Purpose. Methods and Materials

Transcription

1 Comparison of iterative and filtered back-projection image reconstruction techniques: evaluation of heavily calcified vessels with coronary CT angiography Poster No.: C-1644 Congress: ECR 2011 Type: Scientific Paper Authors: M. Renker 1, U. J. Schoepf 1, M. Weininger 1, J. M. Kerl 2, R. Bauer 2, Keywords: DOI: T. J. Vogl 2, T. Henzler 1 ; 1 Charleston, SC/US, 2 Frankfurt a. Main/ DE Cardiac, Cardiovascular system, Computer applications, CT- Angiography, Catheter arteriography, Technology assessment, Obstruction / Occlusion, Calcifications / Calculi, Artifacts /ecr2011/C-1644 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. Page 1 of 11

2 Purpose Filtered Back Projection (FBP) has been the traditional method for CT image reconstruction. However, FBP has limitations for delineating structural contours in coronary CT angiography (ccta). Especially in the presence of high-density calcifications, detection and grading of coronary artery stenosis may be confined by blooming artifacts and image noise. 1,2 Iterative reconstruction has been proposed to be a superior alternative for CT image reconstruction, as it has been shown to reduce image noise, decrease artifacts, and allow radiation dose reduction for various CT applications 3-7 Yet, the effect on interpretation of heavily calcified coronary artery segments has not been assessed. Therefore, the aim of this prospective study was, to compare traditional FBP and iterative image reconstruction for the evaluation of heavily calcified arteries with ccta. Methods and Materials Patients We prospectively included ccta studies of 55 consecutive patients (35 men, 20 women; mean age 58±12 years) with an Agatston score of #400, indicative of substantial calcified plaque burden. Exclusion criteria were a history of contrast media reaction and impaired renal function (creatinine >1.5 mg/dl and/or GFR <60 ml/min). Scanning Technique Calcium scoring and ccta were performed on a 2 nd generation dual-source CT scanner. Prior to contrast-enhanced ccta, calcium scoring was performed in all patients using a prospectively ECG-triggered high-pitch spiral acquisition technique. 8 The ccta scanning technique was chosen individually for each patient depending on heart rate / rhythm and body mass index (BMI), with the goal of minimizing radiation exposure. Scan techniques included traditional retrospective ECG-gating with default use of ECG-dependent tube current Page 2 of 11

3 modulation, prospective ECG-triggering, and prospectively ECG-triggered high-pitch spiral acquisitions. Image Reconstruction Images were reconstructed from the ccta raw data with both FBP and iterative image reconstruction (Iterative Reconstruction in Image Space - IRIS, Siemens) (Figure 1 on page 4). FBP and iterative reconstructions were performed at a section thickness of 0.75 mm and a position increment of 0.4 mm. Corresponding vascular kernels were applied for FBP and iterative reconstructions (B26f and I26f, respectively). Image Noise and Subjective Image Quality Image noise was measured in each data set as the standard deviation of the measured Hounsfield units (HU) within a circular region of interest (ROI) in the ascending aorta (Figure 2 on page 4). In a blinded fashion, two radiologists independantly rated image quality according to the severity of image noise, quality of contour delineation, and general image impression by use of a 5-point Likert scale (1= poor; 2= fair; 3= moderate; 4= good; 5= excellent). Volumetric Analysis of Coronary Artery Calcifications A threshold-based volumetry tool (Volume Analysis, software version VE31A, Siemens) was used to determine the volumes of circumscribed coronary artery calcifications within FBP- and iterative reconstruction data sets. Contours of calcifications were semi-automatically segmented. To improve the results of the segmentation process, pixels with density values in the range of contrast medium attenuation measured in the ascending aorta were automatically excluded from the segmentation (Figure 3 on page 5). Comparison of ccta with Coronary Catheterization All data sets and reconstructions were evaluated by two radiologists in consensus for the presence of stenotic (>50%) coronary artery disease using the American Heart Association 15-segment model. 9 Readers were blinded to the reconstruction technique and reconstruction image series were presented in random order, at least 3 weeks apart in the same patient to minimize reader recall. Cardiac catheterization served as the reference standard for stenosis detection, which was then interpreted for >50% stenosis by two experienced cardiologists in consensus who were blinded to the results of ccta, using the same 15-segment AHA model. Page 3 of 11

4 Images for this section: Fig. 1: Flow chart demonstrating the principle image reconstruction steps with the iterarative reconstruction technique (IRIS: Iterative Reconstruction in Image Space, Siemens) used in this study. Page 4 of 11

5 Fig. 2: Image noise was measured as the Hounsfield unit standard deviation in a region of interest within the ascending aorta for Filtered Back Projection (FBP) and for iterative image reconstruction. Fig. 3: By use of a dedicated volumetric tool, volumes of circumscribed calcifications were quantified for Filtered Back Projection (FBP) and for iterative image reconstruction. Page 5 of 11

6 Results Image Noise and Subjective Image Quality Mean image noise measured significantly lower using iterative reconstruction than FBP (24.9±10.8 vs. 33.4±11.7; p=0.013) (Figure 1 on page 6). Image quality of ccta studies reconstructed with iterative reconstruction was rated significantly higher than of those reconstructed with FBP by both observers. Median image quality scores were 5 using iterative reconstruction compared with median image quality scores of 4 using FBP. Interobserver agreement was excellent for IRIS and FBP reconstructions (Kappa coefficients of 0.89 and 0.84 for IRIS and FBP, respectively). Volumetric Assessment of Coronary Artery Calcifications A total of 142 circumscribed coronary artery calcifications were volumetrically analyzed. Coronary artery calcifications showed significantly lower volumes in iterative reconstruction data compared to FBP (46.2±68.8 mm 3 vs. 56.3±72.5 mm 3, p=0.026)(figure 2 on page 7). Comparison of ccta with Coronary Catheterization A total of 825 coronary artery segments were analyzed with cardiac catheterization as well as with ccta. Cardiac catheterization showed stenoses >50% in 31 (56%) patients with a total of 104 lesions. Both on per segment and per patient analysis there was an incremental improvement in the detection of significant stenosis using iterative reconstruction image series over FBP compared with cardiac catheterization (Figures 3 on page 8 and 4 on page 9). Images for this section: Page 6 of 11

7 Fig. 1: Image noise comparison between iterative image reconstruction and Filtered Back projection (FBP). By use of iterative image reconstruction instead of FBP there was a significant image noise reduction. Page 7 of 11

8 Fig. 2: Volumetric quantification of circumscribed coronary artery calcifications with iterative image reconstruction and with Filtered Back Projection (FBP) resulted in significantly lower volumes by use of iterative image reconstruction. Page 8 of 11

9 Fig. 3: Comparison between Filtered Back Projection (FBP) and iterative image reconstruction regarding parameters of diagnostic accuracy on a per patient and on a per segment level for the detection of significant (>50%) stenosis in patients with highly calcified coronary arteries (Agatston score #400). Fig. 4: ccta study of a 55 year-old patient (Agatston score: 1194). Calcium blooming artifacts result in an apparently greater degree of stenosis in the FBP reconstruction, which led to misclassification of this stenosis as significant (>50%). Based on the iterative reconstruction, this lesion was read as non-significant by both observers. Subsequent coronary catheterization confirmed that this was in fact a 40% non-obstructive stenosis. Page 9 of 11

10 Conclusion Heavy coronary artery calcifications show significantly lower volumes at coronary CT angiography when iterative image reconstruction is used compared with traditional FBP reconstruction, indicating decreased blooming artifacts from calcified coronary atherosclerotic plaques. Compared with FBP, iterative image reconstruction improves accuracy, specificity, and positive predictive value of coronary CT angiography for the evaluation of coronary artery stenosis in patients with heavy vessel calcifications. An increase in the specificity and positive predictive value of coronary CT angiography with iterative reconstruction techniques could reduce the number of unnecessary follow-up studies performed as a result of false positive findings on coronary CT angiography in patients with heavily calcified plaques. References 1. Raff GL, Gallagher MJ, O'Neill WW, Goldstein JA. Diagnostic accuracy of noninvasive coronary angiography using 64-slice spiral computed tomography. J Am Coll Cardiol 2005; 46: Zhang LJ, Wu SY, Wang J, et al. Diagnostic accuracy of dual-source CT coronary angiography: The effect of average heart rate, heart rate variability, and calcium score in a clinical perspective. Acta Radiol 2010; 51: Bittencourt MS, Schmidt B, Seltmann M, et al. Iterative reconstruction in image space (IRIS) in cardiac computed tomography: initial experience. Int J Cardiovasc Imaging 2010; [Epub ahead of print] 4. Brooks RA, Di Chiro G. Theory of image reconstruction in computed tomography. Radiology 1975; 117: Flicek KT, Hara AK, Silva AC, Wu Q, Peter MB, Johnson CD. Reducing the radiation dose for CT colonography using adaptive statistical iterative reconstruction: A pilot study. AJR Am J Roentgenol 2010; 195: Page 10 of 11

11 6. Prakash P, Kalra MK, Ackman JB, et al. Diffuse lung disease: CT of the chest with adaptive statistical iterative reconstruction technique. Radiology 2010; 256: Thibault JB, Sauer KD, Bouman CA, Hsieh J. A three-dimensional statistical approach to improved image quality for multislice helical CT. Med Phys 2007; 34: Achenbach S, Marwan M, Ropers D, et al. Coronary computed tomography angiography with a consistent dose below 1 msv using prospectively electrocardiogramtriggered high-pitch spiral acquisition. Eur Heart J 2010; 31: Austen WG, Edwards JE, Frye RL, et al. A reporting system on patients evaluated for coronary artery disease. Report of the Ad Hoc Committee for Grading of Coronary Artery Disease, Council on Cardiovascular Surgery, American Heart Association. Circulation 1975; 51:5-40. Personal Information Matthias Renker, Medical Student Medical University of South Carolina 25 Courtenay Drive Charleston, SC 29425, USA matthias.renker@web.de Page 11 of 11