Introduction
Two expert reviews on hepatic applications of contrast-enhanced CT have been recently published in Acta Radiologica, a journal of the Scandinavian Societies of Radiology. The authors, all affiliated with radiological departments in Rome, Italy, discussed the use of contrast medium in liver CT in two back-to-back papers: one dedicated to technical issues [ 1 ] and the other to clinical protocols [ 2 ].

Technical parameters of the examination
The first article [ 1 ] examines various parameters of a CT examination that affect contrast enhancement and proposes strategies for their optimization while considering issues of radiation dose. The paper treats vascular enhancement separately from parenchymal enhancement, presumably because of the different needs of clinical applications, as explained in the second paper. Among the parameters considered are those defining the administration of contrast medium, properties of the contrast medium itself and scanner settings.

Starting from the presupposition that vascular enhancement depends principally on iodine flux (expressed in units of grams per second), the authors detail how to calculate several parameters of contrast medium administration. For arterial phase imaging, the authors propose an iodine flux of 1.6 g/s but also explain how this value can be adjusted according to the patient's total body weight and cardiac output. Once an appropriate iodine flux has been determined and given a certain iodine concentration of the contrast medium to be used, it is easy to calculate the injection flow rate. Then, given a desired injection duration, which - the authors explain - is the sum of the scanning time plus the delay before scanning starts (for angiographic acquisitions), one arrives easily at the required volume of contrast medium. Altogether, for a given iodine flux, as the iodine concentration of the contrast medium increases, the required injection flow rate decreases and so does the volume.

If, instead, parenchymal enhancement is important, then it is the total dose of iodine administered that must be optimized. The goal is to deliver a dose of iodine that provides tissue enhancement adequate for diagnostic needs without exposing patients to excessive amounts. Traditionally, this amount of iodine was calculated on the basis of total body weight, but recent research has shown that lean body weight (i.e. excluding adipose weight) is a better variable. Lean body weight may be measured or estimated clinically. This value is then entered into a formula for calculating the required iodine dose according to a desired maximum hepatic enhancement.

Besides these basic parameters of contrast medium administration, other issues that impact on contrast enhancement include the effects of contrast medium osmolarity and viscosity (important to consider but difficult to quantify in practice), the role of a saline flush (which reduces time to peak enhancement but has no effect on the magnitude of enhancement), and the scan timing. This latter parameter must be determined for each patient, using either a test bolus study or automatic bolus triggering software available on modern CT scanners. The remainder of the chapter reminds the reader about the importance of maintaining radiation dose as low as possible and highlights three scanning parameters – pitch, voltage and current – as keys in this endeavor. Automatic tube current modulation methods, which adjust current in accordance with the anatomy of the body area being imaged, are highlighted as important technological advances.

Clinical applications of liver CT and new CT technology
The second review [ 2 ] is rather equally divided between proposing liver imaging protocols for different clinical needs and discussing new liver imaging possibilities offered by the latest CT technology.
According to the authors, CT of the liver is indicated for three main clinical scenarios: chronic liver disease, focal lesions in otherwise healthy liver, and metastasis. As the purpose of imaging in these scenarios differs, so does the scanning protocol, in particular regarding the phases of image acquisition after contrast medium administration. In the interest of maintaining radiation exposure low, non-informative imaging phases are eliminated. Thus, in patients with chronic liver disease at risk of hepatocellular carcinoma, the late arterial, portal-venous and equilibrium phases of imaging are essential while the pre-contrast and early arterial phase scans are not and thus can be skipped. For characterizing focal lesions in otherwise healthy persons, late arterial and portal-venous phase images are required, while the equilibrium phase is used only if an incidentaloma is found. Instead, when patients with cancer have a first liver CT examination to search for the presence of metastases, a pre-contrast scan is appropriate while the choice of post-contrast imaging phases depends on the vascularity of the suspected lesion. Finally, for follow-up of these cancer patients, enough diagnostic information might be obtained with only the portal-venous phase. Choice of imaging phases must be combined with accurate selection of scanning parameters to achieve optimal enhancement at a low radiation dose.
The study of liver lesions is benefiting from recent advances in CT scanner hardware and software. New scanner configurations, using two or more X-ray sources, permit simultaneous imaging with photons of different energy levels and, thus, differentiation of tissues according to composition. The authors propose that dual-energy CT of the liver may help reduce radiation dose of single examinations and also improve visualization of tumor vascularization. In terms of software, new acquisition protocols for imaging over time, combined with algorithms for evaluating time-course CT data, are now permitting the study of tissue perfusion following the administration of contrast medium. Perfusion studies of the liver are having first applications in the assessment of tumor vascularity and response to treatment. How these new CT technologies will impact on the diagnosis and care of patients with liver disease will unfold in the coming years.

References

1. Rengo M., Bellini D., De Cecco C. et al. (2011) The optimal contrast media policy in CT of the liver. Part I: Technical notes. Acta Radiol. 2011; 52:467-472
2. Rengo M., Bellini D., De Cecco C. et al. (2011) The optimal contrast media policy in CT of the liver. Part II: Clinical protocols. Acta Radiol. 2011; 52:473-480