Computed tomography (CT) and magnetic resonance imaging (MRI) are commonly used for hepatic imaging in oncology patients. Both CT and MRI have advanced rapidly over the past 10 years as a result of improvements in computer hardware, software, and other electronic components. These changes have increased the accuracy of each technique in detecting and characterizing focal hepatic neoplasms. However, the improvements in both modalities have led to some confusion over their relative advantages.
Many studies have directly compared the diagnostic efficacy of MRI and CT in patients with focal hepatic malignancies.[1-18] These reports determined that, although the spatial resolution of CT remains superior, MRI has better contrast resolution and has repeatedly been proven to detect and characterize focal liver lesions with greater accuracy than CT.
Despite this obvious advantage, CT remains the primary tool for routine diagnostic evaluation of patients with known or suspected liver malignancies in most centers. The relative underutilization of MRI may be due, at least in part, to a lack of understanding of the advantages and disadvantages of each technique.
This article will review the current literature on state-of-the-art techniques for imaging the liver using both CT and MRI. Direct comparison studies of CT and MRI will be reviewed, and the relative advantages of each technique will be discussed. With this knowledge, the reader will be better able to choose an imaging study for evaluating oncologic patients with known or suspected liver disease.
Both MRI and CT of the liver have various relative advantages and disadvantages (Table 1). Careful consideration of these advantages and disadvantages is warranted before selecting the study that is most appropriate for an individual patient. In many cases, the choice may not be clear, and either study would be considered the standard of care. However, a closer examination will show that each has some fairly specific benefits and drawbacks.
In general, CT is less costly than MRI because it involves lower capital equipment costs and a shorter examination time. Computed tomography is also widely available and often does not have the same backlog of patients as does MRI. Most physicians are relatively comfortable with evaluating CT images themselves, although nearly all still rely on the interpretation of a diagnostic radiologist for a final diagnosis.
The major limitation associated with the use of hepatic CT studies is the need for iodinated contrast material. Both the conventional and the newer nonionic contrast agents are nephrotoxic, and, as a result, their use is restricted in patients with renal insufficiency.[19,20] Iodinated contrast is also associated with a relatively high rate of adverse reactions. In addition, CT uses ionizing radiation, the potential harm of which is poorly understood. Most significantly, studies have shown that CT is less sensitive and specific than MRI in detecting and characterizing focal hepatic diseasea fact that will be discussed in greater detail below.
Magnetic resonance imaging of the liver has several advantages over CT of the liver. Magnetic resonance imaging provides excellent contrast that can reveal subtle variations in tissues of differing histology. No ionizing radiation or nephrotoxic contrast media is used, and the most commonly administered contrast agents, extracellular gadolinium chelates, have very favorable safety profiles. Magnetic resonance images can be acquired in any orientation and, unlike CT scans, are not limited to the axial plane. Together, these advantages of MRI lead to better detection and characterization capabilities than are available with CT.
Magnetic resonance imaging also has some important drawbacks, however. In most regions, it is a more expensive examination than CT. There are fewer MRI scanners than CT scanners, which can result in scheduling difficulties. In general, the MRI examination takes longer and has more contraindications than a CT scan. Finally, MRI has more contraindications than does CT. These include, but are not limited to, the presence of a pacemaker or implanted device (such as a defibrillator or insulin pump), some aneurysm clips and heart valves, and recently placed vascular stents.
Two types of CT technology are currently in clinical use: axial (or conventional) and spiral (or helical) CT.
Axial vs Spiral CT
Axial CT has been available for more than 20 years, although it has been improved continuously. In this technique, the patient is moved a short distance incrementally and a single image is formed, followed by another incremental move and another image acquisition, and so on.
Spiral CT uses a constant linear movement of the patient through the scanner as the x-ray tube revolves continuously 360°, forming a spiral around the patient. This allows for more rapid scanning than can be achieved with axial CT, and in general, overcomes many of the constraints of conventional CT of the liver. Faster scanning is important because it makes possible the acquisition of more images during a single period, reducing respiratory and other motion artifacts. Multiple image sets can then be obtained following infusion of a contrast agent.
Dual-Phase Contrast-Enhanced CT
Dual-phase contrast-enhanced CT is a recently developed improvement in liver imaging. In this technique, a complete set of images is obtained during the hepatic-arterial dominant phase of liver perfusion, followed by a second set obtained during the portal-venous phase. Some lesions, such as hepatocellular carcinoma (HCC) and certain metastases, have blood supplied mostly from the hepatic artery and will have higher attenuation values than the liver parenchyma during the arterial phase (Figure 1). Most metastases have a portal-venous blood supply and are hypoattenuating compared to the rest of the liver during the portal phase of perfusion.
The dual-phase technique offers advantages for both lesion detection and characterization over the single-phase studies, and many dedicated CT liver studies are now performed using this technique.[23-25] Dual-phase examinations require higher injection rates of contrast agent than are routinely used; for this reason, nonionic contrast is customarily administered to reduce patient discomfort and the incidence of adverse reactions.
Angiographically assisted CT is an invasive technique used to increase the sensitivity of CT.[15-18,26-30] In this study, a catheter is placed either in the celiac or hepatic artery (CT hepatic arteriography [CTHA]) or in the superior mesenteric or splenic artery (CT during arterial portography [CTAP]). A rapid bolus of contrast is delivered, which results in a large contrast load reaching the liver either directly from the hepatic artery (CTHA) or indirectly via the portal or splenic vein (CTAP). Although these angiographically assisted techniques are more sensitive than noninvasive CT, they are not routinely used because of their invasive nature.
Computed tomography during arterial portography has been recognized as the most effective imaging technique for the preoperative determination of hepatic tumor resectability, but this distinction was challenged recently by MRI studies using liver-specific contrast agents.[15-18] One significant drawback of CTAP, aside from its invasive nature, is its lowered specificity due to the false-positive perfusion defects seen in the liver. The perfusion defects are caused by altered hepatic vasculature and can appear identical to focal hepatic metastases. Delayed images can help reduce the number of false-positive focal lesions, but CT angiography remains a sensitive, but less specific, technique.