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DOI: 10.1148/radiol.2452061919
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(Radiology 2007;245:577-583.)
© RSNA, 2007

Thoracic Imaging

Image Quality and Radiation Exposure at Pulmonary CT Angiography with 100- or 120-kVp Protocol: Prospective Randomized Study1

Christoph M. Heyer, MD, Patrick S. Mohr, MD, Stefan P. Lemburg, MD, Soeren A. Peters, MD, and Volkmar Nicolas, MD

1 From the Institute of Diagnostic Radiology, Interventional Radiology and Nuclear Medicine, BG Clinics Bergmannsheil, Ruhr-University of Bochum, Buerkle-de-la-Camp Platz 1, D-44789 Bochum, Germany. Received November 10, 2006; revision requested January 12, 2007; revision received January 22; accepted March 1; final version accepted April 18. Address correspondence to C.M.H. (e-mail: christoph.heyer{at}rub.de).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCE IN KNOWLEDGE
 IMPLICATION FOR PATIENT CARE
 References
 
Purpose: To prospectively compare 16-section multidetector computed tomography (CT) at 100 and 120 kVp for image quality and radiation dose.

Materials and Methods: The study had institutional review board approval; written informed consent was obtained. Sixty patients were referred for evaluation of suspected pulmonary embolism with CT angiography. Patients were randomly assigned to a 100-kVp (n = 30; 17 men, 13 women; mean age, 66 years ± 17 [standard deviation]; range, 19–89 years) or 120-kVp (n = 30; 15 men, 15 women; mean age, 62 years ± 15; range, 28–86 years) protocol. Other scanning parameters were kept constant. Contrast medium was injected automatically with bolus tracking. Pulmonary vessel enhancement and image noise were quantified; signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) were calculated. Subjective vessel contrast was assessed by two radiologists in consensus. Effective dose was calculated on the basis of dose length product and volume CT dose index. Results of both protocols were compared by using the {chi}2 test and Student t test.

Results: The 100-kVp protocol had a nonsignificantly higher mean vessel attenuation than the 120-kVp protocol (386.8 HU ± 130.1 vs 317.9 HU ± 112.5; P = .56) and a nonsignificantly higher image noise (16.9 HU ± 5.8 vs 13.7 HU ± 6.2; P = .84), which resulted in almost identical SNR (25.3 ± 11.7 vs 27.0 ± 14.5; P = .37) and CNR (22.0 ± 11.2 vs 22.9 ± 13.1; P = .51). There was no significant difference in subjective image quality between protocols. Mean effective dose for the 100-kVp protocol was significantly lower than that for the 120-kVp protocol (1.37 mSv ± 0.39 vs 2.44 mSv ± 0.97; –44%; P < .001).

Conclusion: Reduction of kilovoltage from 120 to 100 kVp resulted in significant reduction of effective dose at pulmonary CT angiography, without significant loss of objective or subjective image quality.

© RSNA, 2007


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCE IN KNOWLEDGE
 IMPLICATION FOR PATIENT CARE
 References
 
Computed tomographic (CT) angiography has become a major diagnostic imaging procedure in patients suspected of having pulmonary embolism (PE). Modern multidetector scanners with thin-section reconstructions enable accurate analysis of peripheral pulmonary arteries (1) and optimized vessel enhancement by using bolus tracking (2); these practices have led to a constant increase in the diagnostic accuracy of pulmonary CT angiography in the detection of PE (3). Sensitivity and specificity of recently developed CT techniques for depiction of PE in comparison to scintigraphy and conventional angiography are 94%–96% and 94%–100% (4–8), respectively. Pulmonary CT angiography has been demonstrated to result in lower radiation exposure than conventional angiography (9,10). However, CT is the most important source of radiation exposure for the general population and is of special concern with respect to thin-section acquisition of extensive anatomic volumes, which have become feasible with multidetector CT (11).

Results of studies (12,13) have demonstrated an increase in the number of patients examined with CT for suspected PE and an increase in effective dose per patient. On the other hand, several investigators (14–16) have shown that the prevalence of PE among patients evaluated with CT angiography is only 9%–35%. Because of the high percentage of negative results at CT and the fact that young female patients account for a substantial number of patients undergoing CT angiography for suspected PE (17,18), there is a need for reduction of radiation dose. Therefore, the purpose of our study was to prospectively compare 16-section multidetector CT at 100 kVp and 120 kVp, with regard to image quality and radiation dose.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 ADVANCE IN KNOWLEDGE
 IMPLICATION FOR PATIENT CARE
 References
 
Patient Groups
Between October 2005 and January 2006, all consecutive patients who were transferred to the Institute of Diagnostic Radiology, Interventional Radiology and Nuclear Medicine, BG Clinics Bergmannsheil, Ruhr-University of Bochum, Bochum, Germany, for pulmonary CT angiography for suspected PE were considered for the study. Indication for performing CT angiography was based on positive results at clinical investigation (determined with revised Geneva score [19] or Wells score [20]), abnormal findings at laboratory testing (blood gas analysis, d-dimer level), abnormal results at echocardiography or electrocardiography indicative of acute right heart dysfunction, abnormal findings at lower limb ultrasonography, and re sults suggesting PE at conventional radiography. Known allergy to contrast material, hyperthyroidism, renal insufficiency, pregnancy, and being younger than 18 years were contraindications for entering the study. Patients who denied consent for the procedure were excluded from the study group.

In total, 60 patients entered the study. They were randomly assigned to undergo either CT protocol A (120 kVp, patient group A, n = 30) or protocol B (100 kVp, patient group B, n = 30); randomization was ensured by using a computer-generated list. Characteristics of patient group A were as follows: 17 men, 13 women; mean age, 66 years ± 17 (standard deviation); range, 19–89 years. Characteristics of patient group B were as follows: 15 men, 15 women; mean age, 62 years ± 15; range, 28–86 years. Written informed consent for the CT procedure and for the research protocol was given by each patient after the nature of the examination had been fully explained. The study was approved by the ethics committee of the Ruhr-University of Bochum, Bochum, Germany.

CT Pulmonary Angiography
All scans were obtained by using a commercially available CT scanner (Somatom Sensation 16; Siemens, Erlangen, Germany) with 16 detector rows. Patients were examined in the supine position with both arms extended above the head. A frontal scout view was acquired at 120 kVp and 50 mA. The angiography scan was obtained in the caudocranial direction during a single inspiratory breath hold. Scan volume ranged from the level of the right diaphragm to a level just above the aortic arch. A standard collimation of 16 x 0.75 mm was used, with a gantry rotation speed of 0.5 second and a pitch factor of 1.15. Patients were scanned with a kilovoltage of either 120 kVp (patient group A) or 100 kVp (patient group B), while all other acquisition parameters were kept constant, including a tube current level of 200 mA (100 mAs). For all examinations, vessel enhancement was achieved by use of an intravenous injection of 80 mL of iopamidol (Solutrast 300; Altana Pharma, Konstanz, Germany) via a cubital vein, followed by a saline flush of 40 mL. Flow rate was kept constant at 4 mL/sec throughout the procedure. Injections were performed automatically by using a commercially available injector (Injektron CT2; Medtron, Saarbruecken, Germany). Individual contrast optimization was achieved by using bolus tracking (Care Bolus; Siemens) in the right ventricle with a trigger level of 100 HU. An additional delay of 7 seconds was added before every examination.

For further postprocessing, thin-section reconstruction was performed with a section thickness of 1 mm, an increment of 0.7 mm, and a smooth reconstruction kernel (B30f). Final image analysis was performed with transverse images and coronary maximum intensity projections with a section thickness of 3 and 6 mm, respectively.

Assessment of Image Parameters
Signal intensity (SI) (ie, CT number) measurements were determined at a workstation (Leonardo; Siemens) along the pulmonary arteries, including nine different levels: the main pulmonary artery, right pulmonary artery, left pulmonary artery, right upper lobe artery, right middle lobe artery, right lower lobe artery, left upper lobe artery, left lingular artery, and left lower lobe artery. The regions of interest used for these measurements were chosen to be as large as the vessels. On the basis of these numbers, an average pulmonary vessel SI was calculated. If PE was present, average intravascular SI measurement was limited to unaffected vessels. The measurement of background noise was based on assessment of attenuation (in Hounsfield units) within surrounding air at three regions of interest in front of the patient (central, left, and right) with a size of approximately 1 cm2; averaged values were used for final calculation of background noise. In addition, attenuation of the central parts of pectoral muscles and the deep paraspinal muscles were measured on both sides and averaged (muscle SI). On the basis of these measurements, signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) were calculated according to the following equations: SNR = SIMPV/BN and CNR = (SIMPV – muscle SI)/BN, where SIMPV is mean SI of pulmonary vessel and BN is background noise.

In addition, subjective image quality with regard to pulmonary vessel contrast was assessed by two readers (C.M.H., 7 years of experience in chest CT; S.P.L., 6 years of experience) in consensus by using transverse sections and maximum intensity projections. For evaluation of subjective image quality, a five-point scale (score of 5, excellent; 4, good; 3, moderate; 2, still diagnostic; 1, nondiagnostic) was applied.

Assessment of image data for presence of PE was performed, with analysis of central and peripheral pulmonary arteries down to the subsegmental level. The presence of endoluminal clots was considered diagnostic of PE.

Effective Radiation Dose Evaluation
Scan length (distance covered) was documented for every examination. In addition, the amount of extrathoracic fat and soft tissue was individually estimated by measuring and averaging the perpendicular distance between the skin and the anterior margins of the ribs on both sides at the level of the aortic arch (as an image-based quantification tool for obesity). Volume CT dose index and dose length product, which were provided by the scanner system, were recorded, and effective dose was calculated for every patient by taking age- and sex-dependent conversion factors into account. All dose-related calculations were performed by using a dedicated software tool (CT Expo, version 1.5.1; G. Stamm and H.D. Nagel, Hannover, Germany).

All measurements and calculations were performed by three authors in cooperation (C.M.H.; S.P.L.; and P.S.M., 3 years of experience in chest CT).

Statistical Analysis
Results of SI measurements, SNR, CNR, subjective image quality, dose length product, volume CT dose index, and effective dose are expressed as means ± standard deviations, whereas patient age and scan distance are given as means ± standard deviations, with ranges in parentheses. Normality of data distribution was assessed by using the Kolmogorov-Smirnov test. Characteristics of both patient groups (age, sex, incidence of PE, distance between thoracic skin and anterior ribs) were compared by using the Student t test for unpaired samples and by using the {chi}2 test. Results for patient groups A and B were compared with regard to scan distance, SI measurements, SNR, CNR, dose length product, volume CT dose index, estimated effective dose, and subjective image quality. Statistical data comparison was performed by using the Student t test for unpaired samples and by using the {chi}2 test. A significant difference for all tests was set at a P value of less than .05. For accuracy, an adjustment to {alpha} was made for four parameters (SNR, CNR, subjective image quality, and estimated effective dose) by using Bonferroni correction. All calculations were performed with a standard personal computer by using software (SPSS for Windows, release 11.5.1; SPSS, Chicago, Ill).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 ADVANCE IN KNOWLEDGE
 IMPLICATION FOR PATIENT CARE
 References
 
Patient Characteristics
CT evaluation of the pulmonary arteries demonstrated PE in 18 patients (30%). Comparison of the two patient groups did not reveal any significant differences in patient age, sex, amount of extrathoracic soft tissue, incidence of PE, or scan distance covered (Table 1). Thus, further analysis and comparison of SI measurements and radiation exposure were considered feasible and valid.


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  		Table 1. Patient Characteristics and Scanning Parameters

 
Image Parameters
Application of the Kolmogorov-Smirnov test showed that the variables satisfied normality. Mean pulmonary artery SI in patient group B was higher than that in patient group A; however, this difference was not significant (386.8 HU ± 130.1 vs 317.9 HU ± 112.5; P = .56). Likewise, detailed analysis of the distal pulmonary vessels showed higher SIs in patient group A than in patient group B, with one exception (lingular artery); however, these differences were not significant. There was no significant difference between patient groups A and B for mean SI of paraspinal and pectoral muscles (47.1 HU ± 6.7 for group A vs 50.4 HU ± 9.3 for group B; P = .15). Background noise increased from 13.7 HU ± 6.2 (patient group A) to 16.9 HU ± 5.8 (patient group B), which was not a significant difference (P = .84). There were no significant differences between patient groups A and B, respectively, for SNR (27.0 ± 14.5 vs 25.3 ± 11.7; P = .37) and CNR (22.9 ± 13.1 vs 22.0 ± 11.2; P = .51) (Figs 1, 2).


Figure 1
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  		Figure 1: Images in 84-year-old man with dyspnea, positive d-dimer level, abnormal findings at blood gas analysis, and electrocardiographic findings suggestive of PE. Transverse pulmonary CT angiograms (different levels) obtained with 120 kVp (protocol A) demonstrate excellent vessel enhancement down to subsegmental level.

 

Figure 2
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  		Figure 2: Images in 77-year-old man with dyspnea, abnormal findings at blood gas analysis, and pathologic findings at echocardiography suggestive of acute right-sided heart dysfunction. Transverse chest CT images (different levels) obtained with 100 kVp (protocol B) show excellent vessel enhancement of pulmonary arteries and only minor background noise. Note thrombus in right lower lobe artery (arrow).

 
Assessment of subjective image quality with transverse images and maximum intensity projections revealed an average value of 3.78, with a range from still diagnostic (grade 2; n = 1) to excellent (grade 5). No examination was rated nondiagnostic (grade 1). There were no significant differences in subjective image quality between patient groups A and B (3.80 ± 0.93 vs 3.77 ± 0.97, respectively; P = .76).

Radiation Exposure
In comparison of patient group A to B, there were significant differences for volume CT dose index (30.7 mGy ± 10.7 vs 17.6 mGy ± 5.1, respectively; P < .001), dose length product (114.7 mGy·cm ± 39.2 vs 72.9 mGy·cm ± 5.1, respectively; P < .001), and estimated effective dose (2.44 mSv ± 0.97 vs 1.37 mSv ± 0.39, respectively; P < .001). The latter corresponds to an average reduction of estimated effective dose of approximately 44% (Table 2).


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  		Table 2. Measurements of SI, Objective and Subjective Image Quality, and Radiation Exposure

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
ADVANCE IN KNOWLEDGE
 IMPLICATION FOR PATIENT CARE
 References
 
Several strategies have been proposed for effectively decreasing radiation exposure during chest examinations, including modulation of tube current, kilovoltage, scan distance, and shielding. Results of some studies (15,21–23) have indicated the possibility of decreasing the tube current without impairing image quality; current strategies mainly focus on optimizing tube current by adapting it to the patient's weight (24–26). Furthermore, sophisticated technical devices, including modulation of tube current relative to the geometry and attenuation of the scanned object, have been introduced into clinical practice (27–29).

In addition to adjusting the x-ray tube current, pitch, and scanning time per rotation, CT operators can also adjust the tube voltage. Modern CT scanners operate at 120–140 kVp for routine standard imaging because these settings provide good image quality. Investigators (30,31) have shown that lowering kilovoltage settings can effectively reduce radiation dose at pediatric CT examinations without substantially decreasing image quality. Thus, age- and weight-dependent adjustments of tube current and kilovoltage for CT examinations have been established in pediatric radiology departments (25,32–35).

To our knowledge, benefits of reducing kilovoltage at adult chest CT have been reported only twice. In their study, Sigal-Cinqualbre and colleagues (36) compared two CT protocols at 80 kVp with a standard protocol at 120 kVp in slim patients. Qualitative analysis revealed no substantial difference between the images acquired during the low kilovoltage examinations and those acquired during the standard protocol. Moreover, the authors were able to reduce contrast material dose by more than 50% when using a low kilovoltage protocol. Abada et al (37) evaluated the feasibility of coronary CT by using 80 kVp combined with electrocardiographically pulsed tube current modulation. With this strategy, a substantial decrease in radiation dose of up to 88% could be achieved. However, the patient group in this study was small (n = 11), and direct comparison to a standard protocol was not performed.

Reduction of kilovoltage from 120 to 80 kVp leads to a decrease in radiation dose of approximately 40%–60% (36). However, this decrease correlates with increased image noise, which might lead to substantial impairment of image quality (38). Results of our study show that use of a 100-kVp protocol at pulmonary CT angiography induces an increase in background noise of 19% compared with a standard protocol with 120 kVp.

The best criterion for objectively assessing image quality is determination of SNR and CNR. Both values were almost identical at comparison of both protocols used in our study. The main reason for this was the coincidence of increasing background noise and increasing pulmonary vessel enhancement. The latter occurs mainly because the attenuation of iodine-based contrast agents increases with reduced x-ray energy owing to the high relative atomic number of iodine (36). This physical attribute led to an 18% increase in mean intravascular contrast in patient group B compared with group A. Detailed analysis of the pulmonary vessels showed that mean SI was higher in patient group B than in patient group A, with the exception of SI in the lingular artery. However, these differences were not significant. In comparison to the objective criteria for SNR and CNR, subjective image analysis revealed similar findings for protocols A and B: The two radiologists were not able to see the difference between the images obtained with the two protocols and ranked image quality with respect to vessel enhancement almost identically.

Other efforts to reduce radiation exposure during chest CT include modification of scan distance and shielding. A survey (39) of practices and policies for the use of pulmonary CT angiography in pregnant women has revealed that the method most often used for reducing radiation dose consists of a reduction in z-axis coverage. Hopper et al (40) showed that the use of bismuth radioprotective latex manufactured into formfitting garments did not affect diagnostic CT quality but substantially reduced radiation exposure to superficial structures. However, pulmonary vessels were not evaluated in that study, which makes a general recommendation for bismuth shielding at pulmonary CT angiography difficult.

Adequate vessel enhancement at pulmonary CT angiography has shown to be crucial for detection of PE. Results of a study by Jones and Wittram (3) indicated that poor contrast enhancement and motion artifacts are the major causes for indeterminate pulmonary CT angiograms, thus obscuring the presence of PE. However, Engelke et al (41) recently showed that arterial enhancement does not necessarily contribute to missed diagnoses of PE. In their study, which was based on 2412 pulmonary CT angiographic examinations, the amount of arterial obstruction was the only predictor of PE diagnosis. Our data indicate that the use of bolus tracking led to optimal vessel enhancement in the majority of patients, which resulted in an average subjective image quality of 3.80 on a five-point scale. Besides, none of the CT scans were considered nondiagnostic.

There were limitations to our study. First, we did not calculate radiation dose by using phantom measurements. However, data from other studies have shown accordance between exposure parameters provided by the CT scanner (dose length product, CT dose index) and phantom measurements in adult patients (42). Second, we did not evaluate image quality of thin-section CT scans in lung window settings, which can be used to diagnose interstitial pulmonary disease. Although our primary goal was to assess the pulmonary arteries, some authors (43,44) have advocated thorough evaluation of lung tissue even in the context of suspected PE. This is mainly because PE may be mistaken for a variety of common disorders, and no single test for its definitive diagnosis exists. Thus, the ability of CT to depict concurrent or mimicking disease has led to an expansion of its utility. Moreover, we did not focus on patients' weight, a focus proposed by other authors when evaluating low-dose protocols (36,45). However, we believe that our estimations of the amount of subcutaneous soft tissue may be an adequate substitute for total body weight or body mass index and may be an even better reflection of the patient's thoracic shape. Finally, we assessed only proximal pulmonary arteries for objective and subjective image quality. Thus, a potentially decreased SNR or CNR of the peripheral vessels might have been missed by both readers. Nevertheless, Prologo and coworkers (46) showed that increased visualization of smaller, more peripheral, pulmonary arteries did not affect the clinical outcome of patients with PE.

In conclusion, use of a 100-kVp protocol permits a significant decrease in effective radiation dose compared with that of a standard 120-kVp protocol in patients suspected of having PE and undergoing pulmonary CT angiography. Although image noise increased, there was no significant decrease in image quality measured by using objective or subjective criteria. This compensating mechanism was based on the increased iodine attenuation. According to the results of our study, the low kilovoltage protocol with 100 kVp at pulmonary CT angiography is now routinely used in our institution.


   ADVANCE IN KNOWLEDGE
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCE IN KNOWLEDGE
IMPLICATION FOR PATIENT CARE
 References
 


   IMPLICATION FOR PATIENT CARE
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCE IN KNOWLEDGE
 IMPLICATION FOR PATIENT CARE
References
 


   ACKNOWLEDGMENTS
 
We thank Tim Holland-Letz, MSc, Department of Medical Informatics, Biometry and Epidemiology, Ruhr-University of Bochum, Bochum, Germany, for his support.


   FOOTNOTES
 

Abbreviations: CNR = contrast-to-noise ratio • PE = pulmonary embolism • SI = signal intensity • SNR = signal-to-noise ratio

Author contributions: Guarantor of integrity of entire study, C.M.H.; study concepts/study design or data acquisition or data analysis/interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors; manuscript final version approval, all authors; literature research, C.M.H., P.S.M., S.P.L.; clinical studies, all authors; statistical analysis, C.M.H.; and manuscript editing, all authors

Authors stated no financial relationship to disclose.


   References
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCE IN KNOWLEDGE
 IMPLICATION FOR PATIENT CARE
 References
 

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