Coronary Artery Bypass Graft Imaging with CT Angiography and Iterative Reconstruction: Quantitave Evaluation of Radiation Dose Reduction and Image Quality Simon Nepveu 1, Irina Boldeanu 1, Yves Provost 1, Jean Chalaoui 1, Louis-Mathieu Stevens 2,3, Nicolas Noiseux 2,3, Carl Chartrand-Lefebvre 1,3 1 Radiology, 2 Surgery, 3 Research Center: University of Montreal Medical Center CAR 2015 Montreal, Canada
Disclosures Carl Chartrand-Lefebvre: Fonds de Recherche Santé Québec (FRQS): Réseaux de Bioimagerie du Québec (RBIQ) et SIDA-MI; Instituts de Recherche en Santé du Canada (IRSC); Research Collaboration with Philips Healthcare, Terarecon, Bayer Canada. CHUM: Equipment loan by Philips Healthcare and Bayer Canada (CCL).
Background Invasive Catheter Angiography vs Computed Tomography (CT) Although catheter angiography has long been the diagnostic tool of choice, computed tomography angiography is increasingly employed. In 2009, 227 CT scans were performed / 1000 habitants, for 69 million scans per year. 1 CT is less invasive, less costly and more rapid. Efficiency: a. Detection of significant stenosis in coronary artery bypass grafts (CABG): Sensibility: 94% Specificity: 98% b. Detection of occlusion in CABG: Sensibility: 99%, Specificity: 99% 2 However, CT scans have been associated with some of the largest radiation doses per examination, reaching almost 15 msv/scan. 3 1. OECD 2011. Health at a Glance 2011, OECD Publishing. 2. Hamon, M., et al. (2008). Diagnostic accuracy of in-stent coronary restenosis detection with multislice spiral computed tomography: a metaanalysis. Eur Radiol, 18(2), 217-225. 3. METTLER, F. A., JR., et al. 2008. Effective doses in radiology and diagnostic nuclear medicine: a catalog. Radiology, 248, 254 63.
Background Different reconstruction algorithms can be employed to reconstruct CT scan images.
Aim To assess radiation dose reduction achieved by reducing tube current along with the use of an hybrid IR algorithm (idose 4, Philips Healthcare, Cleveland, OH, USA). To compare image quality between IR and FBP reconstruction.
Method - Design Standard Dose Protocol (n=25) 50 patients with CABG (IMA *, sephenous veins) Standard Tube Current FBP Reconstruction Low Dose Protocol (n=25) 41 evaluated grafts - 24 IMA - 17 Saphenous veins 30% Lower Tube Current Iterative Reconstruction (idose 4, level 3) Noise reduction Factor: 0.78 41 evaluated grafts - 24 IMA - 17 Saphenous veins IMA: Internal Mammary Artery Prospective exposure-based sampling Technologists could use less than 30% decrease in tube current after subjective patient overweight estimation. Analysis included IMA grafts to the anterior territory and aortocoronary saphenous vein grafts to the right coronary artery. A total of 82 CABGs were evaluated; 240 graft segments (138 IMA and 102 saphenous vein segments) were assessable.
Method CT protocol CT scanner specifications: 256-slice MDCT (Brilliance ict, Philips Healthcare) Prospective ECG-gating Scanning voltage: 120kV Gantry speed rotation: 270ms Collimator: 2 x 128 x 0.625 mm Scanning coverage extended above the origin of the IMA. Noise, signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) measurements were obtained from the lumen of the proximal, body, and distal segments of the grafts. Post-treatment reconstruction platform: Aquarius Intuition, Terarecon.
Method CT protocol Segmentation of grafts Grafts were separated in three segments to allow image quality evaluation: A. Proximal segment B. Body (middle segment) C. Distal segment Measurements for the proximal and distal segments were taken within the first and last cm of observed graft, respectively. The choice of region-of-interest (ROI) positioning was done as to obtain a plane as parallel as possible to the short axis of the graft without overriding the vessel lumen. For the body of the graft, ROI was chosen halfway from the proximal and distal anastomoses. Graft ranging from the internal mammary artery (IMA) to the left anterior descending artery (LAD).
Method - Image analysis Four quantitative criteria were used: Atenuation Level and Noise Level; Measured as mean attenuation level and associated standard deviation within ROIs respectively, both measured in Hounsfield Units (HU). Signal-to-noise ratio (SNR) Contrast-to-noise ratio (CNR) Measured as: Segment signal - Perivascular signal 2 2 ( + Perivascular noise ) 1/ 2 Segment noise ROI positioned in the center of the lumen of the graft segment. Saphenous Vein Proximal Segment
Method - Image analysis Noise Level: CNR: Segment signal - Perivascular signal 1/2( Segment noise 2 + Perivascular noise 2 ) Noise - Noise ++ Graft Perivascular
Results - Patient and scan characteristics Base Parameters Filtered back Projection Iterative Reconstruction Number of Patients 25 25 P-value* Men (Women) 23(2) 19(6) Body Mass Index (BMI) (kg/m 2 ) 26.61 ± 3.77 29.32 ± 3.75 p=0.014 Mean thoracic coverage (cm) 25.53 ± 2.33 25.03 ± 2.21 p=0.441 Mean metoprolol dose (ml) 71.15 ± 17.22 65 ± 12.91 p=0.399 Mean heart rate (bpm) 58.04 ± 6.70 55.92 ± 8.06 p=0.317 Mean current (ma) 880.24 ± 70.38 689.12 ± 123.71 p<0.001 *Student T-Test BMI between groups was found to be significantly superior in patients undergoing iterative reconstruction as opposed to FBP. Mean current used was significantly higher in the FBP group as opposed to the IR group by 21.71%.
Results - Radiation dose Radiation Dose Evaluation Standard Dose Protocol (FBP) Reduced Dose Protocol (IR) P-value* Effective Dose (msv) 9.90 ± 1.4 7.60 ± 1.3 p<0.001 *Student T-Test The effective radiation dose for the group of patients undergoing the IR protocol was 7.60 msv, ie 23% lower when compared to the group undergoing the FBP protocol.
Results Image quality Graft attenuation Attenuation Level (HU) 490 470 450 430 410 390 370 350 Proximal Segment 460.59 440.63 IMA Graft 448.42 408.38 419.37 383.21 Body Segment Distal Segment Iterative Reconstruction (Low Dose) FBP Reconstruction (Standard Dose) Attenuation Level (HU) 490 470 450 430 410 390 370 350 Proximal Segment Saphenous Vein Graft -12.98% p=0.038 478.1 474.42 459.14 419.92 Body Segment 463.36 441.44 Distal Segment Student T-Test with p<0.05 as significance level No significant difference was observed between attenuation levels for the standard dose protocol (and FBP) as opposed to the reduced dose protocol (and IR).
Results Image quality Graft noise IMA Graft -28.21% -20.24% -27.79% p<0.001 p=0.013 p<0.001 Saphenous Vein Graft -29.5% p=0.040 Noise (HU) 55 50 45 40 35 30 25 20 Proximal Segment 50.54 36.28 42.98 34.28 Body Segment 38.15 27.55 Distal Segment Iterative Reconstruction (Low Dose) FBP Reconstruction (Standard Dose) 50 40 30 41.61 29.31 Noise (HU) 60 20 37.8 36.64 32.53 30.34 Proximal Segment Body Segment Distal Segment Student T-Test with p<0.05 as significance level 172 segments (72%) of graft segments presented a decrease in noise when using the low dose protocol with IR (p 0.040). This decrease ranges from 20 to 29%. No graft segment presented increased noise levels when using the low dose protocol with IR as opposed to FBP.
Results Image quality Graft SNR IMA Graft +36.96% +48.12% +63.37% p=0.001 p=0.001 p<0.001 Saphenous Vein Graft +56.51% +28.65% p<0.001 p=0.013 Ratio 20 18 16 14 12 10 8 Proximal Segment 13.08 14.59 9.55 9.85 Body Segment 16.99 10.4 Distal Segment Iterative Reconstruction (Low Dose) FBP Reconstruction (Standard Dose) Ratio 20 18 16 14 12 10 8 18.86 15.76 15.86 12.05 12.25 12.71 Proximal Segment Body Segment Distal Segment Student T-Test with p<0.05 as significance level 206 segments (86%) of graft segments presented an increase in SNR when using the low dose protocol with IR (p 0.010), ranging from 28 to 63%. No segment presented a decrease in SNR when using the low dose protocol with IR.
Results Image quality Graft CNR IMA Graft +35.09% +45.53% +64.49% p<0.001 p<0.001 p<0.001 Saphenous Vein Graft +32.82% +27.31% p=0.031 p=0.021 Ratio 20 18 16 14 12 10 8 Proximal Segment 14.63 10.83 19.41 17.26 11.86 11.8 Body Segment Distal Segment Iterative Reconstruction (Low Dose) FBP Reconstruction (Standard Dose) Ratio 20 18 16 14 12 10 8 19.67 18.83 17.08 14.81 14.62 14.79 Proximal Segment Body Segment Distal Segment Student T-Test with p<0.05 as significance level 172 segments (72%) of graft segments presented an increase in CNR when using the low dose protocol with IR ranging (p 0.030), ranging from 27 to 64%. No segment presented a decrease in CNR when using the low dose protocol with IR.
Discussion In this study, the use of a hybrid IR algorithm with a decreased current protocol enabled a 23% reduction of the effective dose (7.6 ± 1.3 msv), compared with FBP and standard dose protocol (9.9 ± 1.4 msv) (p < 0.001), with no loss in image quality. In the IR patient group, we have chosen to reduce the current by a 30 % factor, except for patients who were overweight. It is worth noticing that other dose reduction strategies also exist, such as the use of a reduced kilovoltage, more aggressive current reduction, and size-dependent protocols. All of these strategies could also have been assessed, together with IR. Image quality in our study was assessed using quantitative parameters (attenuation, noise, SNR, CNR). Although all CT examinations in this study were of diagnostic quality, visual assessment of image quality, using an ordinal scale, could also have been used. It is interesting to note that improvements in image quality were observed in the IR and reduced current protocol patient group although the BMI was significantly higher in this group, in comparison to the FBP and standard dose protocol patient group. Effective dose is proportional to the z-axis coverage. This explains the higher dose when CT is performed for CABG evaluation, in comparison to native coronary arteries. Z-axis coverage was similar in our two patient groups.
Conclusion Our results show that iterative reconstruction enabled to decrease effective radiation dose by a 23 % factor in 256-MDCT angiography of CABGs while providing superior or similar image quality compared with filtered-back projection.