70 kvp group showed not only higher attenuation and image quality, it also demonstrated a lower radiation dose, as radiation exposure rises exponentially with tube voltage (22). Previous studies have demonstrated a 33% reduction in radiation dose when tube voltage was reduced from 120 kvp to 100 kvp in aorto-iliac CTA (14) and a 30% decline when tube voltage was reduced from 120 kvp to 80 kvp in lower extremity CTA (23). However, in the present study the DLP was reduced by 36% when the tube voltage was decreased from 120 kvp to 70 kvp. This reduction in radiation dose may be attributed to the use of automatic tube current modulation. Use of this system automatically increased the tube current to maintain image quality when the tube voltage was lowered, which is different from the fixed mas settings in previous studies. Gnannt at al. investigated the feasibility of 70 kvp CT studies of the cervical soft tissues and demonstrated improved CNR and an ED reduction of 34% compared to a 120 kvp group when automatic tube current modulation was used (15).
6 Acta Radiologica 0(0) Fig. 2. Lower extremity CTA at 70 kvp and digital subtraction angiography images in our study. (a) Curved planar reformation image; (b, c) digital subtraction angiography images. Normal left peripheral arteries can be shown. There are certain limitations to our study that must be acknowledged. First, because of the retrospective design of this study it was impossible to exactly control several parameters of the two different CT systems which influenced radiation dose and image quality, such as the differences in detectors, pitch, collimation and slice thickness. Despite this, the aim of this case control study was to investigate the feasibility of lower extremity CTA at 70 kvp using dual-source CT with an integrated circuit detector. Moreover, a lower pitch (0.6) in the 70 kvp group indicates the possibility of further reductions in radiation dose if the protocol had used the same pitch as the 120 kvp group (0.85). Second, the body mass index (BMI) of the patients was not compared between the two groups because the height and weight were not systematically recorded when the patients underwent the examination. As an alternative anthropometric parameter to gauge body habitus, we measured the antero-posterior and transverse diameter of each study subject in the CT images at the level of the bifurcation of the abdominal aorta. The study of Duan et al. on 70 kvp lower extremity CTA limited patient inclusion to BMI of less than 25 kg/m 2 (11). Our study indicates that the BMI may not be the main factor affecting image quality of lower extremity CTA at 70 kvp. Third, a relatively small sample size was investigated in our study with only 62 patients evaluated. However, a total of 468 segments vessels were analyzed. Fourth, the diagnostic accuracy of lower extremity CTA at 70 kvp on a dual-source CT systems with an integrated circuit detector was not assessed, as the aim of our study was to evaluate the overall image quality and radiation dose of lower extremities CTA compared to 120 kvp (for a visual comparison to DSA please see Fig. 2). However, in one recently published study, 70 kvp lower extremity CTA had high specificity and sensitivity for the detection of peripheral arterial occlusion diseases on a second generation DSCT system (11). In conclusion, our study shows that lower extremity CTA at 70 kvp is feasible with higher image quality and lower radiation dose when compared to a standard protocol CTA at 120 kvp. Additionally, this protocol showed improvements in the reproducibility of image quality assessment between readers. Conflict of interest UJS is a consultant for and receives research support from Bayer, Bracco, General Electric, Medrad, and Siemens. All other authors had no conflict of interest to declare. Funding This work was partially supported by the Program for New Century Excellent Talents in University (NCET-12-0260). References 1. Martin ML, Tay KH, Flak B, et al. Multidetector CT angiography of the aortoiliac system and lower extremities: a prospective comparison with digital subtraction angiography. Am J Roentgenol 2003;180:1085 1091.
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