Computed Tomography Angiography of Coronary Artery Bypass Grafts: Low Contrast Media Volume Protocols Adapted to Tube Voltage

Abstract

Objective: The aim of this study was to evaluate the potential of contrast media (CM) reduction in computed tomography angiography (CTA) of coronary artery bypass grafts (CABGs) when adapting CM volume to automatically selected tube voltages.

Material and methods: Sixty consecutive patients (mean age, 71 ± 14.5 years) with a total of 176 CABGs (692 bypass segments) underwent contrast-enhanced prospectively electrocardiography-gated high-pitch CTA with automated, attenuation-based tube voltage selection (100 ref. peak kilovoltage [kVp], 200 ref. mAs, tube voltages from 70-150 kVp in 10-kVp steps) using a third-generation 192-slice dual-source computed tomography scanner. Volume and flow of CM (370 mg/mL iodine) was adapted according to the tube voltages using iodine attenuation-curves derived from a foregoing phantom study. In patients, CM volumes ranged from 80 mL (flow rate, 7 mL/s) at 120 kVp to 48 mL (flow rate, 4.2 mL/s) at 80 kVp. Two independent, blinded readers evaluated subjective image quality of the proximal anastomosis, bypass graft, distal anastomosis, and postanastomotic native coronary artery using a 3-point Likert scale. Objective image quality (attenuation of graft and noise) was determined and contrast-to-noise ratio (CNR) was calculated. Volume computed tomography dose index and dose-length product of each CTA examination were noted. Cohen κ was used to define interreader agreement of subjective image quality. Regression analysis was used to determine relationships between tube voltage and vascular attenuation, image noise, and CNR.

Results: Using attenuation-based tube voltage selection, 5 patients (8%) were scanned at 80 kVp, 22 (37%) at 90 kVp, 11 (18%) at 100 kVp, 10 (17%) at 110 kVp, and 12 (20%) at 120 kVp. Agreement in subjective image quality between readers was good (κ = 0.678). Diagnostic image quality was achieved in 679 of 692 (98%) bypass segments in 169 of 176 bypass grafts (96%). Thirteen of 692 bypass segments (2%) in 7 of 176 bypass grafts (4%) were rated as nondiagnostic because of severe artifacts caused by motion or beam hardening (2 proximal anastomoses of sequential bypasses, 3 graft bodies, 5 distal anastomoses, and 3 postanastomotic coronary artery segments). Regression analysis revealed no significant relationship between the automatically selected tube voltages and objective image quality parameters (bypass graft attenuation: P = 0.315; noise: P = 0.433; and CNR: P = 0.168), indicating homogenous attenuation, noise, and CNR across tube voltage levels. Mean volume computed tomography dose index was 4.0 ± 0.9 mGy, and mean dose length product was 135.0 ± 29.6 mGy*cm.

Conclusion: Adapting CM protocols to automatically selected tube voltage levels allows for low-volume CM CTA examinations of CABG grafts with diagnostic image quality.