Original paper Spectral optimization of iodine-enhanced CT: Quantifying the effect of tube voltage on image quality and radiation exposure determined at an anthropomorphic phantom Gunnar Brix PhD a,⇑ , Ursula Lechel MS a , Sonja Sudarski MD b , Christoph Trumm MD c , Thomas Henzler MD b a Department of Medical and Occupational Radiation Protection, Federal Office for Radiation Protection, Oberschleissheim, Germany b Institute of Clinical Radiology and Nuclear Medicine, University Medical Center Mannheim, University of Heidelberg, Mannheim, Germany c Department of Clinical Radiology, Grosshadern Clinic, Hospital of the Ludwig-Maximilians University, Munich, Germany article info Article history: Received 16 June 2016 Received in Revised form 14 July 2016 Accepted 25 July 2016 Available online 30 July 2016 Keywords: Contrast-enhanced CT Spectral optimization Image quality Patient exposure abstract Purpose: To provide an experimental basis for spectral optimization of iodine-enhanced CT by a quanti- tative analysis of image quality and radiation dose characteristics consistently measured for a large vari- ety of scan settings at an anthropomorphic phantom. Methods: CT imaging and thermoluminescent dosimetry were performed at an anthropomorphic whole- body phantom with iodine inserts for different tube voltages (U, 70–140 kV) and current-time products (Q, 60–300 mAs). For all U-Q combinations, the iodine contrast (C), the noise level (N) and, from these, the contrast-to-noise ratio (CNR) of reconstructed CT images were determined and parameterized as a func- tion of U, Q or the measured absorbed dose (D). Finally, two characteristic curves were derived that give the relative increase of CNR at constant D and the relative decrease of D at constant CNR when lowering U. Results: Lowering U affects the measured CNR only slightly but markedly reduces D. For example, reduc- ing U from 120 kV to 70 kV increases the CNR at constant D by a factor of nearly 1.8 or, alternatively, reduces D at constant CNR by a factor of nearly 5. Conclusion: Spectral optimization by lowering U is an effective approach to attain the necessary CNR for a specific diagnostic task at hand while at the same time reducing radiation exposure as far as practically achievable. The characteristic curves derived in this study from extensive measurements at a reference ‘person’ can support CT users in an easy-to-use manner to select an appropriate voltage for various clin- ical scenarios. Ó 2016 Associazione Italiana di Fisica Medica. Published by Elsevier Ltd. All rights reserved. 1. Introduction Contrast agents (CA) are frequently used in clinical computed tomography (CT) primarily for three reasons: to improve the intrinsically poor soft tissue contrast of CT images for better con- spicuity of pathological soft tissue alterations and thereby diagnos- tic yield (anatomical CT), to visualize abnormalities and occlusions of arterial and venous vessels (CT angiography and venography) and to characterize tissue physiology and function (dynamic contrast-enhanced CT) [1]. Accordingly, contrast-enhanced CT studies relate to structures with both low (soft tissues) and high (vessels) CA uptake in almost every part of the human body. For CT patient studies, mainly non-specific tri-iodinated water- soluble compounds are used as CAs. Apart from chemical, pharma- cological and toxicological advantages [2], the current preference for iodine as a radio-opaque element is due to its high atomic num- ber (Z = 53) and K-edge at 33.2 keV resulting in a high efficacy to absorb X-rays with energies somewhat above the K-edge due to an increased photo effect. The energy spectrum of photons emitted by the X-ray tube of a CT system can be controlled by the tube volt- age, U. Decreasing U shifts both the cutoff and mean energy of the X-ray spectrum closer to the K-edge and thus improves the iodine contrast, C, as demonstrated by Keller et al. as early as 1980 [3]. For long, the limited output of X-ray tubes prohibited the use of low- voltage CT. Some twenty years later, the advantages of low- voltage CT were confirmed by a number of clinical studies e.g., [4–8]. However, lowering U is inevitably accompanied by an increase in image noise, N, since the detector dose decreases http://dx.doi.org/10.1016/j.ejmp.2016.07.618 1120-1797/Ó 2016 Associazione Italiana di Fisica Medica. Published by Elsevier Ltd. All rights reserved. ⇑ Corresponding author at: Abteilung für medizinischen und beruflichen Strahlenschutz, Bundesamt für Strahlenschutz, Ingolstädter Landstraße 1, D- 85764 Oberschleissheim, Germany. E-mail address: gbrix@bfs.de (G. Brix). Physica Medica 32 (2016) 999–1006 Contents lists available at ScienceDirect Physica Medica journal homepage: http://www.physicamedica.com