CT angiography of pulmonary arteries to detect pulmonary embolism with low kv settings Poster No.: C-3289 Congress: ECR 2010 Type: Scientific Exhibit Topic: Chest - Your latest results Authors: M. K. Gill, A. Vijayananthan, G. K. Kumar, K. H. Ng, K. Jayarani; Kuala Lumpur/MY Keywords: kvp: x-ray tube voltage, mas: x-ray beam intensity, CTPA: CT angiography of the pulmonary arteries Keywords: Lung, Respiratory system, Thorax DOI: 10.1594/ecr2010/C-3289 Any information contained in this pdf file is automatically generated from digital material submitted to EPOS by third parties in the form of scientific presentations. References to any names, marks, products, or services of third parties or hypertext links to thirdparty sites or information are provided solely as a convenience to you and do not in any way constitute or imply ECR's endorsement, sponsorship or recommendation of the third party, information, product or service. ECR is not responsible for the content of these pages and does not make any representations regarding the content or accuracy of material in this file. As per copyright regulations, any unauthorised use of the material or parts thereof as well as commercial reproduction or multiple distribution by any traditional or electronically based reproduction/publication method ist strictly prohibited. You agree to defend, indemnify, and hold ECR harmless from and against any and all claims, damages, costs, and expenses, including attorneys' fees, arising from or related to your use of these pages. Please note: Links to movies, ppt slideshows and any other multimedia files are not available in the pdf version of presentations. www.myesr.org Page 1 of 32
Purpose General Objective: To assess the feasibility of using a low kvp technique in the evaluation of pulmonary embolism. In developing countries like ours, where 16 slice multidetector scanners are still widely used, this study may be beneficial in improving the efficacy of the available scanners as well as reducing the radiation dose. Specific Objective: 1. To assess the image quality of the low dose (100kVp) protocol as compared to the standard dose(120kvp) protocol. 2. To determine the ideal mas for the low dose protocol and also to calculate the amount of radiation dose reduction using a preliminary phantom study. Page 2 of 32
Methods and Materials Patient selection and period of study: A prospective study was carried in all patients at our center suspected of having pulmonary embolism based on the presence of one or more of the following symptoms: shortness of breath, chest pain, cough and hemoptysis. The patients were divided into two groups; the first group of 33 patients (group A) was scanned with a low dose 100kVp protocol; while the second group of 33 patients (group B) were scanned with the standard dose 120kVp protocol. This study was divided into two phases: 1) a preliminary phantom study to determine the ideal mas for the low dose protocol and also to calculate the amount of radiation dose reduction 2) the main study to evaluate image quality of the low dose protocol with the preset protocol as a standard value. Patient demographics: In total, 66 consecutive patients were recruited and their ages ranged from 19 to 87 years (median age 52.77 years). We measured the antero-posterior (AP) and transverse diameters of the chest at the level of the main pulmonary artery. The patient demographics which included: sex, age, and body weight were obtained from patient records. Eligibility Criteria: 1. Inclusion Criteria Page 3 of 32
All patients suspected with pulmonary embolism from A&E and wards. The weight of the patient should be within 50 to 70kg. 2. Exclusion criteria: Patients who were below 18 years of age, those unable to hold their breath for at least 20seconds, pregnant women and those with severe spinal deformities or had metal thoracic implants. Study Design: 1. Preliminary study (phantom) A phantom study was done to obtain a suitable protocol for optimisation of the low dose scanning technique. Using the 120kVp 90mAs as a standard, different mas ranging from 70 to 135 were compared with 100kVp on a phantom to obtain the most suitable mas for the 100kv protocol without significantly affecting the image quality. 2. Scanning Protocol on patients 16 slice spiral MDCT caudal-cranial direction 5.0mm slice thickness Pitch 1.5 Total rotation time: 18.0 seconds Collimation: 16 x 0.75mm Start delay: Smart Prep HU=100 at right ventricle Contrast media: 80-100mls of IV Iopromide 370 mg I/ml at 3-4mls per second Exposure parameters varied between groups A and B. Patients in group A were scanned with a low-dose, low kilovoltage protocol at 100 kvp and 115 mas(predetermined by a phantom study), while for the standard protocol in group B, we used 120 kvp and 90 mas. (Figure 1) Image Evaluation: Page 4 of 32
Quantitative and subjective analyses were made as follows (Figure 2): 1. Quantification Analysis (a) Vascular attenuation. To evaluate attenuation in the central pulmonary arteries, we measured the mean CT number (in Hounsfield units) in the main pulmonary artery by using a region of interest 2 2 of at least 1.0cm (± 0.5cm ). We also evaluated the maximum attenuation (CT number) of peripheral pulmonary arteries close to the beginning and the end of each scan in a segmental or sub-segmental artery at an apical and a basal section position.the maximum CT number was used as a proxy for vascular attenuation because the caliber of the peripheral vessels was too small to reliably set an intraluminal region of interest to determine the mean CT number. (b) Image noise. Objective quantification of the image noise was analysed by measuring the standard 2 deviation(sd) in three homogeneous regions of interest measuring about 1.0cm (± 2 0.5cm ) in the main pulmonary artery that was free of motion or contrast material-induced artifacts. The SD served as quantitative marker of image noise. Signal intensity (SI) (ie, CT number) measurements were determined in three 2 2 homogeneous regions of interest measuring about 1.0cm (± 0.5cm ) in the main pulmonary artery. On the basis of these numbers the average main pulmonary artery signal intensity (SIMPV) was calculated. The measurement of background noise was based on the standard deviation (in Hounsfield units) of three 1cm² regions of interest in front of the patient (central, left and right). These values were averaged to obtain the final background noise (BN). In addition, attenuation of the paraspinal muscles were measured on both sides and averaged (muscle SI). Based on these measurements the signal to noise ratio (SNR) and contrast to noise ratio (CNR) were calculated using the following equations: SNR = SIMPV/BN Page 5 of 32
CNR = (SIMPV # muscle SI)/BN where SIMPV is mean SI of main pulmonary artery, muscle SI is the mean signal intensity at the paraspinal muscle and BN is background noise. (c) Analysis of segmental arteries Analysis of segmental arteries was done on transverse images only. We used a CT angiography window setting (width, 450 HU; level, 100 HU) and a lung window setting (width, 1500 HU; level, -500 HU) for this analysis. The arteries were named according to the nomenclature of Remy-Jardin et. al. as described by Boyden, and according to the nomenclature of Jackson and Huber. This nomenclature assigns 10 segmental arteries on the right and nine segmental arteries on the left. Therefore, a total of 19 segmental arteries could be expected per patient if no anatomic variant was present. For each patient, we then calculated the percentage of analyzable segmental arteries. Subjective evaluation Two senior radiologists with more than 10 years of experience performed consensus interpretation of the CT images. Images were presented to the observers who were blinded to the patient and scan data in random order. After reviewing several previous studies, the reviewers predetermined the pulmonary artery settings based on consensus and used a window width between 1200-1400 and widow level between 400-700 in order to compensate for increased attenuation within the pulmonary arteries at low kilovoltage setting The mediastinum, liver and lungs were assessed in the mediastinal (WW450 WL100) and lung (WW1500; WL-500) window settings. The following parameters were assessed: (a) Image quality The images were subjectively analyzed for making a diagnosis other than PE and scored by using a five-point scale ranging from 1 to 5 as follows: (Figure 3) a score of 1 : very bad image quality that no diagnosis can be made Page 6 of 32
a score of 2: low image quality that reduced the confidence in making the diagnosis a score of 3: moderate image quality sufficient to make a diagnosis a score of 4: good image quality a score of 5: excellent image quality that enabled excellent differentiation of even small structures Subsequently, image quality for selected anatomic areas such as the mediastinum, the lungs, the main pulmonary artery, and the liver parenchyma were assessed separately. (b) Pulmonary artery contrast enhancement The degree of contrast enhancement was also graded by using a five-point scale (Figure 3), and the subjective contrast enhancement score was determined by evaluating the main pulmonary arteries. After reviewing several previous studies, the reviewers predetermined the pulmonary artery settings based on consensus and used a window width between 1400-1600 and window level between 400-700 in order to compensate for increased attenuation within the pulmonary arteries at low kilovoltage settings (c) Image noise and Motion artifacts The degree of presence and effect of image noise and motion artifacts were similarly graded using a five-point scale (Figure 3). The different anatomical areas were combined to rate the image noise and presence of motion artifacts only once per examination. Radiation Dose Evaluation: The Computed Tomography weighted Dose Index (CTDIvol) is preset by the CT scanner based on measurement of the selected pitch value which was 5.74 for Group A and 7.01 for Group B. The system then determines the scan length from the topogram and combines it with the CTDIvol to calculate the dose length product (DLP) in each patient respectively. The DLP is a measure of the radiation dose delivered to that patient during the scan. Estimation of the effective dose was performed based on the DLP which was converted using a standardized conversion factor of 0.017mSv/mGy for chest CT. Page 7 of 32
Estimated Effective dose = Dose Length Product (DLP) / 0.017 In addition, the Effective radiation doses values for males and females were calculated according to International Commission of Radiation Protection (IRCP). This value was evaluated using a phantom during the first phase of our study. Page 8 of 32
Images for this section: Fig. 0: Flow chart on study design Department of Bio-Imaging, University Malaya Medical Center - Kuala Lumpur/MY Page 9 of 32
Fig. 0: Flow chart on data analysis Department of Bio-Imaging, University Malaya Medical Center - Kuala Lumpur/MY Page 10 of 32
Fig. 0: Five-Point Scales for Subjective Rating of Image Quality Characteristics Schueller-Weidekamm C et al.(2006) CT angiography of pulmonary arteries to detect pulmonary embolism: improvement of vascular enhancement with low kilovoltage settings.radiology;241(3):899-907 Page 11 of 32
Results Study Groups. There were 66 patients used in this study; 25 (36%) were male and 41 (59%) were females with age group ranging from 19 to 87 years with a mean of 52.77. Comparison of the two patient groups did not reveal any significant differences in patients age, sex, amount of extrathoracic soft tissue, AP and transverse diameters of the chest as well as in the incidence of PE. CT evaluation of the pulmonary arteries demonstrated PE in 11 patients (15%). (Figure 1). The mean age in group A was 50.8 years ± 18.9 [range, 17-86 years], while mean age in group B was 54.8 years ± 17.4 [range, 25-86 years]; (P =.372, t test) or sex (there were 13 male patients in group A vs 12 male patients in group B, while there were 20 females in group A vs 21 female patients in group B; P =.250, t test). The mean body weight in group A was 58.2Kg ± 7.1, while mean body weight in group B was 60.4Kg ± 5.6; indicating no significant difference in the body weight between the two groups (P =.167, t test). There was also no significant difference noted between the antero-posterior and transverse diameters of the chest as well as the skin to rib thickness between the two protocols. (Figure 1) Phantom Analysis We matched 70, 80, 90, 100, 115, 120, 125, 130 and 135mAs with the low dose protocol, and concluded that an effective mas of 115 was ideal for the 100kVp protocol as it did not significantly affect the image noise. It also provided a 17.5% reduction in radiation dose in the 100kVp protocol using the 120kVp protocol as a standard (Figure 2) Patient Analysis 1. Quantification Analysis (a) Quantification of Vascular Enhancement Page 12 of 32
Both central pulmonary arteries (measured in the main pulmonary artery) and peripheral arteries showed significantly higher attenuation with the 100-kVp protocol than with the standard 120-kVp protocol as shown in Figure 3 and Figure 4. All differences were significant at a level of P <.001. The average attenuation in the main pulmonary artery was 486 HU ± 112.9 (95% confidence interval [CI]: 70, 172 HU) in group A and 347.3HU ± 93.1 (95% CI: 60, 172 HU) in group B; P <.001, t test. In the peripheral pulmonary arteries at the level of the aortic arch, the maximum attenuation was 503.4HU ± 147.0 (95% CI: 45, 162 HU) for group A and 399.5HU ± 82.2 (95% CI: 45, 162 HU) for group B; P <.001, t test. In peripheral pulmonary arteries at the lung base, the maximum attenuation was 481.6HU ± 98.9 (95% CI: 59, 148 HU) for group A and 377.6HU ± 79.8 (95% CI: 59, 148 HU) for group B; P <.001, t test. (b) Image Noise Image noise measured at the level of the main pulmonary artery was 25.7 HU ± 6.3 for group A and 14.2 HU ± 3.0 for group B. The difference was statistically significant (P = <.001). (Figure 5) Background noise increased from 24.2HU ± 10.2(Group B) to 27.0HU ± 9.8 (Group A), which was not a significant difference (P =.28). There was no significant difference between the patient groups A (100kVp) and B(120kVp) respectively for SNR (18.6HU ± 8.3 vs 16.5HU ± 7.1; P =.26) however there was significant increase in the CNR between the two groups (440.5HU ± 110.6 vs 350.2HU ± 101.5; P = <.001) which was an outcome that was generally expected with the low kvp protocol. (Figure 5) (c) Analysis of Segmental Arteries A total of 792 segmental arteries (396 each in group A and group B:) were analyzed. The percentage of segmental arteries that were considered to have sufficient quality for assessment of PE did not significantly differ between group A (mean, 91% ± 15) and group B (mean, 91% ± 7) (P =.85, Mann-Whitney U test). Figure 6 demonstrates filling defects within the segmental branches in both the protocols. Page 13 of 32
2. Subjective Grading of Image Quality There was no significant difference of inter-observer variability between the two protocols (Figure 7). However a consensual agreement was made and all results were subsequently obtained as follows: (a) Image quality Subjective scores for image quality were lower for the 100-kVp images than for the 120kVp images, while the vascular enhancement was higher in the 100-kVp protocol but the difference between the average scores were not significant (Figure 8). Differences in subjective scoring did not reach significance for any of the five anatomic areas (main pulmonary artery: P =.43; lungs: P =.02; mediastinum: P =.13; liver 2 parenchyma: P =.18; t test). (Figure 9) None of the scans was considered to have such low image quality that it would interfere with diagnosis (grades 1 or 2) for the anatomic regions evaluated(figure 8). (b) Pulmonary artery contrast enhancement The 100-kVp scans were rated as having significantly superior contrast enhancement (mean score, 4.6 vs 3.9; P = <.001). (Figure 9) (c) Image noise and motion artifacts There was no significant difference for the subjective grading of image noise (mean score, 4.6 vs 4.6; P =.659) or of motion artifacts (mean score, 4.8 vs 4.8; P =.804) for the 100kVp and 120-kVp images, respectively. (Figure 9) Radiation Exposure Page 14 of 32
Concerning radiation exposure, the preset CTDIvol (mgy) values given by the scanner were 5.74 mgy for Group A and 7.01 mgy for Group B. There was a significant difference noted in the DLP and estimated effective dose between Group A and Group B; Dose length product (186.3 mgy x cm ± 21.8 vs 247.2 mgy x cm ± 60.5, respectively; p<.001), and estimated effective dose (3.16 msv ± 0.38 vs 6.79 msv ± 1.40, respectively (P <.001). The latter corresponds to an average reduction of estimated effective dose of approximately 53% (Figure 10). Page 15 of 32
Images for this section: Fig. 0: Descriptive statistics of demographic and morphologic data of 66 consecutive patients examined at 100kVp(Group A) and 120kVp(Group B) CT Pulmonary Arteriograms Department of Bio-Imaging, University Malaya Medical Center - Kuala Lumpur/MY Page 16 of 32
Fig. 0: Phantom study where the 120kVp protocol was refered to as a standard to get an appropriate mas for the 100kVp(low dose) protocol without significantly affecting the image quality. We concluded that 115 mas was the ideal for the 100kVp(low dose) protocol as it did not significantly affect the image quality and at the same time reduced the radiation dose by 17.5%. Department of Bio-Imaging, University Malaya Medical Center - Kuala Lumpur/MY Page 17 of 32
Fig. 0: Contrast enhanced CT pulmonary arteriogram in mediastinal window setting(ww 400 ; WL 100) demonstrating the comparison of vascular enhancement at the level of main pulmonary artery (A) showing enhancement of 310.75HU that is close to the average in 120kVp group and (B) showing a much higher enhancement of 513.49HU that was noted in the 100kVp protocol. Circles indicated the ROI of 1.0cm² (±0.5) that was used to measure the average CT number at the main pulmonary artery. Department of Bio-Imaging, University Malaya Medical Center - Kuala Lumpur/MY Page 18 of 32
Fig. 0: Box plot demonstrating the average attenuation in the main pulmonary artery is significantly higher in the 100kVp low dose protocol as compared to the 120kVp standard protocol Department of Bio-Imaging, University Malaya Medical Center - Kuala Lumpur/MY Page 19 of 32
Fig. 0: There was a significant increase in image noise (standard deviation at the level of the main pulmonary artery) in the 100kVp protocol as was expected with a lower kvp. However, there was no significant difference in the SNR while there was a significant difference in the CNR between the two protocols. This result was expected as different behavioural pattern of iodine within the vessels and surrounding soft tissue should lead to a higher contrast between these two structures thereby increasing the CNR. Department of Bio-Imaging, University Malaya Medical Center - Kuala Lumpur/MY Page 20 of 32
Fig. 0: Contrast enhanced CT pulmonary studies in Axial sections at the level of the main pulmonary artery in two patients with pulmonary embolism demonstrating the comparison of image quality at the central and peripheral pulmonary arteries in (A) the low dose 100kVp protocol and (B) in the standard dose 120 kvp protocol. Note the emboli in the lower lobe segmental arteries (arrows) Department of Bio-Imaging, University Malaya Medical Center - Kuala Lumpur/MY Page 21 of 32
Fig. 0: Demonstration of the inter-observer variability for subjective image quality Department of Bio-Imaging, University Malaya Medical Center - Kuala Lumpur/MY Page 22 of 32
Fig. 0: Demonstrating the difference in subjective image quality by consensus between the two protocols. Department of Bio-Imaging, University Malaya Medical Center - Kuala Lumpur/MY Page 23 of 32
Fig. 0: Statistical analysis of subjective image evaluation between the two protocols Department of Bio-Imaging, University Malaya Medical Center - Kuala Lumpur/MY Page 24 of 32
Fig. 0: The preset CTDIvol(mGy) values were given by the scanner. There was a significant difference noted in the DLP and estimated effective dose between Group A and Group B, indicating a significant dose reduction in the low dose 100kVp protocol. Department of Bio-Imaging, University Malaya Medical Center - Kuala Lumpur/MY Page 25 of 32
Conclusion Use of a 100 kvp setting in the CTPA examination results in a significant reduction in the radiation exposure whilst increasing the iodine attenuation level. This actually increases pulmonary artery enhancement Although the image noise was increased, the subjective and objective image evaluation did not demonstrate a significant decrease in image quality between the 100kVp(low dose) and 120kVp (standard dose) protocols. Figures 1 and 2 demonstrate the image quality and vascular enhancement in the 100kVp and 120kVp protocols in two separate patients respectively who were positive for pulmonary embolism. Future prospects of this study Beneficial in other vascular CT studies as well as in the dual source CT scanners because of increase in the attenuation of the vessels with the low kvp protocol. In developing countries like ours, where 16 slice multidetector scanners are still widely used, this study may be beneficial in improving the efficacy of the available scanners as well as reducing the radiation dose. Page 26 of 32
Page 27 of 32
Images for this section: Fig. 0: Contrast enhanced CT pulmonary arteriogram in pulmonary artery window (WW 1600 ; WL 700) in a 58 year old male with history of sudden onset shortness of breath demonstrating filling defects at the segmental arteries (arrows) pulmonary arteries in the 100 kvp protocol in (A) axial sections and (B) coronal sections. Department of Bio-Imaging, University Malaya Medical Center - Kuala Lumpur/MY Page 28 of 32
Fig. 0: Contrast enhanced CT pulmonary arteriogram in pulmonary artery window (WW 1600 ; WL 700) in a 42 year old male with history of cough and breathlessness demonstrating filling defects at the segmental arteries (arrows) pulmonary arteries in the 120 kvp protocol in (A) axial sections and (B) coronal sections. Department of Bio-Imaging, University Malaya Medical Center - Kuala Lumpur/MY Page 29 of 32
References 1. Kalra MK, Prasad S, Saini S, et al. Clinical comparison of standard-dose and 50% reduced-dose abdominal CT: effect on image quality. AJR Am J Roentgenol 2002; 179:1101-1106. 2. Claudia Schueller-Weidekamm, et al, CT Angiography of Pulmonary Arteries to Detect Pulmonary Embolism: Improvement of Vascular Enhancement with Low Kilovoltage Settings Radiology 2006; 241: 899-907 3. Christoph M. Heyer, et al. Image Quality and Radiation Exposure at Pulmonary CT Angiography with 100- or 120-kVp Protocol: Prospective Randomized Study; Radiology: Volume 245: Number 2-November 2007 4. Takahashi M, Maguire WM, Ashtari M, et al. Low-dose spiral computed tomography of the thorax: comparison with the standard-dose technique. Invest Radiol 1998; 33:68-73. 5. Diederich S, Lenzen H, Windmann R, et al. Pulmonary nodules: Experimental and clinical studies at low-dose CT. Radiology 1999; 213:289-298. 6. Szucs-Farkas.Z. et al in 2009. Detection of Pulmonary Emboli With CT Angiography at Reduced Radiation Exposure and Contrast Material Volume: Comparison of 80 kvp and 120 kvp Protocols in a Matched Cohort. 7. Wintersperger B, Jacobs T, Herzog P, et al. Aorto-iliac multidetector-row CT angiography with low kv settings: improved vessel enhancement and simultaneous reduction of radiation dose. Eur Radiol 2005;15(2):334-341. 8. Prasad SR, Wittram C, Shepard JA, McLoud T, Rhea J. Standard-dose and 50%- reduced-dose chest CT: comparing the effect on image quality. AJR Am J Roentgenol 2002; 179:461-465 9. Wintermark, M., P. Maeder, et al. (2000). "Using 80 kvp versus 120 kvp in perfusion CT measurement of regional cerebral blood flow." AJNR Am J Neuroradiol 21(10): 1881-4. 10. Wintersperger, B., T. Jakobs, et al. (2005). "Aorto-iliac multidetector-row CT angiography with low kv settings: improved vessel enhancement and simultaneous reduction of radiation dose." Eur Radiol 15(2): 334-41. 11. Boyden EA. Segmental anatomy of the lungs. New York, NY: McGraw-Hill, 1955 12. Jackson CL, Huber JF. Correlated applied anatomy of the bronchial tree and lungs with a system of nomenclature: diseases. Chest 1943;9:319-326 Page 30 of 32
Personal Information Dr Maninderpal Kaur Gill, MBBS, Associate Professor Dr. Anushya Vijayananthan, MBBS, M Rad, Professor Dr. Gnana Kumar, MBBS, MMed (RAD), FRCR, Dr. Kasthoori Jayarani, MBBS, M Rad, FRCR, Professor Dr. Ng Kwan Hoong, PhD, FIPM, MIPEM, DABMP. Department of Biomedical Imaging, University of Malaya Kuala Lumpur, Malaysia email: mindy_gill@yahoo.com Page 31 of 32
Images for this section: Fig. 0 Department of Bio-Imaging, University Malaya Medical Center - Kuala Lumpur/MY Page 32 of 32